Alternative to the CompCams 252H Camshaft Grind for a Jeep 4.0L or 4.6L Stroker Inline Six

[Moses Ludel]

The owner of a 1998 Jeep Wrangler had several questions about the use of a CompCams 252H grind camshaft in a Jeep 4.6L stroker inline six engine build.  His engine core is a 1998 Jeep TJ Wrangler 4.0L...Here is our exchange.  My comments are in red: 

Keith M.:  I’ve seen some posts, including on Comp Cams’ site, that say the head on the ’98 has different size valve stems than other years and that cams that will work on other years won’t work on this one. I’m pretty confused by what seems to be conflicting and unreliable information. 

Moses:  I’m not clear why there is so much confusion.  CompCams should know parts interchangeability and sizing.  4.0L valve stems are available in both standard size and oversize for a given engine, which may account for the confusion.  Parts interchangeability spans many years.  Exhaust or intake valve head diameters may change while stem diameters remain common.  Federal-Mogul is a well-known reman engine industry parts supplier.  We’ll use F-M as a reference source: 

https://www.fme-cat.com/overlays/part-detail.aspx?brand=SP&PartNumber=V-2527&pt=Intake%20Valve&lu=1998%20JEEP%20WRANGLER&vin=  [Intake valves] 

https://www.fme-cat.com/overlays/part-detail.aspx?brand=SP&PartNumber=V-4554&pt=Exhaust%20Valve&lu=1998%20JEEP%20WRANGLER&vin=  [Exhaust valves] 

https://www.fme-cat.com/Application.aspx?year=1998&make=JEEP&model=WRANGLER&cat=Engine&engbase=4.0L%20L6%20242cid&ga=Y&back=true  [Overview of intake and exhaust valves]

 https://www.fme-cat.com/overlays/part-detail.aspx?brand=SP&PartNumber=VK-216&pt=Valve%20Spring%20Retainer%20Keeper&lu=1998%20JEEP%20WRANGLER&vin=  [Valve retainer keepers]

 https://www.fme-cat.com/overlays/part-detail.aspx?brand=SP&PartNumber=HT-2011&pt=Valve%20Lifter&lu=1998%20JEEP%20WRANGLER&vin=  [Lifters are the same over all inline Jeep/AMC sixes]

A concern with camshaft installations would be the rocker arm ratio.  See the rocker arm interchangeability in this listing.  AMC/Jeep inline six rocker arms are essentially the same with the same ratio:

https://www.fme-cat.com/overlays/part-detail.aspx?brand=SP&PartNumber=R-905A&pt=Rocker%20Arm&lu=1998%20JEEP%20WRANGLER&vin=

Pushrods for 4.0L engines fit the full range of 4.0L years.  They are available in different lengths because the rocker arms are non-adjustable.  I have discussed this at length in the forums and magazine;  see https://forums.4wdmechanix.com/topic/1155-42l-re-build-77-cj-7-project/ and my reply comments from December 25, 2018 and forward.  Read the details on fitting the right length pushrods.  Here is the F-M parts listing for 4.0L pushrods in a standard (OEM baseline) length.  There is selective fit application coverage to compensate for engine block and cylinder head deck height changes, head gasket thickness and so forth:

https://www.fme-cat.com/overlays/part-detail.aspx?brand=SP&PartNumber=RP-3275&pt=Push%20Rod&lu=1998%20JEEP%20WRANGLER&vin=

Keith M.:  I want to use the 252 cam you recommend in your video but I haven’t been able to find a video with specific part numbers. Some of the information I’ve seen indicates that I need to change the valve springs if I go to that cam, other places I don’t see that. I need a timing set but have new lifters so I’m trying to get a package if I can but don’t want un-needed parts. I’m also unsure of which cam works with fuel injection as I have been told this makes a difference.

Moses:  Sounds like you just need the 252H camshaft if your new lifter set is compatible.  If the lifters are OEM replacement, ask CompCams tech if OEM lifters will work with the 252H camshaft.  Typically, the camshaft kit includes the cam and lifters, but if CompCam simply uses an OEM replacement type lifter, you could save some here.  The 1998 4.0L upper valve train (valves, retainers, keepers, rocker arms and such) should be readily compatible with your 252H camshaft choice.  You do need to use the correct length pushrods to attain the right lifter preload as described at the forum exchange and magazine articles.  If lifters are the same, you can see whether CompCams is willing to sell the camshaft by itself.  They may not warrant the camshaft if you don’t use their lifters…Always use engine break-in lube additive (Lucas, CompCams, etc.) with ZDDP to assure proper seating of the lifters with the camshaft lobes.  You still need a timing set from whatever source plus correct length pushrods if the OEM pushrods are not the correct length. 

 As for the PCM compatibility, there is the issue of Coil-On-Plug engines requiring a different camshaft than the 252H.  Your engine is not C-O-P, it has a distributor and earlier PCM programming.  I’ve not heard of anyone getting an engine code from a 252H camshaft installed in a pre-C-O-P engine like yours.  If I were to build my 4.0L 1999 XJ Cherokee distributor type engine into a stroker, I would use the 252H grind camshaft rather than use a stock OEM replacement.  I have used the 252H grind with EFI truck engines as far back as a Ford 300 inline six MPI engine (1987).  The 252H grind has also been tested repeatedly by Tony Hewes on pre-C-O-P EFI/MPI 4.6L stroker builds.

Keith M.:  This package would be fine, under specifications it says it works on years 1964-1998. But I can’t be sure that’s correct really, because of the fuel injection and possible valve stem issue: 

https://www.compcams.com/high-energy-206-206-hydraulic-flat-cam-sk-kit-for-amc-199-258-4-0l.html

If I had to replace valve springs I’d use this kit:

https://www.compcams.com/high-energy-206-206-hydraulic-flat-cam-k-kit-for-amc-199-258-4-0l.html

In specifications it says 1964-1998, but under Installation Notes it states ‘K-Kits will only work in 1964-88 models due to different valve stem diameters”

Moses:  Valve springs and retainer sizes are governed by the valve stem diameter.  This appears to be the reference here.  Logically, you do need to match valve springs and retainers to the valve stems and cylinder head spring seats.  For your purposes, you only need to use the right diameter 4.0L valves, matching springs and retainers for your cylinder head casting and model year choice.  Approach the valvetrain like you’re rebuilding a stock 4.0L engine.  Choose replacement parts for the cylinder head casting and block casting involved.

Keith M.:  And somewhere in here there’s a review for a kit where the reviewer states the kit won’t work specifically on the 1998 engine. That’s strange to me and seems incorrect- I have two heads, one is a 7120 of an earlier (1991-1995 I think) and then there’s the 0630 casting that came off my 1998. Both have 5/16 valve stem diameters by my measurement so I don’t believe there’s a difference. I’m assuming the whole problem with valve stem diameter is the earlier years are a different size so the later heads won’t work with the keepers and other valve spring parts that come in the kit. 

Moses:  My assumption, too.  We’re in accord here…

Keith M.:  I’m just trying to avoid getting the wrong cam and having it fitted to the bearings and then having to get another one. I don’t want to reuse the stock cam really, but I don’t know enough to say a different cam is worth it. This Jeep needs to idle and drive well on the street, I can’t have it be stumbly or rough idling as I will be selling it at some point soon. I do want to learn how to do these builds well as I restore IH Scouts- the 4.2 was an available engine that I think is much better in many ways than the IH engines and if I could find a way to build an excellent and reliable stroker with a 4.0 block and the 4.2 crank I’d do these regularly.

Moses:  Understood, Keith…I’m a Scout buff, too.  If fuel efficiency is an aim and vehicle weight not excessive, a 4.6L build from a 1991-99 (pre-C-O-P) 4.0L block and head could make sense as an alternative to the 304 or 345 I-H V-8.  I-H was wise to outsource AMC 4.2L/258 engines, they offered a high-torque design that tolerated emission controls better than competitors.

Keith M:  Thanks for your help Moses, I’ve done my best to sift through all the info out there and I just can’t come to the right conclusion without your advice...Respectfully, Keith M.

Moses:  No problem…You want to build a safe and reliable engine.  My recommendation for the 252H grind has always been simple:  This grind offers increased lift with moderate duration.  More lift without increased duration means a “bottom-end” camshaft that actually enhances the idle, tip-in response and mid-range power.  This cam is much different than the 260H grind.  Since the 1980s, I have recommended the CompCams 252H for fuel efficiency, quicker torque rise (more diesel-like), superior idle and rock crawling tip-in stability.  This camshaft raises idle vacuum and maintains higher manifold vacuum from idle to mid-range rpm.  This is simply a trailer pulling, rock crawling, high manifold vacuum camshaft for optimal power at low speeds, midrange and to a realistic 4500-5000 rpm maximum shift point.  It will make power to 5,500 rpm in a pinch. 

In your 1998-based pre-C-O-P PCM engine, you should experience no problems.  The lift is not extreme and will not create valve spring “coil bind” with stock ratio rocker arms.  (Valve springs must be new or in good condition and provide the proper spring rates at specified valve spring heights.)  With a stroker crankshaft, the 252H makes even more “stump-pulling” sense.  The gearing of your Jeep should target a 4500-5000 rpm maximum engine speed.  Your single rail EFI/MPI, the 1998 PCM, MAP sensor and camshaft sensor will find this camshaft compatible.  The 302 Ford V-8 injectors described in my articles will make sense. 

C-O-P engine builders should consider the newer grind from CompCams to avoid engine check light issues.  The C-O-P PCM and camshaft position sensor monitors the OEM camshaft valve opening/closing events (lobe valve timing).  The CompCams 252H valve opening/closing events can trigger an engine check light on a C-O-P engine with its PCM programming. 

A roller chain (Cloyes or similar) timing set is always an improvement, though the 252H camshaft will work with a stock/OEM replacement set as well.  Your focus should be selecting the correct length pushrods and setting the valve timing to factory marks.  This is optimal valve timing for the performance gains I have described…Make sure you install the distributor correctly, which will properly index the camshaft position sensor and ignition rotor in the process. 

There is nothing exotic about the 252H grind.  I have installed this camshaft as an OEM replacement.  The valve/lobe timing creates an issue with the C-O-P engines because the later PCM is looking for specific valve opening and closing events in relationship to the crankshaft. 

Let us know how your 4.6L build turns out and your impressions of the 252H camshaft performance... 

Regards,

Moses

[Tawanda]

Moses, 

Love the information you provide. I was reading this article that refers to the 252h Grind cam. The only part number I can find for this is 68-200-4. I can find the product at many online parts stores but can not find it on Comp Cams website. Is this the correct cam? I also notices it says not intended for fuel injection ( I am thinking about running a TBI system like Holly sniper. It also states its a good come for compression ratios between 8:1 & 9:1. I am not sure what my compression will be yet as I just dropped my stuff off to the machine shop. Please let me know your thoughts on this. I will provide my build list below 

1989 - 4.2 crank , 1992 -7120 head, 1998 4.0 block( bore .030), 1998 4.0 rods, IC944 pistions, TBI such as Holly sniper, Howell etc.

Thanks,

 

Kevin

[Moses Ludel]

Tawanda...The 252H grind would be optimal for your build.  Compression around 8.5:1 to 8.7:1 maximum would allow the use of lower octane fuel.  However, I looked through the current CompCams profiles, and the tradition 252H grind no longer exists.  Instead, the popularity of Jeep inline sixes has driven the design of several niche camshaft profiles.

The change in camshaft profiles has largely been influenced by later EFI/MPI 4.0L engines.  From 1999-up, the coil-on-plug (C-O-P) engines with revised PCMs have met emissions demands by focusing more closely on valve overlap and the camshaft-to-crankshaft timing.   As I have noted in several articles, the late engines will throw a code for camshaft timing issues when an aftermarket camshaft has valve overlap, valve timing or manifold vacuum signals outside the factory PIDs.  (In simpler terms, PIDs are the PCM's programmed parameters before a check light code occurs.)  In response to these issues, CompCams now lists camshafts that either will or will not work with EFI (factory EFI/MPI).  Some camshaft profiles can actually perform well but will throw a nuisance engine check code.

Your engine is not a 1999 or 2000 to 2006 C-O-P 4.0L with the late PCM.  Your mentioned aftermarket EFI choices do not depend on Mopar/OEM PIDs or the late factory PCM.  For that reason, you have more latitude on the camshaft selection.  I am guessing that you want a good street and trail camshaft that maximizes low end performance, idle stability (i.e. on rock crawls) and such.  Here, you need conservative/milder valve overlap and good valve lift for torque.  As a rule of thumb, you want maximum manifold vacuum at low engine speeds, especially at altitude.  Milder duration and valve overlap with stronger valve lift is the solution here...If emission legality is an issue, the Howell TBI system has a California (50-State) E.O. number and would work well with each of the camshafts I share below:

https://www.compcams.com/xtreme-4x4-206-212-hydraulic-flat-cam-for-amc-199-258-4-0l.html  "Xtreme 4x4™ 206/212 Hydraulic Flat Tappet Cam for AMC 199-258/4.0L. Excellent torque and throttle response. Great stock cam upgrade in fuel injected applications."

https://www.compcams.com/xtreme-4x4-197-201-hydraulic-cam-for-jeep-40l-1964-98-cpg.html "Extreme 4x4™ 197/201 Hydraulic Cam for Jeep 4.0L 1964-98 (Nitrided). For Rock Crawler applications. Excellent torque and throttle response. Works with stock heads and EFI."

https://www.compcams.com/xtreme-4x4-206-214-hydraulic-flat-cam-for-amc-199-258-4-0l.html  "Xtreme 4x4™ 206/214 Hydraulic Flat Tappet Cam for AMC 199-258/4.0L is a great stock replacement cam. Excellent torque and throttle response. (Not for EFI)"*

*Though "Not for EFI", this camshaft would likely still work with the Howell TBI system and other aftermarket EFI that you mention.  It would provide peppier street/highway performance but not be as dynamic at idle, low speed and light throttle.  For passing emissions, the safer bets would be either of the other two camshafts.  For four-wheeling at high altitudes, the first two camshafts would also be my pick.

The tech support at CompCams could elaborate on my camshaft choices.  My motive for selecting each is based on its relatively mild duration and overlap plus stronger lift than stock.  The stroker does need to breathe but not at the expense of fuel efficiency and slow crawl performance.  To meet fuel flow demands, a 4.6L build of an EFI/MPI 4.0L engine may need the 302 Ford H.O. V-8 injectors that I discuss.  However, each of the camshafts I listed here peak out by 5,000 rpm, way more than enough rpm for a Jeep trail 4x4.  A sensible 4500-5000 rpm ceiling does not demand an excessive amount of fuel.  Since you're considering aftermarket TBI, there should be plenty of fuel for each of these camshafts and the stroker crankshaft.

Moses

[Tawanda]

Great info. I actually came across those articles about the ECUCOP information after I responded the first time. Looks like I can still find the 252H grind at some speed shops even though Comp has discontinued. Would you still prefer the 252H grind over the other current ones listed? If not it seem like the middle one would be the closest match and still be able to use stock valve springs etc. Is it Ideal to keep the compression in the 8:5 range or do you suggest higher compression? Would these cams work with higher compression? 

Side note to this. Is there a big difference between using a 12 weight crank and a 4 weight crank? 

Thanks,

Kevin

[Moses Ludel]

Tawanda...If your intended use is trail crawling and low to mid-range power, the middle camshaft choice would be good and not excessive.  Fuel efficiency would be reasonable if that's a consideration. 

My experience with the 252H dates to the late eighties and the 4.2L AMC/Jeep carbureted six era.  In the late nineties, I expanded its use to a 1987 Ford 300 cubic inch MPI inline six build with great results, followed by using this grind in pre-C-O-P emissions MPI Jeep 4.0Ls, including strokers.  Tony Hewes and I were fully supportive of this camshaft, and like me, he was a staunch CompCams user.  His camshaft choices met needs from 4x4 rock crawlers to Winston Cup cars. 

Times change.  I am certain that CompCams has not compromised on these late camshaft designs.  In fact, they are niche profiles rather than generic "RV cams" as we once called the 252H and 260H camshafts.  As a footnote to the 260H, that profile proved okay for highway use but was too much for crawling for long stretches at an idle or low speed lugging. 

An engine is a vacuum pump:  It's always about maximizing manifold vacuum although the trade-off is what rpm do you need to maximize the vacuum?  Unless off-road racing, sand dragging or in hill climbing competition, I want maximum vacuum from an idle to 3,000 rpm.  Late engines with variable valve timing (VVT) can have it both ways by altering valve timing and manifold vacuum for various engine speeds.  We don't have that luxury with fixed timing sprockets and a chain.

As for compression, often overlooked is the stroker crankshaft.  Typical replacement pistons for a 4.0L are 8.7:1 compression.  Add the additional piston travel/stroke, and there's more A/F volume being crammed into the combustion chamber.  This is a compression boost.  Calculate your compression ratio based on the combustion chamber volume and head gasket swept area (head gasket thickness) plus the new bore/stroke volume.  Your head/combustion chamber volume will be based on the 4.0L casting.  All this considered, stock replacement 8.5:1 pistons might be the right pick if the final math works out to 8.7:1 or so.  Make sure you have the right block deck height for the pistons.  After block and head machining, be certain to use the $20 CompCams lifter preload gauge when selecting your new pushrod lengths. 

The 4.0L does not use a knock sensor, and unless your aftermarket EFI has one, you may be fighting ping or detonation on lower octane fuel.  Many are unaware that the Mopar Performance EFI Conversion Kit for the 4.2L engine recommends 92-octane fuel.  There is minor base timing/curve adjustment available in the PCM if you have a DRBIII or similar high end (i.e. $$$$) dealership equivalent scan tool.  Some owners retard the EFI conversion kit timing curve by offsetting the crankshaft position sensor/pickup to alter the #1 cylinder TDC reading.

Despite lower compression ratios, the 4.2L Jeep inline sixes are notorious for ping when running low octane fuel with stricter Air/Fuel ratios (stoichiometric 14.7:1) and an OEM Mopar EFI/MPI system.  That's also why some 4.2L builders often modify a 4.0L cylinder head and fit it to the 4.2L block.  The 4.0L head is more resistant to knock and a lean A/F ratio.  You have such a cylinder head.

My point is that unless you want to step up for 91-92 octane pump gas or retard the spark timing, you will be much happier with 8.7:1 maximum net compression after taking the combustion chamber volume, head gasket thickness, bore size and piston stroke length into consideration.  Plan your piston choice and compression ratio accordingly.  A 4.5L/4.6L stroker inline six will produce excellent "stump pulling" torque at 8.5 to 8.7:1 compression.  To your question, each of the CompCams camshafts noted would work with higher compression ratios—at a price.  Low cost, high octane pump gas will not be available any time soon.

The 12-weight crankshaft is the "holy grail" for ruggedness if balanced properly.  Approximately 20 pounds heavier, it is also the crankshaft less prone to engine stalling when rock crawling.  (A heavier flywheel is equally helpful here, the reason I always ran 168 tooth GM flywheels on my small-block V-8 trail engines.)  12 weights are less important for an engine driven at lower speeds with a moderate compression ratio.  A common rabbit hole for 4.6L builders is the lure of high compression, high horsepower figures.  Some may be stirred by that kind of performance.  I want rock crawling, stump pulling, usable torque and engine stability (i.e. high manifold vacuum) at low to mid-range engine speeds.  For 4x4 trail running in low range or trailer pulling, horsepower can be irrelevant.

There is nothing "flawed" about 4 weight crankshafts, they hold up well in a seven-main bearing engine.  Again, if you want a build for optimal trail use (best torque, strong vacuum, good fuel efficiency), something other than a high horsepower hill climb or mud bogging competition engine, the four weight crankshaft, balanced with the rest of the reciprocally rotating parts, would work. 

Moses

[Tawanda]

Well said. I did forget to mention this will be a fun vehicle and not a DD, at least for now that is the intention. Fun cruiser, Sand driving, Mild off roading/ camping, probably not real serious of rock crawling and may take it on a trip or 2. Not sure if we will drive it or tow it but would like to be able to take her on the highway if needed. I should have mentioned this before but I do typically go for the lower end torque and towing with my vehicles.

[Tawanda]

FYI - it's also going in a restoration project 1980 CJ7 Renegade Manual T176 4 speed. 

[Moses Ludel]

Tawanda...We're clearly aligned on our concept of a purpose built 4x4 vehicle.  You'll have a great time in the outdoors with your classic Jeep 4x4 designed for utility, recreation and sheer pleasure!  

The T176 will do well with lower rpm power and torque.  Although not the stamina you might get from an SM420, SM465, NP435, an NV4500 or even a T18, the T176 is nonetheless up to the task if in good condition and not pressed to its limits.  Synchromesh on all forward speeds makes the T176 desirable off-pavement.

On that note, without trying to spend your hard-earned cash, each of the five transmissions I mention would be dramatic gains.  The SM420, SM465, NP435 and T18 (Ford truck version) each offer a compound first gear that dramatically lowers your reduction ratio, the quick way to overcome taller axle gearing and oversized tire effects.  The iron NV4500 3/4-ton and larger truck transmission adds synchromesh on the compound 1st gear plus an overdrive gear.

If not now, perhaps down the road or trail, you might find one of these heavy duty transmission options useful.  Be kind to the T176, it will give back.  Many T176 Tremec transmissions have survived behind V-8 conversions.

Moses

[Tawanda]

Moses, 

Thanks for some clarity on the transmissions. After reviewing your notes I think I would like my build to be around a net compression of 8:7 so I can run 87 octane.

This will be a fun vehicle that will primarily be driven on the street with some light off-roading and sand driving so I'm not sure I need a monster crawler. I would like to use stock springs and looking at the cam selection it looks like I would need to upgrade springs except for the 

https://www.compcams.com/xtreme-4x4-197-201-hydraulic-cam-for-jeep-40l-1964-98-cpg.html

That cam seems like it might be really low end RPM torque. Not sure if I need something more all around. What do you think about the following cam numbers below and how they compare/differ? All these cam numbers scramble my brain -

Schneider Cams - 256-62H   Intake- 204   Exhaust 208   lift/ intake .448 lift/ex .448 LSA 112

Clegg Stage 1 -  intake 209 exhaust 213  lift/intake .443 lift/ex .443  LSA114

 

Thanks,

 

Kevin

[Moses Ludel]

I like the Schneider profile for milder duration, good lift and the narrower lobe centers.  That would work with either carburetion or aftermarket EFI as you have planned...What does Schneider share about that camshaft's intended use?

Each of the camshafts we have reviewed offer good street/highway performance.  Despite its rugged design, your 4.6L will seldom if ever see 4,000-4,500 rpm, especially if you gear it properly for the tire size.  (Sand will be the highest rpm use.)  Geared properly, your fuel efficiency should also be good.

[Tawanda]

I have not talked to Schneider -  it states a range of 1,500- 4,500 rpm.  What does the milder duration do? Keep the rpm range lower? Narrow lobe center - is that the lower LSA?

They have 2 others as well.

1,250-4,000rpm

http://schneidercams.com/256HamcI6hydraulic.aspx

1,000-3,500 rpm

http://schneidercams.com/25256HamcI6hydraulic.aspx

or do you prefer the comp cam you listed prior?

Thanks

[Moses Ludel]

Milder duration with stronger lift is a recipe for high manifold vacuum at an idle and low throttle openings plus better bottom end torque.  I shared earlier that the main mission for trail crawling is to have the highest vacuum at idle, tip-in and low throttle positions.  A longer duration camshaft will produce more horsepower at mid- and high rpm range while sacrificing idle and crawl speed manifold vacuum, torque and stability.

LSA is lobe separation angle or, simply put for our purposes, valve overlap.  Here is a quick reference to LSA:

https://help.summitracing.com/app/answers/detail/a_id/4702/~/camshaft-lobe-separation

Again, we're "stuck" with fixed valve timing, duration and valve lift on these AMC/Jeep inline six engines.  The Schneider 1,000-3,500 rpm camshaft would likely be great on the Rubicon Trail.  Control over tire-spin, idle without stalling, immediate throttle response as you tip-in the throttle. 

This camshaft is not for mud bog racing where you want higher horsepower in the mid-range and at high rpm.  The shorter duration camshafts with lift improvement make stronger bottom end torque.  Their horsepower peak and redline rpm will not seem impressive.  When horsepower is the only objective, even with a diesel engine, high horsepower will come at a higher rpm ceiling.

I happen to be a CompCams user and have known the company's R&D strategies since the eighties.  I would likely try CompCams 197/201 rock crawl camshaft.  Schneider's 256H camshaft would be an option.

[Tawanda]

Perfect. Thanks. Lastly when running a TBI on a stroker build. Stock 258 intake or something like clifford intake ?

[Moses Ludel]

Good question, Tawanda...In the late eighties, I covered Jack Clifford's shop at Southern California for OFF-ROAD Magazine.  Jack and I had many discussions about inline six-cylinder engines.  We each appreciated their virtues. 

Jack was among the earliest proponents of EFI conversions on inline sixes.  The first systems Jack used were based on Ford's CPI, introduced on early eighties V-8 engines.  CPI was Ford's response to GM TBI.  An aftermarket company was making TBI conversions for Class A V-8 motorhome engines, replacing the large four-barrel carburetors.  Jack adopted this system to a Jeep 258/4.2L inline six.  This predates Mopar's first HESCO/Electromotive EFI conversion for the Jeep 258, which used an OEM Holley TBI unit from the Mopar V-8s.

Straight eights like Buick's OHV 1941-42 engines optioned two carburetors.  Hudson offered dual carburetors on the straight L-head six 308 Hornet postwar/fifties engines.  Jack successfully raced Hudson drag cars with this engine.  He became the go-to expert on inline sixes, including Chevrolet Stovebolts and the GMC Jimmys.

There were a variety of multiple carburetor manifolds available for inline six hot rods during the postwar forties, fifties and sixties.  Jack, however, concentrated on the challenge of even fuel mixtures to the outer cylinders of an inline six-cylinder engine with a centrally mounted, single two- or four-barrel carburetor.  Jack Clifford designed his "ram" intake manifolds to produce better, more uniform air/fuel flow to the outer cylinders (#1 and #6).  His Jeep 258/4.2L manifold took this approach.  This manifold, originally designed for carburetion, is also well suited for an EFI/TBI conversion.

If you were running Mopar MPI/EFI, each cylinder would have its own fuel injector with the fuel flow uniform and based upon the PCM signals.  The intake manifold would essentially be a plenum for the throttle body to flow air (only) to each cylinder.  You will not have that advantage with Howell or other TBI fuel injection systems.  They will have the same challenges as a carburetor mounted centrally on an intake manifold:  The Air/Fuel mixture merges at the TBI unit and flows through the intake manifold to each cylinder.  Flow velocity and volume are based upon each cylinder's draw (vacuum or negative pressure) and the intake runner lengths.  The outer cylinders have longer runs.

So, yes, there is an advantage to using a Clifford manifold.  The stock BBD Carter manifold is not bad, it's just not as efficient as the Clifford "ram" design.  At low engine speeds and flow velocity, the issue would not be as extreme.  Mid-range and higher rpm would benefit from a Clifford manifold.  Torque would likely be improved incrementally over the entire rpm range from idle to redline...Your O2 sensor will be on the exhaust system (post combustion), so improved, more uniform flow and A/F ratio match per cylinder would gain incrementally from idle to redline.

If you need to smog the Jeep, the Howell TBI system has 50-State legal E.O. status.  Clifford manifolds do not have an E.O. number.  Clifford does list a carburetor manifold for the 1991-up EFI cylinder head that would work with your head and Howell TBI.  You will need an adapter to mate the GM/Howell TBI unit to a Howell/GM TBI unit.  This is an additional aftermarket sourced part. 

A Clifford manifold could be an issue on the visual checklist for a California smog inspection.  That would be nit-picky, though, as the key issue is the EFI/TBI.  From what I understand, Howell TBI is usually an easy smog inspection.  A California E.O. decal (50-State legal) comes with the package.  These conversions always meet the tailpipe emissions threshold for a 258 in good condition.

While on the subject of manifolds, you will need to run the 4.0L "header" type exhaust manifold with your cylinder head.  (Clifford Performance makes an exhaust header—also without an E.O. number.)  A '91-up OEM Mopar or equivalent aftermarket replacement manifold would work.  This would meet your camshaft's requirements.

Moses

[Tawanda]

Good evening,

 

I spoke with Clifford today. They did not seem to like the TBI systems too much and said they wouldn't really work with there intake manifolds. The Holly 2 barrel sniper in particular. They said I would need a 4 barrel TBI and even that would not work well because the computers on them get confused. They suggested to go carburetor and in particular they really like the twin holly set up.

https://cliffordperformance.net/store/ols/products/jeep-40-twin-weber-system-w68-headers

or at least a single carb 

https://cliffordperformance.net/store/ols/products/jeep-40-twin-weber-system-w68-headers

 

Not sure I want to run the dual exhaust headers.

I guess I'm not opposed to running carb/s I was more leaning toward TBI because I thought it would be easier and the computer would make needed adjustments based on certain situations. 

[Moses Ludel]

Umm...Well, Tawanda, the Clifford manifold was traditionally a 4-barrel flange pattern.  Many adapted 2-barrel Holley carburetors to the Clifford intake manifold, using a common 2-barrel to 4-barrel adapter plate.  You need an adapter plate to mate Howell's TBI (GM TBI base pattern) to either a Clifford or Jeep 258/4.2L stock manifold.  Howell's kit does furnish an adapter to the Jeep manifold. 

There are many adapter plates for a GM TBI unit to a four-barrel carburetor pattern like the Clifford manifold.  I found this inexpensive example at Amazon:  Transdapt 4-barrel manifold to GM TBI Unit.  You would need to confirm that this plate would work with both the Howell TBI unit and the Clifford manifold.

From my vantage, the gains of TBI far outweigh the concern about less than ideal air flow to the end cylinders.  If Howell does not support the switch to a Clifford manifold, I would stay with a stock BBD manifold and, by all means, get the emission legal Howell TBI conversion.  It will not produce the horsepower of MPI (Mopar's kit) but will provide a reliable and substantial improvement over a carburetor. 

Carburetors with a float and fixed jets or fuel circuits are sensitive to off-camber (tilted) trails and altitude changes.  In the early nineties, I tested a Holley 2300 two-barrel carburetor on a 258 Jeep six.  Horiba lent me $10,000 (very expensive at the time!) worth of air/fuel ratio testing equipment.  The popular Holley Universal 300 CFM carburetor, jetted and metered for sea level, showed significant enrichment with each 1,200 feet of elevation change.  Each fuel ratio change would require re-jetting to restore a normal air/fuel ratio (14.7:1 "stoichiometric" at cruise). 

Engineer Pass in Colorado is 12,800 feet above sea level.  An EFI system (TBI or MPI) with oxygen sensor feedback can compensate for this.  A non-feedback carburetor cannot.  Even the eighties feedback carburetors could not adjust for this range of atmospheric change.  That, in part, ushered in the era of TBI and port or multi-point injection.

I respect the Clifford products, and the tuning of the dual Weber carburetors is likely well tested.  The system looks clean, complete, well conceived and performance oriented.  Predating modern EFI systems, Weber carburetors earned racing and high performance laurels in European marque cars.  I'm sure the Clifford header complements this induction system, the combination packing a performance wallop.  Add a re-curved aftermarket ignition distributor, a compression bump and performance camshaft, and this would be the ultimate inline six-cylinder AMC/Jeep performer for sand drags or hill climb competition.

However, like other aftermarket carburetors, Weber carburetors are altitude sensitive and do not have an oxygen sensor feedback system.  (The later Jeep/Carter BBD carburetor has an O2 sensor although its function is very limited and aimed mostly at meeting emission requirements—not improving performance.)  An aftermarket four-barrel or dual carburetor conversion is not 50-State legal and comes with an "off-highway use only" disclaimer.  For driving in rough backcountry, Howell's TBI kit offers better rock crawling stability than carburetion, plus the kit is 50-State legal.  It sells for half the current price of a Mopar EFI Conversion Kit.  

The Sniper and MSD Atomic systems perform well, but what about proprietary replacement parts?  Specifically, a Jeep 4x4 needs backcountry reliability, ease of service and ready parts availability at the local auto supply on a Sunday afternoon.  Many of us have made parts runs to Auburn, California on behalf of Jeep 4x4s stuck along the Rubicon Trail.  Beyond carrying a spare electric fuel pump, you can find common GM TBI parts at most local parts houses.  At least the parts would be recognizable. 

To the point, Sniper and Atomic systems are fantastic for street rods, hobby cars, restomods and drag strip performance.  You have a Jeep 4x4.  Decide what will best meet your needs.  Choose the fuel and spark system that will reliably get you home from the woods.

Moses

 

[Tawanda]

Ok I think I have this thing figured out. I actually cam across someone that had a 252h grind in stock so I think I'm going to give it a try. When you refer to keep the compression ratio at a NET 8.5:1 - 8.7:1 are you referring to the SCR or DCR ratio?

 

Thanks

[Moses Ludel]

Tawanda...I use this reference because the longer stroke will draw and compress more volume into the combustion chamber.  A stock replacement piston might be 8.7:1 compression with the stock stroke.  Lengthen the stroke, and you have more cylinder volume (longer stroke) compressing into the same combustion chamber volume.  "Net of 8.5:1-8.7:1" is doing the math for the compression ratio with a stroker crankshaft.  Your cylinder head and the 252H cam would likely tolerate up to 8.7:1 for unleaded regular fuel.

DCR would be the dynamic or rotating ratio that considers the effect of the intake valve opening events.  We don't have to get this specific, especially with stock valve timing and a 252H grind.  (Well, you can if you want.)  The idea here is to account for the added stroke length and its effect on the compression ratio, which SCR (static) will do.  For example, you might find that an 8:1 ratio piston for a stock 4.0L engine nets 8.5:1 with the stroker.  (This is hypothetical, not actual.  Do the math.)

To demystify this, there are manufacturers who now make 4.0L pistons specifically for 4.6L stroker crankshaft conversions. Some sell complete kits with a SCAT crankshaft...These piston makers specify which connecting rods (4.2L or 4.0L) will be used.  If the piston manufacturer is accounting for the correct piston deck height, pin location and combustion chamber size, the piston's compression ratio rating is likely correct right out of the box.  Before these "stroker" pistons were available, the pistons used in 4.6L builds were 4.0L stock replacement or high performance types.

Moses