Thread: 70 429CJ Buildup Questions

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Originally Posted by DwayneD

Paul,

I'm probably going to stick with the CJ heads as this Cobra will end up in somebody elses garage at some point down the road. I think the CJ heads on it will make it more valuable and hopefully, easier to sell. The new owner can always swap the cam out if he prefers a high revver. I'll probably do things a little differently when I build the one that will ultimately stay in my garage.
I'll probably stick with the Offy intake on this one too in order to try to save a few bucks on the build.
I'm about to pull the trigger on the pistons. I'm leaning towards the Probes even though they're about $60 more for the set. I know the Speed-Pros have been used for years and are probably very good pistons but the Probes are lighter and appear to be a newer design then the L2404. Just curious which way you would roll or maybe you have thoughts on a totally different set of pistons?

Thanks,

Dwayne

Dwayne,

I have used countless Speed-Pro/TRW pistons over the years and have been pleased with them. However, since Federal Mogul has moved the Speed-Pro piston line to India for their manufacture I feel that the piston quality has gone downhill big time...and I haven't used them since.

On the other hand, Probe makes a more up-to-date design piston and has improved their pistons over the last couple of years. Compared to the Speed-Pros (L2404), the Probes (12331) will have a weight match that is closer within the set of 8, they will be a lighter piston overall, they are made of 2618 aluminum while the L2404s are made of 4032, they come with a lighter piston pin, they have a 1/16-inch ring pack while the L2404 uses a 5/64-inch ring pack, they have pin clip grooves and clips are included while the L2404s have neither, they put the piston closer to the factory block's deck height than the L2404s, etc. Even if you opt for the budget factory replacement Probe P2404 (4032 alloy & 5/64-inch rings) you will receive a far, far superior piston than the antiquated and offshore manufactured L2404. Now that you know this, I hope that you can easily see that the additional $60 is a drop in the bucket for what you get in return with the Probe pistons. (Usually when I see people trying to save money on cheapo parts, I warn them that they will ultimately end up spending a lot more money that way than had they used the most appropriate/better quality part in the first place.)

I have further thoughts on piston choices: Unfortunately with your combo, you are going to be around 10:1 compression ratio with a P2404, more with the other two pistons noted above (~10.2:1) I think this is pushing the limits for using pump gas without experiencing detonation, specfically with the round-dished pistons that minimize the quench pad's effectiveness. Although there will be a small amount of mixture-motion initiated by the non-dish perimeter of the piston crows, I don't know that it will be adequate at 10+:1 compression ratio. And in this particular case, leaving the piston in the hole, or using a thicker head gasket, in an effort to reduce compression ratio will increase the possiblity of detonation, not reduce it. The best way to run an iron-headed 10:1 engine and maintain the quench pad's effectiveness would be to use a piston with a "D"-shaped dish. This type of piston dish 1) maintains full use of the quench pad, and, 2) minimizes unnecessary flame propagation accross the bore, both of which reduce detonation and also improve torque. Unfotunately, you will not find any brand new psitons with this feature at the price range you are looking at unless you go to a swap meet or find a deal on Ebay. Under these circumstances, I might just suggest the Probe P2404's. Personally I'd prefer the Diamond 40203 but you'll pay good money for these high qualtiy pistons...a path I understand you might not want to take since you'll be flipping the car. By the way, one thing you have going in your favor is that the light weight of the vehicle will reduce the loads the engine will have to endure and this might help detonation control measurably.

Locate the intake on amill and open up the throttle bores on the top of the Offy Dual Port intake manifold so that they accomodate a the Holley 850 throttle blades no problem (assuming they are not that size already). Do this even if you will use a smaller carburetor.

Do the iron CJ heads have the original adjustable valve train that utilizes the stamped steel rocker arms, pushrod guideplates, and polylock adjusters? Or will you be using aftermaraket valve train parts?

Valve train and camshaft might be next.

Paul

Paul,

Thanks for your thoughts on piston selection. The engine came with 2366 flat tops so I'm not sure how the prior owner ran it on pump gas all these years.
The valve train does have the stamped rockers, poly lock adjusters and guide plates and I was hoping to use the stock components with the new cam. Thanks again for your time Paul.

Dwayne

Originally Posted by DwayneD

Paul,

Thanks for your thoughts on piston selection. The engine came with 2366 flat tops so I'm not sure how the prior owner ran it on pump gas all these years.
The valve train does have the stamped rockers, poly lock adjusters and guide plates and I was hoping to use the stock components with the new cam. Thanks again for your time Paul.

Dwayne

Dwayne,

When the 429 CJ came out in 1970, pump gas premium was 98 octane and pump gas regular had an octane rating greater than most of the premium fuels available today at the local filling sations. Further, the OEM 385 Series distributors have next to no mechanical advance whasoever. Install an aftermarket distributor and that bone stock engine might detonate like crazy on today's pump gas, depending on the fuel's octane rating and overall quality.

Within your own observation you already noted the primary reason for ther engine's ability to run on pump gas all these years: it had 2366 flat top pistons. With the little ol' 3.59" stroke, the uncut heads, the undecked block, the flat top pistons (maintaining maximum quench pad effectiveness), the real-world compression ratio was actually around 10.3:1 with those mixture-motion-inducing flat top pistons. Add the lack of advance curve from the OEM distributor and suddenly that engine will have an easier time resisting detonation than any round dish-topped piston with an aftermarket distributor of any kind.

You will be increasing the stroke from 3.59 to 3.85, presumably milling the heads during their reconditioning, decking the block, etc, which will increase compression ratio to a level that is far greater than the 429 engine (assuming the same flat top pistons), and the method with which you want to bring compression back to a reasonable level is by using dished pistons....and there-in lies the issue with detonation. This is why I suggested the "D"-shaped dished pistons. If you are unfamiliar with the differences between the two dish designs and what I mean by "quench," then please let me know. And again, at this point I would recommend the P2404's as they are a piston that is superior in design to the L2404s and will offer a better match to the engine block's OEM deck height, which will allow you to go down to absolute zero-deck and not risk running into cylinder head/intake manifold fitment issues as a result. Detonation will be reasonably under control.

Paul

Paul,

The P2404s are ordered. You are correct, the machine work has been done on the heads and they did need to be shaved slightly and hardened seats installed. I have no idea what effect different shaped pistons have regarding the combustion process so any info you could provide would be greatly appreciated. I'm also not adverse to picking up a book and learning about it that way either if you could recommend one.
Golan's Racing has the block and will be machining it this upcoming week so the pistons should be here in time for them to finish hone it to spec. They're not going to deck it yet per your suggestion to mock build the short block first. Thanks again for the info.

Dwayne