GM Camel Hump Heads (492) & Chevy 355 With TRW L2252A Pistons De-Tuning Queston

Hi folks,

I'm back again with a couple of questions regarding a SBC 355 ci engine that I inherited.

First off I would like to thank all those forum members here who helped out in my other thread regarding some TRW pistons that were identified as 340 pistons with a .030 over-bore. I did end up selling those to a gentleman on a Facebook market place MOPAR forum.

Now I'm in search of some answers regarding what appears to be a 1970 GM 355 ci small Chevy motor with 4-bolt main caps. The motor has the above mentioned TRW L2252A 030 over bore dome pistons with the 492 angle plug heads.

Clearly this is a high compression engine and will not be able to run on 93 octane pump gas. I don't live in area where E85 is available or race gas.

In deciding on whether I will keep this motor or sell it depends on what information I can gather in regards to making it more street friendly on pump gas.

I don't know what the camshaft specs are, but it does have Crane F423 roller rockers with screw in studs and poly locks.

So the question is what kind of replacement pistons would be recommended to bring the compression ratio down and into a more street friendly octane rating on pump gas.

Also if needed I can post up some pictures of the engine and a bore scope image of the pistons and cylinder walls along with heads and valve train.

Kind regards,
Robert

 

Ready for a rabbit hole? Dropping the compression will mean the cam will no longer be ideal.

 

I think it would be okay with 93 octane as long as the total timing isn't over 32 degrees. I'd give it a try before changing everything.

 

should be 11.9:1 with 64cc.

 

So looks like it's at about 11.9 to 1 compression ratio now with the 64 cc heads, a set of 76 cc heads would get you down to around 10.9 to 1 and Aluminum 76 cc heads forgive another point to around 10.0 to one

Like Johnny Gee says the cam might not like the new compression ratio

 

Thanks for the replies guys.

I don't mind swapping the pistons out in order to get a more friendly compression ratio that is closer to 9 or 10. I think that even dropping the the total timing between 30 0r 32 degrees on 93 octane is a bit risky not knowing the piston deck height.

***uming that the block has milled and the pistons are at zero deck height what would be a more suitable replacement pistons. I have a box of a KB 9002HC hypereutectic 2-valve relief flat top pistons that are 0.30. that were sitting on the shelf and I also inherited.

I was checking the specs for these pistons over at Speedway motors web site and they claim the following specs below. Of course they don't mention what type of head gaskets are used and how far the pistons are in the bore so these are just some hypothetical numbers below in regards to their stated compression ratios and head cc's.

C/R 58cc=10.7, 64cc=10.4, 76cc=9.1

So basically I'm here just looking for what you guys would use or recommend for replacement pistons.

Clearly this engine was setup for the drag strip only.

 

Run a compression test If you are below 180 it should work

 

Yes, I can do that. I have the engine on a stand and I could simply pull the plugs, and hook up a compression gauge; and the spin the crank pulley with an impact gun.

So if I'm understanding you correctly then anything below 180 psi would be safe on 93 pump gas?

 

What could I expect if that cam is not happy?

It wouldn't be anything catastrophic to the engine?

 

Sounds like it was a running engine, if so using a degree wheel you may be able to determine the camshaft specs. The cam is likely fairly radical and needs the compression to offset the overlap. You might be able to sell the cam and lifters if you mark the lifter position so it can mate back with the same lobe. In todays world cam break in is a **** shoot, personally I’d buy a proven set up. You could maybe sell cam/lifters, heads pistons and purchase exactly what you want.
Dan

 

I don't know about the impact gun thing

 

I know that the block was machined for roller camshaft and lifters. The bore scope pics show some very clean piston heads with negligible carbon build up. The cylinders have very defined cross hatching and no carbon ridge build up near the top of the cylinder bores.

Unfortunately I don't have a camshaft degree wheel but there may be one in some of the tool boxes back at the garage. That would be a 2 hour drive for me. I may pull the timing chain front cover and see if there's any markings on the cam to help identify it.

 

Yes, I was only making a point on how I could spin the engine over to get the compression readings.

 

Sorry I thought you had a flat tappet cam. A roller cam is even better, my understanding is that the cam can be changed without worrying about the lifters. Someone with SBC roller cam experience would know for sure. The used cam may have value for someone looking for a cam with specs.
Dan
PS
I wouldn’t use an impact gun to roll engine over. I would use a starter motor. If you don’t have a flywheel or flex plate yet, you should be able to get a flex plate at your local junk yard cheap.

 

Doesn't de-tune go against even your username? I can't think of a time I wanted to de-tune something.

On a serious note, I would probably run it as-is or maybe swap heads or something to lower compression. If I was needing to swap cams and pistons, I might just start with a different engine.

 

I can tell you right off that there's no way those pistons with 492 heads will ever run on 93 octane with out some detonation that will eventually destroy pistons! I'm running the same heads with flat top .030" over and my compression is at 11:1 c.r. and my car wont run on straight 93 octane, even with 110% ethanol mix. I have to dump some VP Racing Octanium in at ever tank fill up to make my engine happy. Even then it's not as happy as it is when I take my Austin to the drags and run 110 octane race gas.
As mentioned the cam is going to determine if you could simply swap heads and go to an aluminum head with a larger combustion chamber to bring down compression and have the came still work. A pair of aluminum heads with larger 72cc chambers might help a lot, but probably still need a splash of VP Octanium or race fuel. The early 70's Corvette and Z28 heads were 74cc and have good flow characteristics, and might also help if you're on a budget.

 

You may not even be able swap heads. Unless they are older/used stock style chambered heads. Most of all the newer aftermarket heads, have transitioned to a fastburn style chamber. The popup/domes, on your current pistons. Will have interference issues. Some dished pistons and a cam swap, maybe a better option. If your primarily interest is a street engine.

 

Depends on the overlap, it could turn out to bleed off too much compression making it a real dog.

 

I could bolt up a fly wheel and starter. i just figured it would be quicker to spin the engine over by the crankshaft pulley bolt to get a reading off one cylinder for the compression value.

 

Lol... yes it goes against my forum name, but I'm not going to be using this engine (de-tuned) for a high performance build. If I can get it to be a pump gas friendly engine for very little investment, then I'm going to put this in my 1984 C-4 corvette that needs an engine rebuild.

 

Understood.

 

Yes, that is what I'm after and the reason for this post.

I just want to find a set of pistons and a camshaft if necessary to keep the costs of things down,

So I have the Kieth Black pistons that I mentioned above and would be happy if they would work well enough to make this engine a more street-able option instead or doing a rebuild on my 84 C4 corvette motor.

 

What would you recommend as far as swap pistons (dished) to get this engine down to a pump gas friendly state.

 

Changing pistons will require a re balance unless there the same weight.
64cc heads on domed 350 pistons is not going to work well on any pump gas.
By the time you buy new pistons, hope the cylinder clearances are correct , hone the cylinders, tear apart the bottom end .
IF You have no idea of the actual condition of the engine id pull the heads, pan and inspect the cylinders and bearings .
IF it all looks good buy a set of larger chamber aluminum heads IF they work with your pistons. The cost of the heads will only be slightly more than piston swapping , balancing, rings , likely bearings etc and the cheapest Ebay heads will out flow and perform the best older factory iron headsand allow a bit more compression you cam likely needs .

Im sitting on a similar 11.75 -1 327 with 461 heads and a big solid cam .
Race gas, E85 , methanol or mechanical changes are the options .

You will need a starter on to do a compression test.

 

You can’t detune a Chevrolet engine, they are just too powerful! It can’t be done.

 

I would want to set it aside for a later date when I want something to blow the dust off the parking lot when I ride through lol, but hey thats just me.

 

I saw a guy de-tune one once...by setting a brick on the go pedal and walking away

 

That’s just wasteful.

 

Okay gentlemen.

I'm going to try to get more specifics on the actual build of this engine as far as the camshaft and block in regards to how far the pistons are set in the bore.

Pulling the intake and heads are in order, and then I can make some better decisions on where I want to take this engine. Might be sensible to sell it to someone who could use it in its current form.

Oh, and here's some pictures of this engine.

 

I'm not a Chevy guy so I may be wrong, but those exhaust ports look like some one has really opened them up. If the intakes are ported as well, you can bet it has a really aggressive camshaft to along with the high compression. Bet that was a high rpm screaming monster.