C2- Partial engine rebuild

[Robert M.]

I have a friend with a 65 coupe with an origianl 327/365hp engine. The previous owner had taken out the high compression pistons so it would run on pump gas. The cam and the solid lifters are still there. My friend asked me if he could just pull the heads and the pan and reinstall the proper high compression pistons. Is it as simple as that or would he need to put new bearings in and rebore the block.

[Joe L.]

Re: C2- Partial engine rebuild

Rob-----

Well, there are all sorts of ways to approach this. The best way is to replace the pistons as part of a general engine rebuild. However, it's possible to do it the way you described, particularly if the engine has relatively low miles since the current pistons were installed.

After determining the size (i.e. standard or particular oversize) of the current pistons, a similarly sized set of original configuration pistons could be purchased . Assuming that the bores are in good shape with no ridge, etc., these could be installed on the existing rods and each assembly weight-matched (about the best that can be done for "balancing" given the fact that the crankshaft will be installed in the engine). New piston rings should be installed on the pistons.

As far as bearings go, it's not absolutely necessary to replace them. However, I would replace the rod bearings. These don't cost all that much and they'll be out of the engine, anyway.

The above should do it. However, I've got to ask the question WHY? By doing this, one is going to defeat the original purpose of installing the lower compression pistons in the first place. That is, to make the engine run better on available pump gas. In the process, very little will be gained. If the existing compression ratio is in the range of 9.5:1, or higher, I'd strongly recommend that he leave it as-is. By the way, unless DISHED pistons have been installed, the compression ratio is going to be around 10:1. "Flat top" pistons, the usual type used for reduced compression in small blocks with original L-76 heads, are going to yield ABOUT 10:1 compression.

[Robert M.]

Re: C2- Partial engine rebuild

Thanks Joe, my sentiments exactly. I asked him why. No one really cares if it doesn't quite have the thump of the high compression pistons. Drive it and enjoy it or constanly look for CAm II or avgas.

[Joe C.]

Re: C2- Partial engine rebuild

Rob,

The 30-30 cam has lots of overlap, and NEEDS high compression to produce any appreciable torque/horsepower. There's a lot of info on this subject....look in the archives.
I can say that the dynamic CR with the 30-30 cam will be much lower than, say, an L75 running the same piston/chamber/head gasket configuration. Best way to approach this issue, is to calculate the actual CR, after measuring chamber volume, etc etc. There's a lot more to CR than piston config.
I can also tell you, from experience, that a properly built L76 will tolerate 10.5:1 actual, measured CR on ordinary hi-test gas, without detonation. All of that crap about killing the compression to run on pump gas is just a bunch of HOOEY! That same principle does NOT apply to BBC, and maybe that is what Joe is referring to. It also does not apply to SBC with "mild" cams.
I am about ready to add MORE CR to my L76. Why, because I run about 10 degrees more advance than spec, without detonation. I have run more than this, trouble free. I'll be shooting for final static CR of 11:1, by a combination of thinner head gasket, and milling the heads, to decrease chamber size to 62 cc. At this time, my chambers were polished and unshrouded, and were measured at about 66-68 cc's.

Joe

[Duke W.]

Re: C2- Partial engine rebuild

So it's a dog! High overlap cams and low compression is the absolute worst engine configuration on the planet. (The best are low overlap cam, high compression engines like modern Corvette engines.) On the plus side it will probably run on mid grade or even regular unleaded.

Pump gas that is nearly as high in actual detonation resistance as leaded premium from the sixties has been available for over 20 years. It's called UNLEADED PREMIUM, and most OE high compression engines, especially SBs, will run without detonation on it, and any detonation can usually be excorcised with a little judicious spark advance map tuning.

Yet the myth endures that you "have to have low compression" to run on "pump gas".

In my experience "engine builders", most of whom continue to promulgate this BS, are morons.

To properly equip the engine with new pistons it needs to be disassembled, bores measured, bored, honed as required to fit the OE replacement domed pistons, all measurements taken to compute the CR, which should not exceed 10.5:1 by judicious selection of head gasket thickness, and the entire cranktrain rebalanced because the new pistons are likely different mass than what's installed.

And while you're at it, you may as well massage the heads and replace the weak, spindly OE rods.

So the bottom line is that unless your buddy wants to go through the entire engine and "do it right" just leave it alone and live with it.

Duke

[Robert M.]

Re: C2- Partial engine rebuild

Duke and Joe thanks for the insight. Would the same apply to The L78 Big Block?

[Duke W.]

Re: C2- Partial engine rebuild

Big blocks are more octane sensitive because of the larger bore, but a 396 is not as "bad" as a 427 because of the 396' smaller bore.

For a L-72 or L-71 I recommend limiting the TRUE, MEASURED/COMPUTED CR to about 10.25:1 which should be achievable on most OE machined engines with the OE replacement pistons and proper head gasket selection. Given the L-78's smaller bore I'd recommend a target maximum of 10.35.

For SHP small blocks with the LT-1 cam (for which I have a pretty good body of empirical data) the practical limit on pump premium is about 10.5:1 true, which is easily achieveable on an OE machined engine with the OE replacement domed pistons. In fact, that's about what they were from the plant after GM began double gasketing SHP/FI engines during the '62 model run. Then in 1966 the slightly larger chamber 462 heads with a new thicker shim gasket did the trick. Most production GM engines were built with about a half point less compression than advertised.

By the time your relieve chamber overhang on 461 or 462 heads, and with typical deck clearance being a few thou higher than nominal blueprint dimension, the CR is usually in the target range with the '66-up .026" thick shim gasket, and a Victor Reinz composition gasket of the same thickness is also available.

There will always be variation in measured CR on any given engine, and I like to see the range kept within 0.1 or 0.2 at the most. This is easily done with a little judicious chamber grinding to take out a fraction of a cc on the highest cylinders assuming they are pushing the limit.

Of course, all of this is time consuming and very expensive if you rely on an "engine builder" to do it, assuming you can find one who is competent, which is a BIG IF! But it's nothing that a dedicated amateur restorer can't do himself.

Duke

[John D.]

Re: C2- Partial engine rebuild

Rob, I am glad I listened to my engine builder for my 63. It has the hi-dome pistons and stock cam. Basically pure stock including the oil pump. Sounds mean for sure. Before it had the flat top pistons that were installed during the gas grunch of 1980 and the thing was weak. Didn't have the throaty sound at all. One time an old NCRS judge said-(many years ago) said that your engine just doesn't sound right. He was at the back listening to the pipes. I knew the guy was smart and acted dumb but the old boy had a good ear. All the crap about running flat tops with pump gas is just that. Set it up right and you can run the high domes on 93 octane just fine without a whole bunch of additives. Try a little Marvel Mystery oil in your gas also. John