C3 small block rebuild recommendations

[J W.]

Now that the nationals are over I’m starting my winter project of rebuilding the engine in my 1972 350 4-speed corvette conv w/air. I’m keeping it all stock with its original 3973487 75cc heads, original block , original intake & carb. My goal is to achieve around 350 hp with 9.75 to 10.0 compression. I’m looking at these speed pro pistons- H618CP30 aprox compression 9.4 & H617CP30 aprox compression 10.1. Both using 75CC heads. Also would like your thoughts for a performance cam which would work to achieve my goal. I would like to add that the 487 heads will have the exhaust valve opened up to 1.6 and i will be using hi-flow SS valves. Thanks in advance for your responses.

[Patrick H.]

Re: C3 small block rebuild recommendations

I would make it a 383.

[David H.]

Re: C3 small block rebuild recommendations

Roller cam.

[Duke W.]

Re: C3 small block rebuild recommendations

I assume you have a base engine, and suggest you look at the "restoration threads" at the end of the sticky section and check out the threads on compression ratio, and the Special 300 Horsepower tech support paper and do a Web search "a tale of two camshafts" and "tuning vintage Corvette engines for maximum performance...."

Also search for threads started by me for the latest chassis dyno test of a 327-based Special 300 HP configuration to see what kind of torque and power bandwidths can be achieved.

You might be able to achieve 350 STP corrected HP on a lab dyno with head massaging, a L-82 cam, headers and open exhaust, and no front end accessories, but what makes the car go is power at the rear wheels with the engine installed in the car including the OE exhaust system, air cleaner, front end accessories, and SAE air density correction.

1972 engines carried both gross and net torque and power ratings. Ignore the gross ratings. The net rating of the base engine is 200 HP, and if we assume it's reasonably accurate, peak rear wheel HP given 85 percent driveline/tire efficiency is about 170 with SAE air density correction. You can get this up into the 220-230 range with a Special 300 HP configuration, whatever the stroke. The L-82 camshaft without massaged heads will be about the same and over 250 with massaged heads, but neither will have the stump-pulling low end torque of the Special 300 HP config. If you want a "performance camshaft" the L-82 cam is the ONLY one I recommend.

The Special 300 HP config. will idle just like a base engine and retain OE visual appearance. The L-82 cam will idle like... a L-82, but appear as a base engine unless you want to dress it up to look more like a L-82.

If you have the budget I recommend a 3.75" stoke crankshaft for large journal blocks. Peak power will be about the same as the OE crank (Taylor's Rule), but the increased average torque due to the larger displacement across the low to near top of the rev range will noticeably increase power everywhere in the normal driving range, say up to about 4000 revs.

The internet is full of stories about engine rebuild projects that went off the rails. Most can be blamed on inadequate planning by the owner and lack of properly managing the machine shop. Based on my experience helping guys with engine rebuild projects, the best results are with those who can do their own disassembly and assembly and closely manage the machine shop to do ONLY the machining operations they specify.

I you just turn over the engine to some "engine builder" with some vague instructions of what you want, like "350 horsepower" there's a real good chance that you will end up with an expensive disaster on your hands.

Having a complete list of replacement parts brand/number that MIGHT, but not necessarily be needed and sources is important because the parts landscape has changed a lot in the last few years with companies like Federal Mogul phasing out their OE replacement engine parts lines.

The more comprehensive your planning and management of the machine shop, the better end result you can expect.

Duke

[J W.]

Re: C3 small block rebuild recommendations

I realize that i may have realistically set my H.P. Goal too high and the stock 76cc heads are no where optimal.I would like to know the pros and cons of using a domed piston liked the ones i mentioned in my original post and what to expect with the 487 76CC heads. In the end i may have to sacrifice originality and go with a 64CC head.

[Tim G.]

Re: C3 small block rebuild recommendations

Unless I missed it, you didn't mention the original engine. If so, please don't let them deck the pad (even slightly). Twice in recent years I've seen engines that have been ruined by builders getting aggressive with decking. One case the engine builder apologized, big deal after the damage is done. In the other case, the engine builder had the numbers replaced due to their error. They neglected to mention the error to the car owner.

Get it in writing that they will not disturb the original stamps or pad on your engine.

[J W.]

Re: C3 small block rebuild recommendations

It is an original engine and drive train. I'm very well aware of machine shops decking a block and destroying a lot of value in an original car. this engine has already been bored .30 and is ready for assembly and deck is in tack. Just trying to achieve some more compression without sacrificing the original heads. Although small block 72"s are not that valuable, it is an original Targa blue convertible ( white top) with 4-speed, air and power steering. Trying to make a little more fun to drive.

[Mark E.]

Re: C3 small block rebuild recommendations

Unless I missed it, you didn't mention the original engine. If so, please don't let them deck the pad (even slightly). Twice in recent years I've seen engines that have been ruined by builders getting aggressive with decking. One case the engine builder apologized, big deal after the damage is done. In the other case, the engine builder had the numbers replaced due to their error. They neglected to mention the error to the car owner.

Get it in writing that they will not disturb the original stamps or pad on your engine.

Right. And the operator who decks the block probably isn't the person you instructed, so this happens.

I saw somewhere (maybe here) a jig designed to bolt to the block and cover the pad. I don’t know how commonly used it is or its availability but it looked like it added protection and peace of mind.

UPDATE: Found this CF thread with a couple of good points: avoid decking unless really needed, or find a shop with a CNC machine programed to avoid the pad.

https://www.corvetteforum.com/forums/c3-tech-performance/1956828-decking-the-block-and-retaining-the-vin-number.html

[Duke W.]

Re: C3 small block rebuild recommendations

Now you tell us! In order to determine an acceptable compression ratio we have to know your highest readily available octane fuel, altitude, and maximum anticipated temperature. Some basic valve timing specs for the camshaft are also necessary.

If you read the documents I suggested you should understand that deck clearance should be measured on all eight prior to block disassembly, and I have to assume that was not done.

If the block is bare a machine shop should be able to measure deck height and knowing the piston compression height allows the deck clearance to be calculated, which you input into the CR calculator.

Once you have the above then you can try various piston and head gasket combinations in the CR calculator. It's somewhat of a trial and error process at first, and once you've selected a piston you fine tune the CR with proper head gasket thickness once you have actual, measured deck clearances of the assembled block.

There's no reason to replace the heads. A domed piston should get the engine into the target CR range once you have a handle on what piston dome/head gasket thickness you need. You should also look at Keith Black hypereutectic pistons.

You're already behind the power curve, but it's not too late to recover and avoid the Saber Dance and subsequent crash.

Duke