Keeping It Original

twinpack

Member
Joined
Feb 28, 2011
Messages
9
During my restoration I will get to the engine rebuild. I would like to keep things as close to factory as possible. The L79 had 11:1 CR. I am wondering based on all I have read, can I keep this 11:1 CR and run on 93 oct.? It is still not clear by the articles and comments I have read. Some say yes and some say no. I am confused. This same frame of mind seems to be the same with a couple of engine builder I have talked with. I have been told to install Aluminum heads and I should be able to keep the 11:1 CR. Can I keep my iron heads and address cooling? Please give me your input.
 
I'm far from an expert here (I'm sure CNC will be along shortly) but I think 11:1 with iron heads is just way too much for a street motor. Even with 93 octane and aluminum heads I think you would be right on the ragged edge of livablility. My understanding, and this is not from personal experience, is that 10:1 is really about the outside limit for iron head/93 unleaded. Hardened valve seats will be a must. Most people like it around 9.2-9.5 just to be safe, and that's about what I'm currently running on my 355. With aluminum heads, I think most people like that 10.5 range.
Is there a particular reason why you don't want to drop the CR for a street engine? If you have the $$$, I would lean more toward aluminum heads, you can better control the CR and if returing it to stock is really a big deal you can always throw the original heads under the work bench for later.
 
what's your goal ? An engine that looks original or an original engine ?

11:1 with iron heads is quiet a bit, it takes a huge cam to bleed off enough compression so that you can live with pump gas. Or you could lower CR with dished pistons....
 
You are talking about static compression. What is probably more important with a steet engine is dynamic compression ratio. There are some calculators that will give you this info (you have to input the cam specs also). Quench is important in a high compression street engine also.
 
My goal is to keep the car correct as it came from the factory. It is a L79 motor. I really did not want to deviate from the stock specs. I also want to drive the car. This is why I am asking for advise. I could keep the short block stock with the 11:1 and use aluminum heads. Some say it would run fine and that the rating method of todays fuel is different as it was yesterday. But the octane is the same. Whats your thoughs about the octane ratings?:smash:
 
You are going to need a massive cam to bleed of CR, this will not help low end power and the engine is probably not going to be very street friendly... the least "visible" modification is going to be pistons....

Any details on the engine like cam specs and chamber cc's ??

No idea about the octane ratings but Sunoco sells some 104 octane race gas that should work for you - think it's like $8 a gallon :twitch::eek:

Does the engine run as it is now or is it already disassembled? What size head gasket do you have ?? There are a few thick gaskets out there that might get you to where you can use 93 oct gas.....
 
You are going to need a massive cam to bleed of CR, this will not help low end power and the engine is probably not going to be very street friendly... the least "visible" modification is going to be pistons....

Any details on the engine like cam specs and chamber cc's ??

No idea about the octane ratings but Sunoco sells some 104 octane race gas that should work for you - think it's like $8 a gallon :twitch::eek:

Does the engine run as it is now or is it already disassembled? What size head gasket do you have ?? There are a few thick gaskets out there that might get you to where you can use 93 oct gas.....

Thanks for the input. I will round up spec's and post. I do know that the L79 327/350 HP came with the L79 GM speced cam, 202 CI heads, I think (I will varify) they were 64cc chambers, 11:1 pistons and this set up ran on pump gas. Now is todays fuel the same as yesterdays. That is the question.
 
You are going to need a massive cam to bleed of CR, this will not help low end power and the engine is probably not going to be very street friendly... the least "visible" modification is going to be pistons....

Any details on the engine like cam specs and chamber cc's ??

No idea about the octane ratings but Sunoco sells some 104 octane race gas that should work for you - think it's like $8 a gallon :twitch::eek:

Does the engine run as it is now or is it already disassembled? What size head gasket do you have ?? There are a few thick gaskets out there that might get you to where you can use 93 oct gas.....

Thanks for the input. I will round up spec's and post. I do know that the L79 327/350 HP came with the L79 GM speced cam, 202 CI heads, I think (I will varify) they were 64cc chambers, 11:1 pistons and this set up ran on pump gas. Now is todays fuel the same as yesterdays. That is the question.

That 11:01 is just and estimated compression in a perfect world, If you actually measure the deck height to see how the piston is down in the bore and actullay pour the combustion chamber and figure out the dome volume of the piston I bet you will see that you are under 11:01 compression

If you can keep your dynamic compression below 8.5 you should be fine on 93 octane.

Some times putting seats in those old heads can be a problem as we ran into this last week as we hit water with a .200 thick seat which made the head junk!!!

No matter what you buy for heads they may advertise a 64 CC chamber but it has to be checked to be sure we see the AFR's that are suppose to be 65CC which when poured they are closed to 67CC's

An aluminum head would be the way to go.

We are building a 350 original Corvette engine and the customer wants Dart 180 Pro1's and is going to a hyd roller cam as he down not wnat any issues!

I think we will zero deck the block and save the vin numbers and use a set of the mahle flat top pistons and he wants to go with 6 inch rods as well.

Should be a good little build when done.

Another thought you could set that engine aside and build a 383 or 400 CID engine. This has been done many times!!

Good luck which ever way you go.
 
You are going to need a massive cam to bleed of CR, this will not help low end power and the engine is probably not going to be very street friendly... the least "visible" modification is going to be pistons....

Any details on the engine like cam specs and chamber cc's ??

No idea about the octane ratings but Sunoco sells some 104 octane race gas that should work for you - think it's like $8 a gallon :twitch::eek:

Does the engine run as it is now or is it already disassembled? What size head gasket do you have ?? There are a few thick gaskets out there that might get you to where you can use 93 oct gas.....

Thanks for the input. I will round up spec's and post. I do know that the L79 327/350 HP came with the L79 GM speced cam, 202 CI heads, I think (I will varify) they were 64cc chambers, 11:1 pistons and this set up ran on pump gas. Now is todays fuel the same as yesterdays. That is the question.

That 11:01 is just and estimated compression in a perfect world, If you actually measure the deck height to see how the piston is down in the bore and actullay pour the combustion chamber and figure out the dome volume of the piston I bet you will see that you are under 11:01 compression

If you can keep your dynamic compression below 8.5 you should be fine on 93 octane.

Some times putting seats in those old heads can be a problem as we ran into this last week as we hit water with a .200 thick seat which made the head junk!!!

No matter what you buy for heads they may advertise a 64 CC chamber but it has to be checked to be sure we see the AFR's that are suppose to be 65CC which when poured they are closed to 67CC's

An aluminum head would be the way to go.

We are building a 350 original Corvette engine and the customer wants Dart 180 Pro1's and is going to a hyd roller cam as he down not wnat any issues!

I think we will zero deck the block and save the vin numbers and use a set of the mahle flat top pistons and he wants to go with 6 inch rods as well.

Should be a good little build when done.

Another thought you could set that engine aside and build a 383 or 400 CID engine. This has been done many times!!

Good luck which ever way you go.

correct advice imo.
 
CNC, Thanks for that insight. I feel I am a bit more informed and will take that lead. Good info.
 
Your L79 engine had domed pistons with a raised dome of ~.125". When you rebuild, just use flat top pistons to lower the CR a bit and use a cam with the same specs as the L79 ("nostalgia" cam or aftermarket blue print of the factory cam).

If you are really worried about knock at 10:1, then there are dished pistons available, but IMO, I don't think they're needed.

I run 93 octane fuel in my 11.1:1 rebuilt 327 with stock cast iron 461 heads (L76) - it also helps to retard the timing slightly.
 
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