CR on 1970 454

**Joe L.** · October 16, 2012, 09:31 PM

Re: CR on 1970 454

Originally posted by Pamela H Haus (55255)

Can anyone tell me what was the compression ratio on the 1970 454 390hp?

What's the source?
What's the part # for the piston?

THANK YOU !!!

Pamela-----

10.25:1.

**Terry M.** · October 16, 2012, 09:36 PM

Re: CR on 1970 454

I am pretty sure every one that was built has the CR on the shifter plate on the console. There is/was a place on the interweb to get that kind of data from the GM Heritage Center, but a quick search yielded me nothing for free. Someone (hint, hint) needs to do a more extensive search.

PS: Do you have a bar bet? Is that the reason you ask for the source of the data?

**Pamela H.** · October 16, 2012, 09:57 PM

Re: CR on 1970 454

No, not a bar bet

...Looking for the part # for the piston so we can order 'THOSE" pistons. thanks for the help...

**Michael F.** · October 16, 2012, 10:16 PM

Re: CR on 1970 454

just order some trw or similar and will be cheaper and just as good, ncrs does not inspect internal parts. 1o.25 is correct.

**Patrick B.** · October 16, 2012, 10:23 PM

Re: CR on 1970 454

A 70 454 with 10.25 cr used 3964290 heads with 96.4 cc combustion chambers. A 1973 454 with 8.25 cr used 353049 heads with 110 cc combustion chambers. The 1970 pistons will not bring a 1973 454 to anywhere near 10.25 cr.

**Clem Z.** · October 16, 2012, 11:00 PM

Re: CR on 1970 454

here is the 70 engine specs. http://nhra.net/tech_specs/engine/bl...ts/CHEV-70.rtf

**Joe L.** · October 16, 2012, 11:05 PM

Re: CR on 1970 454

Originally posted by Pamela H Haus (55255)

Can anyone tell me what was the compression ratio on the 1970 454 390hp?

What's the source?
What's the part # for the piston?

THANK YOU !!!

Pamela-----

I missed the part of your question about part number. The original 1970 Corvette LS-5 standard bore size pistons were GM #3976015, left side, and 3976016, right side. The 0.030" over pistons were GM #3976027, left side, and 3976028, right side. However, these were all discontinued without supercession in 1982 and 1984. I doubt that you'll find a set. As Patrick mentioned, these will not result in 1970 LS-5 compression when used with your heads.

You could use Keith Black #203 pistons. These will yield a compression ratio of about 9.8:1

**Terry M.** · October 17, 2012, 09:38 AM

Re: CR on 1970 454

Originally posted by Pamela H Haus (55255)

Can anyone tell me what was the compression ratio on the 1970 454 390hp?

What's the source?
What's the part # for the piston?

THANK YOU !!!

Sorry Pam. I had forgotten about your project when I posted earlier. I thought you had a 1970 and were looking for fuel for your Corvette or were trying to settle a bar bet. Too much water under my bridge since your last post.

**Duke W.** · October 17, 2012, 10:20 AM

Re: CR on 1970 454

Remember that OE specified compression ratios of that era were higher than the actual as-built by up to half a point.

If you run the numbers on the CR calculator using a 119 cc head chamber, .020" deck clearance, and a .039" gasket the resulting true CR with the KB 203 is about as Joe stated above, and the KB 207 piston is about 10.3.

Of course, the above are just estimates based on nominal dimensions. You need to measure your actual head chamber volumes and deck clearance to yield accurate results.

Since there is little data on how much compression the open chamber heads can tolerate on today's unleaded premium, I would recommend going on the conservative side with the KB 203 and retard the CS1093R cam four degrees.

There is another issue with that vintage big block that seriously compromises torque, power, and fuel economy, but it's an easy fix. The OE HEI system has a very short and lazy centrifugal curve, which with the OE recommended intial advance only allows about 30 total WOT advance when the engine needs about 40 to achieve maximum power.

In addition, the vacuum advance should be converted to full time and the installed VAC needs to be evaluated, and replaced, if necessary, to meet the Two-Inch Rule.

Details on the approach are in my San Diego presentation slides including how to download the AMA specs for any year Corvette that contain most of the relavent engine technical details, which allows you to track the changes from the antecedent non-exhaust emission controlled, premium fuel 1966 L-36. The L-36 is an excellent configuration other than having a lazy centrifugal spark advance curve, which is easy to optimize with lighter springs.

**Joe L.** · October 17, 2012, 03:17 PM

Re: CR on 1970 454

Originally posted by Duke Williams (22045)

Remember that OE specified compression ratios of that era were higher than the actual as-built by up to half a point.

If you run the numbers on the CR calculator using a 119 cc head chamber, .020" deck clearance, and a .039" gasket the resulting true CR with the KB 203 is about as Joe stated above, and the KB 207 piston is about 10.3.

Of course, the above are just estimates based on nominal dimensions. You need to measure your actual head chamber volumes and deck clearance to yield accurate results.

Since there is little data on how much compression the open chamber heads can tolerate on today's unleaded premium, I would recommend going on the conservative side with the KB 203 and retard the CS1093R cam four degrees.

There is another issue with that vintage big block that seriously compromises torque, power, and fuel economy, but it's an easy fix. The OE HEI system has a very short and lazy centrifugal curve, which with the OE recommended intial advance only allows about 30 total WOT advance when the engine needs about 40 to achieve maximum power.

In addition, the vacuum advance should be converted to full time and the installed VAC needs to be evaluated, and replaced, if necessary, to meet the Two-Inch Rule.

Details on the approach are in my San Diego presentation slides including how to download the AMA specs for any year Corvette that contain most of the relavent engine technical details, which allows you to track the changes from the antecedent non-exhaust emission controlled, premium fuel 1966 L-36. The L-36 is an excellent configuration other than having a lazy centrifugal spark advance curve, which is easy to optimize with lighter springs.

Duke-----

I think Pamela actually has a 1973 or 74 Corvette with LS-4 but she wants to convert it to near 1970 LS-5 compression. 1973-74 LS-4 had ~113 cc combustion chambers.

**Pamela H.** · October 18, 2012, 09:04 PM

Re: CR on 1970 454

Machine shop will not warranty if we use KB 203...wanted to use KB 257. They said we were building a "performance engine" and wouldn't waranty the parts. Did not want to take a chance..So, we agreed on the 1970 454 390 HP because they were "Stock". (They will only have to go 30 over)
Stock crank, stock rods, Stock OE pistons from 1970 454 390hp to get slightly higher HP than what I have now. Will reconfigure the carb to work with this setup.
Comments?

**Pamela H.** · October 18, 2012, 09:09 PM

Re: CR on 1970 454

Originally posted by Joe Lucia (12484)

Duke-----

I think Pamela actually has a 1973 or 74 Corvette with LS-4 but she wants to convert it to near 1970 LS-5 compression. 1973-74 LS-4 had ~113 cc combustion chambers.

That's right...'74.

**Joe L.** · October 18, 2012, 09:32 PM

Re: CR on 1970 454

Originally posted by Pamela H Haus (55255)

Machine shop will not warranty if we use KB 203...wanted to use KB 257. They said we were building a "performance engine" and wouldn't waranty the parts. Did not want to take a chance..So, we agreed on the 1970 454 390 HP because they were "Stock". (They will only have to go 30 over)
Stock crank, stock rods, Stock OE pistons from 1970 454 390hp to get slightly higher HP than what I have now. Will reconfigure the carb to work with this setup.
Comments?

Pamela-----

So, the machine shop is saying that if you use KB 203 pistons you'll be building a performance engine and they won't warranty it but if you use KB 257 pistons you won't be building a performance engine and they'll, therefore, warranty it? They don't know what they're talking about and I'd be worried about using them because they might not know what they're doing in other areas, too. The KB 257 pistons will result in a compression ratio of about 10.7:1 with your heads versus the KB 203's 9.8:1 with your heads. So, using the KB 257 pistons will actually result in a "higher performance" configuration. I do not recommend going with the KB 257's since there's a risk that at that compression ratio you're going to end up with detonation problems, especially with the camshaft you have. In fact, usually the machine shop should be the very first one to steer you away from high compression because they should know of this risk, too, and the likelihood that you'll be back complaining to them if you end up with detonation.

Also, there's no way that the KB 257's would be considered OEM-equivalent pistons for a 1970 LS-5----no way, at all.

**Pamela H.** · October 18, 2012, 10:55 PM

Re: CR on 1970 454

Joe, No, I must have stated incorrectly...They won't waranty either the 203 or the 257 because they are "performance" We initially wanted the KB257...then went with the "stock" pistons from the '70 390hp beieving they would ultimately give us the extra hp we were looking for....A Pre Emissions engine... Wanted the 10:1 CRNow from what I'm reading it sounds like the stock pistons from '70 won't give us anything different than what we already have in the '74 and we will not have achieved what we were looking for...Am I correct? Are the pistons in the '70/390 the same as the '74/454?Pam