CR on 1970 454

[Joe L.]

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

Pam------

The 1970 and 1974 pistons were different but the crown configuration is about the same. The bigger difference in the engines was the combustion chamber volume. The 1970 was about 100 cc and the 1974 about 113 cc. Therein lies the biggest difference in compression ratios.

If the shop wants to use OEM pistons, where do they propose to get them? As I mentioned previously, these have not been available from GM in years. They could be talking about Federal-Mogul 426P. However, these are a conventional cast piston. This is what the original pistons were made from but it's basically an obsolete material today. I would not even consider using them.

[Patrick B.]

Re: CR on 1970 454

Pam------

The 1970 and 1974 pistons were different but the crown configuration is about the same. The bigger difference in the engines was the combustion chamber volume. The 1970 was about 100 cc and the 1974 about 113 cc. Therein lies the biggest difference in compression ratios.

If the shop wants to use OEM pistons, where do they propose to get them? As I mentioned previously, these have not been available from GM in years. They could be talking about Federal-Mogul 426P. However, these are a conventional cast piston. This is what the original pistons were made from but it's basically an obsolete material today. I would not even consider using them.

Acording to the MVMA specs, a 70 454/390 used pistons with a 12.1 cc dome and heads with a 96.4 cc combustion chamber for a nominal 10.25 compression ratio. A 74 454/270 used flat top pistons with valve notches (a slight negative dome) and heads with a 110 cc combustion chamber for a nominal 8.25 compression ratio. Therefore, the piston change accounted for about half the compression ratio loss. The dome height of the 70 pistons was .094 inch. What are the dome heights of the aftermarket pistons being discussed?

[Duke W.]

Re: CR on 1970 454

It sounds to me like this shop won't warranty their labor with KB pistons, period. They probably had engines come back with broken rings lands because the top ring did not have sufficient end gap as specifically called out by the KB instructions.

The Web is full of such stories, but it's not the pistons fault. It's the fault of the illiterate moron who put the engine together without reading and following the KB instructions on top ring gap.

Duke

[Patrick B.]

Re: CR on 1970 454

The KB203 pistons are essentially identical in dome volume to the 70 LS-5, and the KB202 pistons are flat tops like the 74 LS-4 pistons. The KB calculator gives 8.7 cr with KB202 and 9.78 with KB203 using a 110 cc head. Even though this is for a 0.030 over bore, the calculator is pretty optimistic over the nominal 8.25 cr of the LS-4. The good news is that stock equivalent 70 LS-5 piston will yield the same cr as the KB203 because it has the same 12cc dome.

[Duke W.]

Re: CR on 1970 454

The KB CR calculator is not reliable because it does not specifically state the head gasket thickness, nor the actual deck clearance. These dimensions are the major variables, and small changes/differences can have a signficant effect on the actual as-built CR.

That's why these dimensions must be determined for each individual engine. Otherwise, it's not much better than guessing.

Duke

[Joe L.]

Re: CR on 1970 454

Acording to the MVMA specs, a 70 454/390 used pistons with a 12.1 cc dome and heads with a 96.4 cc combustion chamber for a nominal 10.25 compression ratio. A 74 454/270 used flat top pistons with valve notches (a slight negative dome) and heads with a 110 cc combustion chamber for a nominal 8.25 compression ratio. Therefore, the piston change accounted for about half the compression ratio loss. The dome height of the 70 pistons was .094 inch. What are the dome heights of the aftermarket pistons being discussed?

Patrick-----

Notwithstanding the AMA specs, I have different specs for the 1970 and 1974 combustion chamber volumes. I show the combustion chamber volume of the 1970 GM #3964290 cylinder head at 101 cc and the 1974 GM #336781 at 113 cc. Also, I show the 1970 GM #3976015, 3976016 piston to be flat top with 2 "eyebrow" valve reliefs just like the 1974 GM #340206. I always thought the 1970 LS-5 pistons had a slight dome, too, but that's not what my information shows. With regard to the AMA specs, I have found many instances in the past in which they are incorrect. One would think they would be quite accurate but that's not been my experience.

Has anyone out there had a 1970 LS-5 engine apart who can report on the piston dome configuration?

[Patrick B.]

Re: CR on 1970 454

Patrick-----

Notwithstanding the AMA specs, I have different specs for the 1970 and 1974 combustion chamber volumes. I show the combustion chamber volume of the 1970 GM #3964290 cylinder head at 101 cc and the 1974 GM #336781 at 113 cc. Also, I show the 1970 GM #3976015, 3976016 piston to be flat top with 2 "eyebrow" valve reliefs just like the 1974 GM #340206. I always thought the 1970 LS-5 pistons had a slight dome, too, but that's not what my information shows. With regard to the AMA specs, I have found many instances in the past in which they are incorrect. One would think they would be quite accurate but that's not been my experience.

Has anyone out there had a 1970 LS-5 engine apart who can report on the piston dome configuration?

Joe-- I see that Alan Colvin's Chevrolet by the Numbers shows the numbers you are quoting. Have you computed either compression ratio with these numbers? Its too late to do that tonite, but the difference of 12 cc in the heads does not seem to be enough to account for 2 whole points of compression ratio between the LS-5 and LS-4

[Joe L.]

Re: CR on 1970 454

Joe-- I see that Alan Colvin's Chevrolet by the Numbers shows the numbers you are quoting. Have you computed either compression ratio with these numbers? Its too late to do that tonite, but the difference of 12 cc in the heads does not seem to be enough to account for 2 whole points of compression ratio between the LS-5 and LS-4

Patrick-----


Yes, Colvin's information is also consistent with the other information I have.

I did a calculation for the 1970 LS-5 and 1974 LS-4, each with the flat tops with 2 valve reliefs. Using 0.019" gasket thickness and 0.020" deck height, I come up with 9.25:1 for the 1970 and 8.46:1 for the 1974. So, the 1974 is about right but the 1970 is a full point low. That strongly implies that the 1970 LS-5 pistons did have a dome. I hope that someone that's had a 1970 LS-5 apart can provide info on this.

[Clem Z.]

Re: CR on 1970 454

here are the NHRA specs.

H.P.Disp. Deck Piston Height Vol Valves Cam Lift Springs


90 153 .011 Flat w/n 1.60 cc 1725/1505 397/397 OuterOnly .032
140 230 .011 Flat w/n 1.60 cc 1725/1505 388/388 Outer Only .032 3962084,184,183,180
155 250 .011 Dished .065" 7.67 cc 1725/1505388/388 Outer Only .032 084,183,184,180
200 307 .002 Flat w/n 2.20 cc 1725/1505 390/410 Outer w Damp .018 3927185
250 350 .002 Flat w/n 4.70 cc 1945/1505 390/410 Outer w Damp .016 441
265 400 .015 Dished .077" 12.43 cc 1945/1605390/410 Outer w Damp .034 3951598
300 350 .002 Flat w/n 2.30 cc 1945/1505 390/410 Outer w Damp .016 186,492,041,462
291,292
330 402 .008 Dome w/n .188" 20.05 cc 2070/1725398/398 Outer w Damp* .021 3964290
345 454 .008 Dome w/n .094" 12.10 cc 2070/1725 398/430 Outer w Damp* .021 290,280
350 350 .002 Dome w/n .109" 1.41 cc 2023/1605 447/447 Outer w Dam .016 187,414,492,186,041
462,291,292
350 402 .008 Dome w/n .188" 20.05 cc 2070/1725 461/480 Outer w Damp* .021 3964290
360 350 .002 Dome w/n .109" 1.41 cc 2023/1605 458/484 Outer w Damp .016 414,492,186,041,462
291,292,187
360 454 .008 Dome w/n .094" 12.10 cc 2070/1725 461/480 Outer w Damp* .021 290,280
370 350 .002 Dome w/n .109" 1.41 cc 2023/1605 458/484 Outer w Damp .016 414,492,186,041,462
291,292,187
375 402 .008 Dome w/n .344" 39.53 cc 2195/1885 520/520 Outer w Damp* .021 291,391,840,858,401
390 454 .008 Dome w/n .094" 12.10 cc 2070/1725 461/480 Outer w Damp* .021 290,280
450 454 .008 Dome w/n .303" 31.60 cc 2195/1885 520/520 Outer w Damp* .021 840,291,858,391,401


* Alt spring is Outer/Inner

[Duke W.]

Re: CR on 1970 454

Be careful when interpreting the AMA/MVNA specs. Early on they listed TOTAL combustion chamber volume, which was more than just the cylinder head volume. The total includes volume added by deck clearance, head gasket, and any addition or subtraction from the piston design - in other words, the total clearance volume with the piston at TDC. This and the swept volume is what you need to compute CR.

For 1970 (page 6-Powertrain) total LS5 combustion chamber volume is listed as 6.31 cu. in. so the theoretical CR base on this total is

(454/8 + 6.31)/6.31 = 9.99, and the advertised CR was 10.25.

For 1971 - the year that GM management decreed that all GM engines would run on 91 RON (equivalent to today's 87 PON) gasoline - total LS5 combustion chamber volume was listed on page Powertrains-5 as 7.79 cu. in.

(454/8 + 7.79)/7.79 = 8.28, and the advertised CR was 8.25.

The same data carried over to 1972, but the format changed for 1973. More detailed data was given for each component that affected compression ratio. From page 5 of the 1973 AMA specs.

Cylinder head volume 113.06 cc
Head gasket thickness .028"
Head gasket volume 7.10 cc
Deck clearance (minimum) .020"
Minimum combustion chamber volume 112.06

This last listing is confusing, but I assume it represents the minimum HEAD chamber volume based on casting and machining tolerances.

I could find no data on net piston volume, so the above data is not sufficient to compute CR, however, if one uses the CR calulator http://www.csgnetwork.com/compcalc.html to solve for net piston volume at 8.28:1 it comes out to a net "dish" of 3.4 cc. In other words the piston geometry adds 3.4 cc to chamber volume compared to a flat piston with no valve relief notches.

For 1974 the first and fifth spec above changed to 115.33 and 114.33 respectively, and, again, there was no listed data on net piston volume.

The NHRA specs represent minimum or maximum values that were legal for "stock" engines of the era, and they allow for "cleanup cuts" on the head and block, so head chamber volume and deck clearance ended up less than typical as-built production engines after "blueprinting" to NHRA specs, which in some cases yielded compression ratios higher than the advertised values.

For example, note that the allowable deck clearance for the 454s is .008" while the AMA specs say .020", minimum

Duke

[Clem Z.]

Re: CR on 1970 454

the piston dome volume in the NHRA specs would not change as joe asked if the 70 low HP 454 piston had a dome.

[Duke W.]

Re: CR on 1970 454

I find the NHRA piston volume data confusing.

The 350/350 is listed as 1.41 cc, but my F-M Performance Parts catalog lists 2.4 cc for the L2104F that I believe is the L-46/high compression LT-1 OE replacement piston. At least it's reasonable to assume this is a postive "net dome", so it subtracts from total combustion chamber volume compared to a flat crown with no notches.

The 350/300 lists 2.3 cc, and since it's a flat piston with notches it must be a "net dish" value, so it should be negative.

That forces one to make an assumption if the piston is "net dome" or "net dish" for pistons listed at dome with notches.

Duke

[Clem Z.]

Re: CR on 1970 454

I find the NHRA piston volume data confusing.

The 350/350 is listed as 1.41 cc, but my F-M Performance Parts catalog lists 2.4 cc for the L2104F that I believe is the L-46/high compression LT-1 OE replacement piston. At least it's reasonable to assume this is a postive "net dome", so it subtracts from total combustion chamber volume compared to a flat crown with no notches.

The 350/300 lists 2.3 cc, and since it's a flat piston with notches it must be a "net dish" value, so it should be negative.

That forces one to make an assumption if the piston is "net dome" or "net dish" for pistons listed at dome with notches.

Duke

the volume listed is what the piston would CC in a known volume like droping the piston down .500 in the bore.

[Clem Z.]

Re: CR on 1970 454

i preferred to use this plexiglass cup on the block with a know volume with the piston at TDC and sealed to the cylinder wall with grease.

[Clem Z.]

Re: CR on 1970 454

I find the NHRA piston volume data confusing.

The 350/350 is listed as 1.41 cc, but my F-M Performance Parts catalog lists 2.4 cc for the L2104F that I believe is the L-46/high compression LT-1 OE replacement piston. At least it's reasonable to assume this is a postive "net dome", so it subtracts from total combustion chamber volume compared to a flat crown with no notches.

The 350/300 lists 2.3 cc, and since it's a flat piston with notches it must be a "net dish" value, so it should be negative.

That forces one to make an assumption if the piston is "net dome" or "net dish" for pistons listed at dome with notches.

Duke

the LT-1 is listed as a L2304F with a .109 high dome and a calculated dome volume of 1.41 cc allowing for the valve reliefs