[lhi1]

I recently bought a '67 24217. The restorer said he rebuilt the engine, installing hardened valves, a 0.060" overbore, and a high pressure oil pump. He also lowered the compression ratio from the original 10.75:1 to something less. It has 670 heads.

I note that it gobbles a quart or more of oil for each tank of gas. I would not expect that. The engine does not leak oil.

I did a compression check on each cylinder. Seven cylinders showed a comp ratio around 6.4, but #5 showed a 5.0 ratio. All of the numbers are low, and the fact that #5 is more than 5% different from the others is not a good sign.

My question: How is it possible to get such low compression ratios? The overbore would not get anywhere close to the measured ratios. What the heck is going on?

Thanks for your suggestions.

[steve25]

First off there never was a factory installed 400 of any year that had more than 10.5 comp, and these where latter than 67 motors.
The pistons used in the rebuild where likly the 8 valve notch type made for use with the early 67 motors that still used the 389/421 type head, or the latter 14 degree valve inclination heads like your 670s.
I would need the valve notch volume in CCs of those pistons to figure your compression ratio, but more helpfull would be if you posted your 8 compression readings in PSI.
Your oil usage may be just that the rings have not seated yet.
4 to 5 full throttle runs while bringing the revs up to 5400 rpm should seat the rings in well and then another compression check can be made.
If the cylinder pressure readings even out, but the comp is still low ( 140 psi or less) than I would have to say that the cam is installed retarted from where it should be.

[lust4speed]

Originally Posted by lhi1

...I did a compression check on each cylinder. Seven cylinders showed a comp ratio around 6.4, but #5 showed a 5.0 ratio. All of the numbers are low, and the fact that #5 is more than 5% different from the others is not a good sign...

Actual pressures would be helpful in knowing where you are at. Also, 10% variation is the normal parameter for worrying. Greater consistency is always desired, but not always achieved. Say your readings were mostly around 150, then the flag would be 150 - 15 (10%) = 135 pounds or less.

[Nicks67GTO]

Originally Posted by steve25

First off there never was a factory installed 400 of any year that had more than 10.5 comp, and these where latter than 67 motors.
The pistons used in the rebuild where likly the 8 valve notch type made for use with the early 67 motors that still used the 389/421 type head, or the latter 14 degree valve inclination heads like your 670s.
I would need the valve notch volume in CCs of those pistons to figure your compression ratio, but more helpfull would be if you posted your 8 compression readings in PSI.
Your oil usage may be just that the rings have not seated yet.
4 to 5 full throttle runs while bringing the revs up to 5400 rpm should seat the rings in well and then another compression check can be made.
If the cylinder pressure readings even out, but the comp is still low ( 140 psi or less) than I would have to say that the cam is installed retarted from where it should be.

Ive got an original 1968 Chiltons manual that lables alot of the 400's with a factory 10.75:1

[lust4speed]

Yeah, but if you do the math, that 10.75 compression ratio works out to 10.25 when the numbers are run. What Pontiac claimed didn't come very close to the truth.

[steve25]

Ditto, all the chiltons so called 10.25 compression motors where 9.75 at best.
On avaerage you can knock 1/2 off of all of those factory/Chiltons numbers.
Pontiac back then let room for racers to up the compression by publising those numbers!

[FritzCat]

I once built a 455 that would use a quart or more of oil per tank of gas, it didn't smoke at all. Put in some different rings, no change. Swapped in a set of heads from SD Performance and it stopped using oil. I'm guessing either the guides were bad or the "mechanic" messed up the valve seals somehow.

[android 211]

The 10.75:1 was the nominal ratio, '64-'69. 10.5:1 in '70. The nominal ratio was the ratio the engine would have with the NHRA minimum combustion chamber volume.

[dmac]

Pontiac assumed zero deck, no gasket, and no allowance for valve reliefs in pistons.
They may have also assumed minimum cc in chamber size on heads.

[1967SC]

Originally Posted by steve25

Ditto, all the chiltons so called 10.25 compression motors where 9.75 at best.
On avaerage you can knock 1/2 off of all of those factory/Chiltons numbers.
Pontiac back then let room for racers to up the compression by publising those numbers!

Then why all this fear of detonation @ the stock compression numbers ?
Is 9.75 not low enough to run on today's pump gas, without fear of a failure ?

Thanks

[gtofreek]

I've built and run Pontiac engines on as high as 10.8:1 compression with iron heads and pump gas, without detonation, but the camming, timing curve and fuel management must be spot on or there will be hell to pay. With closed chamber heads, it will be really hard to do at 10:1.

[geeteeohguy]

I haven't been able to run a stock Pontiac GTO on pump gas without detonation for over 20 years, whatever the CR is or was. 91 octane fuel in California's dry hot climate is not adequate. My 9.3 CR low compression '67 GTO doesn't even like 91 octane on days over 100 degrees. My 10.75:1 '65 GTO with a 389 has needed boosted/race fuel for decades. 670 heads on a street driven car are a bad idea, IMO, for the average joe unless dished pistons are used. Mine have been in mothballs for years.