[Fred A]

The 77y high compression at 10.25 seems to be somewhat of a problem with the current fuels. When rebuilding, has anyone used pistons with a slightly lower top to take the edge off this ratio and still try to preserve the original power. I am assuming that the pistons listed for this engine are hi compression, unless the increase comes from a different head. The other area of concern is the timing chain. Two are listed and one is three times the price. What's going on there? Thanks: Fred A

[Fred A]

The 77y high compression at 10.25 seems to be somewhat of a problem with the current fuels. When rebuilding, has anyone used pistons with a slightly lower top to take the edge off this ratio and still try to preserve the original power. I am assuming that the pistons listed for this engine are hi compression, unless the increase comes from a different head. The other area of concern is the timing chain. Two are listed and one is three times the price. What's going on there? Thanks: Fred A

[Poncho_Villa]

Hey Fred, I just had my 71X engine rebuilt and the timing chain I had installed was a "rollerized" chain and was a lot more money. I am guessing maybe that is the other one listed. As far as the compression ratio and fuel conflict, I always use the highest octane fuel that I can find and never have had a problem. Stay away from the regular fuel if possible.

[figbash]

You can't lower the compression and still get the same power. The 10.25 compression ratio should be fine. Mine runs fine on regular, but you can always move up a grade if you need to. Just make sure that your timing is correct and that there is no carbon build up in the combustion chambers to raise the CR higher.

Tom

[mechanickeith]

There are several differences between the high comp. & low comp. engines. The main thing is the pistons on the 10.25 are flat top & the ones in the 8.6 are dished. The cam profile is different & the 10.25 engine has dual valve springs.

The early 60's Pontiac racers would go to the dealership & get the Tempest timing chain to run in their 389's & 421's. The factory Tempest chain was a lot stronger than the stock chain used on the V-8's. I have been running a Cloyes double roller on my 61 daily driver with no problems. I do pull off the timing cover every year to reinspect the chain. The 4 cyl's have so much vibration, they streach the chain & need replacing more often than the V-8's.