Pistons old vs new

[ducenqtr69]

Any 1 Out there know what the LOSS of replacing the stock 10.25 to1 compression pistons (stock) with 9.9to 1 comp, ??? what difference can i expect <img src="http://www.aaca.org/ubbthreads/images/graemlins/frown.gif" alt="" />Loss of h/p ???/ also difference of <img src="http://www.aaca.org/ubbthreads/images/graemlins/confused.gif" alt="" />HeadS need mods??? my 69 430-4 is running great now But some time in the future it will be needen a Rebuild , ao i am lookin at what to expect <img src="http://www.aaca.org/ubbthreads/images/graemlins/frown.gif" alt="" />(pro's an con"s) when that time comes , <img src="http://www.aaca.org/ubbthreads/images/graemlins/wink.gif" alt="" />Thanx for any Help !!!!! TEC

[Turbo Loyd]

[color:"blue"]There are so many variables that it is very difficult to estimate the effect of going from 10.25 to 9.9 to 1 compression ratio.

Some head work probably would off set this change. Good things to do would include:

1. Have Stage 1 valves installed

2. Perform some mild porting and polishing on the heads.

3. Select a cam to go with your objectives, and of course your gear ratio, weight of car and stall of torque convertor.

4. Open up the exhaust (headers or interior honing of manifilds with larger diameter exhaust system) and intake (aftermarket intake or interior honing with less restrictive air cleaner) to enhance the porting of the heads.

A rule of thumb is 93 octane generally will support 9.3 to 1 compression, with variables such as good heads, and choice of cam allowing higher ratios.

Done correctly these changes should more than offset the slight reduction in compression ratio.

[ducenqtr69]

Loyd thanx <img src="http://www.aaca.org/ubbthreads/images/graemlins/grin.gif" alt="" /> for the info , i am lookin at all pos+++ for the 430 before i have to do the rebuild , as with the fuel being what it is an <img src="http://www.aaca.org/ubbthreads/images/graemlins/laugh.gif" alt="" />also the future of these old rides , hope to keep it for many more Yrs to come <img src="http://www.aaca.org/ubbthreads/images/graemlins/smile.gif" alt="" /> , TEC

[Guest]

The only thing I can suggest is porting/polishing to add velocity (make sure you use a reputable head shop with a flow bench and experience on buick heads) If you are changing slugs, spend the extra cash and get forged, not cast. To help bring your Compression ratio back up you can use slightly thinner head gaskets (preferably copper). I prefer gobs of tourqe over gobs of horsepower so I like to keep the CR as high as possible, this usually requires some bulletproofing of the internals, but results in tire ripping power in almost any gear lol. Good luck

[Sgt Art]

I don't think the HP will drop all that much if everything else remains the same. As someone else has already noted, replacement head gaskets these days are generally thicker than the older stock ones. Therefore, you will lose a little bit more compression than .35:1 by going to 9.9 from 10.25:1 pistons. However, in looking at the factory specs of 1968 Pontiac 350 engines:

The same engine going from 10.5:1 to 9.2:1 loses 55 HP or 42.31 hp per 1 point drop. You're only dropping .35 of a point which if my math holds up would be about 15 HP. I'm not certain if my thinking is correct, but it seems reasonable.

The big drops in HP came when all makers dropped from 10.x:1 to 8.5:1. That was 1.5 to 2 full points of compression ratio loss coupled with retarded valve timing, really killed the hi po engines of the 60's.

1968 Pontiac Engine 350 and 350 H.O. V-8 Engines.

The thrifty 350-cubic-inch, 2-BBL, regular-fuel engine is standard on Firebird 350 and optional at extra cost on Tempest, Tempest Custom, Le Mans and Tempest Safari. It has compression ration of 9.2:1 and rated brake horsepower of 265 at 4500 rpm. Engine torque is 355 pound-feet at 2800 rpm. The more powerful 350-cubic-inch "high-output" engine utilizes a 4-BBL Quadra-jet carburetor and premium fuel. This engine, standard on Firebird H.O., is available at extra cost on Tempest, Tempest Custom (except Station Wagon) and Le Mans. Having a compression ratio of 10.5:1, the engine's rated brake horsepower is 320 at 5100 rpm and the torque is 380 pound-feet at 3200 rpm. (See pages M-1, M-2 and M-3 for features and specifications, and "Power Train" sub-section for power team combinations.)

[ducenqtr69]

Thanx to all for the info i am KEEPING all the info in my /shop-BOOK <img src="http://www.aaca.org/ubbthreads/images/graemlins/laugh.gif" alt="" /> so when the time comes, we have some help for the head Shop, <img src="http://www.aaca.org/ubbthreads/images/graemlins/tongue.gif" alt="" /> to offer the right combo , i defn;t will do the forged/hole fillers <img src="http://www.aaca.org/ubbthreads/images/graemlins/tongue.gif" alt="" /> , gotta keep the H/P no subs for cubic inches, HEHEHEHEHe, <img src="http://www.aaca.org/ubbthreads/images/graemlins/grin.gif" alt="" /> Again Thanx