[Peje]

A question that have been bother me for some time: Take a 455 HO 1971-1972.Compression 8,2-8,4:1? What will happen if you raise the comp to let say 10;75:1? Is that possible if you use high octane,let say 98 or 99? Just curious..

[Steve Barcak]

Originally Posted by Peje

A question that have been bother me for some time: Take a 455 HO 1971-1972.Compression 8,2-8,4:1? What will happen if you raise the comp to let say 10;75:1? Is that possible if you use high octane,let say 98 or 99? Just curious..

Sure, anything is possible. The easiest way to raise the compression of any large chamber head is to use a pop up piston. However, that creates other issues.
You posted this is a restoration/original section- not in the race or even street section. Guys here are more interested in mutithousand dollar q jets and number matching, correct battery cables and build sheets than horsepower needs and than how good the car can actually run. Not saying they do not care how they run here, just saying, this is more of a restoration/numbers part of this board. Based on that, I do not recommended 10:75 to 1, as you suggest, on a street engine. The extra cost of using the expensive, high octane gas, will not offset the possible increase in performance.
Most production pop up pistons use the thinner rings, etc. There are also flame travel issues. You could have a custom piston made in any combination you wish but, you are adding considerably to your build cost for a street motor.
As an alternative, it is far wiser to recam a 455HO engine to increase its performance. You could still use regualr gas and have much more power. I also would install better exhaust, retune carb, etc.
High compression in a street car is simply a bad idea. If you have to run the real pricey gas, you will drive your car less. Anytime you put less miles on your car, you cannot enjoy it as much sitting in your carport.
Hope this helps answer your question.
You may get more performance base answers in the race or street section based on your question. Best to you.

[Peje]

Thanks Steve for info and sorry for posting the wrong section. I was just curious about the comp issue.I`m learning everyday! Thanks again.

[Region Warrior]

If money and time no issue, shave the block and/or heads would be the to go.
Shoot for 10.5 cr. Another .25 wont make a diff unless a radical is used.

[Dlivry]

Originally Posted by Steve Barcak

Sure, anything is possible. The easiest way to raise the compression of any large chamber head is to use a pop up piston. However, that creates other issues.
You posted this is a restoration/original section- not in the race or even street section. Guys here are more interested in mutithousand dollar q jets and number matching, correct battery cables and build sheets than horsepower needs and than how good the car can actually run. Not saying they do not care how they run here, just saying, this is more of a restoration/numbers part of this board. Based on that, I do not recommended 10:75 to 1, as you suggest, on a street engine. The extra cost of using the expensive, high octane gas, will not offset the possible increase in performance.
Most production pop up pistons use the thinner rings, etc. There are also flame travel issues. You could have a custom piston made in any combination you wish but, you are adding considerably to your build cost for a street motor.
As an alternative, it is far wiser to recam a 455HO engine to increase its performance. You could still use regualr gas and have much more power. I also would install better exhaust, retune carb, etc.
High compression in a street car is simply a bad idea. If you have to run the real pricey gas, you will drive your car less. Anytime you put less miles on your car, you cannot enjoy it as much sitting in your carport.
Hope this helps answer your question.
You may get more performance base answers in the race or street section based on your question. Best to you.

I think Peje was just asking a brainstorming question. if it's a a 455 HO and it's going in a 70-72 LeMans, Tempest, or GTO/Judge, I think this was a viable question for this section. just sayin....

[The Boss]

Nothing wrong about asking it here, quite a few hard driving, knowledgeable HO owners here.

If it were mine, I would go with a cam change & some carb tuning to start, hard to go back once you've shaved a head or block.

[Loffen]

But what if you take a stock 455 HO and only rise the compression ration, what will you get in HP then ?

[sd7369]

Dome pistons suck in a Pontiac v-8 in my experience. Thats one reason all compression changes were done in the heads by PMD engineering. Better off with edelbrock heads or 6X. You shave a high dollar ho block and heads you can't put it back.

[Baron Von Zeppelin]

Chamber design on our heads don't seem to be very friendly with dome pistons.

I've seen 2 sets of 455HO heads that had been run with dome pistons.
Both sets - on both heads - had eaten/melted a groove between the two center chambers. Where exhaust valves are side by side.

Whether or not the previous owners always used race fuel religiously idunno.
Maybe the cars were sold and driven on pump gas by unknowing new owners.

But domed pistons have this reputation on street/driver type Pontiacs for past couple/few decades. I wouldn't fool with them on a street car.

[Steve Barcak]

Another great possibility for the use of 71-4 HO/SD round port heads is this.

If one had an original 71-4 round port car but, did not have the original engine block- and the budget....

I would use an aftermarket block (MR-1, IA II) with a much larger bore than 4.151". You also would have the option of stroking- 4.5", etc.

This would be the most effective way to increase compression and HP when using '71-4 round port heads besides supercharging.

That would make for a very torquey tire burner, butt kicker!

[Baron Von Zeppelin]

Originally Posted by Loffen

But what if you take a stock 455 HO and only rise the compression ration, what will you get in HP then ?

Maybe 10 horses maximum, if that was the only modification.

I like Steve's suggestion.
Wish i had one of those blocks. (i hope Santa reads on this forum)

[J GLASGO]

I am going to disagree with some of you guys here.Do not mill your heads excessively to raise compression.Once that metal is gone,you cant put it back on. A dome piston which will give you a static compression of around 9.5 ,used in conjunction with a matching cam to keep your dynamic compression in check,will yield substantial horsepower increases in a 455ho. We have done several of these,and they are really strong runners. Look at the et,s of all the 455ho,s that run at the Pure Stock Drags,they are allowed 1.5 compression increase.That translates to about 10 mph difference in trap speeds. My bone stock HO cars ran about 99 mph in the day,a really good one,maybe 101 or so. Now they go 109,110.That is moving a heavy car pretty good. Remember,this is experiance talking,not a repeat of what somebody else wrote.As far as dome pistons,has anybody ever rode in a healthy big block Chevy?? Enough said!!