Increasing bore VS raising compression -- Discuss/debate

[DRIVEN]

So I was on another forum today and a guy posted a thread about an engine he was building for a 4cyl hobby stock race car. His initial question was weather or not to increase the bore (from 85-89mm) on a fresh rebuilt shortblock or leave it alone and just bolt a high compression head to it. Anyway, he got a couple opinions but it eventually turned into a debate on which did more for increasing power.

Is compression or over-boring more responsible for end power? What do you guys think and why?

 

[ERIC87442]

I had my 307 bored .060 over (because it had to be) and I never noticed a difference :lol: I'd say raising compression would do more for you than increasing bore. Also why can't you just do both?

 

[-83MONTESS-]

It was always my understanding that boring a cylinder will increase the compression but I cant imagine it would really be noticable or even comparable to actually increasing compression by changing pistons.

 

[jrm81bu]

Compression, no doubt about it. Increasing the bore would only help in some engines that have small enough bores as to shroud the valves. There's lots of power to be had with compression, you just need a fuel to tolerate it.

 

[GhettoRacingKid]

instead of bore it should be stroke. thats how you really increase cubes.

the thing is its all about the combo and the set up. you can overbore and do more harm then good, you can go to high with compression

Compression is where you see more power then a few small cubes

 

[Evan11]

I agree. For example, if you bore a engine 30 over and put stock spec 8to1 pistons (in some cases) back in, you're not going to have a very noticeable improvement. But if you simply put higher comp pistons in it you would notice a decent difference , which could require use of higher octane fuel. You have to be within reason on compression ratios though.

 

[malibu80kc]

jrm81bu hit the nail right on the head.

 

[online170]

Ditto on the compression.

In the buick world, if you build a 430 and 455 (assuming standard valves, not stage 1) that are identical, ie: same pistons, heads, etc etc. They will run with eachother all day long with negligible difference.

Many people regard the 430 as a more potent engine because it was rated at 10.25:1 compression, and the 455's were rated anywhere from 8.5:1 to about 9:1.

 

[online170]

Just to clarify, its not to say that increasing bore size is a waste of time or it doesnt net anything. But for most real world power levels its the case.

You have to be waaaaaaay up in the power output to notice it. Id say 600hp++

 

[RITTER]

Boring a cylinder is usually more related to cleaning the cylinder wall from pitting and unevenness :idea: