How much horsepower and torque is gained or lost from compression changes? Say I go from 9.5 to 8.5? The reason I'm asking is that I am getting a set of race ready 882 heads to replace my 305 h.o. heads but the compression loss will be about a full point. Will the power gain from the added flow numbers make up for the comp. loss?

 

Since there are no flow gains to be had from those heads, I'd say it will be a significant net loss.

There are few heads you could use that will yield a greater disappointment than 882s. I've only been watching people pretend attempt to build performance motors using those for about 30 years now, since about the time they came out; but I haven't seen anybody - NOT ONE - come up with a motor that ran better with those in any given state of prep than any other heads in the same state of prep. Basically, they are not worth spending any money on.

If you've already bought them, I guess you'll be finding that out. Your car will be no faster, and probably will be slower.

 

Commonly accepted rule is roughly 4% power gain/loss per point of compression.

 

I'm getting them for free, fully ported and polished with 2.02 1.60 valves. I'm just hoping the compression loss doesn't kill the power gains, which I'm kind of afraid they will. But they guy I'm getting them from knows his stuff pretty well and has won quite a few dirt circle track races around here running them. And they are replacing completely stock 305 h.o. heads.

Am I stupid for doing this swap? The only money I'll be out is for gaskets and head bolts.

By 4% loss, you mean I'll see a 4% loss going from 9.5 to 8.5 right?

Thanks for the input! Keep it coming!

 

Better check and see if hte races he won were in the "882 head class".... they may very well have such a thing, in fact I know for certain there's a "186 head class" at some tracks, like some that I built motors for years ago that was basically the "929 cam class" and the "1.25" valve spring" class.

But I guess there's only one way to find out.... swap em, and do a before/after dyno pull or time trial. Butt dyno doesn't count.

 

I think a "butt dyno" will tell you a lot. With those heads you will feel a huge loss in power, even though you might make slightly more hp above 5000rpm. I would advise against the swap, the 416 heads you should have now are far better. Just port your 416 heads and make more power then those 882's.

 

Speaking of the "929 cam class"..... there's a participant.

There are actually "cheater 929 cams" that you can (or at least used to could) buy; they measure .390" / .410", 194° / 204° @ .050"; but the lobes look almost like solid roller lobes. Needless to say, people that ran those (which was just about everybody that didn't have a junkyard sponsor) would have to change springs every race, some of them would even change them every couple of heats. Better chance of finishing. Remember:

"In order to finish first, first you must finish".

Just because a circle track racer wins races, doesn't mean his used parts are any good. His advice might be good, or his parts choices, or his machine work; but I don't want any of them's used parts. They tend to use everything until they figure the odds of falling out of the next race are too high. Since all that stuff costs money, there tends to be not much life left by the time they cut loose of them.

But like I said, I don't trust the butt dyno, especially not somebody else's butt that belongs to the guy that decided to buy the parts on his own. Not exactly an unbiased measurement method. I'd trust only real-world numbers off of test equipment, especially if any gains are claimed.

 

Dissasemble the 882 heads and take the valves and have you're 305 heads rebuilt with the larger valves.
When you install larger 350 sized valves in a 305 head (58cc) the chamber ends up larger to clear the larger valve. (about 61-65cc's is typical) Don't forget to port them first. Just right for a budget 350 buildup.

Find the nearest metal scrap yard and donate the now bare 882's to 'em. A worthy cause....

 

I hope you don't mind me jumping in on this thread. So if my TPI350 is making 320 HP at the flywheel and I go from 8.9:1 to 10.5:1 compression with the same aluminum heads I will add about 20HP using that formula. I'am doing just that plus a few other mods. I will post the dyno numbers when I'am done. Right now I'am at 256 RWHP. Hoping when all is said and done I'am at 300RWHP with all the mods. Allen

 

Unless you plan to put something good on top!