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Higher Compression....

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no1uknow

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Ok I have an 88 K5 with a 454 it has flat top pistons and 781 heads with some port work so from what I have read I probably have about 8:1 compression I was trying to get to about 9.5:1 so my question is what's better to get dome pistons or different heads?
 
You're gonna want dome pistons to make good efficient combustion in a big block. 781s are decent head, especially for a truck. They're traditional oval ports and with larger valves and some work are 600+ horse capable.
 
no1uknow said:
Ok I have an 88 K5 with a 454 it has flat top pistons and 781 heads with some port work so from what I have read I probably have about 8:1 compression I was trying to get to about 9.5:1 so my question is what's better to get dome pistons or different heads?
Click to expand...

Why do you want more compression? That combo works awesome. Also a domed piston will reduce cylinder turbulance making it less effiecent. The 781 heads are the best head you can get for a BBC that isn't for all out drag racing.
 
If you get domed pistons youll be way over 9.5 you can get flattop no valve relief or two valve relief and still run pump gas.
 
77k5blz said:
If you get domed pistons youll be way over 9.5 you can get flattop no valve relief or two valve relief and still run pump gas.
Click to expand...

The 781 heads have a large combustion chamber, approx 119cc. Using a piston like a KB w/ with a small .230 dome will yield around 9.4 compression. This is the pistion / head combo on my new motor.
 
Mine too....I used SRP 212135....yielded about 9.4 static and about 8.0 dynamic with my cam setup.
 
I'm trying to get more compression to get a little more power the cam I am using is comp cams xe268h
 
no1uknow said:
I'm trying to get more compression to get a little more power the cam I am using is comp cams xe268h
Click to expand...

Power is made in the heads and cam selection, compression doesn't do much for power as people think.
 
The reason I want to get more compression is because I want to get a roller cam and as was stated before some cams work better with higher compression. I bought this engine rebuilt so I had no say in the work done to it all I did was change the cam the engine has about 30 miles on it before the transmission took a dump and the tranny is getting worked on already so basically everything will be brand new.
 
Well keep in mind that when a block is bored or honed you have to have the pistons in hand that you're going to use which means that changing pistons might require a fresh hone to proper clearance the new pistons or if the new pistons are smaller than the old ones giving too much clearance then you'll have to bore the block and buy oversized pistons so the block can be bored to fit those pistons with proper clearancing. This also means that a rebalance of the engine would be required to make sure it doesn't vibrate itself to death from a weight difference in the pistons versus the rest of the rotating assembly.

EDIT: I should mention that I was an automotive machinist for 22 years building engines for a living so i'm not just some smuck talking out my ass.
 
for how crappy my compression is in the last 2 454s I have had, I am surprised how well they ran with the timing cranked to 24 initial and the slots welded up. Even with peanut ports on the first engine mine was a screamer. I will tell you the best money is spent on dyno tuning.

If you are going to do it anyway, go dome.
 
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