Dallasdrifts
Active Member
Hey I have a 78 440 and want to up the compression ratio what pistons would be recomended? Prefer not having to bore it out but I'm open to suggestions
78 high comp 440 pistons
- Thread starter Dallasdrifts
- Start date
68GTX4SPEED
Well-Known Member
Same as in a Six Barrel engine http://www.summitracing.com/int/parts/slp-l2355f/overview/make/plymouth
68GTX4SPEED
Well-Known Member
68GTX4SPEED
Well-Known Member
drobertson
Well-Known Member
do you know the compression ratio of those pistons? I only have access to 91 octane fuel in my area. I have heard that the max I could run with 91 octane is 9.5:1 compression.
lewtot184
Well-Known Member
kb184 piston. those 6bbl pistons will be close to 10.5:1 compression and no quench; will never run on pump gas.
moper
Well-Known Member
As much as you don't want to, you should plan for some block machining. New pistons at minimum mean rebalancing and you "might as well" get some other weak areas fixed if you plan to drive it harder. If it were me I'd have the block cleaned, bored, and honed and the rod bolts replaced as the rods are pressed. My choice would be KB hypereutectics. They're light enough to not need heavy metal added to the crank when balancing and will yield a reasonable 9.2-9.6:1 with your factory heads.
Dallasdrifts
Active Member
When you suggest boring do you mean just like 30 over?
lewtot184
Well-Known Member
the easy solution for a compression increase is to yank the heads and mill them.
moper
Well-Known Member
Yes. Boring the cylinder over size, then honing it using a torque plate to the proper size and wall finish for the rings.
That being said - there's a lot that comes with just replacing pistons. There's not a terrible amount of gain exclusively from the compression increase. It might make more sense to simply use a camshaft to work with the low static ratio, than go through the motions and cost to do it right, or the potential issues and messing around with side affects of trying to do it on the cheap. What's the performance goal for this engine?
That being said - there's a lot that comes with just replacing pistons. There's not a terrible amount of gain exclusively from the compression increase. It might make more sense to simply use a camshaft to work with the low static ratio, than go through the motions and cost to do it right, or the potential issues and messing around with side affects of trying to do it on the cheap. What's the performance goal for this engine?
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