Nicolasticity

Decision time on Pistons.

BBC HP425 currently running Speed Pro 14097021 pistons, forged rods, Crane 132561 cam, std 118cc 14097088 heads. Carb is 9022 Holley 600cfm and a B&M 174 blower on top running low boost and it ran great.

Engine deck is 9.8 ins, rods 6.135 ins, stroke 4 ins, compression height 1.645 - not sure if this gives a 0.020 ins clearance from top of piston to the top of the deck but some threads say it does? Gasket is Marine Felpro at 0.039 ins (4.370 gasket diameter)

Compression ratio calculators seem to give different answers for same inputs but searching the internet I cant find replacement pistons that will give me the 8:75:1 CR I would like.

Question is do I replace the std pistons with similar ie TRW 2399 at 30 thou over which gets a higher compression than the standard 8.75:1 at 8.9:1 GR dome is -13,8cc or do I fit a flat top like an ICON IC780-030 which is a lot lower at 8.2:1 CR - dome is 3cc.

I dont want to up the boost its fast enough.

What pistons would you fit?

Rookie

iTrader: (4)

Disregard didn't see you were running a blower.

mike tkach

iTrader: (3)

as long as running 92 oct fuel is ok for you 8.9 compression ratio will be ok.id think about a hyd roller cam while it is apart.are you sure the 9022 is a 600?i think it is an 800.

MILD THUNDER

iTrader: (3)

I would not go with the flat tops, as that will lower the compression too far with your setup.

If you want you can go back with the Speed Pro 2399 .030 piston, and play with head gasket thickness to keep compression close to where it is now, with the larger bore. Like say a .050 head gasket instead of a .040, etc. If you are rebuilding, they prob will surface the block, as well as the heads, to clean them up, so keep that in mind.

MILD THUNDER

iTrader: (3)

Youre right Mike, the 9022 is an 800. Came stock on the 425 Merc 454.

Nicolasticity

yep typo its a 800.

Nicolasticity

Can you machine a couple of thou off a piston crown and set it to 8.75:1?

Unlimited jd

iTrader: (5)

You can but being a blower deal I wouldnt. Just get a cometic head gasket to the thickness needed to achieve the ratio you want. Quench is not a big concern on a blower motor.

sutphen 30

I've taken .200" thick dome to .100" to lower compression.but you really should be looking for flat tops,this will get you around 9-1 depending on head gasket.

dereknkathy

9.8 is a generic deck height number...unless you have checked it. i don't think it can be that accurately measured short of checking deck height with rotating assy installed unless of course you know the engine has never been to a machine shop...