piston clearences

[skizzo81]

So I will be trying to lower the squish on my short block in order to lower knock and raise engine efficiency. I would like to see how many of you have actually checked your piston to deck clearance. as well as your piston to valve clearance.

I will be using +1mm valves and i am running wiseco +.020 pistons 8.5:1 compression.

 

[JacksonAuto]

Most of the time on an 4G63 you end up with a negative deck height. Zero is ideal in most applications to reduce the propensity for detonation and maximize efficiency like you said. The best thing to do is measure your deck heights to see where you're at now then either deck the block or machine the pistons to achieve zero. Piston to valve with that combination of parts wont even be close.

 

[YZFR1]

My piston is about 0.018" in the bore and with the tightest LSA my intake valve is 0.045" from touching the piston. I can't even imagine how people can get away with zero deck not to mention negative deck height?? Do they just leave the cam gears in the straigh up position just for that?

Would the less squish be worth the possible volumetric efficiency gain you can get from adjusting the gears?

 

[JacksonAuto]

Negative deck height is below the deck, which puts the piston farther away from the valves.

 

[YZFR1]

That would make more sence , was almost thinking if i might be missing out on something great

 

[Fehrion_sit]

machining the big end of the rod can sink the piston further down into the bore. but the down side is potentially weakening the rod.

 

[BogusSVO]

machining the big end of the rod can sink the piston further down into the bore. but the down side is potentially weakening the rod.

The rods and caps would really have to be cut a fair amount to change the rod length enough.

If the big end bore is chewed up that bad, that it takes more than a .001-.002 cut off each mating surface, to clean up the big end housing bore. I hope the machine shop would junk that rod and sorce a decent replacement.

 

[JacksonAuto]

^^^This. You don't want to shorten the rods.

The right way is to do it during assembly. Mock up the rotating assembly in the block, meausre the deck heights and resurface the block accordingly. This is why building an engine the correct way takes so much time and costs extra.