blacktop pistons in a silvertop

[ke kid]

The blacktop has higher compression than the silvertop so if I get myself some blacktop pistons for my silvertop this would bump up my compression right? Or is it the chamber thats different that adds to the compression?

[slydar]

youll probably end up with more cr. the bt chamber is bigger than st. only problem is it is significantly different in shape, as much less "squish area" you might find the bt pistons will foul on the squish areas in the st head. there are some pics i think in the wiki that show the difference.

[rthy]

the compression ratio will end up being 11.75:1 if the engine is built with factory specs. However as Slydar says it could well touch the squish area and the blacktop is designed to not have any, well next to none anyway. Just buy a 0.8mm trd gasket

[ke kid]

would touching up the chambers with a mill/diegrinder help with this at all?

[rthy]

ooo tricky job, there is a an easy thing you can do to find out without any grinding/smashing sounds. Put the blacktop pistons in, then without the gasket put the silvertop head on and if it turns without the gasket then your home free.

Even easier if my friend takes his head back off his blacktop engine I can zoom down there with my spare silvertop head and do it for you.

[ke kid]

yeah that would be a great help thanks!

[rthy]

I will call my friend up and ask if he is going to take his head off soon, you might be in for a wait.

Are you sure you want close to 12:1 compression?

[ke kid]

If I run some good quaility 98 octane fuel through it, would it be alright? or will I need some octane booster as well?

[rthy]

with a stock ecu? I would take an educted guess and say so but its not my area

Slydar: this is right up your alley, what do you think?

[ke kid]

nah when the engines finished I will be running some sort of aftermarket management. I havent decided yet what sort/brand

[mech`s blue]

i thought that the angle the st and bt valves sit at are slightly different too. :unsure:

if they are that might have an impact on them hitting the pistons. better confirm this with someone else first tho

[ke kid]

anyone with a blacktop head wanna measure the valve angles for me?

[rthy]

nah they are the same; 25 degrees for the exhuast to the center intlet valve and 37.5 degrees from the exhuast to the outer inlet valves

if your fitting an aftermarket ecu then it will be fine with 98 RON fuel

[ke kid]

excellent then it should just be a matter of whether the piston touches the head then :sweat:

[slydar]

ask on club4ag.com