18cc piston engine build query + full race head
Turbo Mini Forums

hi, i am getting to the point on my project where i need to look at getting an engine together and in. i was going to run a mild turbo motor, as im changing from spi to carb/turbo and mj. i want to make sure i can get it to run before i spend mega bucks on an engine!

i have looked futher along at low compression/high boost builds.

i started pricing up for a build but its quickly running into serious money lowering the cc of pistons (was looking at 10cc omega cast) and head.

i have read a few posts on here recenty with people using the 18cc pistons from med, avonbar etc, and skimming the block/head to achieve a decent compression ratio.

my rough calcs would require 20cc chambers with 18cc pistons to run a cr of around 8.25:1

getting to the bottom of this, i have a hardly used minisport stage 5 head that i picked up cheap a few years back. it has 20 cc chambers. the valves are offset and are 31 exh and 37 inlet.

are these valves too big for forced induction? is there likely to be cracking between the valve seats? i 'could' sell this head but if i cant use for forced induction, i would like to use it on a hot na engine one day.

it just seems a cheaper way to do things with the 18cc pistons if im able to use the highly moded head that i already have, as opposed to shelling out for overbore, pistons and a low cr/modded head.

anyone have views on why this sould/shouldnt work on a turbo motor?

thanks for your time

To be honest, as you already have the head I would be tempted to just use it. It will cost a load to get another one made anyway.

I'm sure others have different opinions though.

Hmmm. Not sure about your calcs. I've got 18cc accralites and 20ccs in the head. My cr is 8.6:1.

What piston to deck clearance are you thinking of? The accralites run 0.8mm down the bore with a std deck height.

Either way. I'd also be tempted to use the head you've got. Although the valve sizes are not best suited to fi.

In the 13's at last!.. Just

Or if you fancy selling it I may be interested.

That will save skimming the crap out my Benross head for a hi comp 1380.

Assuming my machinist hasn't already skimmed it.

thanks but i think i will hang onto if for now. i didnt take into account that the pistons sit that .8mm down the bore. what cc does this give with the standard block deck height?

i guess my cr calcs should be the same as yours.

what psi/power are you running with your motor theoneeyedlizard?

0hp at 0psi as it's not been run yet.

Planning to run 15psi ish. Hopefully it will make more than 100hp.

In the 13's at last!.. Just

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

Very good point Robert.

Although, i'm only at +0.040. So if Dan is calculating this using +0.020 it won't make that much difference.

I reckon he is using the STD deck height. This leaves the pistons 0.8mm down the bore which is about 2.3cc and gives 8.23:1.

Maybe?

In the 13's at last!.. Just

i make it around the 8.3:1 mark with the .020.

and around 8.4:1 with the .040

does this seem right?

anyone else have a view on using this cylinder head with these pistons?

i will probalby look into getting the +0.20 pistons in the future. they look to give a nice low cr with thie head i am considering, also coupled with a ph2 cam.
should make good boost and power with this setup.


thanks

Edited by welshdan on 13th Apr, 2010.