20v pistons in a 16v 4age to increase compression

[AE86fan]

Ive done some rough calculations and think that if I use blacktop pistons in a 16v head I will end up with a comp ratio of about 12.5:1 which is about want Im wanting. This is based on the following....

16v head chamber about 38cc
BT poston dome 8.2cc
Cylinder volume 396.75 cc
Headgasket volume approx 2 cc (TRD 0.

(volumes are all estimates)

38-8.2+2 = 31.8

396.75 / 31.8 = 12.47

Does anyone have actual experience in using 20v pistons with a 16v head?? I know it will become an interference engine and I should go out and buy aftermarket 16v pistons but Im looking for a cheap alternative... open to any opinions and advice

[Rookie]

You would be better off sorting what little squish you can get, rather than chasing some arbitrary CR number.

[AE86fan]

The 12.5:1 comp ratio is based on wanting to run the the car on 98 octane without being impossible to tune. I know I could go higher on the compression but I think that avgas is being phased out for motorsport.

[Rookie]

My point was that you should actually work out what the engine is going to do, rather than making a mash up of specs and hoping that it works out in the end. Have you decided on your cam specs yet? because without those there is no point even entertaining the idea of what CR to run. You could run an engine with 15:1 CR on 91 octane with the right cams.

[Distrb]

Send a PM to GOLDAE86. He did it, and has all the info

[QikStarlie]

agree with Rookie. 20v pistons may give you gain in compression ratio, but will be going backwards in every other way.

should be able to safely get 11.5:1 with smallport pistons and a head skim & trd gasket. maybe more

[AE86fan]

QikStarlie: I know the valve pockets are different on the intake side of the piston but other than that how are they any different to 16v smallport pistons other than a bigger dome and how would it be going backwards?

Rookie: I will be sticking with standard bluetop cams but Im guessing the point is that with bigger cams and overlap the static compression doesnt mean so much?

[QikStarlie]

i was more thinking you were planing on running bigger cams. since bumping up the compression that much. in that case the lack of valve clearance would have become an issue.
you'd want to check that the blacktop pistons are going to retain proper squish area's and clearances also

[Rookie]

Pent roof combustion chambers have really shit combustion so you have to give them all the help they can get, make sure you have the squish bands and the other tolerences right, listen to Kris, he has played with these things many a time and has real world experience.
I'm not super clued up on 16v 4ages, I'd has at a guess that 12.5:1 with blue top cams would be ping central. Regrinds aren't expensive so roll that if you have a budget. There is a whole bunch of factors to consider when building a NA engine, but up there in my book of things to look at are squish band, dynamic compresion ratio, cam specs, and head flow (that's very simplified but if you look at these you will stand in good stead).
Also if you are keen to do it properly, have a look at Engine analyzer Pro, have a play and you will have a good estimate as to how it will go.

[ee904age]

I did it in an engine I was playing with, however plans changed and I never completed it. I will definately become an interference engine and from memory you will need the pistons machined.

I dummy assembled mine with a bare head torqued down without a head gasket and there was interference without even having valves installed, so best case is only a headgasket thickness of clearance.

[GOLDAE86]

Comp ratio 12.5:1
Blacktop pistons, yes it is an interference motor
258 deg cams
Standard head gasket
Shot peened, polished and balanced.
All else was 16v.
90kw atw with more bottom end (can't remember number)
No pinking, was great upgrade in my 86.
Hope this helps

[matt dunn]

The 12.5:1 comp ratio is based on wanting to run the the car on 98 octane without being impossible to tune. I know I could go higher on the compression but I think that avgas is being phased out for motorsport.

Avgas was bough back in again late last year, so is an option.

[Bazda]

Alot of work - Just stick in a blacktop 20v and be done with it

[AE86fan]

Thanks to everyone with their comments, I will be getting out the plasterscene and doing a dummy assemble in the next few days and will keep you posted how I go.

[Flannelman]

check if the rod small end is 20mm to suit the blacktop gudgen and not 18mm

[pc]

should be able to safely get 11.5:1 with smallport pistons and a head skim & trd gasket. maybe more

I did this and got 95KW on standard bluetop ecu with 98.

...and 260 degree cam regrinds.