silvertop 20v to turbo

[adamaw11]

the acl pistons give around 10:1 compression

How is this the case?
On the site, they quote the 20v forgies as 8.5:1 compression
They also say suits silvertop only which I can't see why they wouldn't suit the blacktop also.

EDIT this is what i was looking at: http://www.aclperformance.com.au/Toy...gedPistons.htm

[Sam_Q]

simple, they are wrong and it yes suits both. I have to go out now but if I can I will explain tonight

[adamaw11]

cool, explain it when you get a chance.
forged hi-comp pistons suitable for forced induction for a 20v sound like a good thing.

[Sam_Q]

ok I am back, if you look at the shape of their 16V pistons compared to their 20v pistons you will see the 20v is almost flat bar some valve cut outs and the 16V has a big enough dish for a bird bath. Yet they both are quoted "8.5:1" when both 20v head have a smaller chamber than the 16V. So without a doubt the 20V should have a deeper dish than the 16V.

Ok now the 20v piston:

http://www.aclperformance.com.au/Toy...gedPistons.htm

it's quoted as having a 3cc dish (through the piston cut-outs) and this sounds right. Now I also compared to the compression heights and unless what I measured is wrong and the information I received is wrong they are the same as stock. So 3cc for that, Arias quote 35cc for a silvertop chamber which sounds right and assuming a 1.2mm compressed gasket thickness (some say 1.15) it works out to have around a 10:1 ratio. I can work it out accurately if you like.

I measured a blacktop chamber myself and it was without a doubt 37.8cc. 16V engines are meant to be 39cc

Also the "8.5" 16V pistons end up being under 8:1, I can't remember what I worked out to exactly

Of all these figures vary a lot so this is only a rough guide.

[adamaw11]

That all sounds interesting enough and possible.

Would you say the BT head has bigger chambers than the ST head? meaning if the same pistons were used in both the BT would have a lower CR?

Do you know of any results of people who used these pistons for a 20v build, or any other proof of the CR approximating 10:1 ?

[gixer]

That all sounds interesting enough and possible.

Would you say the BT head has bigger chambers than the ST head? meaning if the same pistons were used in both the BT would have a lower CR?

BT definately has a bigger combustion chamber, therefore lower comp ratio.if using same pistons.

In my Turbo ST 20V, I used Crower rods and Arias pistons. I ported and flow benched the head myself, modding and flowing each time, it was time consuming but enlightening as to where in the port gains are made.

[toymaz]

I have Arias pistons in my 20v silvertop, i ordered 8.5comp and the paperwork that came with them says head cc 38.7. and comp height 1.209 where acl quote 1.201. standard thickness cometic h-gasket and has no problems on 22psi.

[Sam_Q]

Arias quote a chamber size of 35cc for a silvertop, I have seen this quoted elsewhere also. I measured the blacktop at 37.8cc and I have complete faith in the measurement. So a 2.8cc difference.

time to get the calculator out:

lets start with the later model ZE pistons quoted at 8.9:1. This in a blacktop: 9.1:1, silver: 9.6:1

proof of my claims can is by looking at the ZE pistons and they have more dish than the ACL/Mahle piston.

Now using the 3cc quoted by ACL lets assume the gasket is 1.2mm compressed, this is 6.19cc. With the silver this would make it 35+6.2+3=44.2. So the resulting comp is 10:1 exactly

Moving onto the black 37.8+3+6.2=47. So the comp on that is 9.4:1

I would like to do more measurements to double check the compression heights of stock pistons to make sure I am not missing something obvious but assuming they are the same I can't see why I am wrong and why such a large company could make a high school mistake. But hey I could be missing something completely obvious here.

I have to reinforce though that the only figure I trust in all these measurements is the blacktop chamber size, however even that is prone to be different between engines due to casting shifts and that crap. I am pretty confident that the silvertop chamber is 35cc just by looking at it but I can't completely verify it.

I do know of one guy who is running these and his report on them was that the response without any boost was "amazing". He followed through on my suggestion and went from a T25 to a T28 off an S15, he loves it.

By the way check out the weight of these things, they are lighter than a stock 20v piston!

[gixer]

I have some toda paperwork here for BT and they quote the BT combustion chamber at 38.5cc, we took some measurements of one just the other day and it was 38.7cc.

[Sam_Q]

I have Arias pistons in my 20v silvertop, i ordered 8.5comp and the paperwork that came with them says head cc 38.7. and comp height 1.209 where acl quote 1.201. standard thickness cometic h-gasket and has no problems on 22psi.

in their catalouge they say 35? well either way its impossible that its 38.7

[Sam_Q]

I have some toda paperwork here for BT and they quote the BT combustion chamber at 38.5cc, we took some measurements of one just the other day and it was 38.7cc.

what method did you use to measure it? just so you know I am not doubting you at all.

I think it's casting variance, machining variance, valve seats and valves worn down more, seats sitting up higher, take your pic.

[gixer]

Sam, this is BT head I'm talking about. valves in, plug in, clear perspex plate over chamber and burette.

The 38.5cc comes straight from the toda paperwork I have here right infront of me regarding their BT pistons.

[Sam_Q]

oh yeah the old burette, hard to argue with that. There is even the possibility that the head I had had been skimmed at one stage?

[toymaz]

in their catalouge they say 35? well either way its impossible that its 38.7

yes i see that, maybe they had a typo on my page.. well it actually says 34.5 on their web page and refers to black and silver, either way their 9 comp pistons have a cc of -9 and 11.3 comp of 0cc.
and on 16v -4.3cc for 8.6 comp and 7.2 cc for 11.5 comp.
completely different to ACL.

[Sam_Q]

0cc gets 11:1 comp!? thats amusing seeing that the stock piston has dome in it thats about 3 to 4cc and it 10.5:1