Start from the Beginning - Whats my Compression Ratio?
Turbo Mini Forums

Hey Guys,

as you may have seen in different Topics, i bought a Turbo Engine from a previous owner who built it decades ago and - unfortunly - forgot a lot about it. He sayed that the initioal compression ratio of his concept has been 9:1. He reached that by dished forged pistons plus work on the head. The engine was sold and rebuilt twice since then but the pistons and the head is still the same.

Since i really have troubles setting up the ignition with the dizzy now, i may follow metroturbos advice and throw that ancient technology in the bin - but if so i want to do it properly. My Idea is to play a bit with the settings for now and hopefully find a somewhat driveable setting for the summer and also save some money for a bigger investition in winter.

My Idea is changing from the T2/T3 hybrid i am running now to a GT1752 with a stainless steel manifold and a Specialist Components 7-Port head including injection and mapped ignition.
But for that i need to do some research.
First of all i need to find out what my real compression ratio is now and how is it reached. How much my Pistons are dished. And then i have to decide for a compression ratio, so i can order the appropriate head from specialist components.

How do i start my research on the engine, what pistons are used etc.?

Also what CR would you aim for? I would like to run 10psi and aim for approx 130ps running an intercooler.

The only way to know the true CR is to take the head off and measure the chamber size and piston dish, then do the maths.

9:1 sounds spot on for 10psi. If you put an alloy 7-port head on, you could run a higher CR.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

Thanks for your reply, guess i have to open it then after summer.

How does the alloy 7-port head effect the CR? Because it has a better heat transfer and keeps cooler?

Would you still aim for 1:9 with a alloy 7port with the option to run a bit more boost later?

Edited by JackZwiebel on 9th Apr, 2019.

yep, down to better deat transfer due to the alloy

I'd stick with 9:1 and leave the opportunity for more boost

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

Alright thanks! Anything else i should take into account? AS far as i was informed the 7 port is pretty much a bolt on.

Should i consider a different camshaft? According to the guy who rebuilt the engine last time its a MPI camshaft - unfortunly i dont know exactly wich one.

Listen to Wil, he really knows what he is talking about.

Also would advise running a cometic head gasket. I got my 7 port second hand and it had to have a skim as a fire ringed gasket had pushed into it. I would also consider dry decking it as mine had a bit of corrosion around the water holes. Will also help with cooling.

Thanks Jonny, not sure I do know what I'm talking about, but I do have a 7-port turbo.

Good tip on the cometic head gasket.

I don't know about the MPi cam, in my head it is a bit odd-ball, designed to reduce port scavenging on a 5-port injection (happy to be corrected)

My favorite turbo cam is the MG metro cam (or Kent 500). You can't go wrong with the Avonbar PH2 either (if they're still available)

Edited by wil_h on 10th Apr, 2019.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

I thought u got urs all new from Sc jonny?

Ok, thanks for the replys. Is dry decking neccessary for 130ps? I mean it doesnt seem much of a problem....can you order a dry deck ready head at SC already?

Wil, its a very fast 7 port!

Dave, no, was second hand. Matt Argents old head i think. but changed hands twice after him!

Not necessary, seems daft not to though whilst its in bits. I'm sure they could do something. I just tapped and plugged mine.

Edited by jonny f on 11th Apr, 2019.

You’ll get bored of 130ps quickly, you’ll soon want more. For the cost and minimal work it takes to do it you may as well engineer for the enevitable future power hike.

you are right, better do it properly at first. and if i change the cam, i have to put out a lot anyway.