What compression ratio for 200+bhp
Turbo Mini Forums

Hi guys, I'm currently collecting parts for my Morris Ital van that I'm boosting. I will be using a good old T3 turbo.

10 years ago I did the same on my mini, using a genuine 1275 metro turbo block and pistons. I was running 14psi of boost and was a good laugh, it had a modified head, cam, brand new T3. I never had it on the rolling road but from others running a similar spec I guessed it was around the 140bhp mark. Didn't know what the compression ratio was on it.. a bit silly I know.

My question is what compression ratio should I am for, I know that a too low compression ratio will cause it to a bit of a dog off boost, but I don't really mind that, (to a certain extent) I was thinking around 8.1:1 C/R and running around 18/20psi. I have seen others running even lower and obviously gaining more power.

Also what spec cylinder head would you go for regarding what size valves.

So the spec I am going for with this build is -

A plus in-line engine
1293cc
Reconditioned T3 turbo
(Cylinder head undecided on spec)
Plus 20 Forged omega 18cc Pistons
Ph2 turbo cam
Aldon turbo dizzy
big front mount intercooler
Swiftune alloy/alloy duplex
2" maniflow exhaust system
ARP con rod bolts
ARP 11 head stud kit

Final drive 3.6:1/13" wheels

Thanks.

What rods?

8.1 will be fine, but to be honest, the way you fuel and control ignition is far more important. I run 8.6 and make around 180bhp on a gt1752

Standard a+ rods.

I know a lot of people use megajolt or similar now for ignition but to begin I'd rather use a dizzy.

Cheers.

Using a more modern turbo and Megajolt will make a more drivable (and enjoyable) car.

Shane

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Edited by Steve220 on 19th Feb, 2023.

I stuck to a dizzy for ages, belligerently, before going MJ and never looking back.

I agree with the comments above on the FD too. When I had the Metro Turbo 3.2:1 it was far nicer on the road than the 3.75:1 I used with the SC box for competition. Now going to 3.1:1 in the turbo.

Be interesting to see what you get out of that spec, I was at the dyno against a very similar spec in April 2021 (125.8hp @ 5385 & 135lbs/ft @ 4174)

Either way, good luck and let us know how you get on.

3.6:1 is the highest I can go unfortunately, that's the car ratio, the commercial ratio is even worse.

Why is 3.6:1 the highest you can go out of interest?
Is it due to the vehicle being RWD and that's the only diff ratios available?

If that's not the case, comparison below of FD's

A 175/50/13 tyre with a 3.4 diff is 16.43 MPH per 1000 RPM.
With a 3.2 diff gives 18.49 MPH per 1000 RPM.
The 3.4 gives 70MPH at around 4300 rpm vs 3800 of the 3.2.

Shane

What driveline are you using? If standard then it will be very marginal with 200bhp. For comparison I have 8.2 CR, megajolt,GT1752, c230bhp from 23 psi boost with cerometallic paddle clutch, type 9 sccr box and English/Atlas hybrid axle 4.2 ratio. Everything less broke.

'Where does the engine go?'

I know this is off topic but since we are talking about final drive ratios. What final drive do you guys think is best for quarter mile sprints ?

Turbo choice I think can make a huge difference, one of the earlier incarnations of my car I ran a hybrid T2 with a 3:1 FD (.958 drops) and that worked well at 150HP. It used to spool from low revs and launch brilliantly but would run out of puff by the end of the 1/4 but still would run mid to low 14's, I did manage to get in to the 13's eventually with a little more fettling.

I think I prefer that engine to my blown twin cam.

Shane

When you talk about ignition control are you only concerned with improved advance, or are you also saying multiple sparks per combustion cycle to help produce a better burn and gain better emissions?

Own the day

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Edited by stevieturbo on 26th Feb, 2023.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will