Compression ratio's - how low can you go ?
Turbo Mini Forums

Need some advice folks :

I've bought a Metro Turbo engine which has been re-built into a 1986 Metro van block. According to the engine number it has a compression ratio of 8:1 as standard (i've checked this with several people and websites which mostly seem to agree).

I intend to build a well specced motor with this for my clubby but I am unsure about what sort of piston to fit (forged / cast & what dish size) and how much an even lower compression ratio will affect the off-boost driveability of the car.

A basic proposed spec is 1293cc with a stage 2 head and a phase 2 camshaft. I have the standard T3 freshly rebuilt but again i'm not sure about what sort of boost pressure it can cope with.

I'm hoping for around 125-130bhp from 12-15psi. Does this sound reasonable to you more experienced people ?

There's plenty of info around on this and other sites but if someone could set me off with some rock solid advice, i'd be very grateful !

I'll shut up now and let someone else say something !

Clubbyestate : Brown and loud - fast soon !

If it's got low-compression pistons, you can also assume these are 'low quality' and might not be too accepting to a 100+% increase in power than the original (at a guess) 60hp engine had...

I would disregard the metro van and 8.0:1 CR. In order to put up with boost pressure, at a guess metro turbo pistons and cylinder head will have been used, so more likely your CR is probably 9.4:1.
In any case you are going to have to have it in bits, so you can measure and determine what it is you really have got.

When it comes to deciding what pistons etc to use, it really depends how big your budget is. Many people use forged pistons, with maybe 15 or 18cc capacity. However, plenty of people also use cast pistons successfully.
My advice would be to choose your pistons first. Once you have done that, you are then in a position to calculate the chamber size required in your cylinder head to reach the CR you desire. There are two ways to go with this, low CR e.g. 7.5-8:1 allows high boost pressure to be used without risking detonation (assuming the engine is set up correctly) however this low CR will also make the engine less responsive off boost. Going up a bit on CR to maybe 8.5-9:1 will make the engine more responsive off boost, but you are likely to run into detonation problems. Its is your choice.

Personally I have 15cc forged omega pistons, with a Avonbar cylinder head, giving a CR of 7.95:1

Thanks Tom & Dave, that's excellent advice !

As the engine will be fully stripped for inspection before anything is decided I will be able to choose what compression ratio, piston type and boost pressure to run and then purchase parts accordingly.

On the face of it, a lower CR sounds more like what i'm aiming for.

For a start, i've been reading lots of threads on here and most of TurboPhil's website as well as reading the yellow bible a few times. This should hopefully give more words of wisdom and ensure the build is both a success and value for money.

Thanks again !

Clubbyestate : Brown and loud - fast soon !

Tom,

What boost pressure are you running / intending to run on that set up ?

and is that with a BK head gasket ?

Cheers Brocky

BTW - I took my buddy VTEC Pete out for a spin this weekend, and even though I am running her in, he gave it a name, the "Brocket"...cause I'm a brocket maaannn, nah nah nah nah nah nah nah nah nah nah nah....

Build thread - http://www.turbominis.co.uk/forums/index.php?p=vt&tid=7483

Katy Mini Owners Club Thread - http://www.turbominis.co.uk/forums/index.php?p=vt&tid=505552

Let's make sure you embarrass at least one VTEC MINI today !

https://www.facebook.com/groups/17557298589...096464253807802

The engine is running but not under load yet, as the car has no doors etc. Also it is yet to be set up. The head gasket is the Group A item.

I am planning to run around 1 bar of boost and see where I am power wise at that level.

The 125-130bhp will be easily achivable with 15psi with the engine built and set up to avoid detonation.

-MINI CLUBMAN 1380 TURBO-

you should be able to get that with lowering cr to much and run about 10 to 12 psi i would have imagined, use a decent intercooler get set up well, my rr man uses det cans which help no end.

always looking for them bigger bunches of bannanas