Safe boost?
Turbo Mini Forums

Hello everyone,

Sorry if this is a stupid thing to ask, I've done a lot of searching but wanted a bit of advice. I have recently bought my first mini turbo. It is a standard rebuilt metro block (standard pistons I believe) with a rally cam and running with a cosworth intercooler. It hasn't been rolling roaded yet, it is running about 8-10psi currently I believe. I'm going to get it dyno'd in the next week or two when it comes back from the garage.

When it is running ok can anyone give me some advice on how to find the boost range (min and max) as I'm not really sure. Or any other advice on what to do to make sure it is running safely/what to do next?

Sorry for my ignorance but I do not have great mechanical knowledge. Any help would be great.

Thanks

Is the gearbox/diff original? I wouldn't run anymore than 8psi...

Well I believe that it is original to the engine yes.

When I'm driving it seems that the engine revs very high in 4th, e.g. going 55mph it is doing about 4500rpm. Does this sound like the final drive may have been adjusted? It was built to rally as far as I am aware. Any ideas where to go next?

Thanks for replying!

Should be about 80 in 4th at 4500rpm so the final drive has probably been changed, if it was built for rallying it probably had the diff upgraded too (providing they had some common sense) all you can do is ask the previos owner or strip the engine/box. Welcome btw :)

Sounds like its got a very short diff in it then. Or your speedo is under reading by quite a lot.

If something is worth doing, it's worth doing half of.

The speedo does under read a bit as I have gone past speed checkers doing 35 when doing under 30 on the speedo. So I don't know if this would mean it was under reading even more at higher speeds. Just spoke to the previous owner and he said it is the standard box.

Would you recommend getting a compression test also before I start making more modifications?

Thanks

Use a sat nav to check the speed, then you can work out what final drive you're running. But this won't determine what diff you have which is the important part!

If everything else is standard then the compression ratio will be quite high, so this will be the limiting factor of how much boost you can safely run before worrying about the box/diff etc.

The only accurate way to check the compression ratio, is to whip the head off and take some measrements.

If it was me, I'd just take it to the rolling road and get it set up and see what it will run before detonating (a rolling road operator will know when to back it off). Have fun in it, and then when it comes to winter, have a strip down and see whats what.

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1/4mile in 13.2sec @ 111 terminal on 15psi

Ok that's great advice thanks :) That's what I'll do. Is there any way I can find out what diff is in it becuase it doesn't sound like the original owner knows it might not be standard?

I'll come back when I have got compression readings at a later date.

Thanks a lot

http://www.guess-works.com/Gearbox/Technical/ratio.htm