Gti6 Supercharged Boost Question

[ydoidoit 1 1 Cars]

I'm hoping that some one may be able to assist with this.

I'm in the process of putting a home brewed supercharged gti6 engine in my 205.

The engine is standard apart from a head skim following a head gasket failure. The guy who skimmed the head thought it had a previous skim. I know the gti6 head doesn't have much spare meat on it but the head is now level with the valves. Is this a problem?

I'm going to be using standard cams.

With regard to the boost issue. I'm using an Eaton M45 from a mini which will be blowing through an intercooler then a standard throttle body and then into the shortened inlet manifold.

Whilst researching the project I've read that on the Lynx low pressure conversion they use a restrictor on the inlet to reduce the max boost. Is this because they use the rotrex charger or should I be considering some thing similar?

I'm aiming for around 250bhp, but can't afford forged rod/custom pistons etc at the moment.

Any advice would be very welcome

[Tom Fenton 1,524]

The Rotrex charger is a centrifugal supercharger, really it could be described as a mechanically driven half a turbo. Because of this the flow rises exponentially which in practise means that unless you do something to limit it you would end up with more boost than desired hence a restrictor.

The Eaton charger is a fixed displacement Roots blower, this means that the boost pressure is determined by how fast it turns, as it will push out a fixed volume of air per revolution. So in practise boost is determined by the size ratio of the crank pulley to the charger pulley. Need to be a bit careful not to overspeed the charger, the Eaton type does turn into a heat pump if you turn if too fast. However having said all that Allan's M45 Mi16 car uses something like 7psi to make it's 250bhp so you are on the right lines. Do you know him as he lives near Buxton?

Other things to consider are can timing, you want to look to reduce overlap so that the charger doesn't blow fresh charge straight into the exhaust.

[stef205 91 1 Cars]

I have also done this, as much as mine doesn't make as much power as Allens. I run mine on standard internals, but my engine was a very low mileage engine with no skimming on head. Take a look at mine or Allens thread if you need some ideas.

 

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[ydoidoit 1 1 Cars]

Thanks for the advice guys. That all makes sense. Does anyone know what boost it will produce with standard size pulleys or how to work it out?

 

I've spoken to Allan through the forum but don't actually know him. I'm sure I'll probably require his help at some point!

[stef205 91 1 Cars]

Standard pulley is -11% @ 65mm I'm currently seeing 4.5psi but I could have cam timing issues I know Allen uses a -13% pulley which I think is 57mm I know he sees around 6-8psi I think. I haven't had any heat issues with mine as its right behind the grill gets lots of air flow. Mine saw 205bhp @ 3 psi with a boost leak.

 

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[allanallen 528 2 Cars]

Standard mini pulley is 65mm and as stef says mine runs a 57mm pulley and makes 8psi.

The head on mine has also had a fairly heavy skim, never had any issues with it but its been very well mapped to run safely

[Tom Fenton 1,524]

You can get in the ballpark doing some calculations.

 

Work out the ratio of supercharger pulley to crank pulley.

 

Then look up the displacement of the charger per rev, convert this using ratio above to give you charger displacement per one ENGINE rev, and divide this into the amount of air the engine consumes per rev.

[allanallen 528 2 Cars]

You can get in the ballpark doing some calculations.

 

Work out the ratio of supercharger pulley to crank pulley.

 

Then look up the displacement of the charger per rev, convert this using ratio above to give you charger displacement per one ENGINE rev, and divide this into the amount of air the engine consumes per rev.

 

Or just guess

[ydoidoit 1 1 Cars]

Right I've done some man maths and come up with the following based on Tom's above method. Please feel free to shoot me down if I've made a hash of it!

 

The displacement of the Eaton M45 is 0.737 ltrs per rev.

Standard bottom pulley is 126.7mm and charger pulley is 65mm giving a ratio of 1.95.

0.737x1.95 gives 1.437 ltrs per rev of the engine.

1.437 divided by the engine displacement of 1.998 ltrs gives 0.72. So does this represent 7psi of boost?

 

I re did the figures using the smaller 57mm pulley and got 0.81 so would make sense if Allan gets 8psi.

[stef205 91 1 Cars]

Thanks for that now I know I have a big issue haha that's good to know.

 

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[petert 549]

Have you seen the Blower Calculator on the Sprintex site?

[stef205 91 1 Cars]

Never see that before, Peter do you believe boost can be lost through bad cam timing?

[welshpug 1,621]

massively so if you have long duration cams and the charge is just being blown straight through on overlap.

[Tom Fenton 1,524]

Right I've done some man maths and come up with the following based on Tom's above method. Please feel free to shoot me down if I've made a hash of it!

 

The displacement of the Eaton M45 is 0.737 ltrs per rev.

Standard bottom pulley is 126.7mm and charger pulley is 65mm giving a ratio of 1.95.

0.737x1.95 gives 1.437 ltrs per rev of the engine.

1.437 divided by the engine displacement of 1.998 ltrs gives 0.72. So does this represent 7psi of boost?

 

I re did the figures using the smaller 57mm pulley and got 0.81 so would make sense if Allan gets 8psi.

 

Kind of, your displacement per rev is wrong, I guess I haven't explained it that well.

 

First of all the displacement *PER REV* is 1998/2. Think about the 4 stroke cycle, in one complete crank revolution of 360 degrees you will have two different cylinders on induction, one after the other. So engine displacement (air consumed) per rev is 999cc.

 

Your pulley ratio calcs are correct.

 

So now do 1437 / (1998/2) and you get an answer of 1.438.

 

This is the pressure ratio, which I won't explain here, but basically what it is saying is that you should get 0.438 bar boost pressure. Multiply this by 14.7 to convert to psi and you get 6.4psi which seems a credible answer.

 

Alter the pulley to 57mm and you get 9.4psi.

 

Now these are only theoretical calculations, in practise many other things can affect the boost pressure, such as intake temp, air density, cam timing, blah blah blah the list goes on. So don't expect these numbers to be cock on accurate when you've built the engine. But at least they will get you in roughly the right place.

[stef205 91 1 Cars]

That seems more realistic Tom, I think I'll defiantly have to get someone to do my cam timing.

 

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[ydoidoit 1 1 Cars]

Thanks for that Tom. Gives me a rough idea. I'm more interested in making sure I don't get to much boost and destroy my engine as its on standard internals and a high compression ratio.

Cheers