supercharged boost potential?

SuperV8 · Post by **SuperV8** » Mon Sep 26, 2011 8:56 am

Finally got to do some engine tuning over the weekend. Didn't do any WOT mainly cruize areas of the fuel map.
When reviewing the logs of my runs it got to 2.1psi at 50% throttle and 3000 rpm. I'm using a centrifugal supercharger so the boost doesn't rise linearly.

Can anyone have an educated guess as to what boost I could expect at 6000 rpm and 100% throttle?

Tom.

kiwicar · Post by **kiwicar** » Mon Sep 26, 2011 10:48 am

Hi In theory if you double the speed of one of these units then you multiply the airflow through it by 4, in theory you should therefor see about 4.2 PSI. . . however the VE of the engine will change depending on heads and cam so if you are running with a VE of say 85% at 3000 rpm and only 75% at 6000 rpm you will see closer to 5 PSI boost at 50% throttle. There is also a big issue of what 50% throttle actually means as far as cylinder filling and over all VE are concerned but a stab of about 7 to 12 PSI at 100% throttle would be a start. What the throttle plate is doing to the eficiency of the supercharger I haven't even considered and it really is just a guess, if you had figures for 2 rpm pointa and 100% throttle at both then I would be happier to have another go.
Best regards
Mike

SuperV8 · Post by **SuperV8** » Mon Sep 26, 2011 11:55 am

Thanks Mike,
I'm aiming for about 10psi which should see me at arround 400bhp but must admit even at just 2psi and half throttle it was going some! in my 700kg car!

I though I was getting belt slip due to a little black dust near the pulley but not sure I am now.

I will have another go tonight see if I can get to 75% throttle

Tom.

kiwicar · Post by **kiwicar** » Mon Sep 26, 2011 1:14 pm

Hi
the easiest way is to work it out from what the thing is rated at and work back from there using the gear ratio you are drivng it at. If you know max flow at max rated revs and devide by 4 every time you half the revs it should give you a base flow curve to work from using the engine displacment. If you have a VE curve for the engine without the supercharger you can turn that into a boost curve using the engine displacment and the supercharger efficiency map and then I recon you should be able to get within 4 or 5% by calculation. Or go out in the dry, point it up a hill in 3rd gear and stick your foot on the floor and plot 3 points over about 1500-2000 revs and plot it from there. the problen with part throttle readings is you don't know how much restriction to flow there is at any given rev point.
Best regards
Mike

stevieturbo · Post by **stevieturbo** » Mon Sep 26, 2011 6:55 pm

The only way to find out is to try.

Boost will not be linear with rpm.

If you feel it may go too high, you could set a boost cut on the ecu, or just tread carefully with your right foot.

But beforehand, make sure any fuel/timing maps are cautious beyond boost levels you are hoping to see.

SuperV8 · Post by **SuperV8** » Tue Sep 27, 2011 7:26 pm

Well i drove to work in my car today logging all the way.
Peaked at 5.3psi at 5100 rpm and 56% throttle! seems i'm a big gessy!
Really does start to motor above 3000rpm

Same log showed 2.5psi at 56% throttle and 2870rpm.
Wonder how much the nearly half closed throttle effects the psi seen in the plenum?
Going from the above it increased 2.8psi with an increase of 2200rpm so I think it would be fair to 'predict' that I could get another 1.5psi upto 6000rpm so nearly 7psi at 56% throttle! Just need to find some waterproof pants to get more than 56% throttle!
Either way 7psi and should be over 300bhp, still got more rev's potential from the supercharger so looking at smaller pulley when I re-design my belt system oh and some new rear tyres

Tom.

stevieturbo · Post by **stevieturbo** » Tue Sep 27, 2011 9:40 pm

Totally depends on how big your throttle body is. If its' bigger than necessary, then at 50% it could be letting in all the airflow the engine would ever need anyway.

Again, the only way to find out how much boost you'll get is to try