Supercharger Calculation

[Twinky]

Hi guys,

 

Recently I have Fitted a supercharger to my EFI 5k. The supercharger is an AMR500 with a 70mm charger pulley and a 100mm crank pulley.

My first question is about the calculator used in the FAQ/WIKI section relating to the calculation of boost.

Has anyone used the calculation and had a correct answer from it? From my initial calculations the answer came out to be a negative boost number

because my pulley ratio is too low according to the calculator. From memory my calculation was -0.9psi.

 

So for shits and giggles I decided to whack it on anyway to get the ball rolling on the project and then find a bigger pulley later. To my surprise

the supercharger actually put out 3psi.

 

Now I did a second calculation to see how things fit in, I'll actually write it all down.

 

1.5ltrs x 5000 rpm = 7500/2 = 3750ltrs/min (determining air requirements at 0psi)

3psi + 14.7 = 17.7/14.7 = 1.204 (boost ratio)

3750 x 1.204 = 4515lts/min @ 3psi (air requirements at boost)

4515/0.5 = 9000 rpm (supercharger speed)

9000 / 5000 = 1.8 (pulley ratio for 3psi boost @ 5000 rpm

 

So already you can see that there is either a slight error in the calculator or just that the calculator can not take into account all aspects.

I don't know I'm not a mathematician but My ratio is only 1.4 and I get 3psi. at 1.8 I would be roughly getting 4-5 psi in real world.

 

Has anyone else had any experience using this calculator?

 

cheers

[kickn5k]

I used that calculation a few years ago on my sc12 5k. Was only 1-2psi out.

Are you sure the size of the AMR is correct? They are smaller than the sc12(1.2ltr). I'd imagine they'd be 1ltr or less.

[Twinky]

The AMR500 is a 500cc per revolution supercharger, which is the 0.5 in the calculation.

[kickn5k]

The AMR500 is a 500cc per revolution supercharger, which is the 0.5 in the calculation.

 

Serves me right for not looking at the equation properly. Sorry about that.

That's got me stumped then, as it seemed pretty close for me. I'd send Mhmills a pm he's played with an AMR500 before(sorry Jake)

[Twinky]

Yeah no worries, It's just that the calculator could be a bit misleading so maybe a forewarning that it should only bee used to get a rough idea or something

along the lines of that. Thanks for your input though as at least there is a few people who have used the calculator with some reasonable results.

[irokin]

Assuming 100% VE?

 

Is 1.204 (20°C) the right air density for the temperature?

[snot35]

The problem with those calcs to my mind is that they assume 100% volumetric efficiency which rarely happens in the real world and far less likely to happen with a K motor. Those calcs assume that the engine will take in the full 1.5L every two revs. It's more likely that you have 75% efficiency (wild arse guess) and are only taking in 1.125L. In the case the SC has an easier time supplying the required air so you're more likely to get more boost.

 

Try mucking around with the ingested volume and see what answer you get. I'd do it, but I can't be bothered :P :)

[snot35]

Damn you irokin and your short arsed answers :P

[Twinky]

I guess that's the unfortunate nature of equations, they are too specific in where real world values can be so variable that the equation can only be used as a guide.

Lets just play around with it though.

 

Lets say that I do in fact have 75% efficiency:

 

1.5 x .75 = 1.125 x 5000rpm = 5625 / 2 = 2812.5 ltrs / min

3psi + 14.97 (atmo pressure in Adelaide as of Monday) = 17.97 / 14.97 = 1.2 (so only a slight variation)

2812.5 x 1.2 = 3375 ltrs / min @ 3psi

3375 / 0.5 = 6750rpm (supercharger speed)

6750 / 5000 = 1.35 pulley ratio (3.6% difference to my 1.4 ratio @3psi)

 

So after the amendments this seems to be alot more accurate in regards to real world figures.

Maybe that could be added as a footnote to the text of that FAQ/WIKI section?

[irokin]

Damn you irokin and your short arsed answers :P

 

hehe sorry. I started working back to try and figure out the actual VE then decided I couldn't be arsed :P

 

As an apology here's a corrected equation:

 

(1.5 * 0.75) x 5000 rpm = 5625/2 = 2812.5ltrs/min (determining air requirements at 0psi)

3psi + 14.7 = 17.7/14.7 = 1.204 (boost ratio)

2812.5 x 1.204 = 3386.25lts/min @ 3psi (air requirements at boost)

3386.25/0.5 = 6772.5 rpm (supercharger speed)

6772.5 / 5000 = 1.35 (pulley ratio for 3psi boost @ 5000 rpm

 

So 75% VE is not far off the mark. Using the dirty JS I whipped up below the VE works out to be about 78%.

 

 

 

<html>
Capacity (in cc):<input id="capacity" name="capacity"><br>
VE %:<input id="ve" name="ve"><br>
Boost (in PSI):<input id="boost" name="boost"><br>
RPM:<input id="rpm" name="rpm"><br>
<input type="button" onClick="doCalc();" value="Calculate"><br>
Pulley ratio:<input id="ratio" name="ratio"><br>

<script type="text/javascript">
function doCalc() {
capacity = (document.getElementById("capacity").value) / 1000;
ve = document.getElementById("ve").value;
boost = document.getElementById("boost").value;
rpm = document.getElementById("rpm").value;

airreq = ((capacity * (ve / 100)) * rpm) / 2;
bratio = (Number(boost) + 14.7) / 14.7;
atbreq = airreq * bratio;
chspd  = atbreq / 0.5;
ratio  = chspd / rpm;

document.getElementById("ratio").value = ratio;
}
</script>
</html>

[Twinky]

While I am here I might do the calculation for my next pulley up size!

 

5000rpm x 2 x 0.5 = 5000 ltrs / min

5000 / 2812.5 = 1.77 ((1.77 x 14.97) - 149.7) = 11.5psi

Holy shiznit! maybe going up to 2:1 ratio might be a bit too much....

1.7:1 should give me nearly 8 psi. That sounds more reasonalbe.

[Taz_Rx]

11psi won't be too much for the motor, but I would start to wonder if it would be starting to over work the little charger?

[Twinky]

I say that it's a bit much because there will be a turbo going on as well with no bypass valve.

So lets say that my supercharger is putting out 7psi and my turbo will add another 7psi on top of that.

Whereas if the supercharger was putting out 11psi the total boost would be around 18psi, a bit too much for a standard motor.

 

The AMR500 has a safe operating speed at 15,000rpm, at even 2:1 it would only see 10,000 maybe even 12,000 peak rpm.

[Taz_Rx]

Ah well in that case yeah just stick with the 7-8ish from the charger......and make the 11 psi with the turbo! Hehe

[snot35]

Is the AMR500 an always on SC, or clutched like an SC12? If it's clutched then just because it can do 15,000, doesn't mean it wants to do it a lot!