18 replies to this topic

[Verbaliser]

Hi,

I'm anxious that the pressure will be too high and would like to be able to see exactly what it is. So, I'm considering fitting a boost gauge - a mechanical one.

It looks to be fairly straight forward - attaching a pipe with a brass fitting on either end. It looks as though the pipes you can buy are plastic, but if this connects into the inlet manifold under the charger, won't the plastic melt with the heat?

Also, can someone explain to me how to work out what the pressure is likely to be?

I have been reading up on it but just can't piece it together, and I certainly can't read the Eaton performance maps.

I'm charging a 998cc engine with 34cc upswept volume in the cylinder head CR = 8.4:1

Crank pulley 108mm (I think)
Charger pulley 65mm
Drive ratio = 1.66
therefore at 6,000 rpm engine speed, charger speed will be 6000 x 1.66 = 9970rpm

Something else about flow rates but here it where I get lost...

How do I get to psi from here???

Is there a simple method to gauge pressure from relative rpm??

[Sammy D]

There's a boost calculator on the Eaton website buddy. Google the M45, click the eaton link and in that page somewhere is a link to a calculator where you can put in all the variables from above and it'll tell you how much air it will be displacing etc.

I'll try find the link..

Edit: it's at the bottom of this page;
http://www.eaton.com.../Superchargers/

Edited by Sammy D, 07 August 2010 - 08:24 AM.

[Verbaliser]

Cheers for that, I didn't realise there was a calculator tool on the Eaton website. I had a play but its not terribly user friendly (for the non-enginnering minded folk anyway).

I looked up the pressure formula online so I'll post my numbers a bit later on. It came out around 7 psi though at 6,000 rpm (engine speed).

[Wil_h]

Well I calculate 13psi, but you'll lose a few through the increase in temp of the charge, so maybe 10psi

[Verbaliser]

Right, here are my calculations:


VMAX Crank Pulley Circumference = 108mm (I think)
Standard S/C Pulley Circumference = 65.5mm
108/65.5 = 1.65.

Therefore drive ratio = 1.65:1

@ 6000 engine rpm = S/C spinning at 6000 x 1.65 = 9900 rpm (optimum S/C speed is 10,000 above which S/C converts energy into heat rather than HP)

M45 supercharger output capacity = 45 cubic inches of air per revolution = 737cc's = 0.737 litres

So, 0.737 litres for 1 rev
therefore 0.737 litres x 9900rpm = 7296 litres per minute
Convert into cubic feet per minute = 7296 x 0.0353146667 = 258 CFM

998cc engine will displace 499cc’s per revolution (1/2 engine capacity)
therefore 0.499 litres x 6000rpm = 2994 litres per minute
Convert into cubic feet per minute = 2994 x 0.0353146667 = 106 CFM

or using a different calculation:

(cubic inches*RPM/3464) 60.9*6000/3464 = 106 CFM

Excess air = 258 – 106 = 152 “boosted” CFM @ 6000rpm

Boost = (excess CFM / normal CFM * atmospheric pressure) - constant for atmospheric pressure

(152 / 106 * 14.7) – 14.7 = 6.4 psi @ 6000 rpm

Does seem a bit low. Does this look right, or have I missed something? Perhaps I need to control for 85% Volumetric efficiency?

[Wil_h]

Sorry, I calculated for a 1275, for a 998 with a 1.66:1 pulley ratio you'll end up with around 20psi. For a 998 you need 1:1 ratio really.

I don't really follow your maths to be honest, but it is wrong somewhere.

At nearly 10k rpm you'll be pumping enough air for 150bhp. But you'll also create around 20+ psi of boost, which is so far off the published map http://www.eaton.com...t/ct_128483.gif that you'll probably melt your inlet manifold!

[Verbaliser]

I'm lost then! Jonspeed say with their supplied kit, boost is 8psi or thereabouts. I can't believe my set up will create 20+ psi using the standard s/c pulley and supplied crank pulley (same size for 998 and 1275).

My maths must be wrong. Can someone please shed some light.

[Verbaliser]

Spoke to Stuart today at VMAX who told me the boost on a 998cc @ 7,000 rpm (engine sped) would be around 10 psi - a few psi higher than for a 1275cc. It also means the charger will spin at 10,000 rpm - lots of heat. I'm not sure how this figure is derived, so I would be very grateful to anyone on here who could enlighten me.

Furthermore, I research the old shorrock C75 supercharger for comparison and that also displaces 750cc's per revolution and with approximately a 1.08:1 crank to charger pulley ratio, it was generating 10 psi @ 7,000 rpm. With almost a 1:1 ratio the charger is spinning marginally faster than the crank. I know they are different types of supercharger but how can the figures come out so different?

Plugging the shorrock figures into my formula gives the psi mentioned above, so how come the eaton figures are way out?

Seriously confused...!



Shorrock C75b – 750cc per rev

(89/82) = 1.0854:1 pulley ratio

(1.0854 * 14.7 * 750 / (998/2) ) – 14.7

(15.96 * 1.503) – 14.7 = 9.3 psi


Eaton M45 – 750cc per rev

(110/65.5) = 1.680:1 pulley ratio

1.680 * 14.7 * 750/ (998/2) ) – 14.7

(24.687 * 1.503) – 14.7 = 22.4 psi

Whichever way I do it now I always get 22.4 psi.

I'm going to do a compression test on the inlet manifold when the charger is installed since I can't rely on my Maths! Makes we want to abandon the project so my engine doesn't blow up!!

Edited by Verbaliser, 10 August 2010 - 08:36 PM.

[Brett]

i get the same figures as wil,
the engine consumes 499cc per revolution the charger makes 737cc per revolution (@ 1:1) the difference is the pressure

737
___

499 = 1.48


1.48 x 14.7 - 14.7 = 7.01 psi gauge

or with a 1:1.66 ratio ((737x1.66/499)x14.7)= 36.04-14.7 = 21.34psi gauge

Edited by Brett, 13 August 2010 - 10:14 PM.

[Brett]

with the error in the superchargers displacement and the slight increase in crank-charger (1.66-1.68)drive ratio that is where your 22.4psi comes from

if the supercharger can even run that high pressure the temp will be in the clouds soo on conclusion the 1.68 ratio on the charger will end in tears
cr 8.4, no intercooler, suck through fueling?, id rather not

Edited by Brett, 13 August 2010 - 10:18 PM.

[Verbaliser]

I'm glad it's not just me then. Apart from an American firm selling slightly larger m45 pulleys, I am struggling to source a pulley big enough to achieve a 1:1ish drive ratio. More chance of getting a larger diameter m62 pulley (6 groove). Mercedes charge £300 for a pulley alone!

[Verbaliser]

Eaton say displacement is 750cc but u would think M45 implies 45 cubic inches = 737cc

[Verbaliser]

Research continues...I thought people may be interested in this because when I was looking for this sort of info, it was hard to come by.

Because I am running a SWIFTUNE SW5-07 camshaft, I have to account for 25 degrees of valve overlap (calculations pinched from forum posting by AC Dodd Engineering, Nov 2009). The pressure calculation previously discussed, says that roughly there is 5% reduction in boost for every 10 degrees of overlap. Therefore, the psi figure will reduce by 5% x 2.5 = 12.5%.

Apparently different cams will have different valve overlap and is calculated by adding the first inlet figure and last exhaust figure.

E.g. For SW5-07 - Timing: 10/52 (inlet) 47/15 (exhaust). So overlap = 10+15 = 25 degrees.

So, I have amended my calculations;

Using stock 65mm cooper s pulley - 19psi at 10,076rpm (charger speed) 62% thermal efficiency
Using 83mm pulley - 12psi at 7,952rpm (charger speed) 65% thermal efficiency
Using 92mm pulley - 10psi at 7,174rpm (charger speed) 66% thermal efficiency

Maximum charger thermal efficiency is 67% based on my interpretation of the Eaton Performance Map. Basically, thermal efficiency is power you get out / power you put in. Therefore the higher the better.

So, using 92mm pulley (simplified somewhat):

(Pulley ratio x pressure ratio x 1 atmospheric pressure) - 1 atmospheric pressure

Pulley ratio =110mm (crank pulley) / 92mm (s/c pulley) = 1.2
x
Pressure ratio = displacement of M45 737cc / engine capacity 998cc divided by 2 = 737 / 499 = 1.48
x
1 atmospheric pressure, 14.7 psi

So,

(1.2 x 1.48 x 14.7 ) - 14.7 = 11.3psi

minus adjustment for boost lost from valve overlap

=11.3 - (11.3 x 12.5%) = 9.85

final figures based on full values (not rounded as above) and gives a hteoretical boost figure. Actual may be affected by other factors.

Now I have to decide if I want an 83mm or 92mm supercharger pulley - Jonspeedracing can make any size apparently for £60.

Thoughts anyone?

[Wil_h]

Personally I'd go for the largest one and aim for 10psi.

[Verbaliser]

Opted for a smaller crank pulley instead, utilising the stock s/c pulley. According to my calculations, an 82mm crank pulley will give me a theoretical boost of 9psi on a my 998cc engine with cam choice SW5-07. I'm guessing that theoretical boost is likely to be higher than actual boost...better to start lower.

Just to make sure though I have purchased a 52mm Stack mechanical boost gauge which i'm going to plumb into the underside of the manifold with some silicone 3mm tubing. I might get an overbraid kit or heat shield tape for the pipe to protect it a bit more from the heat of the exhaust. Hoses good for 200 degrees - hope that's enough - don't want it to melt

Fingers crossed!

I hope I'm on the right lines....