Centrifugal supercharger sizing?

[SuperV8]

The thread below has got me thining about centrifugal supercharger sizing so I thought I would start a new thread.
http://www.v8forum.co.uk/forum/viewtopic.php?t=6696

I have a procharger C1. I'm planning on fitting it to my re-built 4.6 rover V8. I've been doing some calcs (below) on my supercharger and trying to compare it to some similar Rotrex ones.


(edited)

Procharger don't publish compressor maps. They just state max air flow and pressure, supercharger max supported HP and base range of engine HP before supercharging.

My 4.6 falls fine into these figures, BUT no where do Procharger mention engine capacity/ efficiencies?. Reading through the Garrett website, it says capacity is an important factor in sizing the compressor.
e.g. It states a 2L 4cyl and a 5Lv8 both trying to achieve 400bhp would need the same mass of air but would need different compressors to achieve this. could this be applicable to supercharging?

I'm wondering how/if I can compare the procharger C1 to other compressors where compressor maps are published? Procharger publish inducer and exducer diameters. Would it be enough to roughly size the compressor using these parameters rather relying on their "marketing" blurb?

The C1 has a compressor exducer dia of 107.95mm. This seems much bigger than most turbos. It has an inducer dia of 69.75mm. The trim works out to 41.87 which seems quite low compared to turbos. It has a max rpm of 80,000 which seems quite high for a supercharger.

Really just trying to estimate if this supercharger is suitable for my 4.6. Aiming for 400bhp?

All help/ thoughts much appriciated.

Tom.

[kiwicar]

Hi
It is very difficult (as I found looking at that adapted GT35) to match a turbocharger housing to a supercharger application by looking at anything but maps. Normally the turbo is a free running device, with pressure ratio governed by the waist gate, there is therefor no fixed relationship between flow rate and pressure ratio. On a supercharger set up these two paramiters are goverened by engine capacity head flow (well actually volumetric efficiency) and gear ratio to the supercharger. As far as I can see from the applications lists, and lookang at the range of centrifugal superchargers available, matching is a case of ensuring the device has enough flow to feed the engine, and will operate ar the pressure ratio you require without dropping into areas of very poor compressor efficiency, as these devices are directly driven by the engine then the compressors can be made large enough so that they can have a large high efficiency areas, and there for can accomodate a wide range of capacitys and pressure ratios just by changing th drive ratio. Fortunatly there are three popular engines that are supercharged in the states the 4.6 ford Mod, the ford 302 and the 307 and 350 chevy the two fords are catered for by one size selection of supercharger, the 307/ 350 chevy by one of about 10- 15% larger flow, the net result is that the ones for the 302 / ford mod can cover the 4.6 rover pretty well by gearing them about 10% lower than in the ford application. I don't think you will get one to work with a 3.9 rover however.
Personally if you can't find a compressor map to plot the air requirments for the rover on then you will need to look for similarities in application of similarities with a compressor you have the map for, rather than trying to calculate the flow rates and pressure ration from inlet and outlet diamiters as I feel the flow moddeling will be very complicated.
Sorry not to be more precise.
Mike

[SuperV8]

Thanks mike,

My supercharger spec quotes 1000 CFM max and 24psi max.

I'm guessing that the 1000cfm max would be the flow in open air? 1 bar? 14.7psi. absolute.

I don't know much about the gas laws? if you halve the 1000cfm to 500cfm would that double the pressure to 2 bar absolute, 14.7 psi gauge? I'm sure it's much more complicated?

I've worked out I need approx 550cfm at 10psi for about 400bhp?

Tom.

[kiwicar]

What you have to do is work out what pressure ratio you need, then calculate what the new flow requirment is for the engine (basically old cubic flow times pressure ratio times comperssor efficiency and ajust for any charge cooler losses) and work out what revs on the supercharge that works out at, you need to do this for max revs of the engine and take account of the Volumetric efficiency at those revs. When you have done that you work out what PR and flow you will have at max power revs, again taking into account the VE at those revs. Hopefully you will have enough boost at max power. If not you then need to think about adjusting the supercharger revs and installing an overboost dump valve to take care of the Max revs situation. This type of supercharger produces 4 times the flow for a doubling of charger revs (roughly) so it is important to make sure the heads can flow well at high revs, otherwise heat and pressure ratio go up too fast beyond peak power and you get problems (often this equates to needing extended exhaust cam timing to get over short falls in hthe heads).

Keep in mind the company making kits for engines like the 302 ford have already done this extensivly in selecting the charger for their kit, if your application is in a similar state of tune as the kit was designed for you really only need to ajust the speed of the charger to match the slightly smaller flow requirments of your engine.
Example, a kit using the charger you want is designed to give 9 lb boost on a 302 ford running a standard heads and cam giving a VE of about 75% at max power and needing 450 cmf at 5500 revs on the charger. You have a 4.6 but have stage 3 heads and a bit more cam giving a VE about 85 at max power so your engine will require about 470 cu ft/ min, assuming you are making power peak power at the same revs the change in supercharger revs is the square root of the differance in flow, is square root 470/450 times the old revs, ie 56000 revs and should still give 9 lb boost.
if the peak power point changes, you have to work it out for the new one, and if you want more boost at lower of higher flow you have to take account of that, but if you stick to somthng not too exotic then you can probably use an existing speced kit and adjust to your rquirments of capacity and boost pressure.
Best regards
Mike

[stevieturbo]

Just compare to existing setups used on Similar sized engines.


flow figures wont matter whether free air or under pressure.

Either way, if the charger is spinning say 80,000rpm, then it will be shifting pretty much the same volume of air whether it exits under pressure or not.