I thought these may be useful for someone. I have just finished doing the system calcs for my supercharger. Of course these are theoretical....

2 litre engine (122cid)
Stock power 180bhp @ 6800rpm
Volumetric efficiency = 80% (generally accepted for modern 4 valve engines)

The volumetric efficiency of a centrifugal supercharger is 100%

VE Ratio = Supercharger VE/Engine VE
= 100%/80%
= 1.2

Pressure ratio (PR) estimate = desired BHP/stock BHP
= 340bhp/180
= 1.88

This tells you how much boost you need in theory to get your desired BHP, though its not always that simple!

So thats 13 psi of boost

Next calc the temp gain of the inlet air at that given pressure ratio.

Temp gain = (PR^0.28 -1)x Absolute temp)/Supercharger efficiency

= (1.88^0.28 - 1 x 77+460)/0.8
= 130 deg F

Density ratio (non intercooled) = Original temp/new temp
= (77+460)/(77+460+130)
= 0.8 (this is a 20% reduction in air density at this temp.

Obviously this is where intercooling comes into its own.

Assume an intercooler efficiency of 82% (pretty easily achievable if its got good airflow like mine will have)

Outlet temp of intercooler = 130 x .82 = 23.4deg F above ambient

So:

Density ratio (intercooled) = (77+460)/(77+460+23.4)
= 0.96

So only a 4% decrease in air density with an intercooler

Now you need to find out what your true pressure ratio needs to be (this could be higher or lower than your theoretical depending on your engine VE and whether or not you have an intercooler)

I am using an intercooler so:

Pressure ratio = desired bhp / (stock bhp*density ratio*ve ratio*drive efficiency (90% is a good estimate))

= 340/180*0.96*1.2*0.9
= 1.82

So I need around 12psi of boost to get 340bhp

For non intercooled apps you use the same calc as above, but you need to do another iteration of the calcs with your new density ratio to get an accurate result.

Next thing to do is calculate the peak airflow requirment, first calc the naturally aspirated airflow requirement:

Required airflow (CFM) = (CID*rpm*0.5*engine VE)/1728
= 122*6800*0.5*0.8/1728
= 192CFM

Boosted CFM = CFM*PR
= 192*1.82
= 350CFM

Convert to KG/s divide by 1731.8

So air flow = 0.2kg/s

This will let you pick the right rotrex.

Basically a C30-94 sits at about 80% efficiency at a 1.8 PR at this air flow. This also equates to about 75,000rpm impeller speed.

The stock crank pulley is 132mm on the duratec.

So:

Pulley ratio = impellor speed / charger internal gear ratio x redline rpm
= 75000 / 9.49*7200
= 1.097

So the charger needs a 132/1.079 pulley. Or 120mm

Now the biggest pulley rotrex make is 110mm, so there are 2 options. Hope that your intercooler pressure drop is high (2 psi or more) and use the 110mm pulley (every 5mm smaller equates to about a 1psi increase in boost which is handy) or get a smaller crank pulley from Raceco which is 100mm diameter and use a 90mm pulley.

This is just my ramblings of tonights calcs. So if it doesnt make sense, ask

David

[Edited on 3/9/09 by flak monkey]

Also on the subject of intercoolers, I have had a quote from GRS and Pace to supply one which sits infront of the polo rad. It has a massive frontal area meaning the efficiency temp wise will be high. Sadly the pressure loss will be around 2psi I think because of the small entry and exit areas.

The quote is around £300 from both companies.

If it will fit in your application I might be able to get more than 1 made at a lower price, but I havent asked yet.
Rescued attachment Intercooler sketches.jpg

The maths make sense to me. Only trouble is if you read Forced Induction tuning (Graham A. Bell) he states before virtually every equation that it's only to get you into roughly the right ballpark.... e.g. the equations don't take into account the effect of airflow, valve timing etc etc.

EG an engine with a "tasty" cam with masses of overlap will produce lots of power when NA due to better scavenging of exhaust gases. Strap on forced induction and you won't get much more BHP due to all your lovely boost peeing out of the exhaust valve being kept open for such a long amount....
Equally a poorly ported head with small valves and a restrictive air inlet will produce low power NA but a lot more FI when the flow changes.

Only issue I have with intercoolers is that most OEMs are built very long and wide (ie a few long tubes). Good efficient cooling but also very restrictive....

Got to ask how the fu*K do you know all that stuff!!!

My brain just fliped out looking at that stuff!

quote:

---

disired bhp = 340

---

Favourited!

What higher compliment can be made Flak!

Cheers,
James

It will certainly be interesting to see how close the reality follows the theory.

The duratec std cams are very low overlap (helps with emmissions), high lift and moderate duration, so should in theory be well suited to supercharged applications. Again....in theory!

I am going to lower the CR 1 point (to 9:1) and run 1 bar of boost in total, making the assumption I will loose around 2 psi through the intercooler. Lowering the CR by a point is only going to make a very small difference lower down the rev range, but should make it a lot easier to tune higher up.

David

When I was running high power turbo Audis we used g/s / 0.8 to get the power output as an approximation. My RS4 ran 440 g/s through the MAF so 550 BHP which was validated on a Rolling Road (actually 542BHP). I assume this relates to your 80% efficient for modern cars.

If you run the same calculation on 200g/s you only get 250BHP not 340BHP. Since you can't produce any more power than the air that goes into the engine I think your calculations have gone wrong somewhere. You will need 272g/s to make 340BHP using that calculation or I could be talking complete nonesense



[Edited on 4/9/09 by jeffw]

this is very handy and interesting stuff. Not to sure how mine will turn out yet, want it to be a safely built engine so it is not stressed.
I wuold be very interested in the intercooler if you were having more than one made.
I currently run an escort rad and was planning to have the intercooler connection going round the side but think i shall measure up for a polo rad and the set up you have shown.
cheers again

[Edited on 4/9/09 by goaty]

Just a note:

You calculated the VE ratio as 100/80 = 1.2, but 100/80 = 1.25 when I went to school!

Regards
Hugh

quote:

---

Originally posted by matt.c
Got to ask how the fu*K do you know all that stuff!!!

---

quote:

---

Originally posted by matt.c
Got to ask how the fu*K do you know all that stuff!!!

My brain just fliped out looking at that stuff!

---

Hey flak,

180 --> 340bhp

does the duratec require any uprated internals to run at 1 bar of boost?

interested in doing something similar with my XE after its on the road (preferably supercharged) so I'm interested in your setup. Which resources are you using for the task? I understand its a bit of a headscratcher to get something like this working?

Are you pressurising the standard induction with a single throttle body or running forced ITBs?

[Edited on 4/9/09 by cd.thomson]

quote:

---

Originally posted by cd.thomson
Hey flak,

180 --> 340bhp

does the duratec require any uprated internals to run at 1 bar of boost?

interested in doing something similar with my XE after its on the road (preferably supercharged) so I'm interested in your setup. Which resources are you using for the task? I understand its a bit of a headscratcher to get something like this working?

Are you pressurising the standard induction with a single throttle body or running forced ITBs?

---

awesome, thanks for that, ill keep my eye on it as it progresses .

quote:

---

Originally posted by flak monkey
but to be honest anything over 30bhp is going to be more than enough

---

I recognised the the format of the calculations a I used the same book for my research. My setup is very similar with ITBs and a BOV except I'm using a 1.6 4AGE. I have 270BHP with 1 bar boost initially on 10.5:1 (cast pistons) but currently 9:1 (forged) but ideally you want closer to 10:1 as the Rotrex works better with higher CRs.

If you haven't bought the kit already speak to Andrew at OMEX as they have done a lot of dyno work with Duratecs using the Rotrex. They could save you some dyno time by using their experience but they are a business remember.

Oh and your intercooler looks to be approx 30% wider than my MR2 one I'm using in front of the Polo rad.

[Edited on 4-9-09 by bimbleuk]

quote:

---

Originally posted by bimbleuk
I recognised the the format of the calculations a I used the same book for my research. My setup is very similar with ITBs and a BOV except I'm using a 1.6 4AGE. I have 270BHP with 1 bar boost initially on 10.5:1 (cast pistons) but currently 9:1 (forged) but ideally you want closer to 10:1 as the Rotrex works better with higher CRs.

If you haven't bought the kit already speak to Andrew at OMEX as they have done a lot of dyno work with Duratecs using the Rotrex. They could save you some dyno time by using their experience but they are a business remember.

Oh and your intercooler looks to be approx 30% wider than my MR2 one I'm using in front of the Polo rad.

[Edited on 4-9-09 by bimbleuk]

---

flak,
what management are you planning on using??
i ask as you said you will be running TB's blown through, but i have heard that getting the mapping set up on this can be a task and usually involves two maps unless you use a MAF....
I am planning the simpler 1 TB with a map sensor to keep it in check.
cheers

quote:

---

Originally posted by goaty
flak,
what management are you planning on using??
i ask as you said you will be running TB's blown through, but i have heard that getting the mapping set up on this can be a task and usually involves two maps unless you use a MAF....
I am planning the simpler 1 TB with a map sensor to keep it in check.
cheers

---

does the map sensor not have to be on the engine side fo the butterflys??
how would it work across all 4??

quote:

---

Originally posted by goaty
does the map sensor not have to be on the engine side fo the butterflys??
how would it work across all 4??

---

quote:

---

Originally posted by flak monkey

quote:

---

Originally posted by goaty
does the map sensor not have to be on the engine side fo the butterflys??
how would it work across all 4??

---

Yep, its connected in exactly the same way as in a NA application using the vacuum lines on the engine side of the butterflies.

Pulses from each cylinder are apparently less of an issue when using forced induction.

David

David

---

the issue with tbs isnt the normal fuelling, its the transient fuelling because when you open the trottle you have an intake plenum full of boosted air rather than having to make some.

David,

I took a quick glance at your calcs and you've seemed to have left out the power needed to actually drive the supercharger. At 340BHP@1 bar boost, this is going to be significant.

Or; in the real world: just whack the nearest size pulley on and see what happens

Sebastiaan

quote:

---

Originally posted by sebastiaan
David,

I took a quick glance at your calcs and you've seemed to have left out the power needed to actually drive the supercharger. At 340BHP@1 bar boost, this is going to be significant.

Or; in the real world: just whack the nearest size pulley on and see what happens

Sebastiaan

---