924Board.org

[Euro924S2]

Hi Guys. I've finally decided on supercharging my 931 motor (UK 8.5:1 CR) so I've been looking at blowers. My questions is what size would people recommend for making 12 ish PSI (looking for 240bhp) on a 931 engine??

I've previously fitted an M45 to a 1.6 MX5 but we had to spin the nutz off it to make 12psi.

I've tried searching '931 AND supercharger' and again I just get page on page of random hit's with no mention of size (apparently it does matter fella's!!)

What have people used?
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[ideola]

Ooooh! Haven't gotten to do this in a while:
READING ASSIGNMENT <= there is a subsection in the Forced Induction chapter with many cross references to existing supercharged 924s.

M45 is not a bad match, but IMO, M62 is ideal for 2L. There is a Mercedes unit based on the M62 that is extremely simple with very easy to make shapes where you'll need to fashion inlet and outlet manifolds to plumb into your charge path. Second to that is the M62-based unit used on the first generation Chevy Cavalier SS and the Saturn Ion Redline. I almost bought one of the latter until I stumbled across the Lysholm-stye setup used on Mazda Millenia.

M90 is way too large for a 2L (was typically used on 3.5L and larger applications), and I suspect you'll have challenges fitting it in the engine bay.
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erstwhile owner of just about every 924 variant ever made

[Euro924S2]

Cheers Dan although I've already looked at those links.

Correct me if I'm wrong but only one of those is a 931 motor (Kojab's supercharged 931) and there are loads of fantastic pic's (often lacking in some write-ups) but I can's see any text- was there ever a written write up to accompany that photo album??

As for the supercharger - I think after using the M45 on the MX5 (miata), and having to spin it with a 150mm crank and a 60mm nose (AKA 2.5:1) thats 17,500 RPM at 7,000RPM engine speed (max efficiency is about 15000 IIRC)

The other thing I was thinking was impact the lower compression ratio: 9.5 on Miata V's 8.5 on 932 - does that mean I need a higher SC output volume - I think I need to buy Corky Bell's book!!!!!

The reason I considered the M90 - I once had a Ford fiesta - sure it would do 70MPH on the motorway, but the current Audi 80 2L does it a whole lot easier!!!! So basically I was wondering if rather than spinning a M62 to big RPM figures making lots of heat, maybe spinning a M90 more conservitively may yeild some benefits - this however just late night pondering - not based on any mathamatical calculations and as you say the M90 is rated for engines with twice the capacity - I was hoping this would give twice the boost (maybe 12 instead of 6PSI) with the same (factory set) RPM??????
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[ideola]

931 / 924 long block shouldn't make any difference relative to the displacement you need from your supercharger. The supercharger displacement needs to be sized relative to the displacement of the engine, and has nothing to do with CR. The amount of boost you run on top of the static CR is a completely different discussion that is not directly related to the displacement of the supercharger.

Indeed, time for Corky's book
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erstwhile owner of just about every 924 variant ever made

[leadfoot]

For a 931 roots blower
1200cc or 72cu" for 10psi under applications
1400cc or 85cu" for 10psi above applications

an eaton M62 would be ok if it is the recent model whose efficiency is greater that earlier designs or a whipple unit if you want to go towards the 2bar region.

The second motor I built used the 931 head, flatop pistons (effective compression ratio 8.9:1) and the blower was the same SC12 (1200cc)
more reading for ya
http://www.924board.org/viewtopic.php?t=21953&highlight=engine+pics
http://www.924board.org/viewtopic.php?t=21138&postdays=0&postorder=asc&start=0
Stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[simsport]

I have an EatonM62 series 4 which is not the latest. It makes 11pSI (ish) and 245bhp in a 8.5:1 comp series 2 config engine, std 931 head and std cam.

It is run at approx 2:1 and is a great compromise for power and driveability.

I also have full engine mgmt , and an intercooler.

These last items are a clue to what you need to do to get best power from a Roots type of blower. Spinning them fast makes heat hence the intercooler, and you need better engine management to make full use of the boost.

The M90 is far too big for a 2ltr engine and will be very inefficient, response will not be as good and the power used will be greater meaning that the gain in power made will be small if indeed any at all.

I have run the m62 on the car for many years now, its reliable, smooth and can be found at low prices.

All that said there are the other options such as the whipple, the Rotrex and so on. They all have pro's and con's however its hard to beat the simplicity of a decent Roots setup.

Cheers
Simon
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Blown is always best!

[Euro924S2]

Hi Simon. Thanks for that input. It's great to here from someone who's already walked the line!!

Your setup is pretty much exactly what I was planning. I have a Goteck PRO-X ECU brand new to control fuel and spark and I'm planning a FMIC to keep temperatures under control (I've got an 944S2 style front so a custom wide and thin intercooler is the current plan.

My previous supercharger experience is on the Mazda MX5 and on those the increased throttle volume of an Air/Air IC causes notable lag so the trick they use is to fit twin throttle bodies - one pre-SC and one Pre-plenum - the pre SC is opened a little more than the post SC butterfly - effectivly boosting the intake , so as soon as you plant your foot there's instant responce. (a convensional blow-off valve is used to keep the IC pressures in check)

Just one quick question - what did you do about your exhaust manifold?? My current understanding is Log manifolds are inefficient and the NA is different layout so.......

Thanks again.

John.
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[Euro924S2]

Sorry, one more question - What did you use for the bottom pulley to drive your SC? I drive a Audi 80 as a daily motor (largely similar enigne from the outside atleast!) and I notice that has a 6 rip pulley to drive the alternator, then a standard V pully outside that for the water and PS pump - I was hoping the bolt pattern was the same so I could use a AUdi one on mine ????
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[simsport]

Hi
The exhaust manifold from a N/A model will fit with a little work on the stud holes plus porting to improve flow and match with the 931 head ports.
The only down side is that the manifolds do crack sometimes, I'm on my second.

As for the pulley I had one made up from a solid billet....saved loads of time and cant fail due to flex which did happen to an early version I had.

Cheers
Simon
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Blown is always best!

[leadfoot]

not really necessary to go with twin throttles, a single butteryfly of around 60mm to 68mm is more than adequate, I used a 68mm and it cranked from get go, initially I ran with a clutched pulley setup on the SC (N/A head) but went to fixed to get more boost with the efficiency of the 931 head being greater.
My preference is to also run throttle post SC and use a large bypass valve to recirc air. Almost 3psi pre boost in the system recirculating.
As far a pulley setup if you are not running aircon you can run your pulley behind the alternator pulley, but 165mm is as large as you can go and you still need to grind a couple of cam cover bolts in half, If you go on the outside you can go larger but need to remove this when changing the alt belt.
There are quite lightweight ribbed pulleys out there for alternators, so you might consider switching this and running all 5pk or 6pk combination belts.
Ditto to simons comments about EFI but also adding a decent oil cooler and running the coolant at the lower temp thermostat help somewhat.
Stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[Euro924S2]

Cheers. I've got a 944T rad with twin fans for water but will need a low temp thermostat and thermoswitch (what temps have you guys got?)

As for the EFI - what rate injectors will I need for 250bhp??

Also, I was talking to the machine shop today about machining out the injector ports to mount the injectors lower and closer to the inlet valve - is this something you guys have done and if so can you give me any pointers as to depth etc???

I've been looking at Merc CLK M62's but I don't fancy the ON/OFF nature of the clutch setup and so I'm going to keep looking for a bypass valve type setup - also means I can have a small nose pulley to keep a 2:1 with only a 165mm bottom pulley. I definately plan to keep the SC drive on the inside after I snapped the front of the MX5 crank shaft clean off with too much tension on the SC belt on the front most pulley!!!! New engine job - I wasn't very happy that day!!

If the Audi unit fits then I'll go with that as it's minimal work cos I can leave the water pump and alternator pulleys as is.
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[ideola]

[Euro924S2]

Your not kidding!!!! Certainly not one for the shaky hand club!!!! Should I leave it as it is then??? Does it make much difference drilling it out with EFI injectors?

Thanks for the Pic - says a thousand words (although mostly their just 'DON'T DRILL IT' repeated over and over again!!!!!)
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[simsport]

I'm not a fan of the injectors in the head, I have them fitted to the inlet runners, pointing at the port and held in place by Fiat injector holders and Webber Alpha Injector pipe connections.

An easy and cheap way to fabircate the inlet manifold is to cut the original runners before the bend and to weld an alloy plennum to them. You can then fit your injectors into the runners (but you'll need to have seats made up by your machine shop).

For injector sizes you dont want to go to above 80% of the duty of the in jectors, I think that 951 injectors should be fine for up to 250bhp (I did use 944s2) but check the impedance as not all are the same and your EFI may require a certain figure.

Also make sure the fuel pump is good,( invest in an uprated one now would be my advice).

I read an article written by Porsche who tested supercharging a 951 and they felt that a throttle pre blower was best for response, hence I went that way and bought a remote vac bypass valve from Supercharger UK. I bolted it to the end of the plennum and with the vac level there the bypass works very well, its a butterfly type which flows well even when just slightly open.

Be sure to 'hard pipe' the system, ie dont have blower hose in long lengths or long joints as these will be sucked flat by the vac when you close the throttle! I have very short joints and alloy pipes throughout.

As regards leadfoots advice its spot on, the oil cooler,rad setup and even the flow through the rad via the intercooler are all important as you will make a lot of heat using the blower, however the driving flexibility more than makes up for the extra effort.

Oh, dont bother with the clutch as the M62 will be fine run direct as long as the ratio is correct. Try and get a plain body version as the OEM versions all have castings to suit specific cars.l

Regards
Simon
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Blown is always best!

[fiat22turbo]

With a decent mill, boring the injector pockets out to fit the injectors should be fine.

I'm doing this with a spare head I've got and the milling machine in my father's shop.

Really helps get the injector's spray pattern closer to where it needs to be.

I think there was discussion that a needle pattern spray might work better since it could bounce off the back of the valve and disperse. Not sure if that has been tested yet or not.

They are mounted too close for ultimate power output, but it works well enough for most people's use. That position allows for more clearance between the fuel rail and the intake hose/throttle body on the stock intake.

Oh and if you're having a hard time finding a fuel rail, you might look at something like this:

http://apps.msdignition.com/fuel_9.htm

Each fuel injector gets its own line and is mounted individually.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)