924Board.org

[Kenodog]

So since I have upgraded EVERY single component on my new motor I was wondering if it's worth it to send the throttle body off to open it from 56mm to 60mm ?

I have the 88mm oversize flat top pistons, knife edged crank, EMW big valve kit, cat delete etc. so it would seem to me that the only restriction in the system would be the limit of the TB.

I only ask because I have heard you can possibly lose low end by doing this. (But since the motor is tweeked with all of the mods...)

No bad ideas....


Leigh
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[Rasta Monsta]

My two cents is that an increase that small will make zero discernible difference on a boosted engine.
_________________
Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[fiat22turbo]

Still running CIS?

If yes, I'm not sure it would make a difference.

If no, it might make a difference at WOT, but impact throttle sensitivity at lower openings.

Without testing the changes back to back you'd never really know.
_________________
Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[Kenodog]

It amounts to roughly a 7% increase in opening volume. I was thinking about it because with the larger volume of the cylinders and the larger valves to feed the cylinders I was thinking it just may help at mid to high rpms.

Think I will get her running an broken in and then dyno stock. Then dyno with it done. Can't hurt right...


Leigh
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[fiat22turbo]

Except the CIS flapper is a pretty large restriction in the amount of air in the system (even when assisted with a turbo charger)
_________________
Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[Rasta Monsta]

People make to much of the "flapper impediment," especially the way it works on a 931. It's like saying a wide open door is in your way.
_________________
Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[jason c]

If I calculated it correctly its roughly a 14% increase in volume (60 is 7% larger than 56 but you have to calculate it for a circle).
The AFM is a restriction because of the resistance caused by the control pressure. Once its wide open, there isn't much of a restriction, if any ( depends on engine air volume useage & the rest of the intake system capabilities). The AFM hole & plate can't be modified easily because of its size & shape in reference to the fuel distributor flow.

Unless you want to flow/port everything on a bench and redesign the AFM & fuel distubitor, you can't do much. The easier route is EFI.

[Kenodog]

Jason, not 100% true. Because the WUR and FCU are adjustable it only need to go to someone familiar with these, like the guy that rebuilt it to adjust. It is a fairly simple process to remove both and have them up rated for more flow.
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[jason c]

No, the fuel distributor is not difficult to modify.
I was referring to the discussion of the AFM being a restriction.
The problem is modifying the AFM. It cannot be ported like a throttle body can because of the shape of the bore. So the problem is modifying the AFM, the engineering to do it correctly and then you have to match the fuel distributor.
Even if you achieve it, CIS is not an optimum system for a turbo car. It is limited in power capability and causes lag. EFI solves those problems.

[Kenodog]

Meh, my last 931 had almost no mods excep a fresh stock motor and an air/water i/c and i dyno'd it 170 at the wheels, so 210 ish at the flywheel with 12psi. This car has larger pistons and the big valves and all sorts of other goodies. I will add an i/c in the summer and it should be at least 220-230 at the flywheel. Thats fine for me.


Leigh
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[jason c]

Nothing wrong with that.

[Rasta Monsta]

[jason c]

Must've missed all the "converting to CIS for power" threads.

[Rasta Monsta]

You should look it up. It's near the "parrot cliches so you sound like you know stuff on the internetz" thread.
_________________
Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[jason c]

Never seen that one either.

As you know, I've owned & worked on Porsches & other CIS cars for 20 years now. I never have nor will I ever need "the internetez" to repair a car or for anyone on its approval.

Things do get parroted too often. The fact is, whether you like it or not, what I said is true. This is something I've learned from my own experience. I don't dislike CIS, I have it on several of my cars. It does have its limits though. Porsche made significant changes to the CIS on 931s before any significant increases in power were achieved (gts).
My above posts were in reference to the discussion of modifying the system and the possibility of a AFM restriction.
The point I was trying to make was that it would be easier to covert to EFI than to re-engineer the entire CIS system, especially the AFM. Its not that you cant make good power with cis. Aside from the power capability, EFI provides less lag too.

Have you ever even driven a CIS car before & after an EFI conversion?