MFI Stack Boring

1972_911T · 09-24-2006, 10:15 AM

What size would I need MFI throttle bodies and intake stacks boring to to work on a 3.2 engine? And can 2.2s Mag stacks be bored enough to do this?

Thanks Steve

Shuie · 10:45 AM

The S stacks are worth crazy money unmodified. At best they can only be bored out to 44mm, IIRC. Maybe its 48mm, but either way, they are going to be too small for a 3.2.

I've never seen anyone do it, but I honestly think the TWM-3003 throttle bodies have potential to be modified for MFI injectors. They use a standard weber linkage and manifolds and are readily available at 50mm+ diameters. This entire setup would cost under $2k.

Before you head down the big bore MFI path talk to Henry Schmidt at Supertec.

Here are the TWM's

BURN-BROS · 09-24-2006, 10:49 AM

2.2 mag stacks can be bored 52mm up top and around 42mm down low.
throttle bodies can be matched to around 40mm to 41mm maybe more I forget. They can be run succesfully on a 3.2 short stroke or a normal 3.2 all the way up to redline ( 7300rpm) with agressive cams.

1972_911T · 09-24-2006, 10:56 AM

Hi thanks for the responces, so what size would I need to bore them to to flow right on an otherwise stock 3.2? Do I need to retain some taper on the throttle body of can this be bored straight?

Thanks Steve

davidppp · 09-24-2006, 11:16 AM

Hello, Steve.

The stock 2L S had IIRC 36mm...to get a comparable characteristic for a 3.2 will require in the region of 46mm...

I thought this was possible?..must go measure it!

Kind regards
David

BURN-BROS · 09-24-2006, 11:16 AM

Straight thru is fine and matched to your port. If your budget allows you should really notched the pistons for valve clearance and run a descent cam. Will this be an efi car or a converted mfi?

1972_911T · 09-24-2006, 11:23 AM

Its going to be EFI, I would like MFI but I dont want to get into the relms of modified MFI pumps and the $$$ involved in this. I intend to use something similar to the poweraus injector blocks below the throttle bodies for mounting the injectors.

Steve

BURN-BROS · 09-24-2006, 11:43 AM

Ah, Either way it will be a good runner! Post pics of your upgrade when you start.

304065 · 09-24-2006, 03:32 PM

Why not use the TWMs? Seems to me it would be a lot easier than boring the original MFI stacks, and it gives you a place to put the injectors.

Henry Schmidt · Henry Schmidt's Garage

Max flow across a valve is generally rated at about 80% of valve diameter or as much as 85% for a race engine. The intake valve on a 3.2 is 49 mm. 49x85%= 41.5. Strangely enough the same size as the 3.2 intake port. If the port is 41.5 the max flow through that port would require only a a butterfly a little more than 41.5. The extra will make up for the butterfly and shaft. Now the 2.4 "S" port is 36 mm and the stack is 42 mm at the top. If you wish to keep the same ratio you would end up with a 48.5 mm stack on a 41.5mm port. I was unable to find a 48.5mm velocity stack so we went with the next biggest available stack 52mm. The mag stack when machined by a competent machinist will bore to 52mm.
Any questions? 3.2 MFI

1972_911T · 09-24-2006, 04:12 PM

Thanks for the input Henry, that makes it all a bit clearer. As for why not the TWM's this is an option but I like the idea of using the MFI stacks for the original look, and also if I ever get the urge and the $$ in the future I may want to go whole hog and make it an MFI engine. I also think I could do the setup with MFI stacks cheeper than using TWM's.

Steve

rvanderpyl · rvanderpyl's Garage

As an aside, how much can you bore the Plastic 2.4T stacks?

1972_911T · 09-24-2006, 06:31 PM

If TWM's were to be used what size bore should be used? the biggest is 48mm would this be the best to use then taper the manifold from this to the port size of 41.5 mm?

Steve

1972_911T · 10-06-2006, 04:12 AM

Seems I got a bit of a mixed set, The intake stacks are the earlier mag ones but the throttle bodies appear to be the later 2.4S ones. Can these still be bored out to the 41.5mm port size? It seems tight as the OD of the throttle body is only 45mm measured just above the lower mounting flange this would only give a 1.75mm wall thickness.

Steve

Henry Schmidt · Henry Schmidt's Garage

This is a highly sought after combination and for high performance MFI.
The parts you show in the pictures will make a very nice system when bored.

1972_911T · 04:59 AM

Hi Henry thanks for the responce, so can these later bodies be bored just the same as the earlier ones? I was'nt sure if 69-71 setup was required just because of the plastic stacks on the 72's or if the throttle bodies could not be bored as much as well?

Steve

Henry Schmidt · Henry Schmidt's Garage

We can bore any of the throttle bodies to 41.5mm.

1972_911T · 10-06-2006, 05:29 AM

Henry what size butterflies did you use with thouse bodies 42mm? or can you squeeze 44mm into them?

Steve

Henry Schmidt · Henry Schmidt's Garage

The largest butterflies we build are 41.5 mm. That seems to be the functional limit of the castings.

In most cases that's more than enough.

We have to try and get away from the "bigger is better" mentality of the past. Case in point, F1 engines are now 2.4 liters and some 750 hp without boost.

Things like port velocity, modern cams, piston shapes and ignition control, all offer more quality results than simply bigger.

That said, "I want more & I want it now".

jm993rs · 06:37 AM

I'm subscribed now to this thread because i would mount 2.4 s MFI to my engine 3.0 alu for rally car ......

Now I'm very advanced with the body and have the engine waiting for mount the MFI

But need make the upgrades to this before mount on the 3.0

Please gurus ! Make here your sugestions