My stack has a small diameter- MFI Question

[72_911T]

The velocity stacks on my 72 T-MFI have an inside diameter of around 35mm where they mate to the throttle body. However the throttle body has an inside diameter of around 39mm. Seems to me I could get better air flow if I were to bore out the stack to match the diameter of the throttle body. Any idea why the factory did not match the stack to the throttle body diameter?

With the "T" throttle body the diameter is reduced across the butterfly and is matched exactly where the throttle body meets the cylinder head.

Taking is a bit further is there any potential problem with enlarging the ports on my "T" heads and throttle body to 39mm?

[Grady Clay]

I’ll speculate that some PO installed throttle body(S) from an E or S. Are they the same L-R? Do the throttle bodies match the heads? Is your MFI pump Bosch P/N 0408 126 015 or other?

To function best all the components should be matched – cams, intake ID, throttle body ID, stacks ID and MFI pump calibration. Some mis-combinations will function while others can lead to disaster.

Post the ID top and bottom of the throttle bodies. There are several threads with those dimensions cataloged.

Best,
Grady

[Early_S_Man]

Taking is a bit further is there any potential problem with enlarging the ports on my "T" heads and throttle body to 39mm?

The danger is that you could/would create one giant off-idle bog, and flat spot!

If you still have stock 'T' heads ... the ports are only 30 mm at the lower flange of the throttle bodies. Unless you install Solex, 'E', or 'S' cams ... there isn't really any point to playing with porting the heads, and plenty of amateurs have wrecked the airflow by hogging out the ports with a die grinder!!!

[Zeke]

If you do a search here or on the Early S reg for Bobby's user name boboloo using "chart" and "stacks," you will find the answer to all the sizes.

That having been said, I think all of the TB tops are the same size. The bottoms closely (but not perfectly) matched the heads. The base of the stacks were different as well and I think even the tops of the stacks varied with T E and S.

You are correct that the mismatch will be less than optimum with the T, but at 130 HP, it didn't matter much is the way I interpret it. Anything you do to get more air in the cyl is good. but you need the cams to help out. T cams are only gonna pull so much air thru and you won't get much velocity in the stacks to aid in atomizing and distributing the air/fuel mixture. While that might still run better wide open, your mid range and low range driving characteristics won't be that great, for all I know.

BTW, the sudden change in diameter at the base of the stacks creates what they call an eddy. But again, at just a partly open butterfly, there're a lot of eddys.

You can make much better and smoother flow, but you have to have the cams to really take advantage.

[72_911T]

Grady
The TB's are both the same. Sorry but I don't have access to the car now to read the part number off the pump. Disaster is what I was trying to avoid and it seemed logical to me that everything should be matched to avoid what Milt refers to as Eddy.

Warren
I have been warned by others that opening up the intake would create a bog off idle. It just does'nt make sense to me. Ie the "S" is fully opened up. What causes the bog?
What about changing the cams to say an "E" along with an "E" MFI space cam? Same deal?

[Early_S_Man]

There is an ideal nominal range for velocity in ports ... fall below the range and you risk torque loss.

The 'T' engine flows lower volume of air at a given rpm because the cams have less duration, and lower lift. If you open up the ports to 'S' specs in an engine with 'T' cams ... X volume through a larger port area results in lower velocity in addition to the losses incurred due to the eddys Milt mentioned ... which can cause separation of the flow and 'dead' spots or pockets.

[Zeke]

Sorry. I missplelled "Bobboloo." So here are the stats:
MFI Induction Chart
Look at the last post for a link to the "ultimate MFI resource thread."