'Porting' the intake runners

[JohnJL]

What's the correct procedure for 'porting' the intake runners?

I've got my intake and exhaust ports about to be opened out to 38mm, and as discussed in a previous post I've noticed my intake runners are 33.5MM at the manifold. There is some material to open them out to 38, but I don't see how the rest of the runner can be similarly opened up with a dremel/other tool. There just isn't enough material further up the runners, even if I could reach it somehow.

An answer that occurs to me is to chop the runners off an inch above the injector bungs and use a dremel to open them up from the inside, then run a silicon hose with a 38mm ID from each chopped-runners to the airbox.

Does that sound feasible/like a good idea? What are people using for intake when they port out their heads?

this is a 3.0 on US heads, receiving JE 9.5CR pistons, megasquirt, DR20 cams.

It just doesn't seem right to creat a situation where, even if the difference is somewhat gradual, the air travels through a more constricted area (upper runner) into a more voluminous area (lower intake or port.) Seems to me the effect would be to slow down the air and create less pressure just where you want it most.

thanks all

[asphaltgambler]

So are you using the 3.0 CIS intake runners?

Understand that 'porting' or enlarging that point may actually disturb airflow. Air flow on the intake side only goes one way; throttle body to intake valve (hopefully!)

As you move up in compression, valve lift, overlap and duration the amount of intake flow 'reversion' is increased. Meaning that there is a moment where the intake flow of air is propelled backwards from the valve not fully seated and pressur building in the combustion chamber, obviously impedding flow.

By leaving the intake runner (where it meets the port) slightly smaller overall than the size of the head port it keeps reversion in check acting as a dam for reversion.

The same goes for exhaust port being slightly smaller than the header opening (where they meet) As air flow exits the exhaust port into the header pipe there is a moment of reversion. The exhaust port edge 'hanging over' acts as a dam to keep the flow from reverting back into the combustion chamber.

I would post pics and ask some of the more experienced guru's where (if any) you want to remove material. Porsche engineers are no dummies! The ports are very well designed and are pretty good as they are except for some needed blending behind the valve seat.

[JohnJL]

Thanks, yes I'm planning on using the 3.0 CIS runners to hold the new injectors in position. I agree that the Porsche engineers knew what they were doing, but it is clear that at the time of manufacture, they were having to compromise significantly to meet US emission laws. I am not similarly constrained.

I've looked closely at the port sizes on the Porsche engines with similar set-ups and for markets where the emission laws weren't so constrictive (thanks to Wayne's book!) and at similar displacements and CRs, they went up to 40MM or more. So given the lesser constraints on my setup and the improved fuel & air supply I can provide the engine with, I'm thinking that 38/38 is close to where I want to be in the balance of torque/hp/rpm range.

I don't have access to other porsche engines and intakes to compare the other setups, particularly the RS and SCRS intake manifold-to-port fit. Would appreciate other's views, measurements and pics.

My pics wouldn't be very interesting right now...just a bunch of stock CIS runners...

Also, any comments on the downside of using thick-walled silicon hose as an intake runner material? Would it work to use larger-ID hose in the same approximate shape as the aluminum runners to achieve greater airflow? Would the flexibility of the runner material be detrimental somehow?

thanks

[Steve@Rennsport]

Originally posted by JohnJL
I don't have access to other porsche engines and intakes to compare the other setups, particularly the RS and SCRS intake manifold-to-port fit. Would appreciate other's views, measurements and pics.

Also, any comments on the downside of using thick-walled silicon hose as an intake runner material? Would it work to use larger-ID hose in the same approximate shape as the aluminum runners to achieve greater airflow? Would the flexibility of the runner material be detrimental somehow?

John:

Just a few notes her that might guide you,....

1) The 3.0 RS used early S cams and 2.7 MFI. The port openings matched the stacks. 911SC/RS cars used RSR sprint cams and tall butterfly RSR manifolds. The heads also matched the intake manifolds. Both engines (especially the SC/RS) suffer from reversion but turn more RPM than yours is likely to do.

2) Silicon hose applications in intake manifolds should only be used for joining two pieces of metal or plastic as a gas seal. Using them as manifolding disrupts airflow and the head juncture is a critical area.

Just my 2 cents, FWIW.

[john walker's workshop]

get a 78/79 intake system with larger runners, etc. there's one on the classifieds now.

[BK911]

Originally posted by asphaltgambler
By leaving the intake runner (where it meets the port) slightly smaller overall than the size of the head port it keeps reversion in check acting as a dam for reversion.

That has to be one of the most interesting things I've read on this board in quite a while.

Anybody have any opinions on this? Practical experience?

[Steve@Rennsport]

Originally posted by BK911
That has to be one of the most interesting things I've read on this board in quite a while.

Anybody have any opinions on this? Practical experience?

LOL,..Thats not unusual and it works nicely for certain types of applications.

For racing engines, reversion is contained by other methods but this works fine in street motors.

[asphaltgambler]

I have seen aftermarket (American) intake manifolds with air dams / foils in the runners and the plenum. Somewhat like fish scales. Smooth one way but with slight edges to disturb airflow reversion.

Also exhaust headers with 'cones' at the leading edge of the tube where the flange meets the exhaust port. The cones small end points inwards inside a double-walled header pipe. Reversion is halted because the 'cone' traps the reversing airflow