Gdi Intake Manifold Porting

Go0se

Member
Hi, I'm new to the forum. I've kind of been hiding in the background for a little while now. Reading the incredibly useful/in depth information here.

I have a diy project I'm working on, which is porting an intake manifold for a gm l86 engine. Designed for a truck, low end torque. Runners/ports need nothing except to be cleaned up. But I'm looking at possibly opening it up for a 95mm tb (stock 87mm).

It's designed with the throttle body pointing down at a 15 degree angle with the manifold throat sitting level. So right off the bat the throat has a smaller cross section then the overall tb (not counting the blade assembly). I think this probably enhances performance, but it's an unknown.

My real question is how imperative is shape to the manifold throat?

I understand that as it sits, it's designed to work perfectly for the given application. But I've come up with something that alters the overall shape and direction of airflow in attempt to make are larger throttle body worth while.

Basically I'm opening up the bottom of throat so it has a true 87mm bore. But I can also router down the sides (90 degrees off the tb face). Going from nothing and gradually increasing so that it changes the wall shape to a straight shot to the plenum.

I just have an earlier picture from my test peice, but I think it should be enough to clarify what I'm saying. It also doesn't show the bottom opened up..

Any and all advice is appreciated. If I'm doing more harm then good, I do want to know about it!

Thanks
 

Attachments

  • 20210811_184740.jpg
    20210811_184740.jpg
    87.9 KB · Views: 10
Welcome to Grumpy’s GoOse! Your picture looks like automotive porn :D. Just kidding.
I can’t answer your question but someone who can should be along soon. Any particular cars in your corral? Pics?

Rich
 
with that type of manifold, opening the throttle body throat does usually result in minor improvements,
but keep in mind only one or two cylinders are allowing air to flow into them every 90 degrees of crank rotation,
so the whole cross sectional area of the throttle body throat flows air rather intermittently, swapping flow between intake manifold sides/cylinder banks.
I would not expect the 95mm throttle body to out perform the mildly ported 87mm,
Id concentrate on better headers and a low restriction exhaust,
thus better cylinder scavenging to increase cylinder flow, before thinking of going with the 95mm,
 
Welcome to Grumpy’s GoOse! Your picture looks like automotive porn :D. Just kidding.
I can’t answer your question but someone who can should be along soon. Any particular cars in your corral? Pics?

Rich

Hahaha, thanks. Sadly no, I don't have any cars, and seems like they're on the way out the door...
 
with that type of manifold, opening the throttle body throat does usually result in minor improvements,
but keep in mind only one or two cylinders are allowing air to flow into them every 90 degrees of crank rotation,
so the whole cross sectional area of the throttle body throat flows air rather intermittently, swapping flow between intake manifold sides/cylinder banks.
I would not expect the 95mm throttle body to out perform the mildly ported 87mm,
Id concentrate on better headers and a low restriction exhaust,
thus better cylinder scavenging to increase cylinder flow, before thinking of going with the 95mm,

Thanks Grumpy. Exhaust is opened up with a set of kooks. I can't pretend to know how the headers were tuned but they're definitely more like a mid length header imo.

What I'm doing is really less about hp and more about having a project to work on. I don't have any expectations as far as gains go. Just looking to get the most I can out of it.

The throat can't even be opened up to 95mm, more like 91, 92 max if I reinforce the back with epoxy. But 95 is the only compatible tb.

I'm just concerned if by changing the shape of the throat, I'm changing the overall contour of the velocity profile in a negative way.

Or would it really matter since it all just dumps into a large stepped plenum entry?
 
The Runners are too long for modern Race engine usage.
Truck intake.
You can try and open it up for a Porting Test Engineering project.
May help or may be a waste of time.
Most of the guys here use LSX 3 & LS7.
Found out why LT 4 Is th gen has bad exhaust port design.
Ok use as stock mildly modified.
Darin Morgan hates those Direct injection GM engines.
Also stated exhaust port bad.
Intake port design bad, moves just enough air in and out to make published 425 something power figures.
Need just 200 cfm intake flow @ 28 inches water Depression to do that old school Pontiac V8 Hot cammed.
 
If it's at 15 degree elbow angle there is a short side radius of some sort present.
I would open it up top and sides.
Treat as a High port made and wide port it.
Short side radius best to leave alone, need to flow bench test to get optimal short side radius shape needed.
Just hand blend the bottom short turn.
Some use carbide burrs on to smooth out short turn radius.
I do it by hand no power tools.
Crocus Cloth I use.
 
Last edited:
Anything with a short turn radius can increase or kill total airflow.
Sometimes just. 010" removed ruins cylinder heads .
Usually not that touchy but can be.
 
Does it make sense to add transitioning surface texture (60-120) on the ss of intake runners? Or is it pointless with how well designed these manifolds are.

Or should I just do the entire port 220-240grit. From what I've read it seems to be ideal because it's not so rough that it creates drag, and it's not so smooth that separation is a big problem.

This manifold is glass filled nylon so it does scratch easier then aluminum. Does have some serious turns/twist to it though.

The picture is just the runner cleaned up with 120 grit.

Goes from essentially 4 inches to 2 inches, making more then a 90 degree turn on the ss.
 

Attachments

  • 20210619_131119.jpg
    20210619_131119.jpg
    80.2 KB · Views: 7
  • 2020-Chevrolet-Silverado-2500HD-3500HD-Event-6.jpg
    2020-Chevrolet-Silverado-2500HD-3500HD-Event-6.jpg
    296.6 KB · Views: 6
  • 5c81668b3dfb5_Cannell_2019_ChevyHDenginesFlint-0443.thumb.jpg.b73195307fdbeddb42989adf26f2cca7.jpg
    5c81668b3dfb5_Cannell_2019_ChevyHDenginesFlint-0443.thumb.jpg.b73195307fdbeddb42989adf26f2cca7.jpg
    88.3 KB · Views: 7
Does it make sense to add transitioning surface texture (60-120) on the ss of intake runners? Or is it pointless with how well designed these manifolds are.

Or should I just do the entire port 220-240grit. From what I've read it seems to be ideal because it's not so rough that it creates drag, and it's not so smooth that separation is a big problem.

This manifold is glass filled nylon so it does scratch easier then aluminum. Does have some serious turns/twist to it though.

The picture is just the runner cleaned up with 120 grit.

Goes from essentially 4 inches to 2 inches, making more then a 90 degree turn on the ss.
I would try and finish in 60 grit or 80 grit.
Smooth and shiny does nothing but causes a power loss on the intake side
There is a Boundry Layer rough texture is desired proven to flow more CFM that way.
Its opposite what you think.
 
Appreciate all the input.

I'm taking the advice and sticking with the 87 mm tb. I will pick up a ported unit from soler performance though. Supposed to flow a couple hundred above the stocker, but it's the part throttle improvement that I'm after.

Going with a 60 grit, finish it off with a 80 grit interleaf abrasive/sand.

Any input on the step into the plenum? I think it's just there to keep blow by (from pcv) contained to the manifold. But I don't know if there's some other importance to it, like wave tuning? It's a good inch drop to the floor and I'm tempted to try and blend it in.
 
Appreciate all the input.

I'm taking the advice and sticking with the 87 mm tb. I will pick up a ported unit from soler performance though. Supposed to flow a couple hundred above the stocker, but it's the part throttle improvement that I'm after.

Going with a 60 grit, finish it off with a 80 grit interleaf abrasive/sand.

Any input on the step into the plenum? I think it's just there to keep blow by (from pcv) contained to the manifold. But I don't know if there's some other importance to it, like wave tuning? It's a good inch drop to the floor and I'm tempted to try and blend it in.
Try and blend the step if you want to.
 
Back
Top