I thought I would compare the intake air flow through both LS6 and 90 mm FAST manifolds to get a comparison. I also flowed through a prototype short runner plenum style manifold I am working on with a shop in the UK. The short runner will be for higher rpm, higher horsepower engines.

The stock LS6 did very well considering I port matched the FAST. FAST has a 7.5 cfm advantage at .600" lift. That would translate to 15 - 25 hp depending on camshaft, tuning, header configuration, etc. The short runner flows 14.5 cfm better than the FAST at .600" lift. You can work the numbers. That manifold is still a few months off. I'm too busy to be able to spend much time on it.

 

you do any more work on the short runner from the last time i saw it?

 

No, but I know what needs to be done with it. It will need a new plenum casting that is wider with side walls parallel to the intake face of the heads. The corners need a nice radius instead of sharp corners. Then I think it will work quite nicely with a large oval throttle body.

 

thanks for the info

 

Wow, your short runner didn't loose any low-end compared to the FAST, and still pulled up top. Seems like you wouldn't loose any low-end given the results at hand. Very nice work. Now if someone would work on a ITB set up for F-bodies, we'd have it covered.

 

Steve - What areas of the FAST did you touch up?

 

I'm going to be port matching a FAST manifold to a pair of GMPP CNC heads tomorrow. Comments/questions:
1. It is so great to be able to pop the top and reach down into the runners with a hooked scribe to outline the head ports on the mating surface! I probably spent 5 hours getting the same measurements on a one piece aluminum manifold for my BBC, including patterns to match the vertical and horizontal angles of each port at the junction.
2. Anyone know why there is a large tapered surface on one side of each runner near the manifold face, ending with an abrupt step where the 'mid casting' mates with the lowest one?
3. I can see no reason to leave them intact; they go unless I hear a good argument to the contrary by 9:00 AM EST!
4. We're not going to get crazy just yet, but it appears one could pare back the end of the lower air bell entry to shorten the runner perhaps an inch, for a higher RPM 'tune' point. Anybody have thoughts/comments re this?
5. Steve, did you do a before/after flow test on the ported FAST manifold?

 

Madbill - That "ridge" is usually smoothed to match the contour of the port.

 

Fast fingers, DRK!
Thanks, makes sense. I'll go that route.

 

Nice comparison. However, it seems that the LS7 head/intake combo still outflows everything you tested. Check out this post https://ls1tech.com/forums/showthrea...&highlight=ls7

 

A high port head will almost always out flow a low port head. The short side radius can be made much larger with the high port.

Also keep in mind that the LS7 style heads have larger cross sections as well. This will result in less torque down low especially in smaller cube motors. They will however make a substantial improvement in top end power.

My friend is working on a short runner manifold for the LS7 heads as well. I don't have a prototype yet but will this summer. I'm too busy to spend much time on it now anyway.

Regarding port matching. The lip on the side wall is a manufacturing defect by the looks of things. Blend that out with sandpaper rolls. There is also a hump at the port opening inside the plenum. That should be knocked down and blended in as well.

After talking to Tony Mamo I learned that the best way to match the floor of the manifold to the heads is to epoxy the manifold floor to match the heads. The manifold has a smaller corner radius at the floor than the port in the head. You will lose a little flow if you grind the heads to match the manifold. The corner radius at the floor needs to be kept large.

Steve

 

Good points, Steve!
I used a 3/8" wide mini belt sander on the 'wedges'; worked pretty well. I saw those bumps at the plenum/runner junction, but they didn't look all that critical, and I was sick of eating plastic dust by then anyway. I did polish off the mould lines in the area, mostly because it was quick and easy.

Re the manifold floor, on this installation the port was enough taller than the as-cast manifold that in matching the height I was able to open up the radii to suit the head ports. One area I struggled with a bit though was the contour of the bottom of the manifold ports at the head junction. The angle didn't really match all that well and I had to 'hollow out' the last 3/4" of the floor to create a more uniform joining radius. Also, although the manifold ports were tilted slightly off the vertical, they didn't quite match the slope of the CNC head ports, and so the walls got a bit thin at the top on one side and the bottom of the other in each port by the time they were matched up.
I'm sure the air molecules will be thrilled with their smooth journey, but I was surprised to find I'd spent 6 hours on the job! I guess not everyone is cut out to be a porter...

 

Did you all blend in the TB opening into the plenum?
On my FAST90 there was a 1/4 lip all the way around 1 inch aft. of the TB opening that I blended.
What did you all do there?
I did remove the wedge, the hump at the port opening inside the plenum and radus up the top of the runner closer to the injector.

 

Where can I find a hooked scribe? This would make my life soooo much easier trying to port-match the FAST to my 243's....

 

Well, one way is to just take a straight one and give it a tweak with a pair of pliers. I have had several for years, but I don't remember where I got them. I don't recall any trouble finding them. A industrial tool suppy store would be one source.

 

Thanks for the info. I just bought a Mamo ported FAST from a friend of mine and I cant wait to see what i get out of it. I have a 236/230 ~.600 cam on a stock short block and slightly ported heads so hopefully that will help it breath.
shaun