Porting intake manifold

[pope]

This is a pic of an 81 spider intake manifold. I have been reading Guy Crofts web site up and down. This gasket is much bigger than the bottom of the manifold. By 1mm at the sides, to 3 1/2mm at top and bottom. The gasket is exactly the same size as the ports on the intake side of the head. He has a flow bench to test flow, how do you guys do this with out a flow tester? As you can see from the pic, there is significant improvement to be made to the manifold. Also, the gasket has a strip on the lower right that backs up to nothing. Do you guys trim that part of the gasket to conform to the shape of the manifold?

[pope]

Doesnt anyone do their own work?

[So Cal Mark]

port matching is pretty common. The stock intake has such poor distribution between cylinders that almost any serious performance work incorporates dual carbs

[mbouse]

WOW, is that amount of mis-match a common thing? yeash, i am just learning about this myself...having purchased GUY's latest DVD on the subject.

Mark.... are you saying that there will be negligible improvements if this intake is polished to match the head? as in, go ahead and do it, but it is a waste of time?

or, if he follows through with this grinding, will he experience a noticible amount of improvements...yet not as much as dual downdrafts and a waffle intake?

[pope]

Mike and Mark,
These are all good points and questions. On Guys site, it says that the intake manifold must be as large as the head or a little larger with no inherant problem, but the opposite is never allowed. To see the intake this much smaller is disheartening.
I also found a calculation that said on a f.i. engine with a throttle opening of 55mm (thats what it is on the spider), that engine is capable of 175 hp. Well at least that is up to snuff. The Bosch Air Meter may be another problem.
I figured out that it is a reduction of 15% air flow to the head because of the mismatch.

[So Cal Mark]

no, I'm not saying it's a waste of time. Every lit bit helps, and small improvements usually help towards a common goal. In doing exhaust research I found the intake has really uneven distribution amongst cylinders. I discussed this with Guy, and at the time he was building a single carb motor for a client. He did some flow testing and was able to improve the manifold a little.

[pope]

Mark,

are you talking about the intake manifold on a carb spider or all the intake manifolds in general on spiders? I have a F.I. spider and the intake ports are all the same length, bend, and angle. I have seen carbbed manifolds and they are Irregular port shapes.

[So Cal Mark]

FI is not part of the problem, fuel is injected right at the cyl head port. On the carbs, fuel has to mix with the air and travel thru the manifold. That's the problem with the stock intakes; uneven fuel distribution.

[pope]

Gotch Ya,
I thought you were gonna hit me with a new found problem with the intakes. Less of a problem with the V6,v8 engines as you can put the carb right on the head at the intake. With in-lines like 4's and 6's you had to move the carbs to the side and pipe it in. Some manufactures did it better than others. I had to mess with so many cars that had carbs in my life, and not to step on anyones toes but, I am so glad to have a F.I. spider.

[mbouse]

ok, cuz i wasn't planning on going with dual carbs...ever. i get from point a to point b way to quickly as it is now, just ask the local gendarmes.

but, i could stand to get up to speed quicker, and if i can do that by grinding away some useless intake manifold aluminum, i am willing to giver a go.

[mdrburchette]

Mike and Mark,
These are all good points and questions. On Guys site, it says that the intake manifold must be as large as the head or a little larger with no inherant problem, but the opposite is never allowed. To see the intake this much smaller is disheartening.
I also found a calculation that said on a f.i. engine with a throttle opening of 55mm (thats what it is on the spider), that engine is capable of 175 hp. Well at least that is up to snuff. The Bosch Air Meter may be another problem.
I figured out that it is a reduction of 15% air flow to the head because of the mismatch.

I'm understanding Guy's reasoning different than you are. He says and upstream mismatch, which would be the intake port being smaller than the head port, is okay. This is provided that the intake has a better flow percentage than the head. If it doesn't, then port matching the intake to the head is mute since the intake needs to be opened up throughout. A downstream mismatch with the intake port being larger, will upset the air flow. He does state that on the air horns, if the horns are around 2mm larger than the carb hole, the air will stabilize and follow the main stream of air.

[pope]

Denise, I think you are right in your understanding. The optimal thing to do is to flow the parts and see where you stand. The second pic in Guys article, only shows an intake manifold of about 2" long and straight flow (about as optimal condition as one could get). My FI spider intake is about 9" long and takes a soft 90 degree bend. The opening at the top of the intake manifold is the same size as the intake head port. But the end of the manifold that mates to the head is slightly smaller, together with the bend, I'd think would flow less than the head. The only real way to prove this is to flow it.

Also the intake manifold on Guys pic has a much larger opening on the top side than the head port, mine is the same size as the head (no venturi effect).
If you look at Guys pic, it makes sense, problem is, no one on this site has that setup. I cant prove this until I get it flowed, but I think my intake maniflold flows less than the head. Anyways, Guy always gets rid of the mismatches anyways. Maybe less important on stock, but with my proposed performance increases, I want to get rid of it also.

Denise...... way to go!

[mdrburchette]

I was tickled to see him using the waffle manifold and idf setup as an example since that's what I'm going to use on my vintage racer and what I've got on both my 71 and 72 Spiders. It would be nice to see what the PBS manifold does right out of the box.