Air Flow 101

[Nick]

I have some elementary questions.

Should the intake manifold be able to keep up with the head flow? I tested my heads without the intake, and then with the intake, I lost 6cfm. Is there any reason to not port the intake to remove the restriction?

If the most VE my engine can trap is 100%, and according to pipe max I need about 200cfm head to do that, will a 250cfm head help any? In my mind the bigger head, (provided it is the same basic design) will not make more power because the engine cannot trap it anyways.




Thanks


Nick

[Rick360]

[larrycavan]

Nick

Recently there was a good discussion regarding knife edgeing the runner dividers in manifolds over on Speed Talk. You may have read it already. In case you didn't, you'll want to have a peek at it before you modify yours....

Larry C

[DaveMcLain]

could you post a link to the article on modifying an intake manifold? I've experimented with this quite a bit and sometimes a BIG improvment in the flow of the port + intake combo can be made without a whole lot of grinding on the intake. I did some work on a single plane for a customer's circle track engine not long ago and 90% of the work I did was within about 1 inch of the runner exit and mostly on the center four runners (small Chevy).

I would certainly love to learn more about work in the plenum because that stuff is tricky.

[larrycavan]

[DaveMcLain]

Thank you very much for the info...

[Nick]

There's air in that short turn!!!!! I've been grinding away, learning the hard way. I have found that the short turn is very sensitive to small changes. I have read and heard about this, but I did not realize how sensitive it seems to be. I inadvertently left a small ridge at the apex and killed 10cfm at high lift.

I have found also that I have no idea what the Hel I'm doing!!!
Everything that I think should work usually does nothing (except the obvious) and things that you think should not matter, make a big difference.

This thing has a funny way of humbling you!!

I think life was easier before the flow bench, Ignorance is bliss. I used to think I was good.


Oh well Rome was not built in a day.


Nick

[larrycavan]

[Nick]

Well I got to run my first complete DIY heads at the track.

The heads I ran last year were 302 heads, these are 351 heads a few years newer. I built the 351s myself including the valve job and bronze liners from goodson. I used the same valves, springs, and rockers. The 351s flow more than the 302s but I lost 1 point of compression due to the larger combustion chamber. Here's how they flow.

I ported both myself.

28" 28"

302 351


28.5 30
56 65
88.16 99
119 131
148 157
171 177
180 189
185 199
189 207
192 210
192 210
192 210

If one point is worth about 4% then I'm losing about 15hp there, but I figure I would gain it back with the increase in flow.

I changed nothing else other than the heads.

It runs the same Mph as it did, 113. The et is real close, but the tires are going away and the track has been bad so far. It feels like it wants to run a little higher RPM on the up shift then it did.

So based on the hp calculator I should have 35 more hp, but I lost 15 so, maybe 20 more hp? 1 or 2 mph?


Anyone care to comment?

I thought that they would be just a bit faster. Perhaps they want some more timing or fuel?

Oh I must say I am happpy that my DIY heads run just as strong as the shop built ones I paid to have done. That rocks!



Nick

[larrycavan]

Nick

The good news is..you went just as quick and just as fast. That means what you did worked with regard to porting.

I'm guessing those numbers are in .050" increments up to .600" lift...is that right?

[Nick]

.050 up to .600 is correct. I want to find some more heads just like the ones I'm running now and see If I can improve on them.

I must say this old ford stuff is sooooo out dated. I've been looking at Honda heads for my brother and they seam pretty impressive even in stock form.

Oh well still fun to play with the bench.

Nick

[larrycavan]

Those Honda heads are whole 'nother world. You just don't have the same restrictions that the old Ford heads do. Larry Widmer's site has some really good Honda articles and pics on it.

[Nick]

Yeh I was reading over there on his site.

They make more hp per cu in on pump gas than Most of us can on race gas!



Nick