Intake path cross-section variations / choke points

[BradH]

Background:

1. I have ported heads that retain the factory-size intake port entry dimensions with a CSA right at 2.70 sq. in.

2. Primarily due to the way the heads are cast, the MCSA is right at the intake port entry. The rest of the runner -- up until where it opens up approaching the short turn / bowl -- is right around 2.8 sq. in. The shape of the port is that it gets wider shortly after the entry on the pushrod-side of the port. Instead of a pushrod "pinch", there is more like a pushrod "spread".

4. Wallace's MCSA calculator says 2.81 sq. in. for my combination turning 6800, which is where it used to trap on its best runs. With the updated heads & cam, it "should" gain some MPH, so I'm expecting more like 7000 which bumps up the calculated MCSA 2.89 sq. in.

5. PipeMax 3.9x "says" the best Average CSA for a 6800 HP peak is about 2.7 sq. in.; setting the HP peak to 7000 increases that to approx. 2.8 sq. in. (I hadn't used PipeMax in a while and forgot it gives an Average, not a Minimum, CSA value.)

6. The intake manifold plenum entries are right at 3.5 sq. in., which matches well w/ PipeMax's average plenum entry area calc.

7. The manifold's outboard runners taper down to approx. 3.0 sq. in. before being "squeezed" into the as-cast exit size that's currently smaller than the head's intake entry size. However, the inboard runners are choked down a lot more as cast: they measure as small as about 2.6 sq. in. in about 3" upstream of the exit point.

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Plan of attack:

My gut instinct (not always correct, but always there "talking" to me) is to widen the head's port entries to eliminate the choke point in order to maintain a consistent 2.8 sq. in. CSA from the entry to the short turn / bowl.

This would also involve working the intake manifold's inboard runners to gain more area, comparable to the outboard runners, and eliminating the upstream choke point that exists currently.

I'm not looking at this w/ any intent of increasing flow; it's only to address what I consider unnecessary changes in the CSA of the intake tract.

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Question(s):

Anyone think I'm taking the wrong approach here?

Is there something else that I need to take into account?

Thanks!
Brad

[V Remian]

My guess would be that unless those smaller intake runners showed up as reduced flow on the bench, there probably would be very little gain in H.P.-maybe 5 max and no rpm increase even if they flow a little better. If it were a higher revving deal I think it would be more important.
It's for questions like these that a 1D simulation are very useful and sometimes entertaining.

I hope some of the smart guys see this and reply-I'm just a backyard-er.

[steve316]

Constant port velocity was the theory 35 years ago.I would take mca of 2.8 from intake gasket to 3.8 at the plenum. May need more taper but you can always make it larger.

[mag2555]

If you measure your ports center line lenght from the back of the Intake valve to the ports MCSA, how close is it to the
stoke of the motor?

I find it easy to use a lenght of Solder to make this measurement.

[BradH]

If you measure your ports center line lenght from the back of the Intake valve to the ports MCSA, how close is it to the
stoke of the motor?

I find it easy to use a lenght of Solder to make this measurement.

FWIW ('cuz I don't understand why you're asking) the port length from seat to entrance -- the current MCSA -- is ~ 6.5", vs. a 3.75" stroke engine.

[MadBill]

This seems to be alluding to what's sometimes know as the "gulp factor", which links intake port/runner/etc. volume to cylinder displacement. I have read nothing convincing re its validity.

[wyrmrider]

maybe to correlate short intake paths and longer ones, cc's etc

so you're saying 3.8 up in the intake manifold then tapering to the bowl?
I always loved the heads brought in "matched" to a big intake gasket then a pinch at the pushrod then bigger again

[MadBill]

The perfect port probably resembles a long venturi: the area tapering down uniformly from a bell-mouthed entry to a throat somewhere near* the chamber, then expanding smoothly (the 'pressure recovery' zone) into it. *At it in extreme cases where more valve is needed than will fit.)

[BradH]

The perfect port probably resembles a long venturi: the area tapering down uniformly from a bell-mouthed entry to a throat somewhere near* the chamber, then expanding smoothly (the 'pressure recovery' zone) into it.

That's my understanding as well, which is why I believe the mid-stream reduction in CSA should be addressed. It won't be "perfect", but it will be more consistent in CSA transitions.

Also, I'm a little surprised there weren't more replies by others to my post. Is this such a no-brainer that people didn't feel there was any point? I suppose it is what it is...