Intake length, and air horn length question
 

If I were to increase my intake length by 1/2" would I see a notable change in the power band? Also been trying to get back some info on the effects of different air horns

I'm not 100% on how much 1/2" would effect the power band. I do know that the longer intakes are better for peak power. From port the the base of my ITB's i measure about 13".

Its funny this thread should crop up. I was just thinking if anyone had played around with really long intakes lentghs. I mean like 20" or so?

I think you have it backwards. Longer intake runners move the torque peak lower in the power band not higher. Look at the 787b as an example. The runner started fully extended then got shorter as the rpms raised to give that engine a broad power band.

Yeah thats why folks use the wrap around intakes for DCOE type carb
applications. Make the carbs more streatable by moving the peak power
band down a bit. At least thats my understanding.

As little as 1/8" may have an effect on power. A half an inch might do a little bit. Watch your a/f ratios if possible to see if there are any changes. If there are, and tuning effects may be masked by it.

Yes right you are read the artical wrong!!! derrrrrrrr.

Not picking on anyone, but RB had actual data. They stated that their 48IDA insulator/spacer lowers the peak torque 500 rpm for each added spacer. Each spacer was .500.

So that is some good data. But , that is on their system using their 4 port IDA manifold and they did not state what other mods or porting they used.


Just a note. The rotary has little torque and is not likely to make huge amounts because the manifold is 36" long. The "tuning" only moves it up or down . Other tenets of the porting determine the amount of efficiency/torque available at a given RPM.
The air horns affect the completely wide open throttle performance and don't do a whole lot for the rest of the powerband with the throttle blades limiting their function, but They do play a huge role in the idle performance when a huge amount of overlap is present. The carb needs to be able to control the "standoff.' That is the fog over the air horns that is actually atomized and mixed air/fuel. If you have a baseline horn and there is a standoff of any given amount, then increase to capture that fog.
We used a Holley 650 for some time very sucessfully using a large hand made air horn on only the primaries. This was on a full bridge and it did boost the bottom end over no horn.


GD

I just looked this up, and it say about 300-500rpm, but i don't think those spacers are 1/2" by the looks of things I think they're more 1/4". But either way my power band coming down a bit is not a bad thing. I'm peaking at 8000 rpm right now, would like to have that closer to 7000 rpm.

What about the air horns though? What effect does that give off when changing the length? Is it seen the same as changing the length of the manifold?

I do think it is only having a great effect when WOT. That why the example of the 787 and the variable sliding barrels is a good one. When you have the throttle half open, the blade is acting like a choke or dam. The stand-off is only to get better throttle response as it begins to open. The fog is all mixed and ready to go as opposed to an acellerator pump shot.

gd

Changing air horn length is the same as changing intake manifold length. The engine "see's" the end of the air horn, not the throttle plates. Changing intake length directly influences peak hp rpm. The shorter the manifold, the higher the rpm at which peak hp occurs, resulting in more peak hp. The longer the manifold the less peak hp made, usually with mid range gains, but not always.

Stand-off is due to bad port timing, incorrect exhaust primary length, or excessive exhaust back pressure. Or all of the above. In my experience stand-off is a bad thing. It creates a 'hole' in the power curve and throttle response is usually worse.

Wouldn't the whole area under the curve just shift though making slightly better bottom end? That's the way I would see it and not just a peak being increased or decreased.

Good to know!

Not true. The reason is that the blade effectively changes the cross section of the tube and therefore velocity and volume. Velocity is what carries the resonant wave. No velocity, or movement, no wave. Simple as that. You do not tune your engine to run best at half throttle. Well, most people don't. It's a wide open tube attribute. That is why they mostly use a slide throttle on anything really serious. No blade to reflect or block the given cross section.

It's usually because of overlap. And the other attributes not factored into a new port config. Like installing a PP in a car that has a street exhaust. Not well thought out to the end . Not BAD port timing exclusively. It might be wrong for the length of intake tract. But it is tunable. You must not have had much luck in tuning standoff. David Vizard is an author I highly recommend you take a look at. Hp Books.

gd

This is correct. Eventually you get to a point where the length affects the operating of the engine and then it would reduce hp. You know, where the volume becomes too much to accellerate in the given window of time the port is open.

gd

Ahh, Gotcha. Now when making a plenum, shouldn't the length of the intake be taken into consideration also then as far as how much volume it requires before choking the engine?

For example lets say that the runners are only 3" long, and have a volume of 50 cubic inch, then you make a plenum and realize that you need a minimum volume of 400 cubic inch in that to make it work without chocking power. If that is the same engine, but with longer runners say 12" and have 200 cubic inch in the runners, then could you say that a 250 cubic inch plenum would work just as good? Is it correct to look at things like that? Or is plenum volume not directly correlated like that?

A plenum will never choke the engine, unless it is a restriction. The benefit of a plenum is to contain the homogenized air/fuel mixture in a ready to use state and help to damp the pulses to the carb/venturi. It also serves a function of equalizing the runners, or at least allowing the possibility all ports having an equal length.
The rotary engine config is optimum for a "pig trough" style or "log runner" style.

gd

whats a pig through style?

I think that plenum volume isn´t correlated with runners volume. From point of pressure wave tuning, plenum is acting like open atmosphere. So effective intake length is from port face to runner opening in plenum.

Plenum volume, I really don´t know as there are many engines with plenum and I can´t find any correlation between displacement, operating RPMs etc.
I think that plenum should be sized and shaped to equalize flow to all runners.