Common plenum vs IR intake

[PHRACING]

Does any one have a clue how much to gain making a race engine from common plenum to a IR intake...???

[mag2555]

Well are you 100% sure your 100% maxed out in what I assume is the Plenum type your running already or are you assuming such?
One comment I can make is that the pro stock guys are using a Plenum types to make 1300 hp out of 500 cid so that can't be too bad of a path to follow or be on!

[mk e]

As alluded above I think it depends a whole lot on what the design of both setups is, and probably more on what the rest of the engine/exhaust specs are.....you could very well lose hp if things aren't matched well.

I don't have any real back to back data, but in a dynomation simulation with everything matched IR gains about 5%, on the sims I've run. But those are simulations and that result is baked into the program so there will certainly be exceptions.

[PHRACING]

Let me explain, it`s dragbike whit a Harley based engine on 127cui, 4.6 bore 3.85 stroke, 2.45 intakes 1.9 exhaust now running whit a common plenum whit two 48mm vaccum carburators, the cylinder heads is from a 160" prostock engine and the ports a way to large for the 127" (thats the limit for the class) So now I want to make the ports smaller and Wonder if its worth making it IR whit two 75mm trottle bodies and run fuel injection instead...???

[mag2555]

Harley Exh ports are always way the Hell too large from what I have seen 5 or do with engines/ heads I have had on my flow bench!!

[mekilljoydammit]

Harleys are a special case because it's very uneven flow - common plenum you're probably fighting one cylinder robbing the other, IR if you get everything to fit right you get away from that. May not be easiest to package though, there's a reason they used a second front head on the rear jug of the XR750s.

[Steve.k]

436 cleveland 54 hp. Absolutely zero changes otherwise.

[pcnsd]

It depends.
IMO, the advantages provided by a properly designed for application plenum increase with attached cylinder count and practical plenum volume. In your case, I can't think of system that might provide an advantage over IR that would fit to the area available.

Regarding large exhaust valves on Harley's and other air-cooled motors, I have heard it suggested by credible people that it is to reduce overheating and potential piston damage, but have found no credible data source to peg yet. That said, the concept does make sense to me. It is also my observation that such motors have shorter exhaust durations to go with their larger than typical X-valve.

[peejay]

The main gain with an IR setup is in throttle response. And the intake sound is far more pleasing, which makes the driver happier.

[cardo0]

Plenum volume is critical to reduce (or increase) the reversion pulse of one cylinder affecting another. IR eliminates reversion effects between cylinders but using the same runner length for each runner produces a peak where the Helmholtz reflected 1/4 wave reaches the end of its runner.
Take an intake like a gen II LT1/TPIS mini ram which has very short runners but a generous plenum and you get a low restricted intake with a flat torque curve but it never really reaches any peak - just pulls until it quits breathing.
So now lets look at the LT5 dual runner intake. It has valving that allows a secondary set of runners open up to provide double the air flow capacity - another method to increase the power band. But it does have one same length for all those runners and should behave inline with the Helmholtz reflected 1/4 wave also.
Only intake that has different length runners are dual planes (we're talk'n V8's not Harleys now) and the single plane intakes with outer runners longer than the inner runners. And ya know what I see now is GMPP sells a dual plane intake for the GEN III LSX motors with casting bosses for EFI conversion.

[mekilljoydammit]

Well, I've seen stack setups (not just the old BBC ones) where there were different trumpet lengths, so it's not just dual planes and single planes.

[modok]

Does any one have a clue how much to gain making a race engine from common plenum to a IR intake...???

Where the throttles are.... makes almost no difference when the throttles are open.
IR throttles of course do run a lot nicer at part throttle and idle, but some type of "racing" that may be a moot point.