IR lengths and plenum sizing for carbs

[ptuomov]

Is reducing runner diameter or increasing runner length the more efficient way to boost the average power in that simulation?

[naukkis79]

Define "10% more cam timing". If I have seat-to-seat duration of 315*, are you saying it needs to be 346*

Where does the ICL have to be? Does that change?

Sorry man, I'm not really buying it. Also, you have to look at where the rpm range is. That's what transmissions are for

If max power VE ain't maxed before you can increase intake cam timing. 346 seat to seat timing with tuned intake isn't uncommon.

20" intake track length is very common for engines maxing out at 8000 rpm. Shorter intakes are not tuned, usually only used with V8's single plane manifolds.

[BenE64]

Have been thinking on all this again, purely hypothetically, my manifold has to stay the way it is for now but I have some things I cant fully get my head around, and in the interest of further understanding.

1, All the "tuned lengths" are for harmonics and sound pressure pulses, Is there any formulas, even theoretical ones, that I can look at to figure out how the air itself travels first through the tube and then when piston ATDC inlet valve closes, and the air has to reverse direction in the runner?

2, Does the air as it rebounds to the opening of the runner also have a reversion at the inlet opening the same way the sound waves do?

3, Is it all one and the same? to me it seems to be separate but all effecting each other.

4, All the equations are for a closed end pipe. What effect does the harmonics of the open each end harmonics play? from having a play on the calculator it seems that there could be time for at least 2-3 harmonic waves/pulse whatevers, to be present at inlet valve open, before the vacuum / air velocity in the runner obliterates all harmonics till the next Inlet valve close cycle begins again.

5, Could open each end harmonics have a hand in the reversion of the fuel in the runner, fuel stand off as seen on some engines.

6, Position of the booster in the runner tract, to me there will always be stand off, due to the shit storm going back and forward in the runner but it is hidden by the ram tube/velocity stack if you are lucky, but is there a minimum distance? short stacks definitely seem more likely to have the issue visible, but then some EFI set up get it too? Can the injector being too close to the inlet valve be effected differently? on a carby car with a long runner it has a larger distance between the booster to the valve so fuel in the runner has room to move both ways, an efi injector close to the valve really only has one way for the fuel to go at inlet close?

And lastly, could the increasing frequency of subsequent harmonics in the runner force the fuel in suspension to coalesce back into larger droplets as the nodes seem to get tighter as time goes on? If all the stars align at certain revs could this be a factor?

Wow, a few quick thoughts added up to quite a post! Any thoughts are always appreciated, I went back and re read this thread again and comments that didn't make sense now are much clearer, just caused more questions

[BenE64]

Oh yeah if one were to put some pressure transducers in to data log would having one close to the port as possible and one say 2in from the trumpet opening be a start, I was also going to put one in the header and a speed sensor. probably just the one cyl but at least its a something to look at. position of the sensor could really skew the results of a graph if the position wasn't known?

[mk e]

I’ll try other lengths later. I’ve done some work out at 19”-20” long.

I've run i don't even know how many simulations on my FrankenFerrari over the years of building it. In DM5 I'd convinced myself that longer (2nd pulse) was better even for top end when head flow is adequate ....then DM6 came along and it thinks that is really not true and the 4th pulse is where I want to be because I can't physically shorten to the 5th or 6th....very different answer. I think the normal dyno answer is 3rd or 4th for best hp, 2nd for best tq....and showing up on dyno day with a selection of stack options is the only way to actually know.

[mk e]

From data like this, it's easy to see the allure of 'trombone" runners!

It sure is.....computers just beg you to complicate stuff that was once simple. Stacks that change length, exhaust that knows when it needs to be loud, throttles that keep power delivery smooth and linear as things come in and out of tune.....

[BenE64]

Probably got lost amongst other posts, any suggestions on best location to put a pressure transducers on the runners one before and after the carb?