Tuned intake runner lenght?

[sendler]

I have half of the longest gears I can fit installed with a +1 ft sprocket. The -2 tooth rear is late in production due to the flooding in Thailand a few months ago. this will drop my cruise rpm at 65 mph from 6,500 rpm to 5,900. I should be able to fit a half circle of 38mm plastic hose inside the stock air box to get a 15 inch path length to tune the full throttle pulse and glide.

[gone-ot]

W.O.T @ drag racing equations.

[ERTW]

your engine peaks not only at 8700 rpm, and also at 6450 rpm.

tuning also depends on valve area, and hence port area. waves happen at all rpm, at all throttle positions.

6000 rpm wants a 380 mm intake runner, as measured from the valve to the plenum. the plenum can be as large as 2L, but slightly smaller (~1.5L) will help throttle response.

increasing flow by porting, in other words reducing pumping loss, will boost economy.

imo the biggest thing is the large cam overlap designed for 8700 rpm power. you want minimal overlap so you're not sucking raw fuel out the tail pipe. roughly in the 200* @ 0.050", 112* LSA.
hth

[E4ODnut]

I'm curious as to how you do your runner and plenum calculations.

[ERTW]

i made up a spreadsheet with formulae derived from decades of (someone else's) empirical research. it's more for analysing engines, than for predicting power output. the above numbers are actually very decent starting points. fabbing an intake can be a wonderful learning experience - and minimal economy benefit. can you even get "eco" cams for the 250?

fwiw, 1D simulators like engine analyzer pro aren't accurate or useful. i just got a quote today for professional software that costs $18,000.00 for a 1 year license! (or buy 2 get 1 free the amount of information you need to input is staggering (and what it can do is mind blowing!). not many professionals have this software, and nobody in Canada does. I would love to buy a license this year. It amazes me how much most OE and pretty much all aftermarket miss the mark.

[E4ODnut]

I suppose that obtaining a copy of your spread sheet is out of the question then?

If you were to plug the intake runner cross sectional area, length, and volume of the plenum, could it calculate the RPM at which the system would be tuned? Would it also calculate how much less of an effect would be felt with the harmonics? Would it give you an idea of about how many revs either side of optimum would still receive some benefit?

[oil pan 4]

I dug up an old post of mine (2004) showing the relationship between runner length and harmonics.
Edit: never mind you need to be registered on that forum to view attachments, so I just re attach it here.
In certian RPM ranges its like having free turbo or super charger boost.

I have found a lot of runners like to be right around 24 inches.

[E4ODnut]

Fascinating! Thanks for posting that.

So, If I'm understanding the chart right a 28" runner will peak at ~5100 RPM with the 2nd harmonic at a strength of 10%. A 14" runner will peak at ~3600 RPM with the 3d harmonic at a strength of 7%. A 7" runner with the 4th harmonic will peak at ~2750 RPM at a strength of 4%.

% of what?

[sendler]

Keep in mind I am discussing my motorcycle engine which is an FI single and has the throttle body in the middle of the runner as pictured in the earlier post so a plenum isn't really present as the air box is running with the dirty side lid off right now so it should pretty much always stay at atmospheric regardless of any pulsing.
.

Originally Posted by ERTW

your engine peaks not only at 8700 rpm, and also at 6450 rpm.

tuning also depends on valve area, and hence port area. waves happen at all rpm, at all throttle positions.

6000 rpm wants a 380 mm intake runner, as measured from the valve to the plenum. the plenum can be as large as 2L, but slightly smaller (~1.5L) will help throttle response.

increasing flow by porting, in other words reducing pumping loss, will boost economy.

imo the biggest thing is the large cam overlap designed for 8700 rpm power. you want minimal overlap so you're not sucking raw fuel out the tail pipe. roughly in the 200* @ 0.050", 112* LSA.
hth

[oil pan 4]

The % is the amount of extra air that can be forced into the cylinders.

I have found on quite a few trucks regardless of make modle and year the OEMs like to keep the intake runner length right around 24''.