Optimum Intake Runner Length

[67_6cyl]

Found this website while looking for EFI material - these are the guys who came up with the Megasquirt FI system:

http://www.bgsoflex.com/auto.html

Check out the link to the "Optimum Intake Runner Length Prediction". Authors are saying that the harmonic pulses created by valves opening and closing creates an increased -ve pressure zone in the intake which can, if properly tuned, result in greater volumetric efficiency and therefore power. Makes sense - similar to water hammer, resonant frequency and other control harmonics that are common.

Question: Has anyone ever tried this? Is it worth the effort?

 

[StrangeRanger]

Old news. Seriously, we've been discussing tuned runner length, etc. for years on this board. Check out the book Scientific Design of Exhaust and Intake Systems by Philip H. Smith if you really want tounderstand what's going on.

 

[rbohm]

8) as SR said, this topic has been discussed to death, and not onl yon this forum, but many other as well. one thing about trying to get the optimum runner length though, you need to decide what rpm it will be optimum for. if you look at the old cross ram 413 chrysler engines, it was optimized for low to mid range rpm, where as the injected 327 chevy small blocks were optimized for mid range and high rpms. shorter runner means higher rpms.

 

[67_6cyl]

That's what happens when you're a rookie, on a journey of discovery. I'll check that book out - it seems pretty interesting.

 

[rbohm]

8) forgive us old timers. we have hashed this, and many other, subjects out over and over again, and on a number of different forums. dont be afraid o ask questions though, we are here to confuse.......... er help

 

[Anonymous]

I have looked at the idea of intake and exhaust wave tuning. While I understand the concept I have difficulty deciding how to best apply this knowledge.

Based on the formulas from Blair tuning the inlet lengths produces different harmonics or high pressure waves which occour at different RPMs. Also there are low pressure waves which occour between these high peaks. My question is where should you put the peaks. Should you design for a peak to be at the same RPMs as the camshaft peak or a little higher up the RPMs to try to extend the power band a little.

Also if you are tuning the exhaust where should the exhaust peak be put. (In the middle between the inlet tuning peaks, at an inlet tuning trough, at the same RPM as the cam peak or elsewhere)

If it makes a difference my car will be used for road rallying where a combination of tight bends and long straights mean that high power and good torque are both important.

Best Regards
Tom

 

[rbohm]

8) tom, you are going to have the worst of all worlds trying to tune this combo. tight turns means oyu need low end torque, and long straights means top end power. this makes for a tuning nightmare since you cant use gearing to help without limiting your top speed, unless you are going to use an over drive of some sort. my suggestion would be to tune for as much mid range power as possible, and go a bit stiffer on the rear gear than you normally would, a 3.25 vs a 3.00 for instance. that way you gain a little off the line, but you dont hurt the top end much as the mid range will help get you down the track.

 

[XPC66]

Typically rally engines like Tri-Y setups with one cylinder scavenging a sister port.

Tom, every man and his dog has a secret recipe for the ulimate exhaust and intake system. Whatever you select you need to remember that the lengths are inclusive of the port runners upto the back of the valve.

The harmonic formula is working on the valve closing/bouncing incident. It's much like your garden hose; when the valve rapidly shuts a rarefaction wave occurs that reverses the flow momentarily, building pressure near the valve that then expels some water with a spurt.

I've found harmonic calcs are good for intake sizing, but useless for exhaust. I know this is very unfashionable, but there I said it.