Calculating runner lengths for ITBs by studying your fuel map

I see a lot of people, with a lot of different types of engine fitting ITBs, and then generally ending up with runners that are way too short.

Or some people that realise longer is good... But then still pick a random length with no rhyme, reason, or science involved!

So, what is actually happening in your engine.

Whatever your runner length is, when the intake valves shut a pressure wave builds at back of the valves, and then travels back up the runner. When it gets to the end of the runner, it reflects back down the pipe and meets back at the valves again.

If the valves are open when this happens, you get a boost because it shoves more air into the engine before the valves shut.

(apologies if this is obvious already, but I feel it needs to be reiterated just in case)

Sooo lets say the time period for this first reflection, how ever long it takes. This is "1" time unit.

If the wave reflects back down the runner, but the valves are once again still shut. The wave bounces back up the intake runner, and reflects back down for a second time. If the valves are now open, it will give a power boost, albeit weaker. Since there a wave has now gone up, down, up down... The time that this takes is "2".

And then this same thing can happen again, the time period to get to the 3rd reflection to hit the valves, is "3"

This may sound obvious at this stage, but it has some important implications.

Because the peaks and troughs generated by helmholtz affect your fuel and power, it's very easy to see where they occur by looking at your fuel table.

By studying the ratio of rpm between the peaks in your powerband, you can tell which of the harmonic waves you are looking at.

So if you were looking at the 1st and 2nd waves. Lets just say they happened at 8000rpm and 4000rpm.

8000 / 4000rpm = 0.5, or in terms of our time periods. Thats 1 / 2 = 0.5

Or lets say instead, the peaks and troughs on your fuel map were generated by the 2nd and 3rd waves that happened at 8000rpm and 5300rpm.

8000 / 5300 = 0.666, or, in time periods, 2 / 3 = 0.666

and so on.

So here's a chart of the ratios for each reflected wave:

If you have your air fuel ratios dialled in perfectly across the rev range, you will start seeing these peaks and troughs quite distinctly.

Here are two seperate examples of fuel maps from beams engines:

When you know which harmonic wave you are looking at, and you know the total runner length from tip down to back of valves.

You can do some excel wizardry to find the speed at which the wave travels up and down the runner.

Once you know this speed, you can virtually adjust your runner length in the calculator and see how the powerband will move around.

With my ECU, and using the factory Blacktop 3SGE intake manifold, the peaks and troughs are as per below:

So harmonic peaks at 3265, 4081, 5441, and 8162rpm.

I cannot stress enough, how little change in runner length is actually required to move the harmonic frequencies by a lot of rpm.

If I want the 8162rpm harmonic to drop to 7500, it only takes (from memory) 4-5mm more runner length.

When people go from something like a 120mm runner down to a 35mm runner and they get unpredictable changes, its because the harmonics have changed so much that they're looking at completely different reflected waves in the manifold.

If using an ITB engine, my suggestion for tuning runner length is to get the "EFI hardware" or similar throttle bodies that have the detachable base and a non tapered length that you can cut down.

Buy the longest ones that you can fit, to give yourself the best chance of ensuring that the harmonic waves you want to find will occur SOMEWHERE before your rpm limit for your first analasys.

Once you've established where the harmonics are happening, you can make an informed decision about how much longer or shorter to adjust your runner length by.

So you could calculate this second value, and then trim to that length + 10% to stay on the safe side (As its easier to reduce length than add it...)

Then from here, you can fine tune down to a few mm to get the power working exactly where you want it.

The point of this post is that runner length doesnt have to just be "suck it and see". Once you've got the data from one length, you can work out the others without trying 100 sets.

Within 1 or 2 iterations you should be able to match the harmonics of the runners, to exactly the point where you want your powerband to be.

What's interesting to take away from this though is that its just as important to look at where the "troughs" end up, as well as peak power.

Contributing factors that should be given consideration towards your runner length are your gearbox ratios, and also how high your engine revs.

As gaining 10hp peak power sounds great, but if you end up shifting down straight into a deadspot it's not going to make your car any faster.... Ooooorrrrr you could just fit a turbo I guess and make this all irrelevant hahaha.

Excel sheet is attached if anyone wants to have a tinker.

Coming from a non-engineering background and having been tuning for years I have seen this first hand and knew it was from the pulsations of the intake, just not the terminology, theory and practice behind modifying the runner length. Thanks for sharing.

Thank you for taking the time to post this well worded and comprehensive, very informative, information!

Fantastic explanation of an empirical set of data showing this effect and not a fantasy calculator. Been researching this for years doing porting work and have never gotten to see how the values could be achieved working backwards from the data vs. forward from calculations and design.

Fantastic wording!

Excellent post, and the same thing applies for scavenging from the exhaust.

Most production vehicles are deliberately built to avoid all cylinders having the same critical frequency, to avoid the troughs you mentioned and give a more even torque curve.

As an aside, it is actual 'tuning', which dates back to the early steam engines which had very long plumbing to 'tune' the filling in exactly that way - something the 2-stoke chaps are very knowledgable about.

Suggested further reading - https://www.jenvey.co.uk/support/faqs/

There was a library* book I read many years ago, which should be of interest to some of you folks, called https://www.amazon.com/Scientific-Exhaust-Systems-Engineering-Performance/dp/0837603099 Might have to pick up a copy as, from what I recall, it had some very useful information and tips for making diagnostic/test tools, like manometers to check standing waves in the tracts at different rpm.

*Some of you may have access to libraries with a decent technical section, I'd suggest dropping in and having a look - and they can often get books they don't have from other branches. same with technical institutes and and universities - you won't be able to book them out, but you can read the books, make copies of interesting parts sections, and make a note of books you want to purchase.

@Davidv - Would you be willing to repost a few of the pictures/links?

Thank you!

Forgot in the earlier post, once the manifolds have been built and fitted, don't forget to try swinging the camshaft(s) timing a few degrees either way, to fine tune the pressure wave timings for best power/torque - assuming you checked there is clearance for advancing the intake and/or retarding the exhaust - moving them the other ways increases the clearance, so isn't a concern.

I'm not sure it it's very accurate but i have been using this chart...