ITB Design Help Needed

[tom_p]

Hello, I am currently trying to design a set of ITBs for my M20B25 swapped 320i saloon. I am making my own for a number of reasons; I have access to a CNC 3-axis mill and a manual lathe, along with a heap of scrap aluminium, I am a 1st year mechanical engineering apprentice so this is fun and good experience, and I don't fancy coughing up £1200 for the RHD ITBs. This is not for track use, although I may attend a track day or two, it's for road use. Looking for throttle response and sound, along with it looking cool.

My design is mainly based on the RHD set as I've heard good things, but before I start tweaking things and printing off some parts for fitment testing, I'd appreciate some external input.
Currently a stock, freshly rebuilt late model M20B25, though I am soon to have a better exhaust manifold, O2 wideband sensor, and likely a 272 cam. I am running it with a ClassicDaily Turkey and Cheese ECU (FOME).
I can attach more photos/info if needed, along with an .stl, parasolid or NX part file.

40mm dia, overall height:253mm, distance from valve to head:62.25mm, injector angle:45deg, m50/52 variable TPS.
Vacuum feed from bottom of adaptor block into vacuum plenum, feed then to brake booster, FPR, ECU, input from stock IAC valve. adaptor plate, TB and most parts within to be machined, trumpets to be 3d printed out of TPU(?) and extension tubes to be cut ali pipe with welded flanges.

I am unsure of what runner length I need. The throttle linkage is incomplete, return springs would be sat underneath. Valves currently close with a chamfer against a raised edge, would meeting a flat face be better? What sort of valve would I need to run for the booster?

I have a reasonably good general knowledge of cars, but lack specific technical knowledge. Please scrutinise my decisions and suggest solutions.

Sorry for the scrappy explanation. Any help is appreciated :) Thanks

[tom_p]

Here's my car by the way, his name is Bertie

[ehrawn]

This isn’t a question that’s can be answered here. Please understand I am not trying to be snarky. There’s a lot of math needed to effectively optimize intake runner length. Here’s a qualitative explanation that hopefully helps you understand how complicated the question is that you are asking:

[tom_p]

ehrawn Thank you for the link, I'll give it a read.

I think I'll try and break my questions down into parts after I because I have a fair few, but mainly I just want to know if I'm forgetting something before I get it machined.
I appreciate you telling me this isn't the right place for it so I'm not waiting on a response. I'll do some digging.
Thank you :)

[digger]

Hello, I am currently trying to design a set of ITBs for my M20B25 swapped 320i saloon. I am making my own for a number of reasons; I have access to a CNC 3-axis mill and a manual lathe, along with a heap of scrap aluminium, I am a 1st year mechanical engineering apprentice so this is fun and good experience, and I don't fancy coughing up £1200 for the RHD ITBs. This is not for track use, although I may attend a track day or two, it's for road use. Looking for throttle response and sound, along with it looking cool.

My design is mainly based on the RHD set as I've heard good things, but before I start tweaking things and printing off some parts for fitment testing, I'd appreciate some external input.
Currently a stock, freshly rebuilt late model M20B25, though I am soon to have a better exhaust manifold, O2 wideband sensor, and likely a 272 cam. I am running it with a ClassicDaily Turkey and Cheese ECU (FOME).
I can attach more photos/info if needed, along with an .stl, parasolid or NX part file.

40mm dia, overall height:253mm, distance from valve to head:62.25mm, injector angle:45deg, m50/52 variable TPS.
Vacuum feed from bottom of adaptor block into vacuum plenum, feed then to brake booster, FPR, ECU, input from stock IAC valve. adaptor plate, TB and most parts within to be machined, trumpets to be 3d printed out of TPU(?) and extension tubes to be cut ali pipe with welded flanges.

I am unsure of what runner length I need. The throttle linkage is incomplete, return springs would be sat underneath. Valves currently close with a chamfer against a raised edge, would meeting a flat face be better? What sort of valve would I need to run for the booster?

I have a reasonably good general knowledge of cars, but lack specific technical knowledge. Please scrutinise my decisions and suggest solutions.

Sorry for the scrappy explanation. Any help is appreciated :) Thanks

Runner length is normally determined based on peak hp rpm target with a 3rd harmonic , for an m20 about 12" from bellmouth to head flange is about right for a 7000 rpm engine (this is about 15" from bellmouth to valve). The thing about ITB is they normally allow different runner lengths so no need to be exact.

Pipemax is a useful and cheap tool

BAIN racing have a good induction series on YT worth watching as it goes into air speeds (diameter) and lengths.

[digger]

before you worry about the ITB you need the manifold to be aligned with the ports properly. The port axis is not perpendicular to the head flange.

you'll want individual return springs and you need to think about how you are going to synchronize all 6 butterflies

The other issues is packaging to clear the hood/bonnet and the brake booster as you need to compromise somewhere on the length especially with straight runners

[TobyB]

Well, I have nothing but respect for someone who's willing to dive into the deep end
upon first seeing a swimming pool!

My 02 (we have 2 of them) is not to be too ready to CNC everything-
if it's a 6" long pipe, weld a couple 0.125 flanges to a 5.75" pipe and call it done!

Likewise, if you look at the history of infernal combustion engines, it's
amazing what sorts of crap runs pretty well. So sometimes it's best to
get something hacked together, and then incrementally improve it as you go.
Leaving the ability to modify as much as you can will be important...

From lots of races with DCOEs, your linkage design is crucial. As digger says.
You want one control authority, and you want it to have full travel.
Then the butterflies should all faithfully follow that control path.
You certainly want a progressive action, as in, the first 15 degrees of
butterfly rotation should be easy to modulate with a tall Doc Martin on.

As to the runners, since you're not gong flat out, volume matters a lot,
but a straight shot's not as critical, and section changes might matter a lot less.

This sounds like fun. I wish we all lived forever so I could play, too.

t

[tom_p]

before you worry about the ITB you need the manifold to be aligned with the ports properly. The port axis is not perpendicular to the head flange.

you'll want individual return springs and you need to think about how you are going to synchronize all 6 butterflies

The other issues is packaging to clear the hood/bonnet and the brake booster as you need to compromise somewhere on the length especially with straight runners

Thank you, I'll redesign the throttles. I'll try and make a similar design to the RHD set with the single lever.

I've heard that you can relocate the brake booster to behind the intake-side headlight using an e28 booster. Might that be an option?

I could also angle the throttles a bit.

A wise giant pumpkin farmer told me that roughly 270-280mm was ideal length for intake runner length. I plan on having the 3d printed from TPU so I can go through a few sets until I find what works best.

I will update you when I've made a new design :)

Thank you

[tom_p]

Well, I have nothing but respect for someone who's willing to dive into the deep end
upon first seeing a swimming pool!

My 02 (we have 2 of them) is not to be too ready to CNC everything-
if it's a 6" long pipe, weld a couple 0.125 flanges to a 5.75" pipe and call it done!

Likewise, if you look at the history of infernal combustion engines, it's
amazing what sorts of crap runs pretty well. So sometimes it's best to
get something hacked together, and then incrementally improve it as you go.
Leaving the ability to modify as much as you can will be important...

From lots of races with DCOEs, your linkage design is crucial. As digger says.
You want one control authority, and you want it to have full travel.
Then the butterflies should all faithfully follow that control path.
You certainly want a progressive action, as in, the first 15 degrees of
butterfly rotation should be easy to modulate with a tall Doc Martin on.

As to the runners, since you're not gong flat out, volume matters a lot,
but a straight shot's not as critical, and section changes might matter a lot less.

This sounds like fun. I wish we all lived forever so I could play, too.

t

Ok great, thank you.

I was planning on getting some pipe, laser cutting some flanges and then welding them on (or more likely get my welder friend to). I don't think my gasless mig will be too pretty...

I'll do my best to allow for modifications, and I will change the throttles to follow one linkage. I think I may go for 6 individual throttle units, rather than 3 banks of 2.

Thanks :)

[digger]

Thank you, I'll redesign the throttles. I'll try and make a similar design to the RHD set with the single lever.

I've heard that you can relocate the brake booster to behind the intake-side headlight using an e28 booster. Might that be an option?

I could also angle the throttles a bit.

A wise giant pumpkin farmer told me that roughly 270-280mm was ideal length for intake runner length. I plan on having the 3d printed from TPU so I can go through a few sets until I find what works best.

I will update you when I've made a new design :)

Thank you

The comment wasn't to suggested that it cant work but you need to start thinking about it. And i don't see there is a reason to simply change and copy the RHD design. Surely you want to change it to make it a design challenge.

If you want to use an air box then the headlight area could be occupied by a filter depending on how you go about it. There are smaller boosters for stock location that could allow the runners to be angled more optimally than is possible with the stock booster. There is the Massive booster delete dual master bracket that can make a lot more room. I am making my own curved runners and airbox to suit RHD kit.

If you want a project then a slide throttle set would be good as when you have a 40 mm bore ITB and roughly 6 mm shaft through the middle it is choked off measurably. This would allow a more optimum size throttle size without obstruction

[TobyB]

Oh, Christ on a Capon, slide throttles are
another can of friction- filled rules- skirting nightmares! ;)
Just use 41mm throttle plates!

But seriously, air box design is not trivial at higher RPMs. I went through about
6 iterations with the DCOEs before I found something that was no worse than a large
cardboard box to simply keep the reversion pulses from blowing away.

I remember seeing at least one picture of an intake in development that had
adjustable length runners. The caption said it was being used to tune the intakes
to better suit different tracks. And it was a black and white photo...
There are lots of pictures of DFVs and similar with varied intake runner lengths
to try to widen the power band, too...

t
in for pictures.