M20 ITB's

[digger]

there is not much point running a carb on the street you'll only be better off at WOT peak hp if at all. Carb's (if setup well) often have better fuel atomisation compared to a relatively budget OEM style fuel injected setup and that is why they can make more peak hp, but you'll still need ignition control in which case at worst an AN would be pretty simple to setup. I ran AN with my stock manifold and TB setup and it worked fine and was liveable, albeit not as refined as a properly setup SD system. Using throttle bodies that are too big would also hinder tuning (not just midrange) as you would lose some resolution of the throttle

[SkiFree]

Even a well-choked 45 will be a bit much for street M20's. I've been playing around with hypothetical carb setup for the DCOE40's, will be happy to email the final settings. Digger's caveat holds true **if setup well**

A challenge with 40's is that most ITB manifolds sit at an angle parallel with the engine, starving the float bowel. Solutions are a bit Jerry-rigged. The other challenge is most ITB manifolds are cast/machined for 45's.



As a side note, customer in Switzerland running an IE 3.1 stroker with ITB's, he's coaxed quite a bit out of it.

[clarkson]

Wow, is that 270hp ATW at 6500rpms? Looks like it from that chart. Even if that dyno is a bit generous, that is a very healthy M20.

Got any more info on the build? For example: what head was he using, what cam and what size ITB's...?

[digger]

Wow, is that 270hp ATW at 6500rpms? Looks like it from that chart. Even if that dyno is a bit generous, that is a very healthy M20.

Got any more info on the build? For example: what head was he using, what cam and what size ITB's...?

270bhp actually, being Europe they all convert to crank hp using varying correction factors. using the 16% stated it is 220-230whp which is about 10 more than what a cast manifold can do with a 'streetable' camshaft.

The headflow capability of the M20 head means that much more than 280bhp is a tough ask, i am talking a genuine number not a chassis dyno spitting out fantasy numbers and using bogus % losses.

you can tell bogus numbers by looking at the the BMEP at peak torque rpm and peak hp rpm and comparing to highly developed engines with similar configurations.

[nando]

yeah, they often have BHP converted #s. the problem is it's not easy to measure the real loss, and the curve isn't linear, so if you apply a straight 15 or 20% or whatever you feel like, you can get any number you want.

[SkiFree]

A little more, not just based on internet spatter. It'll be the car-of-the-month for December, will have plenty more info.

It is calculating the lost of transmission when you go lower in speed.. And it gives you directly the power of the engine.. It's not a bosch device being to optimistic.. This one is a real one.. >From the technical engineers school..

And in action, skip ahead to the one minute mark.

http://www.youtube.com/watch?v=L28umKnmIMY

[nando]

it's still subjective and typically optimistic. There are so many more things at play that affect drivetrain loss.

also, what it makes at the crank is much less relevant than what it really makes at the wheels.

[SkiFree]

it's still subjective and typically optimistic. There are so many more things at play that affect drivetrain loss.

also, what it makes at the crank is much less relevant than what it really makes at the wheels.

Certainly, I'm sure you'll agree its equally as subjective to make claims or generalizations about an engine/car that you (speaking generally) know no specs or details on other than the limited information I've listed.

[digger]

i assume it uses coastdown, some only use a fixed percentage. They are approximations and there is nothing majorly wrong using one of these methods as long as the correction used is also shown.

One of the reasons that coastdown is not correct and never will be is that some proportion of the real losses is a proportional to the torque input so when coastingdown as the engine is not loaded in the same manner it can never be perfectly correct

[digger]

it's still subjective and typically optimistic. There are so many more things at play that affect drivetrain loss.

also, what it makes at the crank is much less relevant than what it really makes at the wheels.

not really you want the crank hp to know what changes to an engine actually do. you could make a positive change that results in more crank hp that shows up as a lower wheel hp because of something that has increased the drive losses inadvertantly or by error/oversight in the measurements process. Chassis dynos are not accurate or consistent enough to measure some changes especially if not done immediately back to back.

[tinkwithanr]

New setup in progess: 38mm BMW 1200RS Throttle Bodies






This is still a work in progress but I know another person on here has a completed setup using the same bits, and it's running well. 38mm should be able to outflow even a well worked intake port easily while still maintaining proper port velocity. Another plus is it allows the throttles to be placed very close to the head, taking up less space and allowing for a proper plenum.

[fporro]

Maybe is not the size but the motion.

Everyone more or less agrees that 45's bog down and 38's react faster to throttle changes through mid to upper rpm range, yes?

I think butterflyes are the couse of that assertion, why?, well... their airflow increase does not match the headflow volume increase, realistically the slider type itb's would work better as inlet size volume has a more gradual curve while the buttreflyes go from semi open no load to this upper and lower planes in no time, resulting on bog-down feel., they give more flow than actual intake valve can ingest at given rpm, a more gradual increase would be the trick.
A runner inler profile matched to a head rpm flow requirement would be the ...
visualise it, the inlet port and slider can have any proflile design to match any given cam.


It is posted somewhere, but what's the real max flow rate of any given cylinder allowed per inlet valve/whole head?, say like on a 3.1 stroker, and, what itb diameter will acommodate it ? we're talking n/a now. That should clearify what itb diameter is best.

I've always like the idea of spring loaded exhaust pressure regulators, they are this metal flaps that you make, insert given disired pressure springs and install in pipes., they collaps as pressure decreases hence keeping velocity up.

It works same on inlet side, place it in runner and is collapses as map decreases, hence increasing or as I would like to think, manteining velocity in runners.

Sorry for rant.

[SlamedIAm]

Any info on what is required for those 1200rs throttle bodies? I'm curious to see the results.

[digger]

the reason for not using 45mm is not so much the transient response, it is velocity at WOT, though there would be better throttle 'resolution' with smaller throttles i.e. with big throttle the engine might be at effective full WOT at 75% TPS which might limit the extent of the ECU map usable and driveability would not be ideal

The velocity which needs to be a certain speed to work best without being too fast nor too slow which is determined by the diameter of the runner. I use pipemax which tells the diameter at the head and also the plenum and from that you can work out how big the throttles should be depending on how far along the runner you are.

when i have access i'll run a standard engine and mild stroker, from memory 36mm at the head is a decent starting point so 38mm throttles a bit further upstream is close to hitting the mark or 40mm for a bigger stroker

[tinkwithanr]

When you get a chance I would be very curious to see how 38mm throttles ~1-2" from the head would work on a 3.1L stroker, compared to 40mm or 42mm. Hopefully I didn't "choke" it down to much.