M20 throttle body upgrade

[LowR3V'in]

The limiting factor, assuming an I head, intake and throttle body is the head. Not the intake or throttle body. To make power up top, the head needs to be modified to increase flow and the cam needs to be changed to one that will shift the torque peak higher.

Kpa is measured at the manifold so if there is vacuum, it's because
there is a restriction before the manifold. :)

If say there is 100kpa atmospheric pressure out side, and you see 80kpa at the manifold,
what the engine is doing is sucking through a straw of a throttle body
at the 100kpa outside. It should be 100kpa with 0 restriction. but there will
always be some more or less.

The filter could be an excuse, but take it off and see that the
throttle body is still causing some restriction.
Probably nothing on a stock m20.
3, maybe 5kpa at most? Just a guess.

Nando: Hell yeah make a post when you get the m30 throttle on.
12kpa is ridiculous!

[lennon]

To echo the above my M20B27 motor with a stock 885 head, 325i inlet with 325i cam, 10.2:1 eta pistons, 130mm conrods, stock eta exhaust and motronic 1.3 with a AFM. It also has torque falling off seriously after 5300 rpm (it peaks around 4200 rpm and starts droping after that). This is with a bored out 325i TB. All the big bore TB has done since its been fitted has added some nice induction noise. It did not increase power by itself. After fitting and driving it for while I had it remaped, and that has increased the top end power output as the car was running too rich at WOT.

You have hit the problem enowaffleswaffleswaffleswafflesered by most in tuning the M20. Torque only starting to seriously fall of after 5300 rpm is not that bad. Trying to modify the engine so torque peaks a lot later I would have thought would require a different inlet manifold (perhaps itb's) and maybe a 24v head. But in doing that you are recreating a s50 e.g the euro version with itb's.

You have not posted any power/torque curves with AFR plots so it dificult to assess how serious the fall in torque is. However fitting big bore TB will help induction noise and on that basis it's lovely. I was hoping it would do more than that but it hasn't.

is it running stock motronic 1.3 software or what is your tune? a stock tune and the stock eta exhaust are definitely restrictions. you would be best to change those before the throttle body.

[briansjacobs]

ETA another big thing you are missing is the tuning, my car is tuned specifically for my mods. We have 6 hours of dyno time getting it just perfect, I could have made more power but I wanted to tune for optimal AFR. We made adjustments first to the MAP, and then to fuel pressure regulator as well as making adjustments to the exhaust backpressure (supertrapp) and again more MAP changes. when I do a BBTB I will again tune the car to this modifacation.

It is nice having a dyno at the shop!

[nando]

except you also had a stock 325i cam. it's designed to peak with the manifold. a 288 can easily make peak power at 6500-7500rpm depending on how things are set up. the eta exhaust and AFM don't help either, but that's another topic.

I have several dyno charts of 2.7i engines, which is what you had. All but a one make the same peak HP as a 325i, and all of them have torque curves that die earlier than a motor built with proper pistons and rods. That has nothing to do with the TB - you weren't really making a lot more power than the B25, so you really didn't need the extra capacity of a BBTB. No surprise there!

basically, you're comparing apples and oranges.

you can't upgrade one part alone, and expect to see a big difference. you'd need a cam, higher CR, good exhaust, filter, engine management, etc etc.. Hell, I saw 10whp from ditching the stock airbox (although I still run it during the winter), but I'd already done everything else. The last peices in my puzzle are the manifold (which is much harder to improve) and the TB.

[eta]

To echo the above my M20B27 motor with a stock 885 head, 325i inlet with 325i cam, 10.2:1 eta pistons, 130mm conrods, stock eta exhaust and motronic 1.3 with a AFM. It also has torque falling off seriously after 5300 rpm (it peaks around 4200 rpm and starts droping after that). This is with a bored out 325i TB. All the big bore TB has done since its been fitted has added some nice induction noise. It did not increase power by itself. After fitting and driving it for while I had it remaped, and that has increased the top end power output as the car was running too rich at WOT.

You have hit the problem enowaffleswaffleswaffleswafflesered by most in tuning the M20. Torque only starting to seriously fall of after 5300 rpm is not that bad. Trying to modify the engine so torque peaks a lot later I would have thought would require a different inlet manifold (perhaps itb's) and maybe a 24v head. But in doing that you are recreating a s50 e.g the euro version with itb's.

You have not posted any power/torque curves with AFR plots so it dificult to assess how serious the fall in torque is. However fitting big bore TB will help induction noise and on that basis it's lovely. I was hoping it would do more than that but it hasn't.

[nando]

sure, they work together. The harmonics of the M20 manifold are tuned for a 3500-5200rpm torque range, it's quite obvious from any dyno chart of an M20 using this manifold (it feels like the car is coming on cam, but a huge part of this is actually the manifold). Beyond the 5200rpm range though, it's not working the same way (out of phase), and the TB (and to some degree, the size of the runners) can be a restriction.

If you think BMW sized the TB overly large for a 168hp engine, and that somehow it's not going to be restrictive for an engine making 30-40% more power, and making torque for another 1000rpm past stock, we're just going to be arguing in circles. I'll post an updated datalog after I get the TB and manifold in, and leave it at that.

[jlevie]

that's possible - but the VE map could simply need tuning there. The easiest way to tell is to measure the pressure drop in the manifold.

also, BMW sized the stock TB "just big enough" for the stock engine. Once you've modified it significantly (beyond a chip, basically) that goes out the window.

There's more to it than just the throttle body. The flow dynamics of the intake are also a large factor in how the engine breathes at higher rpm.

[briansjacobs]

I will try to print a dyno sheet next time I go to the shop

[nando]

that's possible - but the VE map could simply need tuning there. The easiest way to tell is to measure the pressure drop in the manifold.

also, BMW sized the stock TB "just big enough" for the stock engine. Once you've modified it significantly (beyond a chip, basically) that goes out the window.

[jlevie]

If the A/F ratio is going leaner at the top (as suggested by your most recent post), the engine isn't running out of air at the top end in the current tune. What I'd really like to see are the curves.

[briansjacobs]

I have a miller chip so I can tune it to a degree, my AFR is between 12.3 and 12.8 at WOT pretty much across the power band. The torque starts at 2500RMP and goes to 5300RPM before it starts to fall, Car is never below 3000 rpm at the slowest corner of any track I go to (the hairpin at Homestead I go thru at neutral throttle and by time the car is turned we are down to 3000 rpm which is great torque)

[nando]

Fair enough...

What do the torque and A/F ratio curves look like?

I know you were talking to brian, but if you look at the green GVE curve, it approximates my torque curve.

[jlevie]

Fair enough...

What do the torque and A/F ratio curves look like?

[briansjacobs]

The limiting factor, assuming an I head, intake and throttle body is the head. Not the intake or throttle body. To make power up top, the head needs to be modified to increase flow and the cam needs to be changed to one that will shift the torque peak higher.

524td crank, 10.5:1 compression,vac cam gear, schrick 288,miller maf and chip, port polish and valve job by extreme heads. im thinking only left is the throttle body

[nando]

The limiting factor, assuming an I head, intake and throttle body is the head. Not the intake or throttle body. To make power up top, the head needs to be modified to increase flow and the cam needs to be changed to one that will shift the torque peak higher.

we aren't talking about stock engines here...



note the MAP value. ambient is 101.1kpa (I live at sea level). The engine is pulling ~12kpa of vacuum at 7200rpm, instead of seeing atmostpheric pressure. The filter is about 2-4kpa of that, the rest of the pressure drop is coming from the stock TB and manifold. That's approximately a 10% restriction - 10% more air is a big deal.