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M20 turbo cam and turbo sizing question

Discussion pertaining to positive pressure E28s.
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Russianblue
Posts: 415
Joined: Feb 12, 2006 12:00 PM
Location: Charlotte, NC

M20 turbo cam and turbo sizing question

Post by Russianblue »

Turbodan, you have to stay out of this one. ;)

But I would like a jury of opinions. You already know the situation. But if you want to interject.....well that's fine too!

Anyway, I am getting ready to turbo my M20, 1988 SuperETA mfr'd 3/87 and wanted to get some thoughts on how my current cam setup will fare with the turbo selection.

I am currently considering a GT28RS and a GT2871R like Dan's. The motor was originally built to simply be an e to i swap, and has a 325i 885 head with 325i TB and a ported 325i intake on the stock bottom end.

My current cam is an ireland engineering re-grind - 272/272 with 50 degrees of overlap. This much overlap is a negative for FI from what I've learned, but in my extensive searches, i cam across this thread http://www.mye28.com/viewtopic.php?t=26596 where Duke's and Russc's comments lead me to believe that it won't be such a bad thing.

In an effort to avoid pulling the head at this time, I wanted to keep my current cam. I plan to shoot for 10lbs of boost on the stock internals. My headgasket is new.

I have an extra stock e30 cam i can use if the 272 doesn't work out. But assuming the 272 will be the cam of choice on the first turbo attempt, as it relates to turbo choice, Dan is pretty much set on the GT2871R with a .86 turbine housing and IMHO, there is very little reason to question anything Dan suggests as I have found his every decision on equipment to be supremely well thought out. However, my tuner has come up with the GT28RS as being more appropriate. The GT28RS is geared more to the low end of the powerband, albeit just a hair.

here’s a good thread on the GT28RS vs. the GT2871 vs. the GT3071. Different car obviously, but i would think many of the same principles apply.

http://www.audiforums.com/archive/threa ... 010-1.html

Do you guys have any opinions on this turbo choice given my current cam setup? I have no idea how it feels for power to "fall off a little" at the top end so there is no frame of reference.

My goals:
This will be a daily driver with 6-10 track days per year. I would like to acheive approximately 275 HP and have a respectable track car (as respectable as a SuperETA can be of course).

The way I look at it is like this: The more I can do to shore up the low end, the better. As TCD once told me, the M20 is never going to be torque monster. However, my agressive cam will need to breathe even more at the top end which may push the smaller sized turbo over the edge.

So i have no idea what to expect if I choose a turbo that's "more geared toward the low end' and whether this blower will simply offset the low-RPM weakness of my aggressively CAM'd motor...OR...if the "falloff" at higher RPM will be dramatic to the extent that it is annoying and/or problematic

Keep in mind that I am running a 3.73 LSD, so i can keep the RPM up in the range most of the time. The problem areas are, for example, at VIR, on the North Course, when you go under the bridge and take a left around the corner, up the hill - my car had NOTHING in that turn. It was completely anemic. I had to climb the hill with momentum only. It did very well on the rest of the course, as long as i could keep the RPM up.

Final note: Catcams.com offers an M20 'turbo cam' with attributes that seem drastically different from both the stock 325i cam (which TDan has proved works VERY well) as well as my IE re-grind, . Any thoughts on the effects of this cam on the turbo and how it relates to turbo selection, and whether its worth pulling the head to swap, would be greatly apprecaited.

clearance [cl] I/E 0.25mm 0.40mm
duration [0.1mm+cl] I/E 272° 248°
duration [1.0mm+cl] I/E 234° 214°
valve lift [cl=0] I/E 10.80mm 9.90mm
cam lift I/E 6.90mm 6.35mm
peak angle I/E 110° 120°
timing [1.0mm+cl] I/E 7/47° 45/-11°
lift at TDC [cl=0] I/E 1.90mm 0.70mm
T_C_D
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Re: M20 turbo cam and turbo sizing question

Post by T_C_D »

Russianblue wrote:As TCD once told me, the M20 is never going to be torque monster.
What I meant, I am sure you understand this, is that the m20 won't make tq at low rpm like the m30.
Jeremy
Beamter
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Post by Jeremy »

The aggressive cam will make the turbo spool later. Using a small turbo will make it slightly quicker, yes, but you still won't make fantastic torque because you're limited by the cam.

That said, you'll still make more torque at any given spot in the rev range than you did naturally aspirated, so you should be able to pull through that spot where you couldn't before.

Essentially, it works much the same way as it does on a naturally aspirated car in terms of cam, but the turbo motor comes on cam MUCH harder. It's all a matter of where you want the power. Since most of us use these as street cars, we like a meaty midrange. If it falls off a bit way up top, no big deal. Your goals may be different.

Jeremy
turbodan
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Post by turbodan »

Yeah mang...360 foot pounds at the wheels. Its not available off idle, but the m20 can make some good torque.

But I'll stay out of this one.
Russianblue
Posts: 415
Joined: Feb 12, 2006 12:00 PM
Location: Charlotte, NC

Post by Russianblue »

Dan, you are a trip! Thanks again for all of your help so far. I am sure I will have more questions later.

TCD, I understood your earlier analogy. I thought it was very good. I'd never heard that put so succintly and it really conveyed an important difference with the designs.

I think Jeremy makes a good point when he says "you'll still make more torque at any given spot in the rev range".

Essentially, this will be a complete overall paradigm shift in the entire power/tq curve shift such that I'll never know (or care) what i might be "missing".

For example, I ran my numbers through Turbocalc at 2300 RPM, the heart of the lull, and assumed I could get 4psi boost at that level on my way to a full 10psi at 3500 or so. I estimated 72.5% compressor efficiency for this rpm level. At 2300, my dyno curve on the N/A motor shows 79 hp / 139 tq. Adding just 4psi boost while the turbo spools up gives me another 15-20 hp and adds a whopping 60 ft/lbs of tq. That's a damn nice torque gain. Plus, with my 3.73 diff, i am only at that level for a split second. the motor revs from 750 idle to 4500 RPM at the top of first gear in probably 2-3 seconds.

do we smell traction problems in the near future?

stll not quite sure what to do about that cam. i suppose if i had them pull the head, swap the e30 cam in, added an MLS headgasket and ARP studs, I'd be looking at an additional $1000 ($500 parts/$500 labor roughly). BUT, this would then allow me to up my boost to what, close to 15psi? talk about a paradigm shift.
Jeremy
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Post by Jeremy »

If it's primarily a track car (or track time is your primary concern in building this motor), high boost is generally avoided. Dave Reid's e24 track sled currently runs only 6psi, but with a 3.91 diff. He loves it.

Jeremy
Russianblue
Posts: 415
Joined: Feb 12, 2006 12:00 PM
Location: Charlotte, NC

Post by Russianblue »

it's not though. it will be a daily driver with probably 6-10 track days per year.

based on my limited track experience at VIR North course, with the NA motor i could basically run the whole course in 3rd gear and then shift to 4th in the straights. 3rd covers a big range in this car and for the most part i REALLY like the way it drives.

it ain't half bad on the street, you just get into that low rpm lull around corners sometimes, or your have to downshift to 2nd or 1st and it sends RPM through the roof.
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