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View Full Version : Help me make heads and tails of turbo selection/compressor maps


BigD
03-17-2009, 12:21 PM
I'm trying to learn about turbo setups and pick components. My goal is 400-450 flywheel HP out of the S52. I don't want to rebuild the whole engine, just MLS gasket and ARP studs, so I'm hoping to get into my ballpark at a reasonably safe 9-10psi.

I've been reading Garrett's site and most things make sense except for the part about picking turbos. I tried to estimate things using their equations but I could be totally out to lunch on the figures. It seems like the ideal turbos for what I want is either the GT2876R or even the GT3582R. My estimated ratio and flow seem to fall into the ideal range for those. But I don't get how the surge lines work. I mean I understand the max. But how do I know if at lower RPM my engine won't put it in the choke area...

If someone has experience doing exactly what I'm talking about, any constructive comments or suggestions welcome.

///tyron
03-17-2009, 01:27 PM
Run an S52 with a Gt3582r. You're not going to make your goal at 10 psi as its not in that turbos efficiency range. You'll more so make it at 15-22 psi.

BigD
03-17-2009, 01:56 PM
Can you explain how you figured that? In my calculations, to get 400-450hp, I'm looking at 45-48 lb/min air at 1.93 pressure ratio. This is right under 10PSI and in the GT35r's meat of efficiency. If I'm wrong then I guess I'll have to go GT28.

EDIT: just to reiterate, this is not wheel HP, but flywheel. Wheel-hp I'm hoping for 350-380.

Axxe
03-17-2009, 02:03 PM
Oh shit, you gone and got yourself on a slippery slope.

BigD
03-17-2009, 02:04 PM
Oh shit, you gone and got yourself on a slippery slope.

How so?

e30_kid89
03-17-2009, 04:59 PM
Looking at the GT35's compressor map I think it'll flow at that rate while staying at 77 -75% efficiency. If Flow is what you're worried about at 400 crank HP the GT35r is just getting started.

I'd go with the GT35 over the 28. I read of some M20 guys making the switch and never looking back.

EDIT: and to add since your're gonna run a MLS and ARP studs you can push more than 9 - 10psi and still be very safe, that is if you have a very good tune and fuel/spark timing all nailed down.

BigD
03-17-2009, 07:26 PM
Cool beans, GT35r it is. Yeah I'll find the best guy for the tune. At this power level I'm going to stick with the 413. I still give BMW the benefit of the doubt of knowing how to make their engines run best - provided nothing's being overwhelmed. I'm told by a source I believe that the 413 will handle anything under 600rwhp.

SickFinga
03-17-2009, 08:01 PM
Run an S52 with a Gt3582r. You're not going to make your goal at 10 psi as its not in that turbos efficiency range. You'll more so make it at 15-22 psi.

15-22psi?? There are people out there with ~450rwhp on Vortech's with just about 11-12psi.

///tyron
03-17-2009, 10:08 PM
15-22psi?? There are people out there with ~450rwhp on Vortech's with just about 11-12psi.

thats irrelevant to anything. Turbo chargers and super charges are completely different types of forced induction. You have to be in the turbos correct efficiency range so it can perform at its best and make the most power available.

You dont see Gt42r's running at 15psi and making 700rwhp+ They are built to perform at 25psi+

To the OP i'd run a Gt3076 with a 1.06 for your power levels.

BigD
03-17-2009, 10:22 PM
Yeah the GT30 looks like it would work too. But I'd still be well in the GT35r's efficiency range, even at 10psi (once again, if you disagree pls provide the math, I'd appreciate it).

///tyron
03-17-2009, 10:28 PM
Yeah the GT30 looks like it would work too. But I'd still be well in the GT35r's efficiency range, even at 10psi (once again, if you disagree pls provide the math, I'd appreciate it).

Whats this car going to be used for? Street? Track?

What a/r would you get if you got the gt35r?

BigD
03-17-2009, 10:37 PM
Whats this car going to be used for? Street? Track?

What a/r would you get if you got the gt35r?

Like I said S52, track whore. (edit: sorry I guess I didn't mention it's for the track, was thinking of somewhere else I asked this)

0.82 looks to be right on the money (or... I guess ballpark since I'm estimating).

///tyron
03-17-2009, 10:46 PM
to answer your original question TRM makes a stage 1 turbo kit that uses a T3/T4 0.63 exhaust and it put down around 300-350rwhp at 15-18psi. I just think you're going to big of a turbo for your power goals. I found a compressor map of the t3/t4 and it looks like the amount of airflow/min this turbo is comparable to a gt28r, so i think the 30r with a 1.06a/r would get you to your goals a whole lot easier.

BigD
03-17-2009, 10:54 PM
It's generically rated at 300-350rwhp at 15-18psi? Regardless if I have an M50 or S52? To get 350-380rwhp out of the S52 requires a little under or around 10psi according to my calculations. Again if you think I'm wrong, please provide the math and explain where I made a mistake.

e30_kid89
03-17-2009, 11:45 PM
.82ar will be good. size for a s52.

///tyron, I'd bet they use that t3/t4 for the m50 and maybe m52...I dunno if its me but a .63 seems on the small side for an s52 (3.2L)

Too much backpressure from small sizing = EGT's go through the roof.

///tyron
03-18-2009, 12:58 AM
.82ar will be good. size for a s52.

///tyron, I'd bet they use that t3/t4 for the m50 and maybe m52...I dunno if its me but a .63 seems on the small side for an s52 (3.2L)

Too much backpressure from small sizing = EGT's go through the roof.

It is small. I was just comparing the t3/t4 to the gt30r 1.06 a/r. If that turbo can make 325rwhp with a .63a/r on a 2.5L the gt30 with a 1.06a/r will flow well .

Big D If you look at the compressor map, you should require around 44lb/min of air. At your boost level of 10psi to make your power goal. that should indicate a 1.6 pressure ratio.

The Garrett turbo tech site says "this means that for any engine, in order to make 400 Hp, it needs to flow about 44 lb/min"

If you look at the map you're completely off the efficiency island. I'd say you'd have to run at least 15psi to be with in both the 30r and 35r's efficiency
http://img25.imageshack.us/img25/2443/gt3582r7145683compe.th.gif (http://img25.imageshack.us/my.php?image=gt3582r7145683compe.gif)

heres a thread for you. S52 t3/t4 Pump gas 12 psi 360rwhp
http://forums.bimmerforums.com/forum/showthread.php?t=578916.

e30_kid89
03-18-2009, 01:06 AM
I see what you're saying now... If the pressure ratio is 1.6 then yea he'll be off the map.
In that case I'd agree with you on a GT30r with a 1.06 a/r housing being a better match.

BigD
03-18-2009, 11:17 AM
Again please provide the math. I think my estimates were all pretty reasonable and I end up at a pressure ratio of over 1.9. You can't just pull out boost numbers out of thin air. 15PSI in an efficient DOHC head (if I remember correctly, I assumed .97 volumetric efficiency, which is pretty conservative) of a 3.2 at 7000 rpm at sea level is going to make a hell of a lot more than 400hp.

///tyron
03-18-2009, 12:56 PM
Again please provide the math. I think my estimates were all pretty reasonable and I end up at a pressure ratio of over 1.9. You can't just pull out boost numbers out of thin air. 15PSI in an efficient DOHC head (if I remember correctly, I assumed .97 volumetric efficiency, which is pretty conservative) of a 3.2 at 7000 rpm at sea level is going to make a hell of a lot more than 400hp.

I used a basic equation with your numbers in the equation, i didnt just make up the numbers.

10 psia + 14.7 psia = 24.7 psi absolute pressure in the manifold

24.7psi/14.7 = 1.68 pressure ratio

BigD
03-18-2009, 03:10 PM
Only according to Garrett, you have to use 1psi pressure loss through inlet depression and 2 psi for a typical pressure loss through piping etc (which, since I'm no expert, is probably even optimistic) for calculating the ratio, so it's 26.7/13.7 = 1.95

everlast
03-20-2009, 10:26 AM
Try this out:

http://www.not2fast.com/turbo/glossary/turbo_calc.shtml

Remember that calculations and math will only approximate things; experience is the only true measure. I'm sure there are guys running different turbos on the S52 that have posted numbers. Ask the Google.

Edit - where did you get this info: ".97 volumetric efficiency, which is pretty conservative"? Race motors can go above 1 at the right RPM range, but most street motors don't approach 1 from what I understand.

BigD
03-20-2009, 10:46 AM
Edit - where did you get this info: ".97 volumetric efficiency, which is pretty conservative"? Race motors can go above 1 at the right RPM range, but most street motors don't approach 1 from what I understand.

From Garrett - they say figure 96-99 for a good DOHC head.

everlast
03-20-2009, 11:06 AM
Wow!

90E30i
03-28-2009, 09:34 PM
Choosing the right turbo is half the battle. Manifold design and intercooler size and design also play a major role. For example a twin entry exhaust housing with a split pulse manifold will give you huge improvement over the same turbo with a single entry manifold. I suppose being a track car you want to have good mid range power to pull you out of the turns.