745 boost

[turbocar]

How much boost will a stock 745 take?

[Jeremy]

The motor can take quite a bit of boost. The stock turbo, however, cannot push enough air efficiently enough to justify turning up the boost past certain levels.

[turbocar]

What is "quite a bit of boost", 15,20,25? Turbo would be a 35r.

[mooseheadm5]

Are you still talking about a K27 turbo using the 745i exhaust manifold or are you asking what the engine can take using a different turbo and manifold?

[BATESY]

Most of us with a modded or stock k27 run in the 15psi to 20psi range. Heard some hitting the 26 range but who knows what was done to the engine upgrade wise and how many miles.

[turbocar]

Are you still talking about a K27 turbo using the 745i exhaust manifold or are you asking what the engine can take using a different turbo and manifold?

]I am asking what the motor will take as I just figured I would get a 35r and put it on as a replacement. I did not want to turn it up to much and lift the head or throw a rod.

[Jeremy]

Do you realize that the k27 and 745i manifold have a specialized flange that doesn't accept a standard t3 or t4 flange turbo? You can't just a bolt a GT35R onto a 745i manifold.

EDIT: I don't know how you could've missed it, but this is a good read in general if you're thinking of upping the boost/power on your motor. http://www.mye28.com/viewtopic.php?t=102708

The stock k27 stops being efficient and producing more power somewhere around 10-12 psi IIRC.

[Golosok]

The stock k27 stops being efficient and producing more power somewhere around 10-12 psi IIRC.

I'm sorry but this is BS. With stock intercooler and exhaust setup you might be right.
But with upgraded charge piping and big front mounted intercooler and 3 inch SS straight through exhaust it can take 21 psi. Cannot speak about actual power, never took it to dyno. But it definitely feels a lot faster then 12 psi.

[turbodan]

The intercooler and charge piping have nothing to do with it. The compressor is a certain size and works efficiently within a certain flow range.

You probably can make 21 psi on spoolup but it won't hold it until redline. The 745i K27 compressor is too small to keep that kind of boost up at high RPM.

[mooseheadm5]

At 21psi at redline, you'd be somewhere in the 50-55% efficiency range of that compressor. The temp gains would be huge (150C or greater) and the compressor would be running pretty much in choke. That would be a great way to damage something.

[Golosok]

At 21psi at redline, you'd be somewhere in the 50-55% efficiency range of that compressor. The temp gains would be huge (150C or greater) and the compressor would be running pretty much in choke. That would be a great way to damage something.

That's why big intercooler helps. In my setup turbo ran to 21 psi until about 5200 RPM and then pressure would go down to about 18 psi. I have a wastegate with just a spring, no vaccumes connected.

[mooseheadm5]

And that is what they wer talking about. You can't even get it to flow at those flow rates, it just doesn't move enough air to make that boost above a certain rpm.

[turbodan]

Nobody sees anything wrong with running with the wastegate disconnected? Its functioning as a pressure relief valve for the exhaust manifold in that configuration. I wonder how much exhaust manifold pressure it takes to force the wastegate open.

[Jeremy]

Dan, I noticed that too. Forget the pressure, I wonder what his IAT and EGT figures are. They've got to be obscene, even a larger than stock intercooler can't effectively cool a charge being created by a turbo running as hard as it can.

[turbodan]

Thats the second time I've seen someone say they're running like that. Its crazy. EGTs have got to be sky high.

I would like to see what kind of IAT a system like this runs. With enough intercooling you could get the IAT down but that would also pull down the boost pressure. There is only so much air mass available from a K27. Thats why they make different sizes of turbochargers. You can't just disconnect the wastegate and run as much boost as you want.

[Ken H.]

Guys,
Let this dickshit run his K27 as hard as he wants. Sooner rather than later it will go BANG---skweeeee. Then he will get to call T_C_D and put something worthwhile on the engine. That is after he rebuilds it.

But before that happens, we will hear any amount of whining about the cost of FI bits and that someone owes him said parts for nada.

In the interim, we will all be told that none of us know jack shit.

[Jeremy]

Ken's right of course, but on a very related topic, does anybody know exactly how much air the stock k27 on the 745i can push? Is there a compressor map available? I had to guess based on memory of prior discussions of the topic, and that's unreliable on several levels. Assuming a stock M102/M106 turbo setup, how much air can that setup push?

The m102 and m106 ran different boost levels, but the resulting power output was the same. How much more air can the k27 effectively provide past the stock 250 hp and 275 tq of stock? Any?

Is there a compressor map readily available for it? I'm not even sure exactly which k27 BMW used. For instance, I know Alpina also used a k27 turbo on the e28 B7, but I'm pretty sure they used one with different specs.

[T_C_D]

Ken's right of course, but on a very related topic, does anybody know exactly how much air the stock k27 on the 745i can push? Is there a compressor map available? I had to guess based on memory of prior discussions of the topic, and that's unreliable on several levels. Assuming a stock M102/M106 turbo setup, how much air can that setup push?

The m102 and m106 ran different boost levels, but the resulting power output was the same. How much more air can the k27 effectively provide past the stock 250 hp and 275 tq of stock? Any?

Is there a compressor map readily available for it? I'm not even sure exactly which k27 BMW used. For instance, I know Alpina also used a k27 turbo on the e28 B7, but I'm pretty sure they used one with different specs.

It's a 48mm compressor wheel designed in the 70s. It just cannot efficiently support more than 350bhp on a m30.

[Jeremy]

Doing some math, that appears to work out to about 15 psi of boost on an otherwise stock 8:1 compression m30b34.

Does that sound about right?

[turbodan]

Its similar to a T04E 40 trim, which runs out of poop at around 35 lbs/min.

[marc79euro645]

Ken h wrote:
Guys,
Let this dickshit run his K27 as hard as he wants. Sooner rather than later it will go BANG---skweeeee. Then he will get to call T_C_D and put something worthwhile on the engine. That is after he rebuilds it.

But before that happens, we will hear any amount of whining about the cost of FI bits and that someone owes him said parts for nada.

In the interim, we will all be told that none of us know jack shit.


Ken,
I respect your intelligence,but I think you went too far.
Maybe we're all doomed to be crotchity old assholes, but not yet.
marc

[turbodan]

If you find yourself on the wrong side of Ken you may want to reevaluate where you're going in life. I'm just saying.

[Ken H.]

This attachment has some data on the K27 turbo--physical dimensions and the compressor map.

http://www.turbodriven.com/files/pdf/K27-3072.pdf

A few historical items, these from wikipedia.
1. The M102 motor ran the K27 at 6 psi.
2. The M106 was moved up to 10 psi.
I don't know if KKK changed internal designs for this; there are several versions of the K27 out there, some of which were used by Porsche on the 944 and the TurboCarrera.

But worth noting is a 6# boost gives a Pressure Ratio of around 1.41 which means the party's pretty much over by 30# air mass delivery.

Moving the boost up to 10 psi and the Pr = ~1.7, you can get out to around 34# at the outside. Go further and you're into serious choke.

34# pretty much agrees with T_C_D's assessment that the most that you could expect would be around 350 fwhp.

With a 2-inch compressor exit, going anything much larger on the compressor-to-IC inlet is going to be a waste of time; ditto on the IC-to-intake plenum tubing.

Given the OP's question, it might be possible to push boost up to around 20 psi (Pr = 2.4), but Dan's correct about air temps--I calculate around 137 deg C out of the snail.
And unless you are using liquid carbon dioxide as a coolant medium, I don't care how big an IC you asre using; you aren't going to get the IATs much below 55-60 deg. C.

Simple physics, girls.

[T_C_D]

A few historical items, these from wikipedia.
1. The M102 motor ran the K27 at 6 psi.
2. The M106 was moved up to 10 psi.

This is backwards. 10psi on 3.2 and 6psi on the 3.4

That map isn't indicative of the BMW k27. BMW k27 compressor inducer diameter is 48mm not 55mm.

[turbodan]

Given the OP's question, it might be possible to push boost up to around 20 psi (Pr = 2.4), but Dan's correct about air temps--I calculate around 137 deg C out of the snail.
And unless you are using liquid carbon dioxide as a coolant medium, I don't care how big an IC you asre using; you aren't going to get the IATs much below 55-60 deg. C.

Simple physics, girls.

If I understand gas laws correctly, a K27 could only support high boost pressures on an M30 without sufficient intercooling. If you managed to cool the IAT down to ~30C, and I imagine you could with enough intercooling capacity, you simply wouldn't have enough air mass available to maintain elevated boost pressures. In that instance, the turbo simply wouldn't be able to make 20 psi. It would peak on spoolup and diminish toward redline as the engine required more flow than the compressor could support at a given boost pressure. I remember TCD dyno'ing an e23 745i did just that.

[T_C_D]

If I understand gas laws correctly, a K27 could only support high boost pressures on an M30 without sufficient intercooling. If you managed to cool the IAT down to ~30C, and I imagine you could with enough intercooling capacity, you simply wouldn't have enough air mass available to maintain elevated boost pressures. In that instance, the turbo simply wouldn't be able to make 20 psi. It would peak on spoolup and diminish toward redline as the engine required more flow than the compressor could support at a given boost pressure. I remember TCD dyno'ing an e23 745i did just that.

This is exactly right. I had a very healthy 745i that was stock. I added a chip, fmu, Tial WG and 3" exhaust. It ran very well but would not hold 1 bar. It would peak at 14.5 psi then slide down to 12.5psi.

It made 273rwhp through the 4hp22.

[Ken H.]

Given the OP's question, it might be possible to push boost up to around 20 psi (Pr = 2.4), but Dan's correct about air temps--I calculate around 137 deg C out of the snail.
And unless you are using liquid carbon dioxide as a coolant medium, I don't care how big an IC you are using; you aren't going to get the IATs much below 55-60 deg. C.

Simple physics, girls.

If I understand gas laws correctly, a K27 could only support high boost pressures on an M30 without sufficient intercooling. If you managed to cool the IAT down to ~30C, and I imagine you could with enough intercooling capacity, you simply wouldn't have enough air mass available to maintain elevated boost pressures. In that instance, the turbo simply wouldn't be able to make 20 psi. It would peak on spoolup and diminish toward redline as the engine required more flow than the compressor could support at a given boost pressure. I remember TCD dyno'ing an e23 745i did just that.

True, dat.
The point I was trying to make is that the K27's map I was able to find defines the compressor's normal "envelope" at significantly lower air mass deliveries than we have come to expect in recent years.
Think Garrett's GT-XX series with their range of capacities and applications.

With this compressor, look for more air mass and Pr needs to increase. Increase Pr and heat of compression becomes a significant issue.
30 deg C. IC outlet temps might be possible, but the IC core would have to be huge and ambient temps well below the desired 30 deg C. Air isn't all that effective as a heat transfer medium.
Not to raise another topic, but A-A ICs aren't all that efficient. In theory, make the core big enough and the compressor might not have enough grunt to push the requisite air through the internal piping.
The efficiency issue is why I suggested ( a bit TIC) using a super-cold coolant medium.

T_C_D: I was aware that the K27 map I found has a much larger inducer diameter than the E23's K27. But better this than nothing.

I guess one of the questions this thread raises for me is why would you want to try to resurrect a K27. If you're going log manifold, Todd has a pretty good record on meeting that demand and at a reasonable price.
And if $$$ is that much of an issue, perhaps you should reconsider FI.

FI:
Cheap.
Reliable.
Powerful.

Pick two.

[turbodan]

This is probably a lot closer to the K27 in question. Hopefully I can hotlink this since I didn't bother rehosting...

[Golosok]

Dan, I noticed that too. Forget the pressure, I wonder what his IAT and EGT figures are. They've got to be obscene, even a larger than stock intercooler can't effectively cool a charge being created by a turbo running as hard as it can.

EGT never went pass 850 deg C

I know because I have sensor and gauge.

[Golosok]

Guys,
Let this dickshit run his K27 as hard as he wants. Sooner rather than later it will go BANG---skweeeee. Then he will get to call T_C_D and put something worthwhile on the engine. That is after he rebuilds it.

But before that happens, we will hear any amount of whining about the cost of FI bits and that someone owes him said parts for nada.

In the interim, we will all be told that none of us know jack shit.

Ken, while I respect your knowledge, with this attitude you can go and fuck yourself with a big GT35R or whatever other turbo you like.