K27 vs K28...

squerly · 09-25-2007, 03:26 PM

What's the performance differance? I have a K28 that was installed at the time of rebuild and I thought that because it was BIGGER than the K27 that it would equate to more power. Is this a valid theory or not?

I'm trying to get more power out of the car and eveyone on this board (with a race prepared 930) appears to be using a k27.

DSPTurtle · 09-26-2007, 02:36 AM

Does the K28 take longer to spool up? Since it is bigger it probably has more mass to start moving but that coudl help keep the boost up between shifts. Plus, you can only put so much air into a 3.3L engine... and once you hit max boost all the extra goes through the wastegate anyway. Then again, your motor may be built to spin 10,000 rpm... is it? So far my research shows the K27 is the quickest spooling turbo that can pump the required amount of air, but I am still looking into this (there is another thread on Turbos that I started a few days ago). It seems that all the specialist stay with the same size frame (K27)and tweak the blades to accomplish more boost or quicker spool. I don't see too many people offerring a K28 option but then again I have only looked at 20 or so different sites so far. Someone had an equation for how many horsepower per .1 bar of additional boost but that seems to just be done via a stronger wastegate spring or an adjustable wastegate. Other then the fully race prep'd motors I haven't seen anything that talks about going beyond 1.2 bar that the K27 makes easily. But, that may just be because they haven't played with a bigger turbo... or they may be a thermodynamisist and know that you can't physically plow more air into the motor beyond that point

RarlyL8 · RarlyL8's Garage

Most use a K27 because it is a "bolt on" and works well for typical applications. Builds specifically for power (such as Juan Ruiz or Goran) don't use a K27 and are not so concerned with power down low. The K28 has its power band shifted towards the big end.

Cobalt · Cobalt's Garage

You might want to do some research over on rennlist. Kevin makes hybrids and Stephen from IA have some great new turbos.

I am running a K27 HF which is a modified K27. It spools quicker, and unlike the K27 doesn't run out of air at 5000 rpm They say it is good for up to 600 hp. they also have other versions that are good for more power or EFI conversions. I find the biggest difference with the K27 HF is that you have much better throttle control and far less of an on/off feel allowing you to feather the throttle gaining much more control over the car.

Unfortunately, the K28 is kind of a dinosaur these days good for track but not much else.

964 T #304 · 09-26-2007, 07:57 AM

Some folks like running the K27 hot side on the K28 to make it spool up quicker and still flow more air on big und than stock k27 I have saw them called hybreds . Do a search here and you should find something about it.

spuggy · 09-26-2007, 08:03 AM

Quote:

Originally Posted by Cobalt

I am running a K27 HF which is a modified K27. It spools quicker, and unlike the K27 doesn't run out of air at 5000 rpm They say it is good for up to 600 hp. they also have other versions that are good for more power or EFI conversions. I find the biggest difference with the K27 HF is that you have much better throttle control and far less of an on/off feel allowing you to feather the throttle gaining much more control over the car.

+5. Everything he said.

My dyno guy said "hmmm" in the preliminary pulls. I said "what?" - as you do.

He said "your turbo is making .5 bar @ 2500 RPM. That's pretty useful". Then he got out and added another strap.

Dunno how much is down to the GHL heat exchangers and how much to the K27/HF, but the car is completely different to drive.

NathanUK · NathanUK's Garage

I heard from one of the 930 boards that the K28 is often a hybrid of one side of the turbo being a K27 & the other aK29, hence the name given "K28".

squerly · 09-28-2007, 03:58 AM

Thanks everyone for your input. How would I know if my current turbo is a HF? Can you tell by looking at it???

Quote:

Originally Posted by DSPTurtle

Does the K28 take longer to spool up? Since it is bigger it probably has more mass to start moving but that coudl help keep the boost up between shifts. Plus, you can only put so much air into a 3.3L engine... and once you hit max boost all the extra goes through the wastegate anyway.

I would have to go out and drive it to be sure but I’m thinking that it starts noticeable spool somewhere around 2500rpm. Also... there has been reference made to the "cool" side of the IC. Which side is that???

(Sorry my knowledge base is very limited. but I'm working on it...) The weather isn't too bad today so maybe I'll drive it to work and get a accurate spool point.

Cobalt · Cobalt's Garage

Quote:

Originally Posted by squerly

Thanks everyone for your input. How would I know if my current turbo is a HF? Can you tell by looking at it???

I would have to go out and drive it to be sure but I’m thinking that it starts noticeable spool somewhere around 2500rpm. Also... there has been reference made to the "cool" side of the IC. Which side is that???

(Sorry my knowledge base is very limited. but I'm working on it...) The weather isn't too bad today so maybe I'll drive it to work and get a accurate spool point.

This is a very complicated science and I don't have enough experience to tell what you have from the pictures. IIRC my HF differed from the stock K27 on the cold side. It had double the number of blades something like 16 vs 8.

The cold side is the cast aluminum side with the non flanged opening. This is what pumps cold air into the intake. The hot side has the flange and is attached to the exhaust. Based on my charts and experience my HF K27 begins to spool quite early. I am at 4.0 PSI at 2000 rpms and at 14.5 psi at 3200. There is a slight creep up to 14.8PSI at 6000 rpms and drops by .1 PSI at 6500 rpms. This is running B&B headers which help reduce the lag and stock cat which help create the lag. I plan on going with a 100 cell cat which should reduce the lag considerably and have me up to full boost by 3000 rpms or sooner. If this were a stock K27 you would see boost falling off at around 5000 rpms and if it were aK28 or K29 you would see a much later spool up and be equally as strong up to redline.

The older design K28/K29 turbos were good for high powered track cars but did not help with low rpm driving. For street use any of the newer K27' hybrids are much more street oriented and offer better overall control with equal power up top. I would contact Stephen from Imagine Auto, he is quite knowledgeable and can set you up to accommodate your specific needs.