Small RS2 hybrid

[S2central.net]

Well its come up before and I could never really find out how this configuration would work in the S2, so I'm gonna do it myself.

I have in my hands an RS2 turbo - complete and refurbed but during the rebuild it was discovered that the hot side had a crack.

So what I plan to do is use a K24 hotside and merge it with the RS2 coldside making a nice 'small hybrid' turbo that will bolt onto S2 or RS2 EM with standard plumbing etc etc.

My rationale for this is to hit a target of 300bhp (more would be nice) but more important to me is a nice fat and flat torque profile with minimal lag. My target is 450Nm (or more if it lets me) with lots of positive boost available from 2500rpm. This lets me keep stock S2 injectors - keeping the conversion cost down. My targets are also without the help of Optimax or Tesco 99 fuel as its impossible to get in Ireland - North or South.

I'm planning on keeping the ABY camshafts for now - to find the limits before investing in 7A or RS2 items.

I'll fit a new RS2 EM whenever I get the money for one and I'll probably have the injectors cleaned and balanced along the way. Gawd knows what the spray pattern on them is like at the minute after 128k miles.

I already have a nearly new RS2 IC and plastic RS2 pressure pipe to keep air temps down. I also have the Bosch mosport fuel pump (Ford works WRC cast off) and new filter sitting in a box.

I'll have a crack at the ECU programming myself when its all bolted together as by keeping the factory FPR and injectors I have a lot less variables to worry about. Doubtless it will get the MRC treatment if I get into diffs with such a custom mapping, but that is ages away.


Reason for the post is to see what the view on the best place to get the turbo built up (in the UK) would be. I have a spare K24 that will be the hotside donor.

Anybody know if there is anything special I need to tell the turbo man about such a build - regards bearings or anythings else ?


Paul

[Fat Audi 80]

There was me imagining you in your shed with turbo in a vice going at it with a hammer and chisel...

[Jamo]

AET turbo's or Turbonetics are two places i've used in the past and have done top notch jobs.

[Ross]

I'm sure Grant @ AET would be very helpful.

[Keeper of The Kitten]

Yep, I was thinking AET as well!

__MDC__

[BRY]

Grant @ AET is the man for turbos.

[Jamo]

Quick question on this, the hotside is the limiting factor for BHP so will the standard S2 hotside hit 300 bhp or will it in reality need the next hotside size up to hit the 300 figure?

[A80Avant]

nice project Paul,good to know that the turbo is up to the job!
cheers
Vasilis

[twoqu]

Paul,

Sounds like a whole new world of pain to me!
Not sure what you are hoping to do. Were you thinking of effectively fitting the back plate, comp wheel and comp housing from the RS2 on to the K24? Or fitting the complete RS2 unit into the turbine housing of the K24?
The K24 has a smaller shaft diameter so the comp wheel would need to be sleeved. Got a horrible feeling the shafts are different lengths too. If so that would mean more machining. In theory the backplate/comp housing should fit as the centre housing are common.
If you are going to try and stuff the complete RS2 into the K24 turbine then you will need to delve more into the relative major/minor dimensions of the turbine wheels. I can't believe that the turbine wheels are the same size in both the K24/26.
I'm guessing that a replacement RS2 turbine housing is expensive. I also believe (although this may be a myth) that the RS2 hotside is sized differently and from the other K26's that Audi used. If the major dimensions are the same on all the K26 turbines and only the minor that differs, it might be possible to modify a more easily obtainable turbine housing to accomodate the RS2. That would be the cheapest option.
So it would be useful to get some major/minor dimensions of the RS2 turbine wheel/ housing. Obviously if you can do the same for the K24 that would be useful. Also post the p/n off the turbine housings.
As you have all the rest of the bits for the RS2 conversion seems false economy not to have the correct turbo. We know the K24 is good for 280bhp which is amazing considering its origins are from the 165bhp 10V MC. If you want more then clearly the RS2 is the way to go for up to 380bhp. All the rest of the bits you have will support that.

[twoqu]

Ok, it would also be useful to understand what the turbine housing designations mean. For example the RS2 is 6.12 and the K24 is 6.13. I though that the 6 was the a/r and the next numeral was the amount of vanes on the wheel. Unfortunately this doesen't tell us much. Pretty sure the K24 has a turbine exit of @46mm whilst the RS2 has @55mm. Of course if the turbine housings are the same casting then it should be possible to machine the K24 out to the RS2. Just worried about the relative size of the turbine majors. Clearly the minors are different.

[merlin's S4]

So what I plan to do is use a K24 hotside and merge it with the RS2 coldside making a nice 'small hybrid' turbo that will bolt onto S2 or RS2 EM with standard plumbing etc etc.

Isn't the RS2 coldside a k24 housing anyway? It just has a k26 coldside wheel. If this is the case, surely all you need to do is modify a complete k24 to get the effect that you're after.

[twoqu]

Isn't the RS2 coldside a k24 housing anyway? It just has a k26 coldside wheel. If this is the case, surely all you need to do is modify a complete k24 to get the effect that you're after.

Nope!
Basically the RS2 is all K26. The only common part is the centre housing which is used on both K24 and K26. Unfortunately it was labelled as a K24 7200. I did have a comparsion pic somewhere. Its clear to see that the RS2 has a larger comp housing!

[merlin's S4]

Ok I can see that. I was quoting from an article on s-cars.org. Here it is:

Is the hybrid K24/K26 turbo the same as the RS2 turbo?

No. The RS2 is all K26 turbo. I've had a couple dozen of RS2 and RS2 Competition turbos apart in the past 6 years. The hot side is based o*n the smallest of the K26 series housings, with a Porsche specified hot side wheel, and the appropriate milling of the housing (which results in a slightly larger downpipe opening). The center section is stock Audi K26 WC, and the cold side is a K24 cold side housing with a K26 cold side wheel. The RS2/Comp both use the larger K26 shaft for the wheels. The RS2 Competition uses a larger cold side wheel and housing, the hot side stays per early RS2. What throws most folks off, is that KKK left the K24 designation in their part numbers o*n the cold side tag.

The actual answer to the years of the K24 cars in the US is up for some discussion. The Audi "claim" is that as of 1989.5 (late MC dual knock MAC-14) Audi switched to the K24 turbo o*n all I5 turbo applications. This switch also "supposedly" came with a revised cam profile, flat flywheel (+ different TOB fork), and 2pc exhaust manifold. IME, 1990 was the ******* year for previous year pieces parts. I've seen 1990's with single pc exhaust, K26 turbos (but all have had the flat flywheel and the revised cam profile).

As of 1991 + intro of the 20vt cars worldwide, all Audi 20vt (I5 o*nly - thought I'd miss that turbo cars except for the RS2, were K24 based turbochargers.

Maximum boost to the K24 is altitude corrected at 16psi. At 3200ft the 1991> stock 20vt cars lowered maximum boost threshold to prevent overspin of the turbo. You guys with modded 20vt computers with K24 turbos should reread that last sentence.

Scott Justusson
QSHIPQ Performance Tuning

[twoqu]

Paul,
Right! I have gone back through some threads. Someone posted the details of their Hohenester 310 turbo as K26-2672GCC 6.92. So that is a RS2 Comp wheel with what I think is the turbine housing from the early 220bhp 944T.
Somebody else posted some details of the MTM small RS2 as having the regular Comp side but using a smaller 50mm outlet turbine housing. Its difficult to be precise as the info I have is in imperial. Audi used two K26 housings, the 6.91 at 1.95" and the 6.95 at 2.20". In metric thats 49.5mm and 55.8mm.
I have a K26 housing that has an outlet of 50mm which I assumed was the 6.91. It still might be given the discrepancy of converting from imperial to metric.
So if MTM used the 50mm outlet turbine than it seems that would be the 6.91. I guess that would mean using the matching turbine wheel/shaft unless its possible to back cut the RS2 one. But then you are going to lose the quicker spool up that you were looking for. However it might also mean that you can source a 6.91 and machine the outlet to the correct size. Hey presto a usuable RS2 turbo!
I also remember someone posting pics of their K26/27 in comparsion with the stock K24. Pretty sure that the casting numbers on the turbine housing was the same. Which would mean that you could replace a damaged k26 housing with a machined out k24.

[twoqu]

Ok I can see that. I was quoting from an article on s-cars.org. Here it is:

Is the hybrid K24/K26 turbo the same as the RS2 turbo?

No. The RS2 is all K26 turbo. I've had a couple dozen of RS2 and RS2 Competition turbos apart in the past 6 years. The hot side is based o*n the smallest of the K26 series housings, with a Porsche specified hot side wheel, and the appropriate milling of the housing (which results in a slightly larger downpipe opening). The center section is stock Audi K26 WC, and the cold side is a K24 cold side housing with a K26 cold side wheel. The RS2/Comp both use the larger K26 shaft for the wheels. The RS2 Competition uses a larger cold side wheel and housing, the hot side stays per early RS2. What throws most folks off, is that KKK left the K24 designation in their part numbers o*n the cold side tag.

The actual answer to the years of the K24 cars in the US is up for some discussion. The Audi "claim" is that as of 1989.5 (late MC dual knock MAC-14) Audi switched to the K24 turbo o*n all I5 turbo applications. This switch also "supposedly" came with a revised cam profile, flat flywheel (+ different TOB fork), and 2pc exhaust manifold. IME, 1990 was the ******* year for previous year pieces parts. I've seen 1990's with single pc exhaust, K26 turbos (but all have had the flat flywheel and the revised cam profile).

As of 1991 + intro of the 20vt cars worldwide, all Audi 20vt (I5 o*nly - thought I'd miss that turbo cars except for the RS2, were K24 based turbochargers.

Maximum boost to the K24 is altitude corrected at 16psi. At 3200ft the 1991> stock 20vt cars lowered maximum boost threshold to prevent overspin of the turbo. You guys with modded 20vt computers with K24 turbos should reread that last sentence.

Scott Justusson
QSHIPQ Performance Tuning

Yep! Guess is was a typo!
He states at the beginning that its all K26 then says it has a K24 comp wheel