7A Exhaust Cam

[Mihnea]

On a real K26 @8 it is 56mm or thereabouts, Doug, what about measuring it so we know once and for all BTW???

FWIW, an RS2 K26 #6 exducer bore is 54mm.

[Mark Halligan]

Mmm.. probably did'nt explain myself very well, what I was trying to say is that a turbo (whatever the make) with an exducer bore the size of Doug's, i.e. around 56mm would spool up a LOT quicker than the present 4.2K rpm with the ceramic bearing option fitted, based upon, mine being 63mm and producing 1 bar at 3k rpm, both with #8 hotside, although mine has been 'altered' slightly , its started out a #8 and the work done to it (bigger exducer bore) would theoretically slow the spool up down?

Mark.

[newsh]

Guys, how about a laymans guide to the numbers and what they mean? and some of the terms, like exducer?
I've only been driving turbo quattros for 20 yrs and I don't know squat.

I take it the stock S2 turbo is a kkk K24. What's the RS2? What are the actual differences?

What would be the best turbo for making say 350 - 380 bhp but still boost up at 3k? (Sounds like Mark's hybrid?)

[Mihnea]

Originally posted by newsh
Guys, how about a laymans guide to the numbers and what they mean? and some of the terms, like exducer?
I've only been driving turbo quattros for 20 yrs and I don't know squat.

ok, here's a first shot:

the turbo is basically made of 2 parts: cold and hot side, the cold side inlet is called the inducer (I guess because that's where the air gets induced) and the hot side outlet (the one that goes into the downpipe) is called the exducer.

The KKK turbos are separated into several families: K24, 26, 27, 28, 29 and there's also hybrids.

The #s refer to the hot side's scroll volume in cubic centimeters, thus a K26#6 hot side supposedly has a scroll volume of 6cc's.

I take it the stock S2 turbo is a kkk K24. What's the RS2? What are the actual differences?

The stock S2 turbo is a KKK24, it's exact denomination is K24-7000 because the cold side's ID is 2470. It has a K24 #6 hot side with an exducer bore of 48mm
The RS2 turbo is called K24-7200, and, although called K24 is a full K26 turbo, it has a larger shaft than a K24, the hot side is a "special" K26#6 (it is special because it is the only hot side that KKK has ever made especially for the 20v turbo 2.2 liter engine. The bearing housing (center part) is a K26 part number (ID= 2672) and so is the cold side. The exducer bore on the K24-7200 turbo is 54mm.

What would be the best turbo for making say 350 - 380 bhp but still boost up at 3k? (Sounds like Mark's hybrid?)

IMO and if it were only for me, the RS2 is your best bet! the cold side can go as high as 370 safe and reliable HP (possibly a bit more though) and as much as 650-700Nm of torque (see SteveH's stock rods, 600 is safer and 700 seems to be the absolute limit for the stock rods) if everything else is done accordingly (exhaust cam, IC, injectors, rod bearings, head gasket and last but not least ECU remapping)

the RS2 turbo gives full boost at around 2800 RPM in the 2 higher gears when accelerating WOT from 2000 RPM or so, and this is due to its hot side mainly.

If it were for me, and this will be the turbo on my next car, it will have the RS2 hot side, RS2/K26 bearing housing and a K27/29 cold side, this way about 400 HP will be available at the top of the rev range, but most important, low end torque will be there as early as 3500-3700 RPM.


HTH,

Mihnea

[newsh]

Thanks Mihnea. Very good

Full boost at 2800 sounds perfect..

I see my parts list developing nicely here....

RS2 Turbo,
(snip) (exhaust cam, IC, injectors, rod bearings, head gasket and last but not least ECU remapping)(snip)

Don't I need the Exhaust manifold too?

[Mark Halligan]

I can certainly vouch for the low end torque and tractability of Steves RS2 turbo, I went for a ride with him at the Castle Combe meet in October, and that car really flew with hardly any lag.
One last thing with regard to the Exducer bore size, just as it effects low end torque and the lag factor it equally effects the high end performance too, at the end of the day its a compromise, I suppose we'd all like almost instant low end torque along with high end power...oh well.
Here's a nice link (from an MR2 site) that shows the effect of the exducer size upon the high end power.

Better go now......I've been banned from the computer today :eek2: .

Cheers,

Mark.

[newsh]

BTW
Whats the story with changing the head gasket? Do you go to an RS2 item on that too?

[sal_s2]

Which exhaust cam is more suitable, the 7A or the RS2 one?

[desmo888]

Originally posted by newsh
BTW
Whats the story with changing the head gasket? Do you go to an RS2 item on that too?

Yes, which gasket set should I get?? I need to get one on order for my 3B.

Martyn???:confused:

[Mihnea]

Normally you can't be wrong with the gasket, there should only one available for the 20vt engines, and it's the metallic one, same as on the RS2. If in doubt, (for Dave) order a headgasket for a '95 S6 with the AAN motor, those ones were metallic from the factory, whereas the original 3B and early AAN ones were paper.

HTH,

Mihnea

[Mihnea]

Originally posted by sal_s2
Which exhaust cam is more suitable, the 7A or the RS2 one?

Sal, it's all a matter of money, the RS2 one is a lot more expensive, but if you want to do things right at 100%, get yourself the RS2 cam as well as the RS2 MAF and then your engine will be almost an RS2.

HTH,

Mihnea

[desmo888]

Originally posted by Mihnea
Normally you can't be wrong with the gasket, there should only one available for the 20vt engines, and it's the metallic one, same as on the RS2. If in doubt, (for Dave) order a headgasket for a '95 S6 with the AAN motor, those ones were metallic from the factory, whereas the original 3B and early AAN ones were paper.

HTH,

Mihnea

Thanks Mihnea - That's what I needed to know. I'll order that kit.

Dave

[Pisobiker]

I would like to point out some aspects about 3B/ABY/AAN vs. ADU vs. 7A cams:

analizing data I have there are some in my opinion interesting things to consider when choosing among different cam's options:




3B/ABY/AAN:

TIMING:
Intake opens 3° after TDC, exhaust opens 42° before BDC

DURATION:
Intake 202°; Exhaust 213°; no overlap -> intake opens 12° after exhaust closes;

ADU:

TIMING:
Intake opens 13° after TDC, exhaust opens 24° before BDC

DURATION:
Intake 202°; Exhaust 205°; no overlap -> intake opens 12° after exhaust closes;

7A:

TIMING:
Intake opens 7° before TDC, exhaust opens 42° before BDC

DURATION:
Intake 224°; Exhaust 213°; no overlap -> intake opens 3° after exhaust closes;

ADU exhaust on 3B/ABY/AAN:

TIMING:
Intake opens 3° after TDC, exhaust opens 24° before BDC

DURATION:
Intake 202°; Exhaust 205°; no overlap -> intake opens 2° after exhaust closes;

7A exhaust on 3B/ABY/AAN:

TIMING:
Intake opens 3° after TDC, exhaust opens 42° before BDC

DURATION:
Intake 202°; Exhaust 213°; no overlap -> intake opens 12° after exhaust closes;


This means that:

1) as a matter of "TIMING" RS2 cams are designed to give best flowing efficiencyes at lower rpm compared to 3B/ABY/AAN. High rpm efficiency is however ensured by a higher exhaust lift of 9.5mm instead of 8.6mm (and by the higher flow and boost rate of the RS2 turbocharger). I belive RS2 cam settings where designed to have best compromise between low rpm and hig rpm performances.

2) using a 7A exhaust cam with a 3B/ABY/AAN intake cam, timing will be the same but you'll have 10.5mm lift instead of 8.6mm which is a sensible difference in final EFFECTIVE FLOW which will more likely be usefull in the higher rpm. Since timing is not changed there are no changes in efficiency rpm ranges.
Compared to the ADU cam you'll have more high rpm efficiency but will have less low rpm torque.


Marco

[Glen Chamberlain]

WOW good info Marco! so whats the best? RS2 cams or 7A ones? is it nessasary to replace both or just the exhaust?

[Pisobiker]

Glen, I don't have a 100% "for sure" answer.

I mean that on the "real stuff" things might be slightly different, so experiences might give better infos.
What I can say is that, "reading" the timing, duration and lift, RS2 exhaust cam has slightly more "retarded" timing and less "duration" (therefore theoretically being better setted to lower rpm) compared to the 3B/ABY/AAN exhaust cam, but having sensibly higher lift that will increase effective average opening area thus increasing flow expecially on higher rpms.

So it seems to me a very well engeneered "compromise" design.

If you just look at top hp and/or track use of the car, the 7A exhaust cam will for sure give higher outputs but will also stress a little more the valvetrain (higher lift).

About using only the RS2 exhaust cam and keeping the 3B/ABY/AAn intake cam, it seems to me (always "reading" from a teorethical poin of wiew) the intake timing will be slightly advanced (10°, thus giving some better flow on higher rpm), while having same duration and (I suppose) lift. This "hybrid" solution will make intake open closer to the exhaust closing (10°) but still not overlapping, so should be fine.

What I cannot "read" anywhere, is that the two cams (intake and exhaust, I mean) could be designed to perfectly work together so that the even apparently small differences between ADU and 3B/ABY/AAN intake cams could suggest to use both the RS2 cams (but I recognize the real difference is made by the exhaust one).


Marco