[200q20v] Re: [s-cars] Power exhaust cams: 3B, 7A
b.m.benz at prodigy.net
Sun Jan 21 10:59:32 EST 2001
Thanks for the S-cars forwards Chris. I don't follow the S-cars list any
more because of time constraints, as I decided some years ago that the S-4/6
was not to be my next Audi because of their major tranny problems. But the
S-cars list is blessed many bright technicians, and inasmuch as cams and cam
timing are of current interest to me in my "3Bing of the 7A" project;
So, Hello John, Tommy, and s-cars listers! Since you asked, here are my
comments about your interesting cam experiences.
First, I don't believe that your expected and noted increase in top end
power using the 7A exhaust cam was primarily due to either its higher cam
lobe lift nor its longer duration, and thus resulting in 'freer breathing'.
Rather, IMO it was due to the more retarded overall cam timing which you
achieved with this cam change, which I'll demonstrate below. Inasmuch as
retarding cam timing increases top end power at the expense of low speed
torque, I suspect that you suffered an equivelent low end torque performance
loss. As you know, this is always a trade-off, and your choice.
IMO, exhaust lift and duration are not sugnificant factors at or below red
line, inasmuch as the exhaust stroke is a positive displacement compressor
and will displace its volume through the given valve opening and duration
virtually uneffected by small changes therein. Valve lift is important in
NA engines primarily on the intake side where one is using the exhaust gas
momentum to help increase the charge mass. But with FI (forced induction)
there is no valve overlap, which would only result in blowing raw charge out
Tommy, you say, "High revving with higher lift is slightly dangerous,
if more valve spring pressure is not added." Not necessarily so, IMO.
It depends upon the valve train max. accelerations and resulting peak
forces. Same max. acceleration, same valve train reliability. Short
duration cams (turbo cams) are always lower lift because all cams are
designed for a max. valve train acceleration at a given RPM. Thus, longer
duro cams have more time to rise higher within the same acceleration
constraint. Works out great for NA engines, and FI engines don't need it.
The Audi valve timing as defined in the relevant Bentleys is measured at 1mm
lift, so is not much good in comparing Audi to aftermarket cams, but is
informative in comparing different Audi valve systems, as follows:
3B: Duro. Lobe Ctr Overlap Lobe ctr
IN:opens 3BTDC @ 1mm timing re TDC
close 25ABDC 208 101ATDC
-3 7.25 advanced
close 9BTDC 213 115.5BTDC
close 38ABDC 224 106ATDC
1.5 3.25 advanced
close 3BTDC 219 112.5BTDC
3B intake,7A exhaust
close 25ABDC 208 101ATDC
0 5.75 advanced
close 3BTDC 219 112.5BTDC
These calculated values above assume that the cam lobes are symeterical,
i.e. the same profile either side of the cam center line.
So, relative to 3B cam timing, your change has retarded cam timing by 1.5
degrees, a not insugnificant amount, and IMO accounts for a large portion of
your high end power increase, and probable loss of low end torque.
The 3B's -3 overlap at 1mm lift virtually insures that there is zero or very
small overlap at zero lift, hence no turbo blow thru and good smog
performance. Your change has increased overlap and thus turbo blow thru,
probably decreasing your mileage and smogability. If smog requirements were
no consideration the 7A could have much greater overlap and longer duration
cams, and thus be much less of a slug than it is.
'87 5KTQ 260K
'90 90Q-20V 150K
'91 200Q-20V 130K
> From: C1J1Miller at aol.com
> Date: Sat, 20 Jan 2001 13:57:23 EST
> To: <b.m.benz at prodigy.net>
> Subject: Fwd: [s-cars] Power exhaust CAMS: RS2, 7A or NM
> Bernie: thought this would be handy btdt info for your project.
> From: John Petersen <petersen007 at earthlink.net>
> Date: Friday, January 19, 2001 5:39 AM
> To: s-cars <s-car-list at egroups.com>
> Subject: [s-cars] more power
> Hi Everyone,
> Under the advisement of Tommy Arnberg I went out and found a 7A exhaust
> cam (from a 20v coupe quattro). This cam has greater lift than either the
> S4 or RS2 exhaust cams. The cam was installed yesterday by my mechanic, and
> now the car is FAST. Full boost comes up 100-200rpm earlier (down to around
> 2400rpm) but the really impressive parts are a) boost at part throttle (it
> spools up QUICK) and b) high RPM power.
> I'm currently using chips that back off the boost after 5k down to about
> 17psi at the redline, but with the new cam the car doesn't slow down after
> 5k when the boost dies off, actually it seems like it might pull harder!
> Immediately after the cam was installed I went out and was bringing the car
> up to 120mph (on a really inappropriate road). The pull is quite
> impressive. I then forced my mechanic to drive it.... The passenger seat is
> really a bit of a scary place to be now. He accelerated from around 50 to
> 90 in 3rd gear and it felt quite a bit like that picture on the urS4
> website, in the 1+ section, where the man's cheeks are being pulled back by
> the G-forces.... Like an amusement park ride, but scarier (is this one
> gonna crash?).
> Installation was pretty painless, nothing special to mention in it, just
> like changing the regular cam with another regular cam. And, the best
> part... the 7A cam is downright cheap at wrecking yards :) I got mine for
> around 100$.
> From: "Tommy Arnberg" <tommy.arnberg at nokia.com>
> Date: Saturday, January 20, 2001 1:08 AM
> To: s-car-list at egroups.com
> Subject: [s-cars] Power exhaust CAMS: RS2, 7A or NM
> Some hidden power secrets
> Original AAN/ABY exhaust cam lift is about
> ~8.6 - 8.8mm
> (difficult to measure without propper tools)
> ADU 9.5 -9.6mm Audi 80 RS2 -94-96, 315 HP
> 7A 10.5mm Audi Coupe -88-91, Audi 90, 170HP
> NM ? Audi Coupe -89-91, Audi 90, 160HP
> According to a story the NM is close to RS2 exhaust cam.
> Could someone measure it ?
> If some extra higher REV power is needed
> you can do it with above exhaust cams.
> Slight drop in low end torgue may be noticed.
> Do not change the original AAN/3B inlet cam.
> It lowers the torque certainly and boost build
> up is slower.
> High revving with higher lift is slightly dangerous,
> if more valve spring pressure is not added.
> - With 7A to 8000 and dead meat with weak springs
> - RS2 to 8000 no problem
> (risk is still there, have no personal experience)
> Above 7000 RPM the 5 cyl really screams well with
> open exhaust system if the engine can get enough air from
> Turbo and fuel also !
> I asked (long time ago) about the 20V turbo std. valve spring
> forces but sadly no official figures found.
> Thanks to those Audi fanatics who participated to that discussion.
> I did my cylinder head preparation with some measurements.
> I can publish some measurements later, if anyone
> is interested in the valve spring forces and valve guide diameter
> Some power cam comments / experiences
> would be nice to hear.
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