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[tech]How to find Shift Points / redline
So to find the shift points, or RPM at which you should shift, we need a
graph of torque at the wheels vs. road speed. We will compare the torque
available in a given gear at a given speed to torque in another gear. The
shift point is where the two intersect. They intersect because at high
rpm's, torque in first gear is beginning to fall off, while torque available
in 2nd gear is rising. By shifting at the intersection, we stay on the
highest possible curve. Some engine tuners mistakenly put the shift point
just a tiny bit past the rpm at which they develop peak torque. The result
is bogging in the next gear from an early shift.
Individual steps:
1. Get a graph of your torque vs. rpm (i.e. dyno graph). If you can't get
one, then you might could use someone else's who has a similar build up.
2. Digitize this graph in something like ExCel. That means turn it into
data points instead of a graph. You could also do the following
multiplications graphically, which would be really simple in AutoCAD,
possible in most photo editors, and doable by hand.
3. Now we'll turn this engine Torque vs. RPM graph into a wheel torque vs.
road speed graph for each gear. Since we are just comparing the gears to
find shift points, we can actually neglect the final drive, tire size, etc.
It also doesn't matter if we started with engine torque or wheel torque for
the same reason. So lets start with first gear. Multiply the engine's
torque by the gear ratio (3.45:1). You'll find that an engine that develops
140ft.lb. of peak torque at the clutch, sends 483ft.lb to the final drive in
1st gear. So multiply every torque point on your original graph by 3.45,
and you get a new graph of torque from first gear vs. rpm. We also have to
scale it to road speed. Again, for comparison of gears, we only need
transmission output shaft rpm, so divide the rpm of each point on the graph
by 3.45.
Now you have found that an engine that sends 140ft.lb. @6000 rpm to the
transmission, also sends 483ft.lb. @1739rpm to the final drive in first.
Scale the original torque curve this way for each of the 5 (or six if you're
lucky) gears.
If you now overlay all 5 graphs on each other, you will see that torque
cascades downward as roadspeed (or transmission shaft output speed as we
have graphed) increases. The shift rpms are indicated by the inersection of
each of the graphs for each gear. If you shift later or earlier, you aren't
getting the full power (area under the graph) out of the engine and to the
wheels.
So you'll find that the shift point for each gear is probably different.
Memorize these numbers and use them well, and you will take a big chunk out
of the learning curve for your setup. The highest rpm shift point would
determine your rev limit, or redline. Give yourself a few hundred rpm extra
in case you want to hold a gear at the end of a straight-away instead of
shifting right before you slow down again, or that sort of thing. Also give
yourself a little extra room for mistakes. 500rpm should be plenty of
leeway for our rpm ranges.
Of course your rpm may not be limited by the amount of power produced, but
might be limited by mechanical limits of the components instead. For
example, our engine in the race car at school is still on the upward slope
of its power curve at 12,000 rpm, but we still limit it's revs there because
we don't want it to explode. We didn't eve ndyno it above that. It's a
nice problem to have, but leads to awkward shifting. When that engine
becomes a spare we might bump it up until it gets valve chatter or so just
to see how much it could realy take. Chances are that that means we have
worked too hard to get high rpm torque from our engine and could trade for
some low rpm torque and possibly get better performance out of the package.
Comparing shift point curves for various setups will help determine how to
make those inevitable high vs. low end power compromises. In our next
version of the racing engine, for example, we are taking what we have
learned and focussing on only adding low end power to it, and not investing
any more in high rpm power (i.e. we aren't going to extrude hone the head
because the head's peak flow isn't holding us back).
Hope that is helpful, or at least interesting to someone out there. I have a
book (on motorcycles) that explains this nicely with great diagrams, which
is much easier to follow that my blabbering. If anyone wants a few pages
copied, let me know.
Sincerely;
Brian Haygood
>From: "Dan Bubb" <jdbubb@ix.netcom.com>
>To: <WaspHwy@aol.com>, <scirocco-l@scirocco.org>
>CC: <mk1gti@egroups.com>
>Subject: Re: How to find your redline???
>Date: Wed, 19 Sep 2001 17:23:28 -0400
>
>Your combination would probably live at 7500 RPM.
>BUT, I'd doubt if the power extends that high. I'd shift at 6800 1st to 2nd
>and 6500 after that.
>My engine is pretty similar as far as the power producing parts and my best
>ET to date was a result of shifting at the above RPM (as opposed to
>shifting
>at 7000!)
>Dan
>
>----- Original Message -----
>From: <WaspHwy@aol.com>
>To: <scirocco-l@scirocco.org>
>Cc: <mk1gti@egroups.com>
>Sent: Wednesday, September 19, 2001 4:37 PM
>Subject: How to find your redline???
>
>
> > how do you decide what your redline will be after you have made mods/
>built a
> > new motor? I have been brooding over this for a few days, and just now
>got
> > around to asking the list. my car will be:
> > 84 gti. stock transmission
> > audi 3a 2.0 .020 over
> > forged pistons
> > ballanced, polished rods
> > lightened stock crank
> > lightly p&p solid lifter head with either schrick 280 asym. cam or
>g-grind
> > 4-2 manifold with tt downpipe, etc...
> > stock injection.
> >
> > where should I stop reving it at?
> >
> > --
> > Email LIST problems to: scirocco-l-probs@scirocco.org.
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> >
>
>
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