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Re: [tech]How to find Shift Points / redline [The easy way!]
Wow, isn't this the coincidence...
Some of you will remember that I wrote a program years back that will do this.
Well, as planned (and promised), I have re-written it for Excel and it's
just about done. All you need to have is your dyno sheet, gear ratios and
some other basic info on your car (Cd, weight, tire size, etc), and the
program will calculate:
- Best shift points
- Top speed
- Time-to-speed acceleration (0-10mph through 0-top speed) (*)
- Time to distance acceleration (1/4 mile, 0-60, 100, 330, 500, 660, 1320,
and 5260 feet)
- Top gear acceleration passing times (**)
- 0 to Top speed time and distance
- Support for up to 6-speed transmissions
- Support for top speeds up to 300km/h (186mph)
- Values in both US and Metric formats
And the best part is with my torque curve, it's accurate to within 1mph on
top speed, 0.1s on 0-60, and 0.2s and 0.2mph on 1/4 mile times when
compared to the average of the three big magazines' (C&D, R&T, Motor Trend)
tests of the Scirocco 16V. :)
Now -- I've talked with Brett about selling this program for a nominal
price (US$7) and donating 50% of the proceeds to www.scirocco.org to help
cover the costs of bandwidth and hardware for the site and the list. If
you guys are interested (and let me know), I will make it available for
purchase online (should take less than a day) and you'll be able to pay
online and get the program instantly using Visa/MC/Amex/Diners, Wire
Transfer, Cash, Check, or Money Order.
Let me have your feedback and if it's positive, I'll throw it on the
website. (***)
Jason
(*) The standing-start acceleration numbers are non-clutch dump numbers,
but nevertheless come up almost dead even with the car magazines. The loss
of time due to the lack of clutch-dump seems to almost exactly outweigh the
loss of the program compensating for the power needed to accelerate (a) the
engine itself and (b) unsprung reciprocating weight, i.e. , wheels, brakes,
tires, and axles.
(**) The only thing that doesn't seem to match up with the car magazine's
tests is the passing time. I assume this is because my calculator doesn't
take into account the time the engine needs to react to the sudden
application of full throttle.
(***) I'm not making _any_ guarantees about the accuracy of the
program. It's use is intended for informational purposes only. There are
too many factors at play to *ever* realistically predict any specific car's
acceleration and top speed. This program tends to be highly accurate
because it is based on actual dyno data, however actual results may
vary. With that said, it's pretty damn close to the actual numbers on any
car I've tested it on, and for comparative data -- such as "how much would
reducing weight by 100lb help), it's fantastic.
All Rights Reserved, Your Mileage May Vary, You May Be a Freak, etc, etc...
At 07:34 PM 9/19/2001 , Brian Haygood wrote:
>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.
>> > To unsubscibe send "unsubscribe scirocco-l" in the message to
>>majordomo@scirocco.org
>> >
>>
>>
>>--
>>Email LIST problems to: scirocco-l-probs@scirocco.org.
>>To unsubscibe send "unsubscribe scirocco-l" in the message to
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>
>
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>
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