Why NEAR rev limiter please?

kepner

Member
Small track, slight banking. Approx 1600 rpm drop. Specific track is K3 Speedway. Senior, 375 lb minimum, unrestricted, aka black slide.

If max torque is 4000 rpm and max hp is 5000, shouldn’t max desired rpm be about 5500? Instead of just below rev limiter?

Thank you.
 

Kart43

Member
TThe crankshaft, clutch and axle are all mechanically locked as one when you stop gaining RPM you stop increasing MPH, so at 5500 RPM lets theoretically say you reach 35MPH your opponent can reach 6100 he may have a 38 MPH ground speed.
 

Jimbo

If you talk the talk you should walk the walk
If you are at 6100 and you loose 1600 rpms in the corner that drops the rpms to 4500 somewhere in the corner.
That's just above your max torque and only 500 rpm below what you are saying max HP is.
That's pretty close to where you would have max power when you come off the corner.
Maybe not perfect but close. It's up to you to find the best set up and sprockets to go the fastest.
Everyone else has the same engine and rev limiter. You all have to deal with the same situation.
Who's the smartest wins the race.
 
Last edited:

jjchat

Member
I find max HP is closer to 5400-5600. Kart 43 also illustrates why we may chose to run past peak HP. Finally, haveing a couple more teeth on the theorhetical optimum takes into account those little movements of the wheel that cost RPM and speed and those extra teeth increase rate of acceleration that helps make up for the micro mistakes a driver may make.
 

kepner

Member
Ya know, I have been on and off this site for many years now. It is really pretty darn awesome to get feed back like this. Thanks fellas.
 

kepner

Member
Looking for sweet spot rpm as Jimbo eludes to I think, with the tach as our guide of course.

Many often say hit the rev limiter barely in longest straight, or just below, but maybe that applies more to ROAD COURSES where rpm's vary in many places on track substantially. I do understand that a higher rpm for a given combination of gears is more mph, that still begs the question doesn't it, whether we want to get around an OVAL track, like above described, with an rpm range of 3900-5500 versus 4400-6000 rpm given the location of torque and hp at 4000 and 5000/5600 respectively. Is my question making sense? :) Perhaps not.

Put another way: ISN'T THE MOTOR THE "STRONGEST" ALWAYS SOMEWHERE BETWEEN MAX TORQUE AND MAX HP???? So on an oval track if we can STAY between the two maxes, if track allows, then in any given 20 foot stretch on straights we are fastest by NOT EXCEEDING MAX HP? I am considering dropping 2,3, or 4 teeth, and will TRY to have same track conditions if possible to watch tac.

AM I FULL OF CRAP?!!
 
My rule of thumb; if you add one tooth to the axle, and we're talking about running a 4.1 gear ratio, plus or minus a few, and we pick up 100 RPM at the end of the straight, we're going pretty close to the same speed.
As far as going over peak horsepower, it all depends on how fast the curve is going down past that peak point. If you go past peak horsepower, and you still have enough horsepower to keep accelerating, although you'll be accelerating slower, you're still going faster, and you have the advantage of the lower gear coming off the corner.
 

flattop1

Dawg 89
What Al is saying is correct .
You have too consider avalible Horsepower not just rpm or mathematical rpm.
You can gear it too do 65 mph at peak horse power of say 5500 rpm .
Now if you have enough hp is another question .
Then consider friction and arodynamic drag must be factored in as well .
 
Put another way: ISN'T THE MOTOR THE "STRONGEST" ALWAYS SOMEWHERE BETWEEN MAX TORQUE AND MAX HP????
As you probably know, HP is a calculation. Work (torque) x time (RPM). At peak HP, you're producing a given amount of work (torue) in a given amount of time. (RPM)
Just a theory, I've never taken any measurements, but as the horsepower increases, and the kart goes faster and faster, some of that horsepower is absorbed by the rolling resistance and wind resistance, which to me means your maximum rate of acceleration is probably, somewhere below peak horsepower, not that you stop accelerating, the rate of acceleration just slows, you stop accelerating when the HP can no longer overcome the forces working against the kart.
 

Jimbo

If you talk the talk you should walk the walk
In sprint racing you are often on the rev limiter a lot more than you might think.
You often select sprockets that get you through a tight infield quicker. As a result you may be on the rev limiter on the straight for a hundred feet or more before you lift for the corner.
Selecting the right sprockets has always been important. With the rev limiter it might just be a little more critical.
For oval racing you have 2 corners. For sprint racing you might have a dozen or more.
 

CarlsonMotorsports

Premium User
On an oval where you lose NO rpm in the corners, if you were to plot 10 points around the track and average them, you should be at peak.
Since that is unlikely, you need to gear the car so that it over-revs the peak (but stays in the meat of the power band) to accommodate the rpm loss in the corners.

@ K3, you're likely losing 800-1000 rpm in the corners. We always found that you want to hit the rev limiter only on your best/smoothest lap of the race.

Depending on how smooth your driver is, chassis set-up, tire program, etc will determine the rpm needed for your kart.
One thing for certain is that the stop watch never lies.


-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
30 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
 

kepner

Member
👍. Thanks for input and thoughts guys. Myself, son, and another had some interesting discussion and debate on the way to the track last night on this subject. I will share all the input above.
 

Ted Hamilton

helmet painter and racer
Plot the Torque curve. Using the scale, measure the RPM loss, and slide it along the scale until the area under the curve is maximized. This will be the fastest combination as you're using the most available power. See below:
 

flattop1

Dawg 89
Nice read Ted . I could not find any interactive graph to plot .
 

Ted Hamilton

helmet painter and racer
I'd use a "standard" LO206 torque plot and import it into a CAD program and calculate area "automatically." Here's a theoretical one for example...
Red area has more "available power" under the curve for the same width of RPM...even though the torque is "falling off". Somewhere halfway between the two is probably the best.

1571084070426.png
 

Ted Hamilton

helmet painter and racer
PS -- you also need to know more than RPM "spread." you need to know HOW LONG at WHAT RPM..... for example, your RPM spread may be 1200, but you may stay in the upper end of that spread 80% of the time. In such a case, if there's an area of the curve with radically more power, but less area underneath, I'd chance it by gearing "sub otimally" over the whole spread to gain the speed in that very narrow 80% range... eg. It doesn't matter if you're good off turn 1 hairpin if you spend the other 12 turns at almost full RPM...
 

Legend

Premium User
I see these gearing/torque/hp discussions multiple times over the years here on Bobs. Everyone has good comments and it is great discussion. BUT, I think you are all missing one important factor that has everything to do with the end result. Unless I've missed it why doesn't anyone ever factor in the gear ratio when they look at available HP to accelerate? Your engine has X amount of torque at any instant, that torque times your gear ratio determines the ability/rate to accelerate. If the torque doesn't change but the gear ratio does, your ability to accelerate changes. Something to think about?? I think this explains why you can't gear strictly off the torque curve, you need to factor in the gear ratio to determine if you have a gain in available torque or a loss.
 
BUT, I think you are all missing one important factor that has everything to do with the end result.
The gear ratio multiplies the torque, but it doesn't multiply the calculated horsepower, the calculated horsepower at the engine, and at the axle, once the clutch locks up, are identical. Maybe not identical, but that's only because there's always a bit of slip in the clutch.
The formula for calculating horsepower is RPM X torque / 5252.1. RPM of the engine or RPM of the axle. The gear ratio only compresses or extends the period of time when that horsepower is available.
Think of it this way; you're in your car, you're in 1st gear, your engine is producing 120 hp, you accelerate very fast, you shift into 2nd gear, the exact same amount of horsepower is available, but you accelerate slower, shift to 3rd gear, same amount of horsepower, slower acceleration.
 

Jimbo

If you talk the talk you should walk the walk
1571404626914.png
1571404693794.png

Please determine the gear ratio from the torque of this engine.
 

flattop1

Dawg 89
NICE .
A safe bet you could add a couple too the driver and remove a few off the driven .
 
Top