24 replies to this topic

[Gaz66]

I wish I understood all this.

[Shooter63]

As long as you have constants it's not that hard to calculate, I've got a set of excel spreadsheets that I knocked up which calculate such things. If you want to know the available torque at the wheels then it's just a case of multiplying the know value by the ratios, afterall all a gearbox is, is a torque multiplier. Generally the more torque you have the longer gear you can run, to give you an example my next door neighbour has a drag car, when he started it had a BBC ( Big block chevy) N/A max rpm is around 8.5k and to hit that at the end of the 1/4 he needed 4.10 gears. Since then the engine is now BBC blown alcohol, because of the massive power increase the car now runs 3.23 gears which is still too short as its on the limiter before the lights but the car is now 20 mph faster and needs chutes
If you have less constants ie weight, drag etc then complications come into play, if anybody is actually interested in this area unfortunately you do need a pretty decent understanding of physics and maths although once you get into it a whole new world appears.

Shooter

[IronmanG]

As long as you have constants it's not that hard to calculate, I've got a set of excel spreadsheets that I knocked up which calculate such things. If you want to know the available torque at the wheels then it's just a case of multiplying the know value by the ratios, afterall all a gearbox is, is a torque multiplier. Generally the more torque you have the longer gear you can run, to give you an example my next door neighbour has a drag car, when he started it had a BBC ( Big block chevy) N/A max rpm is around 8.5k and to hit that at the end of the 1/4 he needed 4.10 gears. Since then the engine is now BBC blown alcohol, because of the massive power increase the car now runs 3.23 gears which is still too short as its on the limiter before the lights but the car is now 20 mph faster and needs chutes
If you have less constants ie weight, drag etc then complications come into play, if anybody is actually interested in this area unfortunately you do need a pretty decent understanding of physics and maths although once you get into it a whole new world appears.

Shooter

Yep sounds 👌
Using the gears as a torque multiplier in a mini then and using say 3.4, 3.1,2.9 and 2.7. Every time you drop a ratio you need approx 10 torques more to accelerate at the same speed?
Ignoring drag etc etc

[Steam]

Use a stopwatch nd the seat of your pants

[Spider]

 

i find there are many variables,changing gear is the main loss.Being able to hold longer/go further before changing is sometimes advantageous.Of course you might only be interested in top gear acceleration.A calculator can only do so much,is the torque/hp peaky?is it flat?does it come in early or only at the top?Curious myself,Steve..

Absolutely there are lots of variables. There must be a nerd who has figured it out.
If you have 120 torques with a 2.9 but can only rev to 6000 vs a NA 7500 screamer with 90 torques and a 3.4 which will have the fastest 1/4 mile.

 

 

They are peak numbers and almost meaningless in these calculations. You need the whole playing deck, not just the Ace Card here.

[timmy850]

I’d say the best way to compare 2 different engines and eliminating as many variables as possible would be to do a speed run in the same gear - say 3rd gear and 20mph to 60mph. Then there’s no gear shift or wheel spin that needs to be taken into account

A draggy or vbox etc could measure this.

[Ethel]

What we're all saying is the gearing is critical.

 

As Spider says peak torque is nothing more than a rough indicator. Though, another "peak torque" will be more meaningful: that's the one where your tyres lose grip.

 

To get the best acceleration you'd want to climb your way through the gears  with the average value being as close to wheel spin as possible in each gear. As its easier to exceed tyre grip in the lower gears, and you'd spend less time in them, they'd be less critical than the higher gears, so the final drive ratio will matter most.

 

Know your engine peak power rpm, multiply it by your overall ratio in 3rd for any prospective final drive and that'll be your speed to change in to 4th. Decide what your target top speed is & pick the final drive that sits best astride your peak torque rpm  while matching the change point & top speed revs.

 

Of course, whatever you chose, another ratio would have worked better in the situation you find yourself in when you factor in variables like road gradient, head winds...

[sonscar]

drag racers lose in the first sixty foot,after that it is mostly catch up,usually,Steve..

[DeadSquare]

For this, we require a piece of white string, a tape measure, a vine eye, a couple of dusters, a lipstick, some glue and most important, a wind-up gramophone, probably available at Aunty Wainright's shop  (Last of the Summer Wine), 

 

It has to be a wind-up Gramophone because their speed of rotation can be varied, so the first thing to do is to adjust the speed of rotation to 60 RPM and then glue the lipstick, upright, onto the edge of the turntable.

 

Next, tie the dusters to one end of the string and threading it through the vine eye, tie the other end to the rear bumper.

 

Finally, glue the vine eye onto the case of the gramophone so that it guides the string at just the correct height to be touched by the lipstick each time it rotates.

 

Line up the car, the gramophone and on the end of their 1/4 mile piece of string, the dusters to act as a drogue, and start the gramophone.

 

As the car accelerates, the lipstick marks become further apart.  At the end of the 1/4 mile, measure and note the distance between these marks, and plot them on a graph.

 

Looking (proudly) at your graph, you may be none the wiser, but to your motoring friends, you will be better informed.

[sonscar]

So is it 0-60,quarter mile?in gear 50 to 80?I suspect there is no suitable formula due to all the variables.Steve..