Help with calculating different diff ratios

corazon

corazon

Well-Known Member
Hi everyone. I've been thinking more about jag differentials for my future spec.
I've been offered a 3.54:1 from a TVR which I think is probably not tall enough for my application.
I've tried online calculators but obviously something is amiss because I only get Na results.
The zf4hp24 box has 0.73:1 in top overdrive I believe.
My wheel/tyre size currently is 205/60/15;
So can anyone help me with calculating speed/rpm with a jag diff of 3.54:1?
Thanks :D
Jim
 
corazon said:
The zf4hp24 box has 0.73:1 in top overdrive I believe.
My wheel/tyre size currently is 205/60/15;
So can anyone help me with calculating speed/rpm with a jag diff of 3.54:1?
Click to expand...

2500rpm @ 71mph
 
Thanks Harvey :)
That's not too shabby actually is it..and acceleration should be improved over stock P6 V8 diff correct?
I just want to be careful I don't defeat the point of the overdrive gearbox upgrade!
How did you calculate that out of interest?
Jim
 
corazon said:
How did you calculate that out of interest?
Jim
Click to expand...

Trade Secret.....

Actually I calculated it having done the same calculations dozens of times for conversions, but there are sites online that you just punch the numbers in and they give you the result.
 
Using your calculation, if that were changed to a 3.07:1 diff it would be more like 81mph@2500.
Is that right?
 
harveyp6 wrote,...
2500÷0.73÷3.54x77.56÷36÷1760x60=
Click to expand...

Hi Harvey,

What do each of those values represent, and what units apply to each? Having a guess I can see diff ratio, overdrive ratio, possibly yards in one mile, engine rpm,...but I don't like guessing.

Thanks,
Ron.
 
engine rpm, o/d ratio, final drive ratio, tyre circumference in inches,inches in a yard, yards in a mile, minutes in an hour.
 
Thanks Harvey.

A formula that I located provided the following,....vehicle velocity in inches per minute = (tyre circumference X engine rpm) / (transmission gear ratio X diff gear ratio). To determine mph, multiply the answer by 60 to give inches travelled per hour, then divide that result by 63360, which is the number of inches in one mile.

Confirms 71mph for 2500rpm.

Ron.
 
I have combined and simplified the formula, so for speed S in mph, it is...S = C.R/(1056.tg.dg), where C = tyre circumference in inches, R = engine rpm, 1056 = simplification cconstant, tg = transmission gear ratio and dg = differential gear ratio. N.B : the full stops within the formula are used in place of multiplication signs, which is a normal mathematical method of representation.

I checked my Rover's road speed for 2500 rpm by plugging into the formula which gives 61.0 mph. By rearranging and making dg the subject, you can work out the diff ratio for a given road and engine speed. In this case, dg = C.R/(1056.tg.S)

Ron.
 
Next question.
Is there a way to estimate acceleration?
I've already plotted a graph of 1,2,3 and 4 overdrive ratios with corresponding road speeds/rpms with a 3.54:1 diff.
I know the 4.6 engine's max torque in stock form is around 2600rpm, but compared to the 3.08:1 rover diff there would be more torque at the wheels at lower rpm too correct?
Is this going to be useable? ie from a traction point of view?
I've calculated at 5200rpm in top overdrive it could make just short of 150mph, at a theoretical 6000 rpm- 170mph :shock:

2500rpm
1st 22
2nd 35
3rd 52
4th 71

5200rpm
1st 47
2nd 73
3rd 103
4th 148

Jim
 
That's correct, the 3.54 is a low ratio diff compared to the Rover - the engine will be spinning faster at a given road speed.

If those speeds are correct, then I'd say that's pretty damn near perfect gearing for a fast road car. You might even find it a tad too high geared for Surrey/Sussex roads. So you might find yourself using second more than you'd imagined when you have a real head on.

Please try that SD1 Club link - it allows you to vary absolutely everything, including the tyre size - because that's the other big unknown - what size tyres are you going to finish up running? There's no reason to stick to the "official" oe rolling diameter as you have to arrange a new means of driving the speedo anyway. And there is considerable advantage in coming down a few profiles from the 205/65 X 15's I assume you are running currently? I reckon you could get away with 225/50 X 15's - the 50 profile allowing you to shrink away from the pinch point in the D post.

Chris
 
Thanks Chris, I had somehow missed that link.
I'm currently on 205/60 tyres but I was thinking I might go down a size when I lower the car.
I did try entering some figures into this, but it was more for fun really. I doubt it could be very accurate with so little details, however I've seen 4.6 range rovers do 14 second 1/4 miles..
I fiddled the "rpm through lights" a little and the ideal ratio came up as 3.54 :wink:

http://hotrodworks.net/hotrodmath/quarter.html

Drag Performance Calculator
Vehicle Weight (in pounds): 2862

Vehicle Horsepower: 220

RPM through lights (finish line): 4650

Tire Diameter (Height) in Inches: 25.49



1/4 Mile ET: 13.07
1/4 Mile Top End Speed: 99.58
Ideal Ratio: 3.54
 
Hi Jim,

In terms of putting power to the ground, I know that my Rover's 4.6 delivers to the rear wheels at just over 30mph (48kph) nearly 100% more power, ie almost double, what mine and typically what most 3.5 P6B's will deliver at that same speed. At this speed alone, my 4.6 delivers more power that the 3.5 will do at maximum rpm, so when you give it some stick at low speeds, even in second gear, you'll very likely break traction. If you run a 3.54:1 diff, the acceleration will be astounding, assuming you're able to maintain traction.

In terms of outright speed, certainly where the 3.5 litre engine is concerned, it won't pull to 5200 in 4th gear, indeed the maximum speed will be less than in 1:1 3rd. My reference is the 5 speed SD1, which in top gear with a ratio of 0.79, maximum rpm is only 3950 and speed 117 mph. Still nothing to be disappointed about in terms of speed.

Ron.
 
That's good to hear Ron.
Since you're the only person I know running a 4.6 on carbs in a P6, you're experiences are most useful :)
All in all it's sounding rather good to me.
Very strong acceleration coupled with a 2500rpm 71 mph cruise is really what I wanted, and definitely falls under the Grand Tourer category :D
The diff is a salisbury 4ha from a TVR and like I said a very good price, I'm just negotiating posting it here.
Jim
 
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