Lets talk about diff ratios...

[450Nick]

Hi all, I'm starting to think that my 1997 Chim 450's diff ratio is a little short. After noticing that the car is now able to very rapidly reach 6k revs in 5th and sit on the limiter at 'only' 155 leptons (on the gps, agreed by the speedo), I think it may be time to get a slightly taller diff.

I was at TVR Power yesterday and they checked my diff numbers, verifying that it is a standard 3.45 and should be able to do '155 ish' which seems to add up. It was suggested that I maybe opt for a Tuscan diff or similar as it would suit the car a little better. I think this is the same one they use on their SC500 and they've reportedly had 180ish out of that before bottling it as the roof looked like it was about to come off!

Is there a table of diff ratios and their equivalent top speeds anywhere as I'd like to see what I'm looking for and what cars they may have been fitted to. I don't want to go too high so as to ruin the acceleration, but some slightly longer legs would be nice I think. Has anyone tried altering these ratios? What does it do to the acceleration feel?

Cheers,

[TVR_owner]

The Tuscan Ratios are either the same or shorter than what you have - 3,46 or 3,73?

Are you sure you want to take the edge of something you probably use often (acceleration) for something I assume you use rarely (top speed)??

[450Nick]

No, to be honest I don't want to lose much in the way of acceleration, though I am aware that this will do just that. I would quite like to just give it slightly longer legs though as I seem to be running out of gears rather quickly at the moment. I agree that I will rarely use top speed, but some slightly longer gears might be a bit nicer. In a perfect world, I would rather raise the rev limit to perhaps 7k to get the best of both worlds, but I'd need to do all sorts to the engine internals, and with the current pulley, the charger's rev limit is 6200rpm.

ETA, another reason is that I am currently unable to use some of my acceleration anyway as the car is unable to put the power down in 1st, 2nd and to some extent 3rd. This will change on the track as I'm ordering some R888's, but my theory is that I may not lose any noticeable acceleration. I'll probably do a few track days first with the new tyres and see if I still want longer gears.

Out of question, what do the Chim/Griff 500's run ratio wise? The reason being that I'm sure they have higher quoted top speeds than the 450's

Edited by 450Nick on Tuesday 27th January 12:28

[spend]

I have note that ratios available for the BTR are 3.46,3.73,3.91,4.11, which doesn't really help you. Have you considered asking Bernie what ratios he could put in your box?

[itsallyellow]

Nick how about a T5 from a Cerb?

Mike

[450Nick]

Well I asked about changing the ratios at Power and they seemed to think that all T5 ratios are basically the same, and that the diff decided the final drive. The Cerb I imagine has a slightly longer diff but also the benefit of a higher rev limit so it can stretch the gears out further anyway.

Who's this Bernie chap then Dave?

[itsallyellow]

Nick you are using the same ratio diff as i am. Also i obviously have a T5.

If i ltd my car to 6200 rpm it would get to 165.]

Mike

[spend]

Nick, Bernie is the gearboxman

gearboxman.co.uk IIRC Competition Transmission Services...

[450Nick]

Oh right then maybe its almost entirely down to the rev limit then? Hmmm maybe I just need another gear... Wasn't some guy talking about a kit to put a 6th gear on a T5 a while back? Feasible? Maybe I should just leave it as it is and worry about other things

[SC500]

Start this thread on Monster Nick or just give Andy a call, that man knows his diff ratio's.

[RichardD]

450Nick said:

Well I asked about changing the ratios at Power and they seemed to think that all T5 ratios are basically the same, and that the diff decided the final drive. The Cerb I imagine has a slightly longer diff but also the benefit of a higher rev limit so it can stretch the gears out further anyway.
...

If TVR Power (Doms Power?) have had a RV8 car to 180mph then it must have a different differential, so why hasn't this been offered to you?

What you need is transmission similar to the Cerb of Brummie who's 600bhp 6.9 Chevy monster wanted taller gearing.
Hence he has a 3.08 diff which allows a higher top speed in each gear and of course a higher top speed (as he like you has the power to achieve one)...

[Gooner]

I hope I don't sound like a smart arse but with all that torque you have I'm surprised you keep running out of gears. In my Griff (OK it is a 500 but not so different), 3rd gear is good for 20 - 80+ if I feel like it. I got some track day coaching once from a guy who pointed out that these cars don't necessarily go faster if you rev the nuts off them; 3500 to 5500 revs is a pretty useable range and in every day driving 1500+ is OK. You could just try doing your normal route a gear higher than you are at the moment, see if it works for you.

Meant to be helpful

gooner

[RichardD]

Nick, do you have RR graphs from before and after the supercharging for your car?

[trackcar]

If you're already traction limited in the lower three gears then upping your final drive sounds a very good idea to me .. you'll find yourself slipping the clutch a bit more in stop start traffic as tickover in first will be slightly faster, but other than that it sounds a very good idea.

[rev-erend]

Hi Nick,

I have a nice and cheap solution for you.

Change the rear tyre profile ..

Gearbox man has a really neat gear speed calculator (Excel) - you input the tyre and wheel sizes, final drive, gear box ratios and shift rpm.. it calculates the rest..

http://www.gearboxman.co.uk/inforatios.html

I took a guess at your wheel tyre sizes :

225/ 35 X 17 inch gives a rolling radius of 6.08 feet
and top speed in 5th of ... 155 mph.

If we just change the tyre profile to :

40 - top speed is 161
45 - top speed is 167
50 - top speed is 173

[TVR_owner]

rev-erend said:

Gearbox man has a really neat gear speed calculator (Excel) - you input the tyre and wheel sizes, final drive, gear box ratios and shift rpm.. it calculates the rest..

http://www.gearboxman.co.uk/inforatios.html

Now I'm worried, my old 3000m has a theoretical 158 mph top speed

Spa in October is now officially a worry...

[rev-erend]

TVR_owner said:

rev-erend said:

Gearbox man has a really neat gear speed calculator (Excel) - you input the tyre and wheel sizes, final drive, gear box ratios and shift rpm.. it calculates the rest..

http://www.gearboxman.co.uk/inforatios.html

Now I'm worried, my old 3000m has a theoretical 158 mph top speed

Spa in October is now officially a worry...

Many older cars had a true over drive final gear .. does not mean it has the power to pull it to the red line though..

I seem to remember some yankie cars like the Corvette had a 6th gear that had a 0.5 ratio .. that would have given a top speed of about 250 mph but was just used for ecomomy.

[TVR_owner]

Alan,
The car used to run out of revs with a 4 speed box (1:1)

New box is T5 with a 0.87 5th gear and I suspect it will pull the red line in 5th. That speed in an old M is a worry...

[rev-erend]

TVR_owner said:

Alan,
The car used to run out of revs with a 4 speed box (1:1)

New box is T5 with a 0.87 5th gear and I suspect it will pull the red line in 5th. That speed in an old M is a worry...

I did not think you racers every worried..

even you sweat is supposed to be pure oil.. 0/40 or something like that

[TVR_owner]

rev-erend said:

TVR_owner said:

Alan,
The car used to run out of revs with a 4 speed box (1:1)

New box is T5 with a 0.87 5th gear and I suspect it will pull the red line in 5th. That speed in an old M is a worry...

I did not think you racers every worried..

even you sweat is supposed to be pure oil.. 0/40 or something like that

I'm sure real racers worry less, but I'm just an older sort having fun.

Sweat is normally closer to a good red wine than oil..