Board logo

Long gearing ruining cars performance?
alex1991 - 3/5/15 at 03:47 PM

I've just bought a kit car (formula 27) with a Rover 1.8 VVC (143bhp).
Assuming the engine is healthy the car feels quite mediocre in performance. I dare say its slower than a typical tin top hot hatch (civic type R etc).

I was wondering if the long gearing of the diff would be blunting performance, the tag has snapped off so I've not found out what final drive it is yet.
Would long gearing really slow down a seven that much?

Ugg10 - 3/5/15 at 03:57 PM

If you know what gearbox you have (guessing a type 9 with a 0.82 5th gear, tyre size and the revs at 70 mph then there are loads of calculators out there that can work out your diff ratio. Alternatively, again assuming it is an open diff, stick the car in neutral, lift one back wheel (other must be on the floor, put a mark on the input flange and the rear wheel and turn the wheel counting the number of prop revs, from this you can work out the diff ratio. Must admit I had a zetec with 160hp, a type 9 and an open 3.38 Sierra diff and this still felt quick but was doing 2500 rpm in top.

wylliezx9r - 3/5/15 at 04:24 PM

Even with long gearing I would expect it to be quicker than a type r. And often 7 type cars "feel" quicker as they are so low. Is the VVC system working properly ?

daniel mason - 3/5/15 at 04:30 PM

Why not get the car on the jack and find the ratio? I think the caterhams run a 3.92 ratio with the k series

Sam_68 - 3/5/15 at 05:33 PM

quote:
Originally posted by wylliezx9r
Even with long gearing I would expect it to be quicker than a type r.


It rather depends at what speed range. Bottom end acceleration should certainly be better, but a CTR will do 146mph whereas the worse-than-bricklike aerodynamics of a 'Seven' means that even with optimal gearing, you'll be flat out at about 125mph - with progressively blunter acceleration at anything above 70.

And on the basis that you ideally want to be gearing so that you run out of revs at the same time that you run out of power (assuming high cruising revs don't bother you), then a 143bhp K-series 'Seven' should be geared so that it hits 125mph at its peak revs of 7,250rpm, so about 17mph/1,000rpm in top - but that means you'll be cruising at over 4000rpm on the motorway.

If you're geared taller than that, though, then yes, your final drive is blunting your performance to some degree.

bi22le - 3/5/15 at 07:48 PM

I noticed this one day when i lost a rolling start drag race at 70mph against a golf GTI.

Then i realised how unsuprising it was that a 200+bhp super slippery hatch can out drag a 150bhp brick!

I have come up against many hot hatches on track and leave them for dust. I exit the corners with 30% more speed and they have no chance. On the longer straights i drag them and still get to 120 at the same time.

Its a case that if i nail the corners right a 911 or exige level of car cant shrug me off unless we are on a long straight.

Dont worry about it. Get on track and then you will notice the benefit of less weight is more speed.

alex1991 - 3/5/15 at 08:58 PM

I'm thinking its slow 0-70 mph.
It accelerates smoothly with no flat spots but I'd struggle to beat my old car in a straight line which was a 150 bhp honda crx.

Currently the tacho doesn't work, which isn't helping, checked the wiring today and that's fine so new gauge time.

I'll check the wheel rotation to input shaft rotation. I'm presuming if one rotation of the wheel = 3.6 turns on the input flange I have a 3.6 diff?

Ugg10 - 3/5/15 at 09:09 PM

I've git a feeling you have to double the number of turn, numbers you will be lookin for are 3.38, 3.64, 3.92 iirc for a Sierra diff.

Sam_68 - 4/5/15 at 08:32 AM

quote:
Originally posted by alex1991
I'm thinking its slow 0-70 mph.


Then you most definitely have a problem somewhere... a 143bhp 'Seven' should be able to outsprint a 150bhp hot hatch, off the line, in no uncertain terms!

mcerd1 - 4/5/15 at 08:41 AM

quote:
Originally posted by alex1991
I'll check the wheel rotation to input shaft rotation. I'm presuming if one rotation of the wheel = 3.6 turns on the input flange I have a 3.6 diff?

that's fine if both wheels turn together - so if you can count the propshaft turns while pushing the car in a strait line this is the easiest way


If one wheel is off the ground and the other cant turn then 1 turn of the prop will give you twice as many turns of the wheel as you would get with both wheels turning - so doing the reverse means one turn of a single wheel will only give you half the number of turns of the propshaft you'd get if both wheels were turning

coozer - 4/5/15 at 10:10 AM

As said, back one wheel up, put a line on the top of the wheel and the input flange, turn the wheel one complete revolution the check the mark on the flange.

If it stops at about 2 oclock its a 3.38, just after 6 its a 3.62 and just before midnight its a 3.92

Mine started life with a 3.62 and I swapped it out for a LSD 3.92. This made the car much more drivable but no noticabe difference in acceleration.

Gearing with the 3.62 was 140mph in fourth, 1:1 at 7200rpm. (Rolling road 166bhp)

Highest speed I managed on the road was 125mph with the road and wind resistance.

alex1991 - 4/5/15 at 03:53 PM

counted and I've got a 3.6 non lsd diff.

So putting a 3.9 in going to make enough difference or should I be looingk at engine health?