14 replies to this topic

[charliefarlie37]

Is anyone clued up on diffs and engine revs?
I don't know what diff I'm running but id like a little more top end, seem to be having to drive it hard just to get 55mph...
I'm running a 998 engine with 10"wheels fitted with 145/80/10 tyres and currently I'm getting
30mph @2000rpm
40mph@2600rpm
55mph@4000rpm
Any idea what diff I'm running, what would give me a better top end? Really want to stay on 10"wheels but just want to drop the revs slightly at speed

[KTS]

i would say have a look at the guessworks website, but it seems to be having some issues...

[nicklouse]

The server is down at the mo. He is aware and it should be fixed....

[unburntfuelinthemorning]

Assuming your speedo and tacho are accurate that looks like a 3.76:1 final drive ratio.

 

I think you need to look for other reasons for the lack of performance as a 998 with that ratio is quite quick up to 70 if you rev it a bit.

 

How high are you revving before changing up gear?

 

You should eventually get up to about 80 with a 998.  A final drive ratio that drops the revs at speed will also reduce your acceleration.

[nicklouse]

Is anyone clued up on diffs and engine revs?
I don't know what diff I'm running but id like a little more top end, seem to be having to drive it hard just to get 55mph...
I'm running a 998 engine with 10"wheels fitted with 145/80/10 tyres and currently I'm getting
30mph @2000rpm
40mph@2600rpm
55mph@4000rpm
Any idea what diff I'm running, what would give me a better top end? Really want to stay on 10"wheels but just want to drop the revs slightly at speed

Changing wheel size hardly changes the rpm for speed ratio as the overall size is near as dam-it the same.

[Cooperman]

Wheel size is unimportant because the rolling radii are almost identical to within a few percent.

 

On your numbers you have a 3.76:1 FDR and with that it should rev to well over 5000 rpm in top. in fact that was the gearing for the original 850 and was carried on in the 998 for many years. The original Cooper also had a 3.76:1.

 

It should do about 60 mph in 3rd and up to about 80 in top.

 

You could go to a 3.44:1 FDR, but you would lose some acceleration.

 

There is clearly some other issue preventing it from driving as it should which needs to be investigated. It could be as simple as poor engine settings (ignition timing or carburation).

[Spider]

Just be aware that going to a taller differ will make the car less lively.

 

Personally, I wouldn't go any taller than 3.44 under a 998.

[charliefarlie37]

Hmmmm more investigation is needed then, I'm struggling to get over 55/60 this is around the 4000rpm mark, engine sounds like it's absolutely screaming tho....
Carb has been refurbished and by the plug colour it's pretty much set up spot on, from memory timing is where it should be according to haynes...compression on all cylinders is good cold and hot

[Cooperman]

An 850 or 998 will cruise all day at between 4000 and 4500 rpm. That's how we used to drive them in the 1960's.

[charliefarlie37]

It's probably just me, to comfy with the creature comforts of a modern 4x4 lol

[unburntfuelinthemorning]

Hmmmm more investigation is needed then, I'm struggling to get over 55/60 this is around the 4000rpm mark...

Is that with your foot to the floor and it's still struggling?

 

Check for binding brakes and wheel bearings - that can make quite a difference.

 

Is it a standard engine? 

 

What needle is in the carb?

 

Worth checking timing if you're not sure.  Check maximum advance timing.

[Ethel]

... and air filter and tyre pressure.

 

What mpg you get is a good indicator of general health. You should easily top 40mpg if you're not driving it like you stole it.

[charliefarlie37]

Hey again...
As far as I'm aware it's s standard 998, hif 38 carb, AAU needle, standard airbox and filter.
Ignition is supplied by a Lucas gold sport coil and powerspark ignition module, plugs are mum bp7es.
I've fitted a maniflow freeflow exhaust which is coupled to a rc40 twin box system.
Brakes are adjusted as they should be and the car is easy to push back and forth so no binding there...tyre pressures are spot on...

Mixture was set up using a colourtune and I'm pretty sure it's running where it should be, plugs are a perfect colour.
How do I set up the maximum advance timing?
Advance mechanism is working ok in dizzy, have sucked on vacuum pipe to see if it's working.
Speedo has been checked against a speedo app I have on my phone that uses GPS, this app has been checked against my daily and it's pretty accurate...
4000 revs seems to be my limit, it goes through all gears /revs pretty quickly, no labouring of the engine at all when driving.
Clutch is ok, no slipping etc.
Mpg is unknown as I haven't really been far in it, most it does is a 6 mile run every now and again when I've been working on it.
Cheers

[unburntfuelinthemorning]

Worth checking for any induction air leaks too from gaskets or breather system.

 

You ideally need a timing gun with an advance feature like this https://www.amazon.c...=gateway&sr=8-4

 

You turn the dial on the gun to keep the strobe light on the top dead centre mark on the engine and then read the advance off of the dial on the gun.  You can get a rough idea with a conventional timing light by seeing how far round the strobe light moves when revving the engine.

 

Timing is normally set at idle and when everything's working properly that's fine but it's the advance rate and the maximum advance which is more important than the idle advance.  As distributors wear these timings get messed up and often different distributors to original are fitted which may or may not have the correct characteristics.

[nicklouse]

plugs are too cold for a start should only need standard ones. 6ES's