Speed Ceiling

[Guest salty_monk]

Jim,

Problem was there before the RR. Timing is set up at 34 degrees full advance on RR.

Haven't got any points any more - magnetronic ignition now.... (made little difference) & timing has been altered on RR.

 

Doesn't make any sense! Why would it perform in the lower gears but not 4th & 5th??

 

Only thing I changed was the diff - doesn't make any sense to me either!

 

Cheers,

 

Dan

[IanS]

My imediate reaction is that the diffs are not the same ratio.

Did you ever check your speedo reading against another car?

You should be able to check the ratio of the diff you took off easily, mark the output flanges and check that they are both rotating together when you turn the input flange. Count the input turns to turn the output exactly once.

If the outputs will not both turn together then lock one and count the input turns to turn the free output exactly twice. This will give you the diff ratio.

You can do the same to the diff on the car its just a bigger job, jack one wheel off the floor, mark the bottom of the tyre and with the gearbox in neutral you should be able to turn the input by hand and again count the turns to turn the free wheel twice (the other rear wheel is on the ground and will not turn).

 

From your rolling road you should have a power out curve. If the power is the same for 4k and 4.5to4.8k revs (4th and 5th) then the top out speed will be the same and peak speed will be reached with an overall gear ratio between 4th and 5th.

 

Ian

[Big Jim]

It's still not the diff. that diff has done 125mph, and was good for more1 but i was running out of tarmac and bottle!

[Guest salty_monk]

Jim - Glad you're back! Was hoping you could shed some light! Have you been welding your car or off rallying again??

 

As this will mean more to some of you lot than me...... (see pic)

 

If we're convinced it's not the diff & that both are 3.92 (the rpm versus speedo is identical... but I will have a go at the test above next week) then I don't know where else to look, it can only be the engine but the RR would suggest not.

 

I guess I'll replace plugs, leads, cap etc & if they doesn't work the fuel pump & see what happens!

 

[Grim]

If we're convinced it's not the diff & that both are 3.92 (the rpm versus speedo is identical...

that's incorrect - you speedo could well be reading 70 at whatever rpm, as the speedo is taken off the gearbox, so the gearbox output shaft and engine speed are still moving at the speed corresponding to 70, but the diff is turning more revolutions per input revolution, so you are travelling faster than your speedo says.

 

I'd check the markings on the diff to see what ratio it is (no disrespect to the intelligence of those that sourced it ) and see what that says.

[Guest salty_monk]

So you're telling me that the Rev counter is driven by the same cable as the speedo from the gearbox rather than being driven electronically??

[ibrooks]

So you're telling me that the Rev counter is driven by the same cable as the speedo from the gearbox rather than being driven electronically??

No but they are both measured before the diff has any effect. The speed of the engine relative to the output shaft of the gearbox will only change if the ratios in the box have changed. If the diff is a different ratio to the old one then the engine will still show the same rpm per mph as shown on the speedo but the mph shown on the speedo will be wrong.

 

Are you sure your speedo was correct before?

 

Is it correct now?

 

If your car is pulling to high revs in the lower gears but not in higher gears then the only factor that can be effecting it is the velocity. This would be shown if you did several power runs in different gears by the results being parallel lines on the graphs. The only reason velocity will affect your max speed is when your torque matches the resistance of the vehicle moving forwards from aerodynamics (or lack of with a seven).

 

I'm suspecting that the diffs are actually different ratios and your actual "speed ceiling" is no different to before but your speedo is showing something different. Are you 110% sure that the old diff is a 3.92? Like Jim says they will either work or they wont - there's a large area in between where they will work but complain that they're not happy.

 

The rolling road plot states 99bhp at the wheels and says 121bhp at the flywheel (you can see the coastdown at the end of the graphline for him to work the difference out) - respectable numbers for a standard injection Pinto. It also shows the top speed reached was 105 mph (170kph). What did the speedo show at this point?

 

Iain

[Guest Mike Rhodes]

My bog standard 2L pinto with flares and 3.62 diff turns in 105mph in 4th and only 90 in 5th but I've got 20Bhp less. Rev limiter packed up yesterday, was pulling 6700 in 2nd before I noticed.

Mike.

[Guest micky bigthumbs]

It may sound completely stupid that my only answer to every problem on ford engines is check thefuel filler cap, if the engine is taking fuel faster than the tank vents and causes a vacuam it restricts fuel to the pump , when the demand reduces the tank pressure equals out and the vacuam in the tank drops

[Guest salty_monk]

I've actually added venting to the tank (it did 110 before I did that with no venting) rather than taking any away...

[Bob Tucker]

Hi Dan,

Just read the whole saga now!!

dont forget changing the diff meant taking off

& replacing the rear brakes.

If theyre binding now, (was it you who had trouble releasing the handbrake cam?)

the effect will be much more obvious in high gears.

You could easily lose 15mph in 5th.

Jack it up & give the rear wheels a spin.

Bob.

[Guest salty_monk]

Checked that one straight away.. not the brakes. Not me with the handbrake Cam. Just about to go double check the 2 ratios

[Guest salty_monk]

Ok, have just been out with Jack in hand to try out IanS' method.... here's what I found.

 

Old diff with both outputs turning freely just under 4 turns to one of the outputs (3.92 as expected then)

 

New Diff on car (one wheel locked, other turning freely) 3.5 turns to 2 revolutions of the free wheel.

 

Using Ian's method this means the new diff is actually a 3.62. This would ring true with this comment from Bob Tucker who has run both ratios:

 

"With a 3.62, I get 110 in 4th, 95 in 5th!!!

the 3.92 gives 75 in 3rd, 95 in 4th at about 6250 & 115 in 5th at 5750

Thats with 136bhp"

 

The fact that my speedo actually thinks it has a 3.92 in it also means (and I'm not sure exactly how to calculate this so have taken a guess & based it on percentages) that the speedo is approx 8% out.

If my guess is right then I'm actually getting about 100 -105ish in 4th & about 95 ish in 5th the same as Bob.

 

I'm convinced this must be the answer as it now all falls into place & even explains why things got a little better after the RR tune... Glad it's not my engine in any case!

 

I guess my next move now then is to source a 3.92 (offers welcomed!! ) or have the original reconditioned as the teeth look pretty good & I think it is probably only the input bearing that is making all the noise. (Will also investigate possibility of slightly different ratios whilst I am there, what do you peops think of a 4.11 or 4.22 which I've heard are perfectly possible in one of these... I don't think we'd want to get too much lower as ideally it would be the ratios in the box I'd change not the diff)

 

(that's me happy cause problem seems solved!!)

 

Big Jim - I think you've lost your touch (no seriously, this diff has done me a favour anyhow & provided lots of us with hours of finger tapping amusement on here so thanks once again )

[Guest chris brown]

salty I have a 3.92 but it is still in the car at the moment have got a LSD to go in and am at present (when not tapping away on this thing) cleaning up all the bits (shafts callipers etc) but if you can wait awhile it will be available . I’ll see you at Barkston over the weekend. If weather good will be there Friday night

[Peter Bell]

Salty why not stick with the diff you have? it will be nicer on motorway runs to have the slightly higher ratios. If you go to this link http://www.rhocar.org/download.php?view.5 on the main website you can sort out your revs/speed/gears. as long as you know how far out the speedo is there is not too much of a problem.

Peter.