Fiat 2ltr twin cam

[moggyman64]

Hi All,

I have recently fitted a rev counter to my traveller and am getting high readings, 3 to 4000 rpm at 50 to 60 mph. I would have expected 3500 rpm at say 70 to 80 mph.

it is a 2ltr Fiat twin cam which was recently rebuilt with new bearings etc. including a full engine balance. The engine, gearbox and back axle were all from the same car so the ratios are matching. I had the gearbox overhauled with all new bearings etc.

I dont know what the rpm was before the rebuild so have nothing to compare with.

I have thought that maybe it is to do with the shape of the car ( not very aerodynamic ) that is making the engine work harder hence higher rpm.
just an idea.

Static timing is set at 10 deg BTDC with standard Weber 34ADF carb
and that engine purrs

What are your opinions of why the rpm is so high relative to low speeds.

What rpm is your 2ltr running at?

Regards

keith

[chrisryder]

Unless the clutch is slipping, or the wheels are spinning, engine revs and road speed are inextricably linked.

Rebuilding the engine or gearbox will not change the engine rpm to road speed ratio, unless gears were replaced for different ratios during the gearbox rebuild.

The only other thing could be tyre size. If you've got smaller wheels than the donor vehicle had originally, this would make the engine rev faster for the same road speed.

[MarkyB]

Rev counters often have ways to change between different numbers of cylinders, any chance yours is set for an 8 or 6 cyl engine?

[moggyman64]

Hi Marky,

It is set at 4 cyl, it is the first thing I checked.

keith

[dp]

If I understand correctly, the engine, gearbox, back axle and rev counter are all sourced from one donor car? If that's the case, the revs /mph ratio should all match the original. Is it possible that when your gearbox was overhauled the gears were changed for ones of different ratios? Or did you get back the same gearbox you gave in or was it an exchange system where you give them yours and they give you one already rebuilt off the shelf?

See: http://www.guy-croft.com/viewtopic.php?f=6&t=1802 as an example of differing ratios.

Fiat 131 gearboxes are normally overdriven in 5th gear but not the Abarth versions.

Actually, have you got a 4 speed or 5 speed box?

How are you measuring speed, the original Minor gauge? If so, - is it recalibrated? Or sat nav which will be quite accurate?

[moggyman64]

HI dp,

1) The gearbox was stripped and all gears were ok they just reshaped a few teeth on the reverse gear idler. They ordered and replaced a complete sets of bearings. So all gears are same.

2) It is the original 5 speed box.

3) I have used an original new fiat speedo which I re-shaped and installed it inside the original moggy speedo clock housing. I used an original Fiat speedo cable and mated it to the original gearbox. I was conscious of keeping everything Fiat mated together which is why I bought the entire car to use as a doner.

4) When checked with a sat nav the speedo reads 4-5mph faster. So that is not causing the problem.

Chrisrider pointed out that a slipping clutch could be causing or contributing to the high rpm, the complete clutch is new and although some slipping is not immediateley noticeable it could be slightly slipping so when I get a chance I will slacken off the adjustment and make sure that the clutch is full disengaged, then check the rpm readings again.

Regards

keith

[moggyman64]

The diff ratio 3.9 has been checked, which gives 3520 rpm at 70 mph. but I dont get that.

Keith

[Innovator]

What does the tacho read at tickover?

Where does the tacho pick up the signal from and what ignition system are you running?

My first thought would be the tacho is faulty.

[chrisryder]

You still haven't said whether you've got the original fiat wheels and original tyre size too?

IF you've fitted smaller tyres then that would give you your high revs at lower speeds.

[moggyman64]

Chrisryder hit the nail on the head, I slackened off the clutch cable and adjusted it up bit by bit (3 times) and tested it each time and hey presto 3300rpm at 70.

For your information I marked the cable nut and locking nut prior to undoing them and I have only adjusted the nut by 3.5 turns. This small amount of adjustment wasnt enough to realise it was not fully disengaging the clutch but it was enough to allow the engine to loose drive and rev more than necessary.

This is one to be remembered.

Thanks for all the input.

Regards

Keith