normal mph and rev limits for a 4.2

[DBSV8]

On a rebuilt engine that has just been run in for 1100 miles oil pressure at 60 psi .

idle speed around 1200 rpm .

What are the rev limits through the gears for 1965 FHC 4.2 .

I have an acurate GPS monitoring speed an in 4th gear she was indicating 4200 rpm on the rev couter with actual speed of 115 mph .
this seems a low speed for the RPM indicated ?

Is this normal and what would be the likely top end speed with out over stressing the engine .
The car is fitted with wider 6j wheels

[Heuer]

What diff ratio are you using? Try this calculator to help work out what the revs should be: http://wahiduddin.net/calc/calc_speed_rpm.htm

Be aware the tach generator can also begin to fail which may give erroneous rpm indication. It should produce 1v/100rpm; more info here: http://etypeuk.com/forum/viewtopic.php? ... tach+board

[christopher storey]

Assuming the car has the normal Uk 3.07 diff , and that it is using standard wheels and tyres, the mph should be about 24.8 per 1000 rpm. Thus at 115 mph , without making any allowance for centrifugal force expanding the tyre ( I do not know how much this happens with radials) , you would expect a reading of about 4600 rpm . The workshop manual gives figures making allowance for the effects of tyre growth on the RS5 tyres originally used, which suggest that 115 mph will equate to 4344 rpm. It seems therefore that your tachometer is slightly slow by between 3% and 10% , and it is probably prudent to restrict yourself to no more than 5000 indicated rpm . In any event, most 4.2 engines begin to sound a bit harsh beyond about 4500 rpm and there is not much advantage in real terms in going beyond that point

It seems that many needles eventually get rotated on the spindle, usually as a result of people fiddling with them . A clear indication of this is if the apparent error remains fairly constant in absolute terms throughout the rev range . A fairly good check for this is to check the indicated rpm at a true ( GPS ) 30 mph, when I would expect on a standard 4.2 to see a reading of about 1200 to 1250 rpm

[DBSV8]

Looking at these figures for 115mph at 4600 rpm , you would be hitting 125 mph at 5000 rpm. Im a bit confused with these figures for a car that can top around 148mph ? How would you then reach top speed without over stressing the engine , looking back at Norman Dewis' famous drive to Geneva he must have been driving flat out in the first production E type ?

Planning a continental run which will include unrestricted sections of the autobahns dont want to mess up the rebuilt engine.

[Heuer]

The high speed runs were done with 9600 HP whose engine can best be described as 'experimental'! It was also a 3.8 which can rev to 6,000 rpm without problem whereas the later 4.2 is not comfortable over about 5,400 rpm. I just used the above calculator for my car and it shows 3,000rpm = 100mph (with a 2.88 diff and 5 speed box) which is exactly borne out by my on-road observations..

[graemea]

I have no doubt that my 4.2 has and will easily pull 6,000 rpm, just not for an extended period.

The English 4.2 road tests I've seen showed 6,050 at the 150mph maximum with the 3.07 rear end.

Your 1,200 rpm idle sounds far too high. I would have thought 650 to 700 rpm about right.

Cheers,

Graeme

[Moeregaard]

I would be very reluctant to take a stock 4.2 to 6,000 RPM. The 4.2 does its best work in the mid-range anyway. Your 1,200-RPM idle is high. If everything is where it should be (timing, unworn carb spindles, etc.), 600--700 RPM is normal. I would also check the fast-idle pushrods where they pass through the carb body. I've seen them stick and not allow full return when the choke is turned off.