Prima engined lightweight with LT77

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tathoarder

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I’ve recently acquired an ex raf lightweight. A previous owner has fitted it with a perkins prima engine and an LT77 5 speed gearbox. I have a prima in my 109 high capacity which works brilliantly and with an overdrive cruises very nicely, and will touch 70 with a hill and fair wind.

The lightweight is great on back lanes but tops out at 59mph, am i asking for trouble by fitting a set of range rover diffs to gear it up slightly?

Cheers Ian.
 
Don't know much about Prima engines but I would imagine RR diffs would make it better on a Mway but you mite struggle on hills and slow roads but an o/d mite help aswell.

I'm doing what you have asked on my S2a I'm in the middle of rebuilding but I'm using a 200tdi, H/R gearbox, 3.54 diffs and an o/d hopefully the 200tdi should cope pretty well :)
 
I have RR diffs on my LWB S2a with the Perkisn 4203. They work well on the move and with overdrive it will cruise at 60 at about 2000 rpm. They are ok on hills and cross country but you need to be in low range. If I'm towing I set of in low 1st then straight into high 1st. The 3-4th gap is big so I use the D/D to split it. The problem is first and especially reverse are too high, even in low range. Reversing pretty much has to be done slipping the clutch, mine has a 10" clutch (Jaguar I think). Reversing up a hill is very tricky.
 
rob1miles said:
I have RR diffs on my LWB S2a with the Perkisn 4203. They work well on the move and with overdrive it will cruise at 60 at about 2000 rpm. They are ok on hills and cross country but you need to be in low range. If I'm towing I set of in low 1st then straight into high 1st. The 3-4th gap is big so I use the D/D to split it. The problem is first and especially reverse are too high, even in low range. Reversing pretty much has to be done slipping the clutch, mine has a 10" clutch (Jaguar I think). Reversing up a hill is very tricky.
Click to expand...

That is why the Ashcroft High Ratio transfer box works well with 4/203 and 4/236.
It keeps low range the same, but raises high range 20%.
 
Blackburn said:
Have you tried entering the details into Ashcrofts ratio calculator to compare both vehicles?
Is the throttle set to give the same max RPM from the engine. Tyres are certainly big enough.
http://www.ashcroft-transmissions.co.uk/calc/ratio_calc.html
Click to expand...

Interesting, i’ve looked at filling it in, but as i didn’t build it in the first place i dont know what model The gearbox is out of. I’ll do some research and see if theres any way of identifying it. Also don’t have a rev counter!

I am tempted to try the range rover diffs though, she will pull off and climb steep hills in 3rd.
 
They make a big difference, especially if you use the motorway, its a case of doing each thing, tyre dia, diffs, O/D to get a comforatabel cruise. The speedo will be way off, I rely on the sat nav and some marks on the face unitl I can find somewhere to recalibrate it..
 
rob1miles said:
They make a big difference, especially if you use the motorway, its a case of doing each thing, tyre dia, diffs, O/D to get a comforatabel cruise. The speedo will be way off, I rely on the sat nav and some marks on the face unitl I can find somewhere to recalibrate it..
Click to expand...

I have a gps speedo, so have that base already covered!
 
I learned an expensive lesson, before I got the sat nav I thought I could use the speedo as is and save the £75 I'd been quoted for a recalibration. Got caught in a speed camera at 33 in a 30 and had to fork out £95 for a speed awareness course. Worst of it was I was only doing 33 because its a hill and I wanted to hold some speed to avoid a gear change on the steep part.
 
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