Ho much bhp can a Javelin diff take?

[Jack]

So, as part of a series of decisions I am trying to make about the potential Javelin project, I could do with knowing how much bhp a Javelin diff can work with on a day to day basis.

I don't want to kill the thing, and know that standard 50bhp is entirely within design spec - how far could the bhp go before I risk breaking the diff? I don't really want to damage the thing, the brief is to try and complete the project without doing any modification that can't be undone, so destroying an original diff seems a bit out of line with that. Keeping the original, however, would solve a number of potential problems and probably cut the length of the project (and potential problems) in half.

Any real world experience of what the diff can handle much appreciated. I know a few engine swaps have been done, though many seem to have gone with alternative options for the rear axle - not sure whether this was by choice or necessity.

Jack.

[Forumadmin]

It is really torque not bhp; that is required. I should think it was OK up to about 180bhp based on a normal torque curve from a petrol engine. Need to do some research on other cars it was fitted such as Morgan and Jag. The half shafts are probably the weakest. Provided you do not drop the clutch with sticky tyres they should survive. Drag racing would break it though.

[Jack]

It is really torque not bhp; that is required. I should think it was OK up to about 180bhp based on a normal torque curve from a petrol engine. Need to do some research on other cars it was fitted such as Morgan and Jag. The half shafts are probably the weakest. Provided you do not drop the clutch with sticky tyres they should survive. Drag racing would break it though.

Good news, looking at approx 100bhp with standard Javelin type wheels. Not masses of torque, no turbos etc and certainly not drag racing anything

Nothing drastic, just upgrading from the standard to something that can sit at 70mph comfortably and that pulls away like a modern car.

Jack.

[Jack]

So nobody concerned with a Javelin diff taking 100bhp and 120 ft lbs of torque?

This is promising, I think I have found the engine to use in that case. It also happens to be rather smaller than the 2.2 and 2.5 engines previously discussed, and readily available in a number of donor cars.

I think we should be able to reveal a bit more at the weekend.

Jack.

[Alastair Gregg]

The Salisbury Axle was used with basically the same design in mk 2 Jag with a 3.4/3.8 Litre engine. Jowett used a specialised to them 3HA and Jag used either a 4Ha or 7Ha, so no concerns, it is not a component that people complain about either.

[Jack]

The Salisbury Axle was used with basically the same design in mk 2 Jag with a 3.4/3.8 Litre engine. Jowett used a specialised to them 3HA and Jag used either a 4Ha or 7Ha, so no concerns, it is not a component that people complain about either.

OK then! A good strong diff we have. Lovely. I think I've decided to tame down the project a little with a smaller engine, partly because it will fit more easily in the engine bay, but also because if we get to much more than 150bhp we're going to need some serious brakes, suspension starts to be a concern, and the power means more wear over time - this way we can keep the car almost original except for the lump under the bonnet and gearbox, which would be easily swapped back in the future.

I think I found the donor car last night, and it is going to give me some pleasure sending the rest of it to the scrapyard in the sky

Jack.

[David Morris]

Hi Jack,

Good luck with the new donor car!...It all sounds exciting and very mysterious?..By the way, as has been mentioned, it is probably the half shafts that are the 'weak link' on the Jowett axle. The early axles with hydro-mechanical brakes had really 'weedy' halfshafts and should probably be avoided. I have had touble with the keyways on the tapers with the early axles, although this was probably self-inflicted!

I am not sure who mentioned this to me, but I have been told that Jowett's bought in the Salisbury diffs and then added the outer axle casings etc. themselves, rather than buying-in the whole axle assembly. This sounds pure Jowett to me...always looking for the most cost-effective solution!

All the best, and I look forward to hearing which donor car you have selected!

David