Poor performing 1275

[mugsy]

Having read recent comments about performance and top end speed i appear to be in the same dilema and am faced with a moggy mechanic confusing me with his reasons. Basically i've had a 1275 engine fitted followed by a 5 speed sierra gearbox which as resulted in a loss of performance/power. It has gone back to the garage, who is a minor specialist, to try and resolve the problem. With the old 4 speed and 1275 engine it would reach 70-80mph on the flat and that's with a commercial diff fitted. Now with a saloon diff and 5 speed box it seems to just loose power above 50 - 60mph. I don't want to tear up and down the streets but expected to visit the 3rd lane of the M62 now and then if i wanted to with this new set up. Mechanic phoned this afternoon to say he had rectified a slight problem with distributor but also suggested a larger exhaust being fitted as the standard pickup one might be too small to cope with the bigger engine. Does this makes sense to anyone as i explained the reasonable speed achieved prior to 5 speed box and new diff fitted.I have read two different psi ratings for the 155/R14 tyres fitted, one saying 22psi for all the tyres and another 26psi front and 28psi rear. These are Toyo tyres without a recommended psi stamped on them and any advice on correct rating would be appreciated although i don't think this will solve my problems to any extent.
My mechanical knowledge is not the best so i realise there could be a million answers but any reply is greatly appreciated.
Thanks to all those that take the time to reply.

[alex_holden]

You must have been really thrashing it to get up to 80MPH with a 4.55:1 diff! Fitting a lower ratio diff and using the 5th 'overdrive' gear of the Sierra box will greatly reduce the engine RPM at a particular road speed, so should allow quieter/more comfortable/economical cruising, but your engine needs to have plenty of torque in the middle of the rev range or it will struggle. Changing down to fourth may well let you get up to a higher top speed, though that would make it a bit pointless having the Ford gearbox. I don't have a 1275 or a Ford gearbox but I suspect you should be able to do better than 60MPH in 5th gear.

Tyre pressures are completely unrelated to this issue, but if your vehicle is a van I'd suggest trying 30PSI front and 32PSI back (unloaded) and increasing the back ones to maybe 34 when carrying a heavy load.

[mugsy]

Thanks for input. I did have my foot down the other day as i was on about my 100th trip back to the garage so probably not in a good mood.
It's a pickup so maybe the canvas tilt is holding me back. Thanks for tyre info.

[alex_holden]

It's a pickup so maybe the canvas tilt is holding me back.

I'd probably start with less pressure on the back tyres then - maybe 30PSI or even 28 unloaded.

[PSL184]

What speedo have you fitted. It may be under reading with the sierra box and your indicated 50/60mph might be 80/90mph !!!

[charlie_morris_minor]

What speedo have you fitted. It may be under reading with the sierra box and your indicated 50/60mph might be 80/90mph !!!

and the change of diff will affect the speedo reading.

as for tyre pressure i run my toyo tyres at 30 all around.. as suggested by bmc if i recollect correctly

[bmcecosse]

Compare your performance in 4th gear on that box - it should be a 'straight through' ratio and so should be exactly the same as your old 4 speed box. My 1098 engine is slightly warmed up - and with a 3.7 'overdrive' final drive it can easily reach the national speed limit - and perhaps beyond ( ) if no-one is looking! It cruises beautifully at 60/70 with only a whisper of throttle on a flat road. there is something sadly wrong with your 1275 - but this has been discussed recently on another thread - and I do believe some 1275 engines are quite low powered - maybe only 50 bhp which is scarcely any better than the 1098 rated at 48! The original exhaust WILL be too small - and it begs the question - are you using the original Minor manifold - and perhaps also the small original carburetor? And as others have suggested - check the speedo for accuracy!

[mugsy]

Thanks for the input. With regards to the engines performance it seemed to have a reasonable amount of grunt when I ran it for a month or so with the 4 speed box prior to fitting the sierra 5 speed box, only then did it seem to perform worse. I don't think it was lacking in power and it seemed to have more than an extra 2 bhp than the 1098. The speedo is the original one at the moment. Someone is mocking up a new one for me but in the meantime the original one has stickers showing the correct speed which has been calibrated using a sat nav for accuracy. The manifold and carb have not been changed but again this wasn't a issue when the 4 speed box was being used ( and the pickup diff ).
All being well i am attending the national rally so anyone wanting to have a nosey can do so. I won't be hard to find i'll be the one with the sad face and the empty bank balance ( and possibly wanted for murder if the mechanic doesn't sort it out ).
Thanks again for advice, will post any conclusions for you to put in the memory bank.

[bmcecosse]

The original standard Minor carb and manifold (and exhaust) will be SERIOUSLY limiting a 1275 engine! Depends what final drive ratio that 'pick-up' diff had - if it still the same diff that's in the car ?

[mugsy]

With all these experts available on this site and these times of mechanical uncertainty wouldn't it make sense to form some sort of coalition garage, ideally in West Yorkshire so i have somewhere to take my car ?

[bmcecosse]

Depends if you want simply to have some Labour available there to work on your car - or some experts to give you a Conservative view of what's wrong with it................

[RobMoore]

be sure to inlude a liberal amount of british nationals that are independant. ^^

[bmcecosse]

[MarkyB]

But don't you kip on the job.

[barrie102]

Best set up I can suggest is 1275 engine HIF44 carb correct needle (winsu will tell you) big bore exhaust with lcb and 3.9 differential. I have previously used this (Ran out of speedo !!!) but do make sure you got the brakes to stop it with. Barrie

[bmcecosse]

Don't rely on Win SU too much..........