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best diff

bodyshell2

Freshman Member
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Hi all i am thinking of fitting a 649 cam in my 1340cc S fitted with twin 1 1/2" H4 carbs 1.5 roller rockers , could i get away with a 3.7 diff ( for limited road use only ) i know the works cars used 4.1 but this would make it a screamer especially with straight cut box and drop gears . Your thoughts please .

Best Regards
 

dklawson

Yoda
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I can only offer conjecture since my car still has the standard gearbox and a 3.44 final drive.

With straight cut gears and a 3.7 final drive your car will certainly be driveable around town and it should accelerate quite well. It is also going to be very loud if you ever take it on the highway. Consider that a Mini on 10"s with a 3.44 FD turns about 4000 RPM at 65 MPH. You'll be closer to 4300 with the 3.7 FD and the straight cut gears are just going to be that much "louder".
 

dklawson

Yoda
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You asked "could I get away with a 3.7 diff" if you fit the 649 high-lift cam. That all depends on what you are looking for and asking about. You mentioned limited road use. Are you concerned about how driveable the car will be on the street or how it will perform on the track?

The higher the FD number, the more revs it takes to go a mile. That means the available torque will allow faster acceleration. Your 649 cam is supposed to have a power band between 3k and 7k RPM. Kent considers it a race cam. When driving at lower speeds and around town it will benefit from higher final drives... but as I said earlier, you will have those higher revs at a lower road speed. Starting from a stop and driving at low speed (RPM) may be less than smooth. As I mentioned earlier, it will be loud.

I suggest you contact the folks at Kent regarding using high-lift rockers with this cam. Long ratio rockers have their own problems, not the least of which is/are higher contact forces. Make sure your valve springs don't collapse to solid using the high-lift cam and long ratio rocker arms.
 

Jerry

Luke Skywalker
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My mini is a 1380 with 2 1.5 inch SU, a 288 cam and dyno at 5800rpm to be 95HP. My rear final drive is 2.9.
things I would do differently if I took the engine out:
1. The cam has moved the power band to far for street driving. Not much power below 2500rpm. I would use the next cam down, (around 275)
2. The final drive is great on the freeway, At 3800rpm, I am going 70mph. But, loss of acceleration around town due to the final drive ratio and cam.
3. I have tried about 10 different needles in the carbs using the MInty program. Those 2 1.5 inch carbs are a little too much at lower RPM. Perfect on the freeway.
Still, it works pretty well. Once you hit 3000rpm, it takes off like a rocket. Fun to drive.
Jerry
 
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