49 replies to this topic

[Ethel]

Let's start simple,

What's the ignition system. Has it been on the rollers to check fuelling & timing. It's in about the right ball park for points bounce.

[ministar]

Its a recurved electronic ignition by H&H ignition specialists. Yes fully set up on the rolling road by Peter Burgess.

Points bounce? I have never heard of that, always learning!!

[Ethel]

Not that it applies to you but any reciprocating spring has its limits, the one that closes points is no different to the ones on your valves.

It still seems a tad low for maximum rpm, does it fair better when it's doing less work - through the gears, downhill...?

[ministar]

Due to the long final drive it really doesn't like being off the power band of the cam, It performs best between 3.5k - 6k. With max power found at 5750rpm.

How do you mean, how does it fair?

[ministar]

What he means is does it pull past 6k in the gears (1st, 2nd & 3rd)

I want to extend the power band of my modified 1275. At just over 6k it looses all power and really falls off cam.

Basically no, even trying to keep it going past 6k rpm to get into the power band in the next gear is impossible. It really gives up at over 6k

[Ethel]

Not only that but how quickly does the acceleration tail off?

If it needs "X" amount of HP to drive it at a certain speed it will by like hitting a wall if that's all the power you have when you get there.

A lower gear, tail wind, or downhill road would take you to higher rpm without any significant drop in acceleration, but if it was down to internal losses or other limitation of the engine it would bog down much the same at the critical rpm.

[ministar]

Not only that but how quickly does the acceleration tail off?

If it needs "X" amount of HP to drive it at a certain speed it will by like hitting a wall if that's all the power you have when you get there.

It tails off very fast. I would describe it as a clutch slip? If that makes sense? Just can't go anymore.. It trys but nothing there to pull. Which makes sense with what you saying you need X amount of power to go Y amount of speed.

So, the camshaft is doing its best with the components it has around it. The limitation would be head, and being the most restrictive in regard to more power?

[mk3 Cooper S]

Sounds like it isnt breathing well.

Does it have a large bore LCB?

Was the CAM timing done with a vernier wheel

[ministar]

Hmm. How would I improve breathing? You mean internal breathing or carb breathing?

It is a Maniflow LCB. (not the stage 2 one though)

Yes Cam timed into 106degrees. Duplex timing chain.

[mk3 Cooper S]

Flowing/porting /matching the inlet manifold.
The same with the head.

to be honest 77 ponies does not sound too much off the mark for that spec.

[newnham500]

^{well accoirding to the yellow bible a stub stack/ ram pipe wil move the power up and down the rev range depending on lengh. if i remember correct longer is lower, shorter is higher.}

[ministar]

I agree, its not a bad number for the spec. But just wondered if I could do something to increase the power band at the topend. Ie. make it go over 6k rpm.

The answer seems no, and the head needs more work to let more horses free.

[Ethel]

Not sure that you got my HP to max speed point. If you did something that reduced the HP requirement for a particular speed then it ought to go past that speed until it finds a new "wall" where the speed matches the HP available. In a higher ratio gear it may well reach the rpm limit of the engine first because the gear ratio means there's always excess power available for the speed you get relative to the engine rpm.

This is just thinking aloud really - if the gas velocities somewhere are approaching supersonic at that 6k rpm it will be like hitting a wall. The valve seats would probably be the most likely suspect.

Another thought, what plug gap are you running? As the rpm goes up there's less dwell for the coil to recharge itself, perhaps it can't fire across your plug gaps above that rpm? I wouldn't really expect it to be a problem at 6k, but it'd be easy enough to try reducing the gap a little.

[carbon]

Special tuning did a conversion kit for Cooper S back in late 60's. This kept the standard cam, used modified head and twin 1.5 SU's. Power output was allegedly over 100hp at flywheel @ 6,800 rpm.

I doubt the 276 is the weak link. Are you sure exhaust is clear?

[ministar]

Exhaust was brand new, so was the manifold(s).

So, can anyone get a 276 over 6k?