11 replies to this topic

[tim]

I'm on with a rebuild of my 1275 gt engine A-series.

I have a choice of conrods and was wondering which to use.

The engine will be  a fast road 1330 with 731 cam and lightened balanced flywheel /crank and ported big valve head.

The concods I have to fit have been ground down and look like lightened but have a big square end cap.

The part number is 12g1507/8 on them,the others I have 'which has the piston still on' I think are from a 1275 mg metro A+ engine.

Which are better for what I'm building????

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[Cooperman]

The original rods are horrible as they have heavy 'lumps' on the big-end caps to aid smooth running at lower revs.

 

The best con-rods are either A+ rods or Innocenti 1300 Cooper rods. A+ rods are fairly easy to find, but in your case it looks like you have them already.

 

Just a word about the 731 cam if I may. It was never very well-regarded, even back in the 1960's, and is very dated now. You would find an MG-Metro, Kent 266 or SW05 better cams for a 'not quite so slow' road Mini. I found that a 1275 'S' performed better with a well-built engine with an original 510 cam than a 731 as it retained its excellent mid-range torque and, with a well-flowed head at around 10:1 CR, the overall performance was better than with a 731 whilst retaining the standard 3.44:1 FDR and Cooper gear ratios. That's just a personal view and others may have found they got a 731 to work very well.

[tim]

So which is the better rod?

The one with piston or without?

 

Cam wise I have thought about fitting.

 

http://www.minispare...|Back to search

 

But my engine already has 731 fitted so thought stick with it,also has 1.5 roller rockers.

I have never driven this engine so never knew how it drove.

[ACDodd]

by the way the reason why the 731 cam is so poor is because it has less lobe displacement from split overlap to full lift. Ie from tdc to full lift it lifts the inlet valve less than a stock 850!!!!

Ac

Edited by ACDodd, 25 October 2016 - 05:38 PM.

[greenwheels]

by the way the reason why the 731 cam is so poor is because it has less lobe displacement from split overlap to full lift. Ie from tdc to full lift it lifts the inlet valve less than a stock 850!!!!

Ac

So is that why the OPs engine has 1.5 roller rockers i.e. to compensate. I always understood that the 731 should be run with 1.5 rockers. Some comparative rolling road figures would be interesting.

[grizzler73]

The A+ rod has a much more rounded small end, that's the one you want. Much better to put a good modern profile cam in it than any other mods, that should not be a compromise.

[Cooperman]

The one with the piston fitted looks like the A+ rod which is exactly what you want.

 

You would be better off running an SW5, 266 or MG Metro cam with standard lift (c.1.3:1) rocker ratio than a horrible 731 with 1.5:1 rockers.

 

There is no point in have a super-hot cam for a road car only to find that the torque and power bands are so high that a change of gearing is needed to keep the engine 'on the cam' ad the engine wears out quickly because of the need for very high revs to 'make it go',

 

What is needed for a road car is optimum torque rather than top end bhp at very high revs. Too many owners 'over-cam' their cars and later regret it when they have to lower the gearing and thus use more revs to cruise at a given speed. 

[ACDodd]

You can't polish a raw turd, and that's what the 731 is. Putting 1.5 rockers on it is like adding to the failure. The lift rate is wrong, its still wrong with 1.5 rockers fitted. The 731 valve movement with 1.5 rockers is about 0.285" between tdc and full lift. The sw5-07 have more like .350" ish and 30degrees ish less duration. The 1.5 rockers magnifies the difference and actually looses more ground to a modern profile. If you want to make a 731 work, simply regrind with 6 degrees of advance and increase the lobe lift to around 0.315" that will work much better and be on a par with most modern profiles for the same duration class.

This fundamental point is the limiting factor on a lot of cams, especially old factory grinds, these items should be consigned to the tuning lore of yesteryear.

Ac

Edited by ACDodd, 25 October 2016 - 08:25 PM.

[carbon]

The optimised 731 timing sounds like the AC Dodds RS+ profile?

[ACDodd]

Nope my RS+ has more overlap lift and is a hotter cam.

Ac

[Fast Ivan]

So which is the better rod?

 

the ones that measure up right and are within spec

check this out -

 

http://www.theminifo...ne-build/page-2

[tim]

Thanks for above Fast Ivan interesting article.

Good responses all round