mk1-forum.net

I have been offered a 731 cam which I could fit to my 1275 S. Anybody have any experience of them? I seem to recall they were a good Road Rally cam in their day but that is my lot. What difference will it make to a 1275 S? What are they worth nowadays? Good condition and looks pretty well unused. Cheers. Mark

just thinking out loud here but isnt the 731 the lesser used of cams, i seem to recall no one having anything good to say about it

what you using the car for - road, rally or ???

i think if youre going to spend cash on a cam , it needs to be a wise choice you make

pad

It's not a case of not having anything good to say about it.

The thing is that the 731 was designed specifically for the 997 Cooper engine. In that long stroke / small bore application it works pretty well, but as I am sure you appreciate there aren't many people tuning 997's nowadays. When this cam is shoehorned into other more high revving applications it can be disappointing, giving a pretty lumpy tick over & no appreciable increase in top end either!

The good old SW5 is a much better option in a 1275 or even a 998 engine for that matter.

M

I seem to recall it worked OK in a 1275 but lots of people put them in 998s where it didn't really.

Modern cams are undoubtedly better though.

From the main MK1 site.

Hard to read, but interesting.

Simon776 wrote:I seem to recall it worked OK in a 1275 but lots of people put them in 998s where it didn't really.

Despite being officially called a "rally" cam by BMC, the 731 never seemed to be used by the works S rally cars, which tended to use the later standard S cam, the 510, whenever the 544 or 648 wasn't used.
I co-drove a 998 Cooper with a 731 and all appropriate other mods on the Scottish in period and it was gutless on the road sections where I was in the driving seat.

In about 1979 (!) I did what was probably one of my first 'mods' to a mini, I put a 731 into my Damask Red 10" 1275GT. I kept the single 1 1/2 and 'de-coked' the head ( ) THe other essential mods were the yellow fibreglass 'whip Arial' and 10 X 6 KN Star Mag Alloys with W+P arches!

Result, no discernable performance difference what-so ever! But Man it looked the Bizzo!

I used a 731 in a 1293 'S' that I drove on road / occasional stage rallies, I was not disappointed with it, but on the other hand it was not outstanding, bearing in mind I wasn't looking for power at 8000 rpm, it was geared more for torque, which it possessed in bucket loads! I was initially concerned that the 731 had 3/8" wide lobes instead of the 1/2" which most other camshafts seemed to have, however it turned out not to a problem, showing no serious signs of cam or follower wear after many miles of a mix of competition & road use. I don't know what all you guys will think of this, but I had an Austin Healey Sprite 1275cc, so the first thing to do was to remove the engine strip it, balance & lighten, port & chamber the head, & someone (an experienced builder & driver of rally Minis) suggested a 948 (88G229 for a spline drive) camshaft as fitted to the 997 Cooper. It went like a scalded cat & I've been a great advocate of that cam ever since, I wish I had tried one in the 'S'. It also works very well in 998's. To be honest, I have no idea what a slightly used 731 would be worth, but what I would say, examine it carefully, an original is identified by 3 rings, 3/8" lobes & pin drive.

The reason that most of the BMC rally cars used the standard cam is that most of them ran in Group 2 & they HAD to use the standard production cam.

Interesting thread, is the 731 profile really that mediocre?

For example if you look at the timings for the AC Dodds RS+ cam then it looks awfully close to the 731 timing with 5 degrees of cam advance and a lot more lift. Granted the acceleration rates are also likely to be faster on the RS+.

Could the 731 actually work well in a 1275 with 1.5 lift rockers? I've not tried this yet, has anyone else got practical experience?

PS. There are factory 731's with 1/2 inch lobes as well

Interesting discussion on here as I was considering to put one into my 998, but now I think that the 88G229/2A948 (the std 997 cam).

mk1 wrote:The reason that most of the BMC rally cars used the standard cam is that most of them ran in Group 2 & they HAD to use the standard production cam.

Most did run in Group 2 - where the camshaft was free. Group 1 required the standard cam - but there were very few works entries in total in Gp1 in comparison with Gp2 (or even Gp6).
Lets not have capitals (ie. shouting) when you are factually incorrect?

I was considering to put one into my 998, but now I think that the 88G229/2A948 (the std 997 cam).

I tried that permutation as well and it seemed to make very little difference.

I have a similar letter around here someplace from Downton dated in the early 1970s that says much the same thing; i.e., they could find no reason to use a 731 in a Cooper S.