11 replies to this topic

[rally1380]

Hi.

 

I've been researching Cam's in order to bung a new one in my rally mini.

 

One thing that is apparent in tinternet world, is that there are loads of 'opinions' out there, with 'expert' A saying cam profile such and such is the best ever, or 'expert' B saying brand new cam profile X is best.

 

I noticed on another mini website a 'Our personnal cam opinions' section - with them slating the cam i was (perhaps still) going to buy.  But then it got me thinking, the title clearly says 'opinion', so this is just someones 'opinion' of what they like in a cam's power and torque delivery.

 

Soooo....I was wondering if there is a source out there in tinternet land that has dyno'd the popular 'go to' A series cams for a clear and easy to read comparison.

 

One thing I don't want to do is have to work my way through cams to find the best one for me.... I was hoping a more scientific approach could save me the money and time involved with experimentation.

 

[ACDodd]

What do you need? I do cam shaft consultancy and have custom cams made for customers and also perform camshaft measuring and comparison for some other big name engine builders, what can I do for you?

Ac

[Dusky]

In a rally mini i'd go for a 643 or a 649 cam. Have a 643 in my 998 cc ( love it) and a 649 in my dads 1330 and it works well, not drivable on the road (maybe due to superlight flywheel).

[Cooperman]

Dave,

 

I run a 286 in my 'S', but I have to say that for the sort of road rallies you are doing it is a bit too cammy. That is with a SC CR gearbox and a 3.9:1 FDR. On the tests it can be difficult keeping it over 4500 rpm, which is where the torque is and if I were just going to use it for Historic road rallies I would switch to a 276 or, maybe, even a 266 where it would always be 'on the cam'. Look at the driving profile on the sort of tests you are doing, with stopping and starting, handbrake turning, etc, where good low-end torque is necessary and a 266 makes a lot of sense really.

For the sort of 'all out' night events which I like a lot of power is great and on Welsh mountain roads in the dark with the navigator reading every bend accurately, it is possible to keep it at over 4500 rpm all the time with over 6500 quite often, which is where a 286 is best. Similarly on special stages on tarmac with full pace notes a 286 works well as, again, you can guarantee to be keeping the revs right up very high. I pulled 6700 rpm on gravel in top on a long forest straight - blooming frightening! - but it might have been better with a 276 and a 3.76:1 FDR.

Rallying is as much about gearing and torque as ultimate power. And about driver & navigator working as a team and learning how to carry the speed through a corner.

Just remember, on gravel the 'works' used to use a standard 510 'S' cam (except Timo who liked a 649 and a 4.3:1 fdr which is why his car often blew up!), which is slightly milder than a 266 and they won the RAC International with that spec. A high torque cam with peak power at about 5600 rpm.

[carbon]

Dave,

 

Other point to consider is looking at the cam profiles, nominal timings and LDA is all well and good, but what really matters is how far and fast the valves move as this is ultimately what affects cylinder filling. My point here is if you take a fairly mild cam like a 266 and add a set of 1.5 rockers it will give it a lot more top end sparkle, and depending on your set-up may even give a better power curve than a 276 running sintered rockers.

 

I don't think you will find much reference 'back-to-back' data with cams from different manufacturers running in a similar spec motor. There is some good stuff in the Vizard book which gives pointers, but there are a lot of more recent grinds out there now which were not around 20 years ago.

[Captain Mainwaring]

In a rally mini i'd go for a 643 or a 649 cam. Have a 643 in my 998 cc ( love it) and a 649 in my dads 1330 and it works well, not drivable on the road (maybe due to superlight flywheel).

 

 

Just out of interest why would the super light flywheel make any difference on the road - surely the inertia of the car is hugely greater than the inertia of the flywheel?

[Ethel]

Perhaps it makes it hard work to keep it idling in stop start traffic?

 

Anyways, opening valves quickly matters, especially if they don't have much circumference, That's where high ratio rockers can pay dividends as they multiply the ramp rate of the cam, the extra maximum lift is less important, if at all. 

[Captain Mainwaring]

Perhaps it makes it hard work to keep it idling in stop start traffic?

 

Anyways, opening valves quickly matters, especially if they don't have much circumference, That's where high ratio rockers can pay dividends as they multiply the ramp rate of the cam, the extra maximum lift is less important, if at all. 

 

You'll find that most of the flow is generated in the first couple of mills of valve opening....as you say high ratio rockers and abrupt ramps will all help with this.

[Dusky]

In a rally mini i'd go for a 643 or a 649 cam. Have a 643 in my 998 cc ( love it) and a 649 in my dads 1330 and it works well, not drivable on the road (maybe due to superlight flywheel).

 
 
Just out of interest why would the super light flywheel make any difference on the road - surely the inertia of the car is hugely greater than the inertia of the flywheel?

The idle of the car is ruined especially with very hot cams, ofcourse it is do able but surely not on a daily driver orso;) The car also brakes quicker ' on the engine' (if that is a right translations )

[Captain Mainwaring]

 

 

In a rally mini i'd go for a 643 or a 649 cam. Have a 643 in my 998 cc ( love it) and a 649 in my dads 1330 and it works well, not drivable on the road (maybe due to superlight flywheel).

 
 
Just out of interest why would the super light flywheel make any difference on the road - surely the inertia of the car is hugely greater than the inertia of the flywheel?

The idle of the car is ruined especially with very hot cams, ofcourse it is do able but surely not on a daily driver orso;) The car also brakes quicker ' on the engine' (if that is a right translations 

 

 

Idle of the car may be less smooth - but that's about it. As for engine braking - I don't think so.....engine braking against friction and compression losses would be many times greater than difference in weights between the two flywheels would make.

 

Maths is easy enough t do if anyone can give a flywheel weight and COG

[Earwax]

The MLC motorsport forum had a section that did report builds and dyno results etc , so AC dodd would have a lot of that info at his disposal. You could go the other way, and ask people for their torque sheets, and what build... .  a thread on ausmini recently, had a few responses , and RE cams were featured, but other good cams out there.. also presuming N.A. build , so ask around for builds producing 100ft of torque

[Cooperman]

The problem here is that different cams suit different motor-sport applications. Remember, the OP (Dave) wants a rally car, not a sprint or race car.

Before advising anyone it is necessary to fully understand and have experience of the type of competition being considered. Here we are concerned with Historic Road Rallies. These contain a mixture of reasonable slow road navigation sections and short 'auto-test' type speed tests with some stop astride a line then continue forwards.

For tests like these a cam providing a wide torque band is ideal so I recommend a Kent 266 or 276 which have the right profiles and still give an adequate top end at about 5700 rpm/6000 rpm respectively.

I have done many of these tests with my car which has a 286 cam and a very light flywheel. It is easy to 'fall off the cam' and lose a couple of seconds when it 'bogs down' and all one can do it give it loads of throttle and push the clutch down and up again, not the ideal thing really.

But on a tarmac special stage with no stopping my engine with 286 & 1.3:1 rockers is fantastically quick.

So build an engine for its application, not just for peak power at high revs.

Anyway, simple power does not win rallies. It is the ability of the driver to carry the speed through the corners and around the hairpins and of the navigator to supply the necessary route instructions in good time to enable the driver to drive smoothly and accurately. A couple of bhp or a few extra ft.lbs. of torque will make no difference to the final result.

Edited by Cooperman, 02 April 2014 - 09:27 AM.