Cam vs Cam, why bigger/longer is not allways better.

A very short bit on cams as I ran some simulations and you have to do something after 10 hours of calculations šŸ™‚

It is a 1380 with a 44mm SU and big bore manifold and LCB using 1:1,5 rockers


Lots of people look at cam specs and think… okeey MD310 is for race.. MD286 is kinda lumpy,Ā  the mg metroĀ  252/268 but that did only 72 bhp so I’ll get a either a 266 or a 276.. better take the 276 cause maybe I think that the 266 is too tame.That’ll get 100bhp no problem..

In practice it is far better to under cam the engine than to overdo it, especially on an A series.

A few things: Cam timing even if it is all measured in exactly the same way (which it is not) does not quite tell you all you need to know.

It does not tell you how much dwell a cam has (not necessarily better btw) , how much lift on overlap, the LCA. etc etc.

Nor does it tell you how it will mate with your car’s weight, use, final drive, cylinder head, induction system, etc etc.


the bodge picture illustrates that you can have two different lift profiles with the exactly the same timing figures . The other lines are acceleration curves , and further derivatives, that are not of interest for this time.

If you have a less than good head that has trouble breathing at high rpm’s , mating it to a cam that needs to rev it’s pants off, will in all probability cause it to lose a load or bottom end andĀ  never gain it back up top. The more cam you put inĀ  the less tolerant it becomes for bottlenecks in the entire chain.

Just to illustrate..

Swiftune racings SW5 and SW10 simulated profiles based on measured (not by myself) numbers (which are probably in the neighbourhood unless they use very funky profiles). The reason I think they are in the neighbourhood is that you can’t really go nuts with the profile as there is only so much acceleration you can use before you start to break things. So I just used the standard ramps and a rather arbitrary 5 degrees of dwell. And I must make a cam measurement setup on of these days.

It are simulations, but the results are not all that far off from what I have seen on actual engines although the peaks are lower down on my setup.

The SW5 ( which AFTER people read that it was 244 degrees was immediately dismissed at a soft road cam) ,” Will not rev past 4500 rpm”Ā  Well IMHO if it does not rev past 6000 you are doing something wrong. If anything is wrong with it it is that it is a bit dull.. in a modern turbo diesel way.Ā  Virtually no overlap, yet pulls quite a bit more rpms that this would suggest.

The SW10 is a MD 286 bracket cam with better manners and better emissions using 280 degrees of seat timing.

sw5-vs-sw10The only thing that has been changed is the cam and it starts at 1000 rpmĀ  so it isĀ  a bit more useful for comparison than Vizards rather eclectic mix of configurations an graphs, even if it are simulated values.

Note that the sw5 will spank the SW10 all the way up to 5500 rpm (Mr Swift mentioned 5000 by email a few eons ago so seems about right) and most people will not rev a road engine past 5500 for more than a few seconds.

While in this simulation you don’t have valve float or broken conrods to deal with ( and it will thus rev to 15.000 rpm),, in real life this will have turned into an A-series cuckoo’s clock or grenade by then), in my experience a halfway decent engine with the sw5 it will rev pretty happily to 6500 rpm although it will produce peak power a bit sooner most of the time.

Swiftune get’s about the numbers for the sw5 you see here ( a bit higher even), but I think they used a pretty high spec for the rest of the engine and they have been building race engines for 40 years plus. The sw10 numbers are a fair bit higher than what you see here but you’d need a better head than I have got .

If the exhaust port gets better, the engine will hold on past peak power a bit longer ( at least in this simulation).

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3 thoughts on “Cam vs Cam, why bigger/longer is not allways better.

  1. Bob says:

    Great blog! Thanks for keeping it updated.

    I was part of a conversation about rod ratios in the A-series engines. One thing that was being discussed was reducing the rod ratio in the 948cc engine to ‘rock’ the power curve back down a bit. Could it be possible to try this experiment in your A-series model in EngMod4t? I have the program, but I don’t have engine parts to measure to complete an A-series model.

    Here is a link where the conversation took/is taking place:

    • mowog says:

      Thanx Bob ! I should get back to fiddling a bit more with the model. I’m not fully happy with it as it is but it is reasonably close to what I can do without having full cam profiles. My gut feeling is that the differences will not be large, but we’ll have to test it first.

      • Bob says:

        Mine as well. I changed the connecting rod length in the G50 model in EngMod4t (only takes a few minutes to run) by the same % and didn’t really see much, but it also doesn’t rev as high. I might try to build a single cylinder ‘model’ of the 948 for curiosity. I have tried to start a thread in the advance section of speedtalk, but either I’m not clear enough or people have poor reading comprehension. Oh, well.

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