Hitchcocks Forum

OK! I have checked few blogs and found that Enfields cannot take higher rpms, anything higher than 6000 and so is it because of Enfields having longer stroke? What can we do to make it fail proof? I mean.. I do have some plans to buy high lift cams for AVL but I don't want to blow up the bigger end after every 5k kilometers. I know few racers who rebuild their big end pretty frequently.. like a friend who rebuilt it at 10k then 15k then 22k then 24k. But isn't there any way to make the lower end survive a bit more longer? With the "S" Cams..Enfields go till 7000 rpm it seems... which is dam yummy but what can we do to make them last longer? I got a Machismo 500 aka Electra X and I am kind of worried about this

Hi Dampking,
There are mixed reports about big end failiures in the AVL and it would seem to be a bit of 'potluck' regarding the length of big end life for these machines. I will probably get contradicted here, but I believe the worst treatment for any big end is to 'slog' it, high rpm's rarely see one off, usually it is those who like to make their machines shudder at 40 mph in top up a long, steep hill that will see off the big end rapidly.
The standard cams in the AVL engines would lead you to believe there is a rev limiter fitted and set to 5,800 rpm, but this is actually inlet valve bounce and I caught one on the dyno once, in the proccess of dropping an inlet valve as a result of this.
If you fit the 'S' cams, it doesn't mean you HAVE to rev it to 7,000 rpm, just that you CAN, but you will gain power right through the rev range over standard.

I find one has to be very carefull about taking Fully Floating white metal big ends up in to those rpm's , I have a rev limiter which cuts in at about 5800 , sounds like you need the race RE 15 roller big end and a forged steel con rod , ask our hosts as they race Enfields and know the engines limits and what other parts are needed , it needs to be remembered that this is no Manx Norton or BSA gold star engine , Jawa speedway engines need there cranks pulled apart and inspected about every 10 meetings or so .

I am in total agreement on this one.Want to hammer out your mains or bigend just lug your engine.Anyone who is that unsympathetic to a motor deserves the cost.Have seen a couple of similar failures on modern v twins from the same treatment. Bomber

Yep I agree, the earlier square case Ducati motors had a stepped crank pin and these were supposed to be prone to failure but mostly because they pulled so well at low revs and people couldn't resist giving them welly at low speeds to hear the lovely sound. It's all fun and games till someone loses an eye as they say and the lovely sound of the L twin often came to an abrupt halt as the big ends pack a sad. You shouldn't have trouble with high revs unless there is a balance issue.

Phil,
We race Enfields too and have done for several years now, with many good plcings and a good few wins in both 350 and 500 classes.
We ran a plain, floating big end in the 350, for four seasons racing. It was fitted second hand as part of a used crank we bought from our hosts as a stop gap replacement for a failed roller big end. The plain one quickly died when the oil pump spindle lost its' teeth and the oil flow stopped. That engine was pushing out over 30 bhp and seeing 7,500 rpm frequently.
The plain big end is fine if fed with plenty of clean oil.
Paul.

Bullet Whisperer, how does the S cam allow you to increase revs from 5800 to 7000 without the valves bouncing? Cams will seriously alter the valve timing, valve lift and overlap, however valve bounce is exactly what it says. The valves when closing at high speed bounce of the seats and then hit the piston coming up at them. Surely you must introduce some much heavier valve springs to prevent bounce.

Hi MadMike,
The 'S' cams are not particularly 'hot', but I tried all sorts of cams and valve springs in that AVL I tuned a few years ago, with no joy. Watching the engine closely during a dyno run, I noticed that just when the metalic noise / brick wall for revs appeared [at around 5,800 rpm], there was a large vapour cloud blowing back out of the carb. This was the inlet valve bouncing back open near tdc compression, borne out when the engine later lost power and stalled due to the inlet valve's collets losing their ridges. the valve spring cap hadn't quite slipped right off the stem and collets, but it was up enough to foul the rocker enough to hold the valve open a bit and stop the engine.
After trying all sorts of cams and timing combinations, I only had some old 'S' cams lying around and thought I might as well give them a go. I couldn't believe the difference, the top speed was just over 100 mph, as opposed to just under 90 mph before hand [other tuning work had been done.
The standard [and 'Classic' type] cams cause this valve bounce, I don't know why, they don't look very aggressive, neither do the 'S' type, but these latter ones work very well in the AVL engines.
Paul.

Thanks a ton BW for the detailed information. By the way, do you think that using CastIron 500 cc cams on AVL 500 will have any bad effect? how hard is it to time them and get them running? Basanti was telling me that he tried those cams and those were great, way better than AVL 500 cams and suggested me to go for "S"Cams whenever I can afford them
At moment I can't afford "S" cams but in the near future I can but for now I wanted to test those STD 500 cc cams on mine.

I once read some articles by Peter Williams and also Colin Seely as regards cam profiles. As far as I can remember, it's all to do with acceleration factors determined by the profile. If the cam lifts the valve with a very high initial speed it has time to slow down before the peak opening is reached. I think Colin Seely found he could install weaker valve springs and still rev to something like 8000rpm on his Triumph Twins (I'll have to find the article to check details) This might explain why the valves float with standard cams but are ok with sportier fast lift cams. "Valve bounce" is not always the valve bouncing, it is sometimes the cam follower and valve shooting past the cam profile and loosing contact with the cam. this is why it is often called "valve float" rather than bounce. The most obvious characteristic is the sudden clatter from the valve gear due to the big clearances that occur......What Dampking should know is that the load on the big end increases at the square of the difference in engine revs. So revving to 7000 instead of 6000rpm is an extra 1.44 times the upward inertial weight created by the piston and as there is approx 1 ton of inertial weight at 6000 rpm, an extra 0.44 ton of upward force is created on the big-end just by adding an extra 1000rpm to your driving style, that's why big ends or con rods break when you over rev them. Better to go bigger if you can rather than rev it faster.