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MG MGB Technical - Camshaft Choice
Well really I'm after your choice of secret weapons
I have a friend's MGB that get's fiddled with regularly
It's a 1950 with a good ported big valve head with multi angle valve seats ;alloy flywheel good sump etc
It's on 1 3/4''su's lcb exhaust recurved dizzy
It's a real good performer and is used regularly in club racing and revs are self limited to 6500 through the gears but allowed to spin out to nearly 7000 in places to save chopping through the gears but it doesn't necesarily need heaps of strong power in the 6500-7000 range as long as it's real strong to 6500
We have experimented with quite a few camshafts from mild to wild and higher ratio rockers etc
My question is has anyone on here found a magic camshaft or camshaft /rocker combo that really really works from 3500/3700 through to 6500
We are quite happy with the thing as it is but always want to try things that might just make some horsepower that could be hiding somewhere
Anyone got any suggestions
have you tried a scatter cam?
A Kent 718SP has a power band from 2500 to 7000...
I guess one of the racers will be along soon..
|We use a Piper 755/756 @ 104 for rallying, which is good in that rev range with 1.6 rockers. |
Lift at cam 0.351 unlet, 0.341" exhaust
duration at .016" cam lift (=BMC timing clearance) 305 deg inlet, 300 deg exhaust
duration at .050" 260 deg inlet, 263 deg exhaust
(yes they are the right way round)
Tractable from about 13-1500 on part throttle, 2000 on full throttle
Pulls well from 3000-7000+
Best from 4000-6500
In a nutshell it's a bigger cam that the Kent 719 yet I can autotest comfortably with it, and it pulls well to 7000 and beyond with ease.
|Mike - Paul|
Thanks for responding
Mike , your cam looks good but I'm a bit of an angry cam sort of guy , probably wrongly so but---------
Paul, that looks like it's worth a try.
The best results we have had so far have been from an old sprint cam 75.45/60.80 that I had my camgrinder mate take the back out of nos. 2&3 inlet lobes so they closed early at 65deg to make it act like a scatter cam sortof - I'll look into the755/756 thanks It must have interesting lobe shapes to get the duration swapping over like that -- I fall for weird stuff (hope the wife doesn't read that) --thanks willy
Anybody else want to spill their beans --
I'm all ears --
|Willy, I've been puddling round town in the last few days in the rally car, and should clarify the tractibility of the above camshaft:|
Pulls away happily on part throttle 1300 RPM in 1st gear, in top gear it's more like 1600 rather than 1500.
Otherwise as above.
But from your comment I don't know that it's all that relevant. What I do know is that it puzzles me how such a big cam works like that, but it does.
Looking at the detailed analysis of the lobes, the "crossover" in timing appears to be because the inlet lobe has a longer opening ramp.
|Hi Paul, I trust you are well? It was good getting to say 'hello' to you.|
I asked Piper for the spec sheet on the 755/756 as I had not come across it before as far as I am aware. The cam lift is quite high do you fit MGC diameter followers for that lift?
I haven't joined the topic as I have not 'found' the ideal cam for high lift rockers as yet. The STR930 on a B cam works nicely with high lift rockers but may be too mild for race use?
|Peter Burgess Tuning|
|Hi Peter, all good here thanks, yes it was good to meet up. |
Whenever I get a different cam profile I get campro run over it so I can see exactly what it's doing. In particular I want to see what the maximum lift velocity is, as that determines the diameter of follower required. It's a bit easier now that campro also spits out the minimum follower diameter required (0.79" in this case) - so yes, it just sneaks in on the standard followers.
I haven't tried the STR930, but have used the VP4 (290 deg x 0.325" IIRC) in a B and found that a bit mild for rally use. The 755/756 does nicely better whilst still retaining enough tractability to retain control in the odd misread corner.
Interestingly I never saw it listed on Piper's site, found out about it by word of mouth. I can email details if you like.
I had another camshaft before this in the car, with about the same lift (0.353" at the lobe) and another 10 deg duration at .050" lift, and similar ramps, perhaps slightly lazier. It was usefully better at the top end, but not as good between 3-4000 as you'd expect. Given that in the multi-day events with about 700 miles of special stage I limit engine revs to 6500, it wasn't really the right cam for the job. But it would be a good race cam. I kept a copy of the specs but the person I got it from didn't let on who produces it.
|Interesting you get to use standard followers. Yes please for any additional info on the 755/756|
|Peter Burgess Tuning|
|Will do. The 755/756 has now done 5 years use now, both on the road and about 2000 miles competition, with no ill effects on the followers or the lobes.|
Yeah it's interesting that as you get further up into cranky camshafts that they get to a stage where they start getting easier to handle low down again
One of the cams I tried in my old midget was way over the top but you could drive it round like an old tractor
It had that much overlap that it wouldn't start on su's as it had no suck, It had to have a weber with accelerator pumps to get it cranked up and when it did start you could easily stall it by putting your hand over the exhaust Didn't stay in long--but worth a try
This thread was discussed between 15/11/2014 and 19/11/2014
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