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MG MGB Technical - Camshaft choices

Just about to head into a winter project V8 conversion, The engine is a low mileage 3.9 hotwire injection Landrover, Not real sure what the comp.ratio is but it is what it is.We'll find that out when it comes apart.It'll get rings and bearings, gaskets etc and want to upgrade the cam. to something that buries in better from around 2000 to 5500 maybe 6000 max. but not too lumpy to wreck bottom end power--I've got a cam in mind but am asking for suggestions from those that might have a favourite grind or know of something special that does the job well in these engines------------
willy
William Revit

Be very careful Willy. I designed a few cams for the Rover V8 succesfully improving performance, emmisions and fuel economy. None of them proved durable - pushrods bent before end of the test. I'm working from memory here but the cam that the engine came with from Buick was a 272 deg period and quite a moderate lift but is bullet proof. I attemted to shorten the perioc and increase the lift. I came up with a very good 264 deg cam, the performance guys loved it but ended in tears. A colleage of mine eventually designed the cam for the P38a engine but he didnt shorten the period but did increase lift. It was successful in test and in the field.You have to remember the engine has very small valves for such a big bore and its a short stroke. Yours doesn't sound like a P38A engine and I don't think the cams are interchangeable.
Paul Hollingworth

I'm not 100% sure what this engine is but it's supposedly Landy. It's 3.9 with the manifold cut down to fit the std.MG bonnet It's got a distributor but that's going ,It'll be cop. The hotwire will go as well and run straight off throttle sensor and maybe manifold vac but probably not
I've got a pair of worked 300 Buick big valve heads left over from another car so will probably use them up, maybe/or not
There's a blank billet cam ready to go and my camgrinder mate wants me to use the grind I've used successfully in the past but that was in a 5.4 stroker motor and I think it'll be a bit too angry in the little 3.9 (for normal driving round town use)but then the injection will flatten the idle out a bit compared to the other on carburettor--just undecided and was hoping someone might say-
--I've got this cam that's magic in a 3.9 it's a XXXX drind ---around the 285 degree mark depending on lobe centres etc
It can have up to .500 valve lift if the Buick heads get a run but as usual I have to keep reminding myself not to get carried away, it's a road car after all -

Anyone got a secret to share--------??
William Revit

The original Buick cam has 0.25" lift, so assuming a rocker ratio of 1.6 then the lift at the valve is just over 0.4". The timing period is quoted between 0.040" valve lift so 285 deg. Its not really this, the opening and closing ramps are much longer and higher to allow for the tappet sink down. This vagueness makes it difficult to compare cams.
Paul Hollingworth

When I rebuilt my Rover V8 3.5 it was bored out to 3.6 to take Vitesse pistons, raising CR to 9.75.

I rang Piper, told them the specs of the engine and they ground a cam to suit - it has done over 100k miles now and is still fine.

Produces over 200 bhp. I did have a set of stage 3 heads to fit it, but the power is more than enough for me so ended up selling the heads for a tidy profit!
Chris at Octarine Services

Cheers Chris
Had a look on the Piper website and their V8BP285 reads up scaringly similar to what I've run before, slightly less duration but same lift at tdc same overlap and similar full lift--I'll see if my bloke here has a master for that and go that way, will be getting new lifters anyway so I think I'll go for a 5degree set so that'll tame it a bit more anyway-Sometimes i wish all cam makers would quote the same figures like my US mates do at .050" at the lobe and at .003" valve lift, life would be a lot easier
William Revit

You are starting to give me nightmares again now. The problem with the V8 is that it has a very compliant valvetrain. The hydraulic tappet can get airated, the pushrods are small diameter and there's a overhung rocker. All this means it requires longer and higher opening and closing ramps on the cam. The 'Austin' way is to measure the period from the top of the constant velocity ramps i.e. the bit of the cam where the follower is accelerating. The function of the ramps is to allow for tappet sink down on the opening side and to control the velocity that the valve hits the seat on the closing side. So how high do the ramps need to be on the V8 ? Obviously higher that anything in my experience.
All around the industry various people measure period above a set valve lift but if the valve is only open a few thou then its not going to flow anything. What this set valve lift is varies wildly amongst manufacturers and consultants. So comparing cams periods becomes very difficult. Best to plot out the lift, velocity and acceleration on top of one another to compare, that's if you can get the data.
Log period cams are a doddle to design. The challenge for production engines these days is to maximise lift on a given period. That's usually on the short side and can be anywhere between 230 and 252 deg. Thankfully modern engines have very stiff valvetrains.
Paul Hollingworth

Just noticed an error in my second post. Should read The timing period is quoted above 0.004" valve lift so 285 deg.
Got my decimal point in the wrong place, damn imperial units.
Paul Hollingworth

Nightmares are good Paul ,they keep your blood flowing
You'd have a big one if I showed you what goes in my Chevs- I'm not blurting the specs out here but just roughly they're Roller, 320 advertised duration (@.003"valve opening)and
.690" valve lift
-295 duration @ .050"
The lobes are nearly square on top,
up, wait,wait,wait then down
This is where I have issues picking a road cam, I'm brainwashed into these big boys and get carried away
Comes from being a horsepower nerd
William Revit

This thread was discussed between 06/06/2020 and 08/06/2020

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