Lotus Elan

Mismatched Camshafts

PostPost by: type26owner » Wed Sep 29, 2004 8:09 pm

Kind of in a pickle at the moment and lack the practical experience on how to proceed. Bought a single replacement camshaft awhile ago to replace the original intake 104hp one which has a flattened lobe on #2 cylinder. Have removed the head to replace a head gasket which sprung a water leak into #3 and only now have compared the camshaft profiles for the first time to find they are different. Yikes, hadn't counted on that! Old cam has a base lobe diameter of 1.2" with .35" of lift and the lobe radius measures .125". New cam has a base lobe diameter of 1.14" with .35" of lift and the lobe radius measures .281". Both have no markings on the sprocket flange just a casting number. Can I run this new cam without creating a problem? Can't remember the actual piston type but it has a slightly raised dome only about .06" high and it has valve clearance pockets. How much valve to piston clearance is there typically and is this something that has to be checked? Have gotten advice from two experts which contradicted one another on whether it's okay to mismatch the cams or not. I suspect the new cam is a 'sprint type'. My car is an S2 with the stock 4 into 1 cast iron exhaust manifold. Did the sprint spec engine use the same type of exhaust by any chance?

Got to stay with a .35" lift cam so there's room to fit the teflon positive type valve stem seals. My engine billows out clouds of blue smoke without them.
-Keith
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PostPost by: type26owner » Thu Sep 30, 2004 12:48 am

Just did a 2D cad profile of the two lobes. The old cam has a 300 degree duration. The new one is closer to 285. The opening and closing rate of the new cam is definitely more aggressive. Ring any bells?
-Keith
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PostPost by: rgh0 » Thu Sep 30, 2004 12:53 am

Keith

I am not sure what sort of cam you have there. The sprint cam had a .360 lift so its not that. With its 1.14 base circle which is less than the standard 1.2 it is obviously a regrind. The question is what profile has been ground onto it ?? I presume by "lobe radius of .281" for the new cam you are measuring the radius of the tip of the cam lobe. The bigger radius compared to the standard cam suggests a longer duration cam. There are many profiles out there with longer duration and the standard .35 lift. Most of them are worthless as you loose torque and dont gain anything significant in the way of power if you run a 6500 rpm rev limit. The twin cam needs lift, it does not need much in added duration.

I would take it to a cam place and get its profile properly checked to positively identify it. If its timing is different in a big way from the standard cam then you need to decide if you want to go down that path or not.

With any rebuild with new cams, valves or pistons and despite doing all the measuring and analysis of the assembly to determine if they would fit. I would ALWAYS do a dummy assembly and check valve to valve and valve to piston clearances. Its too expensive if it goes wrong to trust what the supposed experts say ! However I would not expect any problems with a .35 lift cam of any reasonable duration when matched to standard valves and and what sounds like a reasonably standard piston, provided the piston deck height is still standard.

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PostPost by: rgh0 » Thu Sep 30, 2004 1:38 am

Keith

Your old cam sounds like a L2 profile which was a common profile 30 years ago.

Kent cams quote this profile as .35 lift and 302 degrees duration at .010" checking height. The new cam does not match anything I have listed.

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PostPost by: type26owner » Thu Sep 30, 2004 1:55 am

Rohan,
I was recalling the dimensions which I measured last Sunday. Remeasured the lift of the new cam again and it's closer to .36". I just rounded down before. Besides the potential for valve to piston clashing is there any reason this cam will cause a problem. I realize the any potential increase in power will probably be mullified by the exhaust cam and the restrictive stock exhaust system. For instance if the engine will not idle that would qualify as being a bad trait. Thanks for the input!

Yup, the radius on the sharp end of the lobe is 1/8" versus 9/32". It's like a night and day difference.
-Keith
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PostPost by: rgh0 » Thu Sep 30, 2004 2:24 am

Keith

Still cant find any good match for the new cam at .36 lift. Sprint cam timing was 270 degrees so 285 degrees is bit beyond that. I see no reason why it would not run and be Ok in day to day driving if your current .35 lift 300 degree cams were OK. The shorter overall duation and more aggressive cam acceleration rates should if anything make the cam more tractable than your old one.

Measuirng duration of cams is not easy without specialised equipment. The advertised durations are typically measured at .010" lift above the base circle at which point the cam has taken up the follower clearance and you are quoting duration of actual valve opening from seat to seat. Measuring duration at .050 valve lift ( .060 cam lift) is another common quoted point and is even harder to measure as the contact point between the cam and the follower is well off to the side of the vertical from the follower surface to the cam centreline at this lift.

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PostPost by: steveww » Thu Sep 30, 2004 1:09 pm

Can't help you to identify the cam but if it does have 300 duration then it is way too long for an every day road engine, it will loose most of its torque below 4500rpm.

I would see if you can get the pair reground to something more sensible. The standard Sprint/SE profile is very good and there is no real gain going for anything else unless you are building a racing engine.
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PostPost by: type26owner » Thu Sep 30, 2004 1:23 pm

I'm actually doing a crude approximation when rendering this profile by 2D cad. I'm measuring with dial calipers and projecting points on the surface where I think they are and connecting the dots. Best I can do with the cams loose in hand so that the duration values are most accurate relative to one another more rather than as absolute degrees. I understand the finer points and limitations of camshaft metrology though. Without the knowing the exact measurement technique the duration data is virtually useless.

Really only swapping out the old damaged cam because it makes an annoying clacking noise. My wife asked me recently when stopped at a traffic light if we were going to make it home after she noticed it too. It's been getting slowly louder. Think I'll run the old cam out and have it reground just in case. Perhaps the new one will be noisier so I'll still have to the listen to it clanging about anyway? :unsure:
-Keith
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PostPost by: rgh0 » Thu Sep 30, 2004 11:49 pm

Keith

If you have not done so you need to check your cam followers for wear also as the clacking may be coming from one of them. Also if replacing the cams you should either fit new followers or at least have the followers checked and the top surface reground if required.

The worn lobes on your existing cam can be rebuilt with hard face plasma spray welding. In Australia it costs about $50 per lobe and I have done it successfully a number of times to get racing cam profiles out of old standard cams. The hard face steel overlay is more prone to galling on initial startup especially if running against the newer steel followers rather than the original cast iron followers. You need to aim for a 10 Rc hardness difference between the cam and follower and its a good idea to phosphate treat both the cam and followers to provide added protection during break in, in addition to using a good moly assembly grease and synthetic lube oil.


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PostPost by: twincamracing » Fri Oct 01, 2004 3:28 am

While the cam is out you can lay it in a pair of V blocks. Using a dial indicator to follow the lobe and a degree wheel on the cam to derive the correct timing values in crankshaft degrees.

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PostPost by: type26owner » Fri Oct 01, 2004 4:25 am

Rohan,
I'm running up my parts bill again on this rebuild. Eight new buckets and a full set of cam bearings are already purchased. I talked to Rich Kamp and got his cam grinder's name and phone number the other day.

Scott ,
That does not replicate the actual angular duration of the full diameter bucket running against the camlobe. Not obvious at all though so please don't be offended. Best practice is to measure these type of dynamic systems in place to reduce these type of induced measurement errors. The spherical tip of the indicator does not contact the camlobe in the same way as the planar surface of the follower bucket. Even if you had a flat ended tip that replicated the bucket on a the linear motion type dial indicator any skewing misalignment out of plane to the camlobe's rotational plane will also give inaccurate measurements.
-Keith
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PostPost by: steveww » Fri Oct 01, 2004 9:33 am

If the followers are not badly worn they can be "buffed up" for a new cam with some fine wet'n'dry and WD40. Place the wet'n'dry on a flat surface, a bit of glass is a good flat surface, using a good quantity of WD40 rub the follower on the wet'n'dry in a circular motion, changing your grip regularly.
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PostPost by: type26owner » Fri Oct 01, 2004 1:36 pm

Steve,
I've got a tool post grinder that mounts onto the lathe for doing those types of repairs. Just contemplating hand working the tops of the buckets is giving me a carpel tunnel anxiety attack! :(
-Keith
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PostPost by: steveww » Fri Oct 01, 2004 4:37 pm

Of course if you just happen to have a full machine shop in your garage then there are better ways of doing this ;)

The wet'n'dry method only really works for light wear not when they are pitted.
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PostPost by: types26/36 » Fri Oct 01, 2004 5:50 pm

But as far as I understand it the camfollower (bucket) should have a slight raised raius on it and flatting with wet & dry on a flat surface could not produce this, it would actually destroy the radius.
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