what are the advantages and disadvantages of cam reprofiling on a GT cam?

also what reprofiling would you agree would be best and is there any more work need than just reprofiling the cam

cheers jack

when you say reprofiling you mean having the back of the lobe ground to a smaller diameter so the lift is greater?

to a certain degree your better off with a billet camshaft ground to certain specifcations rather than a reprofiled oem cam, you can gain about 1mm of lift with a reprofiled cam but sod all increase in duration. i have a reprofiled cam in my new (untested) engine as it was done for free as a favour, i also had the valve seats reprofiled and cut into in the head by the same amount as the reprofiling, then the head was skimmed to keep the CR to a maximum. long story short, its a pain in the ass. newman do cams for polos from 150quid, they are a reputable firm and the cams are a known quantity, reprofiling isnt a cost effective way of doing it unless you have the connections.

re-profiling refers to increasing lift/duration on an OE cam by adding material to the lobes and grinding it to the desired shape. an easy process with the right tools.

without the right tools, near impossible.

i spoke to the guy from kent cams, he mentioned nothing about adding material, only grinding out the base circle. like in this picture

the black outer line is the original cam, the red one is the reground profile, the lift is calculated by the difference between A and B, if B get smaller it increases the lift.

the machines they use are extremely expensive CNC machines. not something found in the average workshop.

you can alter duration by machining the lobe to be flatter and then machining more off the back circle as well. But you are very limited to what you can achieve depending on the profile of the original cam being reground. I don't know what you mean by "add" material, how would that be done?

Since off the shelf cams for polos are readily available for cheap prices, just buy one of them. You probably wont save much (if anything) by regrinding the original and the end result will be a compromise. Unless you know a friendly machine shop nearby who has the equipment to do it and will do it for you on the cheap.

I'm just trying to work out how by re-grinding the back of the lobe will give higher lift...

The top of the cam stays in the same place and the bottom gets smaller so increasing the lift. Whatever you take off the diameter adds half to the lift. I.e. 1mm grind adds 0.5mm to the lift. Decent cams are made from chillcast blanks. You don't add material.

ah my bad.

surely changing the cams bearing diameter will occur issues with the bearing faces on the head and bearing caps as the diameter has changed and it won't seat properly.

the bearing faces are left alone, the lobes are skimmed. its fit a treat, you could take the risk and hope the tappet takes up the extra distance or do as i did and has a 3 angle valve seat cut into the head meaning the valve sits closer to the centre of the camshaft, and then skimming away as much as possible. if i wasnt getting this done extremely cheaply i'd have bought a newman. its alot of hassle and head scratching, especially when it comes to decking the pistons and block to raise the CR, then cutting pockets in the pistons to the correct depth. vernier pulley will sort the 75thou reduction in distance between the centre of the crank and cam.

what are the advantages and disadvantages of cam reprofiling on a GT cam?

Advantages: Higher lift, maybe increased duration.

Disadvantages: Cost, other profile cams are avaialble (i.e newman), finding a good machine shop to do the work, other engine machining may be needed, more valve train wear.

Also what reprofiling would you agree would be best and is there any more work need than just reprofiling the cam

I don't know what reprofiling would be best you'd have to speak to the machine shop about that, but the options would be quite limited.

Other work may include; cutting 3 angle valve seats, raising the CR, cutting pockets in pistons, adjusting tappets (easy with solid or bucket tappets, not so easy with hydraulic lifters)etc (all as polopete had to do).

A lot of work.

I'd just get a cam off the shelf.