Cam Regrinds

[Coonper 0]

How can they regrind a cam and get a higher lobe to hold the valves open for longer??

 

Surely they can only take metal off the cam, and not extend the lobes?? I dont get it!

 

And is a re profiled cam as good as, say, a catcam or kent?

[brianthemagical 1]

They can remove some metal from the base circle, there fore giving a higher lift.

[Coonper 0]

They can remove some metal from the base circle, there fore giving a higher lift.

 

Cheers mate! Nice and quick and very clear! Im with you!

 

Who does it and how much do i expect to pay??

[welshpug 1,622]

PeterT does it for a very reasonable price, IIRC around £90 exchange plus carriage.

 

Details of the camshafts here - http://www.taylor-eng.com/index.html

[whizzer71 0]

Or Kent Cams (Ashford Kent) £150 for the pair,always had good results with these

 

[petert 550]

Somebody with a Kent regrind sent it over to me for an upgrade! Naturally I measured it then reground it to my Stage I. It had serious lack of duration.

 

There is a limit to hydraulic regrinds. You can only remove from the base circle the limit of the hydraulic lifter. If the lifter doesn't have enough preload it will rattle and eventually die. With solid grinds you can do what ever you want, within the limits of the lobe material available. The adjusting shim is altered to take up the clearance.

[ALEX 98 1 Cars]

I thought regrinds were to increase the duration time of the Cam and the lift isn't changed.

i.e. the cam becomes more pointy, hence lumpy idle which is not factory ideal, but the valves stay shut longer giving an increase in power!

 

If I'm wrong here, could someone clear this up for, I'm a little confused myself now?

[ALEX 98 1 Cars]

This is my understanding of a re-grind!

 

If it makes sense?

 

 

[welshpug 1,622]

Removing material there would make the duration very short, not what the aim of a regrind is!

 

this is my awful rendition of what I think a regrind is

 

[Doof 11]

^^^^^ - HAhahaha!

[ALEX 98 1 Cars]

I thought the idea was to open and close (edit: or either just Open or Close) the valve as fast as possible, The engine still turns at the same speed!

 

What you've shown looks like the lift of the cam is reduced! i.e the valves wont open as much.

In fact it looks more like a worn cam i replaced once no offence

 

I'll admit I don't know a great deal about cam and profiling, but that pic Welshpug put up doesn't make much sense to me!

Could some explain what I'm missing?

Edited April 23, 2008 by ALEX

[welshpug 1,622]

you've missed the reduced base of the circle, cams like PeterT's open the valves earlier and close later, but don't increase lift dramatically.

 

the point of uprated cams is not to open the valve faster, but for a longer duration, i.e for more of the stroke

 

open the valve too fast and you're likely to induce valve bounce and cause more stress to the engine.

[GLPoomobile 958]

Whilst I understand duration, to an extent, can someone explain lift, and how changing it affects things?

 

I'm assuming that greater lift will allow greater flow (up to a point) but I guess it probably affects flow speed (does that even make sense? ) too. Explain please.

[ALEX 98 1 Cars]

I think I understand how a regrind can increase the lift now

I had to draw it out though!

 

[welshpug 1,622]

spot on substitute shim in your 8v for a hydraulic tappet in an Mi/gti6

 

a regrind basically changes when the valve opens and closes by changing the profile of the lobe, regrinds can also change the lift as you illustrated excellently there

 

GLP thats how I understand it, there's a few flow charts on PeterT's site, though I think it'd be best for Pete to try and explain it

[ALEX 98 1 Cars]

Glad you liked the diagram, I was going to show the spring and add some colour but I didn't have the time!

[ALEX 98 1 Cars]

One last question, is this correct showing the increased duration from a regrind?

 

[welshpug 1,622]

not quite, the lines should be over the nose of the lobe, i.e the bit that opens the valve.

[ALEX 98 1 Cars]

So where's the duration measured from?

I thought if you had a 360° cam duration, It would just be a round bar, and obviously not work!

Edited April 23, 2008 by ALEX

[welshpug 1,622]

[ALEX 98 1 Cars]

Well I think your right that duration is measured over the time the valve is open, not the other way round like I thought.

But I still don't understand where you got the angle from, must be just because I'm thick as pudding!

Edited April 23, 2008 by ALEX

[petert 550]

Great drawings!

 

The duration angle is measured in crankshaft degrees. For example for a 30/70 grind, the inlet valve would open 30 degrees before TDC and close 70 deg. after BDC. The total duration would be 30+70+180 = 280 degrees. The lobe centre line, or the maximum opening point, would occur at 110 deg. after TDC. ie 280/2 - 30.

 

Without getting too technical, advertised duration, like 30/70 or 280deg. is rather flimsy, as it doesn't specify at what valve lift it was measured. This is why cam companies should quote a duration at 1mm, or better still, 0.050" lift. It makes comparison of camshafts more accurate.

[ALEX 98 1 Cars]

Thanks Perter

 

So the crankshaft angle is measured to give the cam duration figure!

 

It makes sense I suppose as the crank rotates twice as much as the cam/s hence the 280° duration angle I couldn't measure on the cam!

 

So how would you measure the duration of a cam if you don't mind me asking?

[petert 550]

Either put it in a head with a bucket/spring/valve, or set up a test tappet geometrically the same as a bucket.

[ALEX 98 1 Cars]

So just to clarify, (sorry about this, I should really check the web)

 

If you had the set up as mentioned above, head built up on a bench, you could work out the cam duration by rotating the camshft to the point the valve starts to open, to when it fully closed, then measure the angle it rotated by then multiply it by 2 to give the crankshaft rotation i.e. Cam duration.