Cam Regrinds

[petert 550]

Yes, provided you have a degree wheel on the cam and a dial indicator on the bucket. Naturally it's harder to measure the angles as accuracy is decreased.

[ALEX 98 1 Cars]

Cool thanks for you help

[RalliArt 0]

So, I'm looking in this page http://www.taylor-eng.com/xu9j4/16v_overview.html and I'm thinking is there somewhere the same info about 8V engines.....

P.S. the cam timing I'm talking about

Edited June 27, 2008 by RalliArt

[kyepan 291]

was chatting with dad about this last night, specifically relating to the need for different pully's or verniers to advance or retard the cam and have a question.

 

Let me get this straight - and please correct if not..

 

The reason for changing the timing of the opening and closing of the cam in relation to the tdc or bdc of the piston when you install a longer duration cam shaft is because..

 

The cam opens the valve earlier and closes later, you want to add an even ammount of duration onto either end of the cycle. Without adjusting the timing of the cam it may open at the standard time but close much too late, or open too early and close at normal time.. so keeping the central point even is the key to this..

 

or is it slightly more muddy than that, and opening and closing at different times affects different parts of the rev range?

 

also are cams always centered in the same place, or does it vary, and if it's a regrind, can the peak point / max lift point move, therefore requiring a different cam timing / pully to be used to time in the maximum valve lift to the center of the cycle?

 

is there a book on this?

Edited June 27, 2008 by kyepan

[welshpug 1,622]

the way I understand it you're right on both counts

 

as in with your second point, its illustrated quite well with the different inlet cam positions on the 1.9 MI16's where engines with a certain cam pulley/cam lobe centreline seem to have more at the top of the rev range than the others, but not the "shove" midrange".

[kyepan 291]

Looking a bit further at the timing points, the first one we see on the diagram is the exhaust opening point. We have all noticed the different sounds of performance cams, with the distinct lopes or rough idle. This occurs when the exhaust valve opens earlier and lets the sound of combustion go out into the exhaust pipes. It may actually still be burning a little when it passes out of the engine, so this can be a very pronounced sound.

 

The next point on the graph is the intake opening. This begins the overlap phase, which is very critical to vacuum, throttle response, emissions and especially, gas mileage. The amount of overlap, or the area between the intake opening and the exhaust closing, and where it occurs, is one of the most critical points in the engine cycle. If the intake valve opens too early, it will push the new charge into the intake manifold. If it occurs too late, it will lean out the cylinder and greatly hinder the performance of the engine. If the exhaust valve closes too early it will trap some of the spent gases in the combustion chamber, and if it closes too late it will over-scavenge the chamber; taking out too much of the charge, again creating an artificially lean condition. If the overlap phase occurs too early, it will create an overly rich condition in the exhaust port, severely hurting the gas mileage. So, as you can see, everything about overlap is critical to the performance of the engine.

#

from http://www.compcams.com/technical/TimingTutorial/

 

so changing the inlet cam probably won't change the sound of the engine, because the exaust cam is not affected and therefore combustion noise probably won't escape out of the exhaust.

 

when he's talking about the inlet opening too early the piston is still emptying through the exaust vavle, and so the exaust valve opens too early air will get forced up the inlet manifold. if it's opening too late how does that "lean it out" lean out the mixuture? or just hinder complete cylinder filling? wouldn't imagine it could change the air fuel ratio, so it must mean that the cylinder does not time to fully charge.

[benlilly 0]

No expert but I think there are two issues with the inlet opening too late

1. Valve may still be closed when the piston begins it's downward stroke so filling is less effective

2. If it opens late, it will also close late so may pump air/fuel mixture back out of the inlet

 

There is some good stuff on cams here Guy Croft - Camshafts

[iamjackiechan 0]

hi

i think the second picture is what a re-grind would look like allowing the valves to open for longer and therefore allowing more fuel to flow.

[welshpug 1,622]

just remembered about a great picture I saw a while ago;

 

left IIRC is a PeterT spec stage 1 gti6 inlet cam, right is the standard inlet.

 

as you can see the standard item is quite pointy meaning fairly high lift but sod all duration, flatten the nose and remove material from the opposite side to give the same lift but more duration.