Ive heard that you can lower or raise the dynamic compression of a motor by tuning the cams.. Im trying to get an understanding on how this would work.. Im thinking by advancing an intake cam, you are starting the process of combustion sooner so you would also have to advance the ignition timing.. then the total amount of combustion that goes on would also increase... Can someone please explain this? You should get more power but is this a safe thing to do?


[Modified by DIRep972, 3:38 AM 1/16/2002]

 

Cam gear tuning affects dynamic compression because it changes the amount of time that air/fuel can fill the cylinder during each combustion cycle. Advancing the intake opens the valve sooner, but that also means that it will close sooner. The same with the exhaust side. The complex part is overlap when both are open... which also directly affects dynamic compression... since the mix can leave the exhaust ports before being compressed and burned.

Note that this affects both the intake/compression and power/exhaust strokes.

The fact that the distributor is connected to the intake cam is just coincidence, and adjusting your cam timing the same amount you adjust your ignition timing is surely not what you want to do... which is why you can physically adjust your timing by rotating the distributor. Igniting fuel earlier does allow for more power to be extracted from each cycle (in general), but go too far and you get max cylinder pressure too early while the piston is still traveling up. This is known as detonation... and it will hammer your ring lands and your head gasket.. The cyl is at max cyl pressure but the piston is not able to use it ... it is still going up! This also hammers your main bearings... You want peak cyl pressure about 12-14deg ATDC... But fuel mix burns at different speeds when it is pressurized... thus the need for ignition retard under boost. If there was no retard, the fuel would burn faster so peak cyl pressure would occur earlier... and detonation follows. Max cyl pressure before 12deg ATDC will just damage the motor... so practice safe timing curve selection.

Dustin

 

I'm going to name my kid after you, yes, yes I will.

 

Ahhh, I see.. so when tuning the cams to make more power, you dont have to do anything to the ignition.. Is tuning the cams to max power bad for your motor in anyway? or is just making it more efficeint

 

You must Tune as a System....

Accomplise your Fuel Tuning, then your Cam Gear tuning...then fine tuen your overall Spark Timing and you will have tuned as a system...All things must work together to achieve the most powere, reliably.

Suprdave

 

dustin
your my hero.
that was worthy of websters.

 

It's important to know that although it does affect dynamic compression, it does not affect your air/fuel ratio. Common sense to some of us, but worth pointing out.

 

When you do adjust the intake cam gear, remember that the ignition timing is affected because the distributor is attached to it. So remember to retard the ignition timing if you advance the intake cam.

As far as tuning the motor for max power.. It's hard to say whether tuning the cam gears to MAX power is necessarily bad for your motor or not. I'd proly say no. Ignition timing is more critical. If you set the timing where it makes max hp, there may be no detonation but the burn may happen too soon and increase cylinder pressures dramatically, with only a gain of a few hp.

-Mike

 

politely I wish to disagree with this statement, eg keeping Ex side open while intake opens will create scavenging effect if this is done to long partial air fuel mixture will be lost through exhaust, too much overlap on turbo cams will cause massive lean and backfires, overlap must de dialed out to 90% degree in order to maintain proper A/f otherwise who knows waht your )2 is really reading raw unburnt fuel that was kicked out of the cylinder before combustion or a real rich condition.