What kind of things happen when you advance the cam. I think we all know that advancing it will improve low end torque. But what about things like......vacuum, spark timing, a/f ratio, engine temp? Also, how far does one advance it normally? let's hear some experiences.
TJ

 

Years ago I bouught an item called the Ed Hamburger adjustable cam timing gear. It allowed you to change the cam timing without pulling the cover. You accessed it though 2 holes in the timing chain cover and their special tool. It had limits in either direction to the amount of advance/retard. I once advanced it to the point where my cammed L48 pulled like a big block off idle but wouldn't hit 5000 if you held it down all day. I also went the other way where it sounded like a dragster idling. It took about 10 minutes to change settings at the track which was a great tuning tool. I liked to check the position afterwords with a degree wheel but that was a pain on an assembled/installed engine. Personally I found a few degrees made a big difference in the engine's personality. I wish somebody still made those.

 

So what do you think, 2...4...6 degrees? My cam is installed straight-up now. The motor will run-up to 6500. How many degrees till RPM's start to drop off?
TJ

 

I would like to know more.... I just installed new timing chain/gear and it has 4 deg adv/retard in it. Right now it's factory location I used, but if I were to advance it, how much of a gain are we talking about? My cam is supposed to quit after 5500 rpm anyway. I am looking for more bottom end to make up for my stall converter and 3.08.

 

Be careful here. Many...actually most are ground with 4* or so built in. Many cams that have say a 110 LSA, actually are ground so that the Intake centerline is at 106* BTDC and the exhaust is at 114 ABDC. A "straight up" one would have the intake centerline at 110* and the ex centerline at 110* too. Without actually degreeing them, you can get crazy. My current roller is ground "straight up" with no advance built in...the flat tappet I play with occasionlly in it has 5* built in. If I was to just stick it in with the 4* advance slot on timing gears, it would actually be 9* advanced. You really have to watch the timing cards.

All that said, the common # is to move it in 4* increments. 2* usually doesn't do much either way. You are generally better off with a slightly larger cam installed 4* advanced, than a smaller cam installed 4" retarded (assuming you're trying to hit similar rpm peaks). Typically 4* will shift powerband 350-500 rpm max.

Also watch Piston to valve clearance. As you advance it, the intake gets closer to piston and ex gets further away.

You are essentially making the intake lobe *act* like a smaller cam. You are opening it earlier and closing it earlier. That gets it open further as the piston starts down and closed earlier trapping more mixture in cylinder. Cranking pressure will go up as you advance it and that is an indication of how it makes more power..be careful about detonation if you are already close. If you advance too far, as noted, you will end up with valve closing way too early andnever getting cylinder filled at high rpm. That's where the idea of a "bigger" cam that is advanced comes in. It's big enough to hold valves open a long time for power, and then advancing it *tricks* it into acting a little smaller.

You are also opening ex valve earlier (less pressure pushing piston down...don't want it open too early).

As you can see, you really need to think out what you're trying to accomplish and make sure of the cam you are dealing with.


JIM

 

Youy need to pay close attention to the cam card/specs. Another thing is I always check everything with a degree wheel before taking for granted the keyways and cam are in harmony with each other.

 

I was contemplating a cam advance myself,and was playing around with simulater software the other night. I found with no other changes but a 4 degree advance,the ET would pick up almost .2 seconds. Trap speed is almost the same,as is the estimated horsepower.The shift points are 300 rpms lower with the advance also.Since I'm not into drag racing,and went with a solid cam for more top end in the first place,I think I'll go with the straight up cam timing.That'll let it rev a bit more on the top if I want it to.