Advancing/retarding cam: idle quality effects?

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The Artist formerly known as Turbo84
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I'm curious what happens if I change the cam timing. IIRC, retarding the cam is supposed to help upper RPM breathing a touch. I've got more bottom end power than my tires can normally handle, so it would be nice to move a bit of it up the RPM range some. But I'm curious, what does it do to the idle quality? I'm just going through the mental picture of what's all going on (regarding valve and piston positions) at the moment, but I haven't worked through all the issues yet. I like a fast car, but I'm just growing tired of any modifications that makes the car a PITA to drive on the street.

Any opinions or experiences to share?
 
If you have a solid cam you can experiment. If the stock lash is .020 increase it to .030 and then vice versa. A .010 change in lash is several degrees of advance or retard. In my experience one way or the other is not going to behave the same way for every carb and distributor so I just try it both ways. You probably should not run the car long with too little valve lash or you could burn a valve. Use a manifold vacuum gage to see the difference on the signal to the carb and will need to go around and adjust the idle speed and mix screws with each setting.

Hydraulics you can play too by changing the lash but I am not expert on hydraulic cams and I think the hyd lifters can compensate across a wider range.
 
It's an hydraulic cam. Big block, so uber RPM is not a big requirement.

As I think about it, I'm conflicted about what the idle quality impact is going to be. The intake valve is going to be close a touch later, meaning more of the intake air will be pushed back out the valve, crapping out the idle quality. However, the exhaust valve will close later meaning more of the exhaust is removed out of the cylinder, so hopefully there would be a reduced "puff" of residual exhaust gas burping into the intake manifold. This should improve the idle quality.
So, which action will override the other?
 
I think (not know) that changing the valve timing slightly by reducing the valve lash changes the pumping characteristics of the engine. When the intake valve is open for less time (degrees) the engine works against the closed valve or closing valve and creates more vacuum in the intake runner. More vacuum makes for a stronger signal to the carb and distributor which makes for a more stable idle and bottom end. I have changed valve lash .005 on a BBC and picked up almost an inch of idle vacuum. With more idle vacuum the engine behaves like it has a smaller cam, shakes less and the pbrakes work better. You can always back the nuts off a half turn to see if you like it!
 
Changing the lash on a hydraulic cam won't change lift or duration, and won't produce any results, unless you back the nuts off so far that you create actual lash on the hydralic cam. But this is not recommended, because you can push the plunger retainer out of the lifter body and cause severe engine damage.

Changing the lash on a solid cam changes duration slightly, but won't simulate a change in lobe center - rather, it can tell you if your engine wants more or less duration. It won't tell you if your engine wants more or less cam advance.

I've played a bit with cam timing. Generally I've found that advancing the cam 2 to 4 degrees will improve manifold vacuum, smooth out the idle, and pick up a little low and mid range torque. Retarding the cam timing has the opposite effect, but can shift the torque curve slightly up the rpm scale, this allowing you to produce a little more top end power. But you have to be careful with advancing the cam, since many cam manufacturers are now grinding their cams with a 2 or 4 degree advance already in the cam, so if you install it "straight up, you already have 4 degrees advance. Putting an additional 4 degrees advance on a cam like this will often produce a power loss across the rpm range, whereas a slight retard can produce measurable high-rpm gains.

Lars
 
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Here's my personal experience. It's kind of long to explain what happened and what I was trying to achieve. This was some 20 years ago, so some of the numbers are a little fuzzy.

Okay. I use to build BB engines for several teams, all were single carb, 1/4 mile engines; all ran 8s in the 140s, except for the roadster I drove that ran 175+ with a tunnel ram.

A car with one of my engines was going to a meet. There was a meet in Michigan we wanted to attend, and I wanted to try a new Crane Inverted Flank SR. So I degreed it in according to Crane's cam card and next day we headed for the four day meet.

Engine sounded awesome, sharper that ever before and throttle response was wicked. Cranking compession jumped a LOT so I backed off the timing 2 degrees (?)

Two passes were made during Thursday time trials and the driver came back complaining that the power seemed to go flat at the first MPH stripe. Since the IFR was to critical on valve lash, I couldn 't play with that, so I ended up re-phasing the cam to retard it four degrees.

On engines running Cloyes chains, whenever I'd degree a cam, I'd always install a bushing so I could re-phase the cam at the track if needed. Of course this called for re-setting the ignition timing and a tweak to the carb idle mixture.

With that move, Skip reported power hung on till the finish line, but he wanted even more. He wanted to be able to push his competitor to break-out in case he needed to. This told me he wanted even more top end.

We kicked it around for a while and I finally convinced him that we could try getting what he wanted by going a different route.

So we upped the pill in the 7AL allowing him to leave a 7k/7.5k (?) and raised the pill in the rev limiter to 9.2k.

Anyway, we entered into an 8.90 index and he went three or four rounds before he red-lighted out. I don't recall the numbers, but after the retarding, his 60' times were down a little, but his half track times were better, the engine continued to pull through the traps and he never got ran over.

Hope some of this helps.

Jake
 
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