cam timing and compression and ign timing
 

i was on another board and someone said they changed the cam timing, dont know if it was advanced or retarded, but their compression got better. they also wanted to know about ignition timing. my question, has anyone here ever checked compression vs cam timing? would cam timing even effect compression? if it did, i would think advancing the cams might raise the compression. would this be why advancing the cam increases bottom end? i am talking compression (psi), not comp ratio.

would ignition timing follow the cam timing or would it still be set the same?

compression ratio, like 10:1, is measured by the volume of the cylinder with the piston from bdc to tdc. cylinder pressure, in psi, is measured with the valves closed and the piston at tdc (max psi).
don't know why these numbers would change by advancing the cam timing. Unless a valve starts to open prior to tdc. that would not be good for power.
Advancing the cam effects WHEN the valves open and close. several degrees advanced start opening the intake valve sooner and retard the opening of the exhaust valve adding a little more air/fuel into the cylinders. But the exhaust valve will open at the same normal time just a few degrees sooner.
at least that's how I figure it out. It will help but not like getting bigger cams with bigger overlap. timing will have to be adjusted also. Check on the engine site many more smarter folks there.

As I understand it, the compression is affected by the cam timing at bottom dead centre as the compression stroke begins. If the inlet valve is still open then, at low rpm, the flow into the cylinder can reverse and compression will be lost. In effect, the compression does not begin until the valve is closed so the later it closes, the more compression is lost. In the Miller cycle engine, this is taken to the extreme to reduce power loss during the compression stroke and the overall loss in peak compression is compensated by forced induction (turbo).

yes, so advancing the cam, it opens sooner to let air in sooner, it closes sooner to begin the comp sooner. i can see this at lower RPM's, but what happens at higher RPM's? does the power drop off sooner?
someone else said he did a test and found advancing tha cam raised the comp numbers (psi).

Im no pro by any means...but I just got finished setting the cam timing on my SC. I set it to 1mm as per Waynes book. According to what I have researched, this is the most "retarded" setting and gives the best performance at the upper end of the RPM range. I had a few sleepless nights over this. I believe that compression can be affected by cam timing. Here is my novice, layman, inexperienced, and probably not quite 100% take on it ;)

The deeper the valve is in the chamber at the begining of the compression stroke, the sooner it will close. Instinctively, this makes me believe there would be more compression. Alternately, a less advanced setting would have the valve not quite as far into the chamber, at the begining of the compression stroke, would close later and not have as much piston travel to generate compression compared to the advanced setting.

hopefully im not too far off with this theory!

Ok. basic chemistry. PV=nRT so P=nRT/V. R is constant(gas constant), T lets assume the intake temperature is constant, V is constant(volume of cylinder is not changing) so the only why to increase pressure is if n-or mass of air fuel mixture increses.(I know n is moles but moles is a mass measurement) So if changing cam timing increases the amount(more mass) of air fuel mixture then PSI will increase. But notice that means the pressure is more at all stages of the compression stroke ie at BDC also, because the compression ratio is not changed at all - as has already been explained. The higher amount of fuel/air is what would give you an increase in power not the psi increase which is a result of more fuel. How much you can increase the fuel/air amount by changing cam timing depends I think alot on cams, port sizes, intake sizes etc. I don't think messing with cam timing is worth it. If you go too far then piston hits valve. Not good.

This is my longest post in 9 years. I must be really bored at work.

As rpm rises, so do gas velocities through the ports and the pulses through the runners get closer together. This introduces effects from inertia of the gases and resonance in the intake and exhaust runners. As I understand it, these effects, together, allow the intake valve to remain open after bdc on the compression stroke without the flow reversing. By advancing the cam timing, I would expect the compression (psi) to improve at low rpm (such as cranking speed when you are measuring) at the expense of high rpm power. This is because an advanced cam fails to take full advantage of the residual intake flow after bdc that is available at higher rpm.
This answer ignores the additional effects caused by the advanced exhaust cam but that doesn't appear to be the question here.

Yes, the timing of the closing of the intake valve effects compression pressure. There are two different compression ratios, static and dynamic. The static is based on a full cylinder and ignores valve timing, the dynamic is based on the stroke after the intake valve is closed. The timing of the intake valve closing is the most important thing in determining the power band of an engine. If you delay the closing of the intake valve, you can increase the static compression ratio at the same octane requirement. Ignition timing is a different issue, the ignition is timed to produce peak cylinder pressure at 14 degrees ATDC regardless. If the engine is knock limited, delaying the closing of the intake valve may allow more timing for a given octane, but only because it was retarded.

I think you are talking about 2 different things.

Compression ratio (as you know) is the ratio of the volume of the cylinder when it is at the bottom of the stroke, to the volume at the top of the stroke. This is the static compression ratio.

The problem is that the intake valve is not closed when the piston starts the compression stroke, so pressure can escape back out the valve. This is Dynamic compression. As the intake duration of the cam increases, the intake valve closes later in the compression stroke, making less of the stroke available to compress the charge.

A compression tester does not measure static compression, it measures dynamic compression, which can be changed by advancing or retarding the cam timing.

If you advance the cam timing the intake valve will close a little earlier in the compression stroke, raising the dynamic compression ratio slightly, which should be observable on a compression tester.

This is an interesting topic!

We did some experimenting with our 2.7 9.5/1 weber 40's running DC-40 cams.

The stock overlap was 4.5mm and static compression, cranking it over with open carbs was about 80lbs. We wanted a little snapper engine so the first thought was to get the compression up a bit by setting the overlap down to 4.0 which gave us about 100# compression. Well the engine was even more sluggish and even more at higher RPM where is really started to peter out. On the advise of Camgrinder we set the cam overlap at 4.75mm and he even heard of others going up to 5.0mm and with it set at 4.75mm it made a huge difference, much snappier, never ran out of top end. He explained, its all how the engine breaths!

Now I wanted the same results for my 3.4 10.5/1 running super cup cams which are now set at 2.0mm his advice was to drop them to 1.6 to 1.8mm which we are in the process of doing.

Hopefully someone can explain why, all I know it works and can make a huge difference.

craig, im a little confused. if you go from 4.5mm to 4mm, isnt that retarding the cam, which should give better high end?

while you are in there, how much effect can valve adjustment have on cam timing? if a mm or so of cam timing seems to have a noticable effect, what effect would 001 (.005, or .003 setting) valve adjustment have on cam timing? my point or question here is, can a good valve adjustment (everyone set the same) be noticable over one that is a little sloppy? (this is kinda a spinoff on the other thread about temp and valve adjustments)

If I remember correctly the common thought on this was to retard cam timing to increase torque at lower RPM, but there is a trade-off at high RPM.

Reducing valve clearance from 0.004" increases the risk of burning valves, mostly the exhaust valves. :(

I am still learning but this is a great answer.

If we want the peak pressure to be at 14 deg ATDC to get the most power we have to time the spark and the intake and exhaust events in a way to make this happen.

On our motors we can move the spark with changes in RPM but we can not actively move the cam timing.

We can move where a given cam will operate most efficiently a little by advancing or retarding a given cam.

We can move the timing of intake an exhaust events a lot by how the cam its self is designed.


As to spark, at low rpm we have more time to build pressure so we start the ignition closer to TDC to get our peak pressure at 14 deg. At higher rpm we need to spark earlier.

One top of that the more air and fuel you pack into the cylinder the faster the burn so we have to start the spark later at WOT than at cruse to achieve peak pressure where we can use it best.

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Cam timing is the same thing to a degree. However on our motors we can not vary the timing of the cams with rpms. Thus, we much pick one point where we want our motors to work at there most efficient point.

That point is where TQ peaks.

Most motors of a given size and design are going to make about the same peak TQ number but by moving it to a higher rpm level gets us more HP. By keeping it at a lower rpm we have a more drivable car.

That is, if we make about 190 ft lbs at 5500rpm on one motor and 190 ft lbs at 6500 on another motor, the HP goes from 200 to 236hp.

It takes a lot of changes in cam timing to move where the cam works its best from 5.5 to 6.5k. However, we can move it a bit up or down by advancing or retarding the cam.

I always heard the opposite.

However, my 964 cams are timed at 1.4mm (1.26 is the "norm" so mine are advanced) and I have noticed that my power isn't dropping at high rmps nearing redline. It honestly feels like it want to keep going and going. Doesn't run out of breath.

Yet I haven't noticed any change in low rpm power.

Anyhow, just thought I would share some off topic, non-scientific data.

To summarize, advancing the cam timing increases the effective power at lower rpms at the expense of high rpm power. Retarding the cam timing increases the effective power at higher rpms........

It's a compromise. You might notice that PAG specified slightly different cam timing settings for the same camshaft on different MY engines.

Sherwood

I think there must be different factors involved to determine cam-timing.

SC-engines have essentially the same engine and cam and yet, the '78-'79 non-Lambda overlap is 0.9 - 1.1 mm but the '80-'83 with Lambda is 1.4 - 1.7 mm.

I checked various opinions from credible people on the issue of losing/gaining power with advance/retard settings and it ranged from:" not noticeable" to "very little" if any for street-applications.

For my SS 3.2 CR 9.8 with 964 cams non-Lambda CIS, I timed the cams 1.4 mm and like mca says, the engine really comes to life after 4k and loves to rev.
1.4 mm for my set-up was recommended by a couple of experienced engine-builders and I am happy that I followed their advise.

I would recommend this overlap for the 964 profile to anyone building exactly what I have but, if there is any difference in fuel management, CR, etc., I wouldn't dare to recommend anything.

There was a Carrera 3.2 club racer here some where that did dyno tests of the stock cam set for stock v for more low end. He saw about a 3% improvement in HP earler when he did.

The state of ones intake and exhaust will have some effect. Time the cams for high end on a small port CIS car with poor exhaust and little will probably be had. Time them for more low end and there might see some improvement.

Again, one will probably only be able to move there TQ peak up or down about 200+/- RPM by retiming a given cam.

A lot more can be done by choosing a cam that is timed differently (over lap, lift, and duration).

The stock cam is actually a pretty good cam. A 3.2 with webbers and race exhaust can make 270hp. There are stock cam blueprinted 3.0SC's with custom intake EFI making in the 260's fwhp.

There is probably more to be had from other areas like getting the air in an out better, getting the AFR & timing right.

Unless one goes to a very high rpm cam (that will need close ratio gears) changing the cam is only going to be good for maybe 10hp.

Just my opinion.

Very timely for me. I'm getting ready to tear my engine down to the case and put on my new Mahle 3.4L/98MM 10.5:1 compression ratio pistons and cylinders. :D

I have Dougherty DC19's already in, and I'm going to talk to John about where to set in his recommended 1.8-2mm recommendation for the best top end power. I have to look at my notes, but I believe I split the difference and went 1.9 when I built the engine 2 years ago. I want the top end power over the low range. The relatively small window may be somewhat insignificant compared to other grinds with larger tolerances.

Very nice. :)

3.4L/98MM P/C's sounds like a "long-stroke".
Are you using a Carrera 3.2 engine with a 74.4 mm crank?
10.5:1 CR meaning twin-plug?