School me. Theory of Cam Timing 101: Advance, retard, cam lope, RPMs, torque, HP

[sugarwood]

I've always been curious what this is all about.
I did some reading today, and wanted to see if I understood correctly.
I'd also like to hear any comments in the context of a 911.
Let’s see if I got some of this right?

This video explained how varying the cam lobes will change the timing of the intake/exhaust valves,
which changes the duration of the compression and power strokes,
which ultimately impacts low RPM torque vs. high RPM HP (But, it doesn’t explain why...)

There is a trade-off of varying the duration of compression and power strokes,
and resulting impact on low RPM torque vs. high RPM HP
COMP CamsŪ - Sorry...

So, at low RPMs what does advancing the timing accomplish?
Are you trying to maximize the compression? This is what affects torque?

At high RPMs, there is a physical limitation of how fast air can travel into the combustion chamber.
So, what does retarding the timing accomplish? Keeps the intake valve open longer?
This allows more time for the air/gas mixture to travel into the cylinder at at higher RPMs?

How does the trade-off between advancing and retarding work in terms of give and take during low vs. high RPM?

Advancing the camshaft timing in relation to the crankshaft will lower the power curve range on your engine. This means that the power curve will start at a lower rpm. Retarding the cam timing, increases the power curve - the power curve will start later at a higher rpm.

Advance the Cam Timing - Route 66 Hot Rod High

In racing you would retard a cam for high RPMs, they also could breath and had no restriction in the exhaust. The power may come in at, lets say 3000 - 6500 RPMs and advancing a cam for more torque and power, that same cam may produce power at 1000- 4000 RPMs and after all, who drives over 4000 rpms on the road.

Cam timing

Effects on Cylinder Pressure: The greatest impact an advanced cam will have is to close the intake valve sooner in the compression stroke. This means that a greater volume of air and fuel gets compressed before being ignited and has the effect of creating more torque and power. However, if too much pressure builds up, extreme heat can cause pinging (detonation).

What happens when you advance camshaft timing? | Answerbag


And that brings us to cam “lope”....that funny car dragster sound. The rumble of 60's big block muscle.
It’s not clear to me if this is actually a good thing, or a symptom of an inefficiently tuned cam?

Cam “lope” is caused by the overlap when the exhaust valve is opening and the intake valve is closing. The more of an overlap, the longer and lower the sound of the “lope” will be. This is usually a result of a longer exhaust duration. .....valve overlap causes misfires at idle and that makes the lope or choppy idle. It causes a lot of intake reversion which causes the engine to basically choke on it’s own exhaust during idle. The more the overlap the more the lope will be.

Comp Cams on Cam Lope | Track Time Audio

The earlier the exhaust valve opens, the more pressure is remaining in the cylinder. This leads to the exhaust note having more of the sharp “bark” that is very pleasing to the ear. Another characteristic people associate with performance is a “lopey” idle. This is largely a function of cam overlap. Cams with a large overlap have increased exhaust reversion into the intake at idle and low rpm.

Big Cam Sound - Kennedys Dynotune

[Bob Kontak]

Originally Posted by sugarwood

So, at low RPMs what does advancing the timing accomplish? Are you trying to maximize the compression? This is what affects torque?

Just talking here and hoping it kicks off more knowledgeable input.

I think you are correct regarding maximizing compression. Maybe the term utilizing max compression at a lower RPM with the trade off being less heavy breathing at higher RPM?

This is so out of my league it's not funny. Not that I cannot conceptualize the pistons and valve train working in concert with the physical limitations of air flow and desired rpm punch, but because how the hell can a regular guy with a socket set and a multi-meter know where to dial it in?

[WideRide 86]

Interesting thread. I've been wondering how can timing affects the power curve after recently adding 964 cams. I understood that the curve would move up the rpm range a bit but the max torque seems slightly lower.

Subscribed.

[T77911S]

do a search on the internet about cam timing. dont look at the videos, there are some good sites. there even some that have cam calculators that let you enter numbers and it gives you compression.

the most important part of cam timing is the when the intake valve closes. that deteremines dynamic compression. this compression is variable. think about this. the earlier the intake closes (advanced) the higher the compression because the more time to build comrpession.
the later it closes, (retard) the less compression due to the short time til TDC. (ignition actually).
this is why you cant put a cam with a long duration, due to the late closing of the intake in a stock engine. suppose you have an 8:1 comp engine and you close the intake at 90* ABDC. that does not leave much time for a low compression engine to build up comp to make any power.
there is an idleal dynamic compression ratio that builders shoot for with cam timing so just closing the intake sooner is not the cure all.

here is one i like for playing around with timing and boost.

https://www.rbracing-rsr.com/comprAdvHD.htm

[LakeCleElum]

I did this once on a 4 cylinder 1,000 motorcycle engine. Had read a great article in the days before the internet. The key was moving each cam independent of each other and just a few degrees.

Was looking for "lobe centers" of 111 degrees on one and 114 on the other, I think? Accomplished this by slotting the holes in the cam sprocket so the sprocket could be rotated in relation to the cam.

Was time consuming work with a degree wheel and a dial indicator. The theory is that modern engines are designed with emissions in mind and a compromise between low end and high end power.

Results?....At the same time, I had added a 1075 cc piston kit that bumped compression. I already had bigger carbs and exhaust. I was very pleased with the results. It pulled so hard, I had to go with stiffer clutch springs to prevent slippage.

[sugarwood]

During the compression upstroke, as the piston starts to move towards the spark plug, the cam can vary the intake valve closing. If the cam closes the intake valve early, it will maximize compression, since more fuel/air is trapped in the cylinder. If it closes it late, you will reduce compression, since there is less fuel/air to compress. So, why would you close the intake valve later in the compression cycle? What is the upside? I am having a hard time linking this to the idea of leaving the intake open longer to allow more time for air during higher RPMs. By the time the piston is moving upwards, no more air/fuel is entering the cylinder anyway. Varying the closing time of the intake valve during upstroke is only a matter of how much you compress. Whatever was taken in, was done during the downstroke of the piston, not the upstroke.

[HawgRyder]

There used to be A device available to retard the cam during operation.
The idea was to have the cam advanced at idle...and slowly retard as the RPMs got higher.
It worked quite well...but like everything that you add...it becomes one more thing to go wrong.
If you want to see the effects or numbers of different cams..try looking at some of the old cam grinders sites...like Isky...Crower...just to name a couple.
The "old guys"...and I'm one of them ...had NO money...we did things that cost nothing...or right next to it.
Changing cam timing...changing rocker ratios...building weird exhaust systems...cut outs in the exhaust...long stacks in the intakes...all sorts of cheap things.
Nothing is really new....just recycled.
Oh...and check out anti-reversion plates for the exhaust too.
Bob

[Bill Douglas]

That's why VVTi on new cars like Corollas is so good. Sorry, VVTi = varible valve timing injected. So at low revs the cam is advanced and becomes a torque slogger sort of engine then retards to be a high .rever

[304065]

Thinking about intake valve closure is really the third step in the thought process

so think backwards

think about volumetric efficiency-- what you need to do to fill up the cylinder to generate maximum cylinder pressure at around 20 ATDC on the power stroie

then think about port and valve sizes, and how they affect the flow of air into the cylinder when the intake valve is open. For the same piston velocity, a smaller port will require a higher air velocity. Correspondingly, with a larger port and valve the velocity will fall off.

Think about the difference between a 911T cam and a 911S cam. . . get the specs from the little white book and map them out on a piece of paper with 360 degrees marked on it. . . show the opening and closing point for each cam and compare the two.

the 67S motor at lower RPM is not as volumetrically efficient as the Normal. . .those large ports and valves inhibit complete cylinder filling. . . but once you get to 5300 RPM. . . and the column of air flowing into the cylinder is moving fast. . . that is where the 67S motor makes more torque due to greater VE

you close the intake valve when the cylinder has had time to fill. . . at ONE rpm that you are tuning for with the entire engine

this is why variable valve timing was invented. . . .

[sugarwood]

I'm not actually trying to tune my car, I just want to understand the theory.

I think I understand the idea behind advancing timing
If the cam timing is advanced, it closes the intake valve early,
it will maximize compression, since more fuel/air is trapped in the cylinder.
More torque and this is generally suited for low RPMs

But, I still don't understand the idea of retarded timing helping during high RPM.
What difference does it make if you close the intake valve later during compression?
There is no extra time for fuel/air to intake, b/c the piston is already moving up, and pushing air out of the open valve.

[snbush67]

I think that retarded timing at high RPMs helps to avoid detonation.

[T77911S]

closeing it early allows more time to compress what is in there. closing it late gives you less time to compress the air. that is why you need a higher compression ration when you close the intake late.
duration or the amount of time the intake is open determines if more or less air is in the cylinder. also closing it later lets you have a longer duration so you can pull more air in.

[psalt]

There is no extra time for fuel/air to intake, b/c the piston is already moving up, and pushing air out of the open valve.

The inertia of the incoming charge continues to fill the cylinder even when the piston is moving up.

[sugarwood]

psalt,
Are you sure? That doesn't really make sense. If air is still coming into the cylinder after the piston is moving up, then what is the benefit of closing the intake valve early? You're saying more air enters the cylinder if you close the valve late, b/c of inertia. This would be contrary to everything I've read that says the whole logic of closing intake valve early is to trap more air.

[Bob Kontak]

Originally Posted by sugarwood

psalt,
Are you sure? That doesn't really make sense.

Yes it does. In fact reading through the posts from my last one, I had pictured this, as at least, the partial answer.

What you are not embracing is that intake air at 950 RPM flows at a fairly lazy CFM rate. At 5,500 RPM the CFM rate is astoundingly more aggressive and stuff happens differently.

Advancing ignition timing is because an air fuel mix explodes at the same rate, whether at 950 or 5,000 RPM. It does not have a clue that you are spinning the "air-pump" a squillion times faster. Your piston is moving upwards five times faster at 5000 RPM than at 950 RPM. The explosion expansion rate remains constant.

So you have a mixed bag of variables. You fire way earlier at high RPM, yet the valves are still open. By the time the flame front has expanded a wee bit and extra air and fuel has been "rammed" in there from the fifty hundred extra CFM's of flow, when the valves finally seal for compression, it's in the not understood by a layman "happy place" for the bang to have close to the best impact.

If you retard the cam timing a bit you have chosen to capture/lock the compression at a point that maximizes compression for higher RPM efficiency.

Edit: Also, psalt is someone you can trust, absolutely. He has been on the board three years longer than me and has very few posts. He does not have the "windbag" gene like I do. I respect his input, again, absolutely. Perfectly ok that you challenge him, but the dude is the gold standard.

[bgyglfr]

Ok. I am no expert by any means but I think you also have to consider that the intake valve doesn't just open and close in a split second. It's rolling on a curved cam and gradually closes at the end of the intake stroke. So, with that in mind, late cam timing could help with flow into the cylinder. If the valve closes exactly at bottom dead center, it would be closing through a portion of the intake stroke gradually choking off the flow. Later cam timing will allow full filling of the cylinder and start choking off the flow more on the compression stroke and less on the intake side. As said before, cam timing needs to be set for what your peak rpm goal is. I would assume that most cams are timed with the cam beginning to close before bdc and finish closing after bdc. So the cam timing is somewhat dynamic and would have to be set to maximize the compression in a specified rpm range as said earlier in the thread.

[psalt]

Are you sure? That doesn't really make sense. If air is still coming into the cylinder after the piston is moving up, then what is the benefit of closing the intake valve early? You're saying more air enters the cylinder if you close the valve late, b/c of inertia. This would be contrary to everything I've read that says the whole logic of closing intake valve early is to trap more air.

Yes, I am sure. If what you are reading implies otherwise, throw it away and start with an intro to IC engines textbook. Start with the definition of volumetric efficiency. Longer duration cams work and improve output at high rpm by closing the intake valve later and increasing VE. Compare the valve timing of three examples. The VW 1200 closes the intake valve at 35 ABDC and makes peak power at 3600 rpm, the 911 3.0 closes the valve at 53 ABDC and peaks at 5500 rpm and the 917 4.5 closes at 104 ABDC and peaks at 8400 rpm. There is a loss of low speed output with longer duration cams, but usually this is offset somewhat by increasing the static compression ratio. This compromise is the "why" of variable valve timing.

What bgyglfr said is important, there are many things going on at once and the intake manifold is a swirling mess of back and forth pulsing, with a mixture of air, fuel and exhaust that is hard to understand. Output or Hp is rpm dependent, but torque is displacement dependent. Even a pukka 800 hp (NA) F1 engine that peaks at +14,000 rpm can only produce around 80 ft/lbs liter.

[HawgRyder]

I think what is missing is the idea that air/fuel mix is a FLUID!!
Fluids have mass.
Once a mass is in motion it tends to stay in motion unless acted upon by an outside force.
So...at higher RPMs...the slug of fuel/air is travelling down the intake tube (Lotus...way back in the 50s figured it out to be something like 300 feet per second) and until the intake valve closes...that slug will continue to travel down the tube.
When the intake valve finally closes...it does so with some effects.
Think of when you quickly shut off the tap in the kitchen...and you hear the "bang" from the pipes...this is the mass being stopped by the valve.
So even though the piston is rising (compression cycle) the slug of air/fuel has enough inertia to overcome the pressure being built in the chamber for a short time.
The timing and overlap of the cam now come into play.
Really high performance engines have both the intake AND exhaust open at the same time (overlap) so the incoming charge will assist the purging of the cylinder of old burnt gasses.
The timing (overall) is what you are concerned about...welcome to the engineering part of cars.
In an ideal world...we would be able to change the timing of all the parameters to get the most power/fuel economy/torque out of the engine...we can...now the technology exists to do all of that...its called electronic control of valve movement!
Some race engines have no camshafts at all!
They use coils to open/close the valves!
This allows ALL the parameters to appear at your fingertips (on the computer).
Now...only your brain is the limiting factor.
Have fun.
Bob

[sugarwood]

Originally Posted by psalt

Start with the definition of volumetric efficiency. Longer duration cams work and improve output at high rpm by closing the intake valve later and increasing VE.

Yes, but why does closing the intake valve later increase VE?

To reiterate my question, my confusion was doesn't closing the intake later simply give extra time for the piston to push air OUT of the cylinder? Wouldn't this reduce power, since it's the opposite of closing the intake valve early, which supposedly gives more power during low RPM since more is trapped in the cylinder.

I think Bob's answer above drove the point home for me. Do I have this right? I think we're saying the low RPM's "close early = more mixture" logic no longer applies during higher RPM. ie: During high RPM, close early does not mean more mixture, since it has not had time to enter the cylinder yet. So, I guess closing later during high RPM is a trade-off between less mixture trapped (low RPM close early logic) and more time for mixture to enter (even though the piston is moving up and forcing mixture out of the cylinder)

[Bob Kontak]

Originally Posted by sugarwood

Yes, but why does closing the intake valve later increase VE?

I don't think it increases VE. Max VE is fixed on a non VVT car and it generally coincides with the torque peak. (Yes I stole this - see fifth para in link below)

Volumetric Efficiency (and the REAL factor: MASS AIRFLOW), by EPI Inc.

What I think you do if closing early, is to short-change max VE, a bit, in exchange for nice peppy response where you normally drive in day to day traffic, lower RPM's.

To close the valves later brings the car closer to where it breathes the best given all that science fiction (well, fact) stuff that all but the experienced can't get their head around when it is spinning hella fast.

On an SC, the difference will be less noticeable as the torque peak kicks in at a low RPM number.

Originally Posted by HawgRyder

Some race engines have no camshafts at all!
They use coils to open/close the valves!

Are desmodromic valves still used?