Changing just the CAM pulleys / installing adj cam gears

[NitroDann]

Just to jump in here and vouch for you both, cos I probably know you both better individually than you know each other.

I reckon neither of you are taking any offense here and just misunderstand each other a bit.

We should restart this talk here, but perhaps go into it with more detail so no one misses anyone point.

I think a big part of misunderstanding about cams, and why its looked at as a black art is that people learn about how cams work and how the cycles work and they learn slowly. That is to say, they think about it at the speed it says to say the parts of the cycle, where as in reality it happens dozens of times a second.
Things forgotten include the fact that air has mass, and that resonance is created in the inlet manifold due to the fact that the air has mass. And what about turbulence?
All of these things mean that when considering performance modifications 90% of what you know is totally thrown out the window.
The other thing that people forget is that at top dead centre the piston may move 1mm total for 15* of crank revolution.

Anyways all of these things mean that a basic understanding of what moves and where the air goes when, does not give much insight as to the actual flows and timing at 8000rpm.

Im not familiar enough with this stuff to be able to eloquently explain it online. Although with a sketchpad and half hour in person I can.

Dann

[gslender]

Thanks Dann - fair points.

Interesting aspect is that how is it that bolting on a turbo to an engineered engine designed to operate naturally aspirated is ok, but if I take 2 deg retard or advance to the exhaust cam I'm entering a dangerous zone that only a dyno can truly figure out for me?

There are a bunch of people just bolting up their turbo, adding some more fuel and retarding spark and apparently every other aspect of the engine's cam design is fine and suitable?

My main point, and motivation is that this can't be true, and certainly not anymore equally invalid as reducing overlap (not removing completely but reducing it) so that the charged air has more chance of doing what it was designed to do. Both actions are better served by time on a dyno, but most folks do neither.

Where I've arrived is still a healthy amount of overlap. The intake opens later as it doesn't need to be as early as the air is charged at 28psi instead of 14 (almost double the force). The exhaust, now opens a little earlier being advanced probably doesn't do anything other than heat the valves a bit, but as there is still a bit of overlap, they get a blast of cool air at the end, clearing the chamber and cooling the valves. The exhaust being slightly advanced reduces the amount of blow through... reducing wasted fuel. If I was running a turbo, the earlier extra hot exhaust gas being expelled might help spin up a turbo quicker, and the less cool air slowing it down (previously due to too much overlap) would be a benefit too.

Perhaps returning the exhaust cam back to 0 deg factory would be more sensible for my SC setup, and a touch more retard on the intake to compensate.

I'm agreeing that it is hard to know if there is any true improvement in raw HP, but fact is, seat of the pants tells me it has changed the dynamics of the engine in a positive way.

G

[NitroDann]

so that the charged air has more chance of doing what it was designed to do

I think this is the part that wozzah is a little frustrated about.

You dont take away overlap to gain power.

110% NO.

Its done to offset the effects an improved volumetric efficiency has on torque curve.

Overlap works when the engine is spinning so fast that the engine cannot possible breath properly, both removing 99% of the exhaust as well as inhaling fuel/air without it.

With no overlap the engine cant breath fully at any RPM above a few dozen RPM. Fact.
Understand this first.

With a lot of overlap the engine wastes air and fuel at low RPM. Fact.
This is because at low rpm the engine CAN breathe fully, there is enough time for the vacuum to equalize pressure inside the cylinder and out, and the overlap results in wasted fuel and lowered dynamic compression.

The more overlap you have the more RPM it takes for the overlap to perfectly time full cylinder filling. Fact.
Because at lower rpm the valves are open too long, this is what you argue about when boosted.

The higher your Volumetric Efficiency (VE) the faster cylinder filling happens. Fact.
This is what Volumetric Efficiency means.

The higher the RPM, the less time there is for cylinder filling to occur, duration and overlap allows for more time per cycle. Fact.
No explanation needed.

So heres the go.

You have an n/a car and some overlap.
The overlap is inefficient at low RPM, open at the same time, and theres enough time for intake to go straight out the exhaust.
At higher RPM, there is less time, the overlap gives the cylinder time to fill, as the extractors pull a vacuum on the exhaust valve.
When you increase VE (with boost, portwork, better exhaust) the cylinder filling is easier, and faster.
Faster filling means it happens in less time.
This suits higher RPM, which of course has less time for each cycle than lower RPM.
So now with increased VE your torque curve moves up the RPM scale.

Are you following now?
The overlap increases rpm at which torque occurs, this in turn INCREASES PEAK HP.

You decrease overlap simply to reduce RPM that the overlap is now timed for with the higher VE and faster cylinder filling.

IT ONLY KEEPS THE TORQUE CURVE THE SAME.

Dann

[gslender]

I'll answer in full later, but answer me this..... What is overlap for, exhaust or intake? Think it through and decide what benefit it provides to what part/s of the engine cycle.

At extremes with zero or negative overlap, which part would still work effectively and which part suffers?

Think this through and then answer only this question.

G

[wozzah1975]

so that the charged air has more chance of doing what it was designed to do

I think this is the part that wozzah is a little frustrated about.

You dont take away overlap to gain power.

110% NO.

Its done to offset the effects an improved volumetric efficiency has on torque curve.

Overlap works when the engine is spinning so fast that the engine cannot possible breath properly, both removing 99% of the exhaust as well as inhaling fuel/air without it.

With no overlap the engine cant breath fully at any RPM above a few dozen RPM. Fact.
Understand this first.

With a lot of overlap the engine wastes air and fuel at low RPM. Fact.
This is because at low rpm the engine CAN breathe fully, there is enough time for the vacuum to equalize pressure inside the cylinder and out, and the overlap results in wasted fuel and lowered dynamic compression.

The more overlap you have the more RPM it takes for the overlap to perfectly time full cylinder filling. Fact.
Because at lower rpm the valves are open too long, this is what you argue about when boosted.

The higher your Volumetric Efficiency (VE) the faster cylinder filling happens. Fact.
This is what Volumetric Efficiency means.

The higher the RPM, the less time there is for cylinder filling to occur, duration and overlap allows for more time per cycle. Fact.
No explanation needed.

So heres the go.

You have an n/a car and some overlap.
The overlap is inefficient at low RPM, open at the same time, and theres enough time for intake to go straight out the exhaust.
At higher RPM, there is less time, the overlap gives the cylinder time to fill, as the extractors pull a vacuum on the exhaust valve.
When you increase VE (with boost, portwork, better exhaust) the cylinder filling is easier, and faster.
Faster filling means it happens in less time.
This suits higher RPM, which of course has less time for each cycle than lower RPM.
So now with increased VE your torque curve moves up the RPM scale.

Are you following now?
The overlap increases rpm at which torque occurs, this in turn INCREASES PEAK HP.

You decrease overlap simply to reduce RPM that the overlap is now timed for with the higher VE and faster cylinder filling.

IT ONLY KEEPS THE TORQUE CURVE THE SAME.

Dann

gslender and Dann,

this is the best way i have seen it described so far in the thread,

Cheers
Woz

[gslender]

You know what, I'm tired of debating with you two (cheers erupt from the forum members).

There are countless articles that recommend reducing cam overlap when supercharging, many include lengthy explanations similar to what I've provided.

In addition, both DIYAutoTune and FlyinMiata both recommend you do this too. Both of those organizations I trust with them knowing forced inductions and MX5 engines.

A common approach for FI cams is to have twice duration on the ex cam than the intake cam, with very little if any overlap. I can even find cam shops with product codes for cam shafts that prove that view.

Finally, I'm convinced that both of you are thinking NA with respect to how all this works. Overlap is purely designed to support intake filling the cylinder chamber as a function of NA head design. It has nothing to do with exhaust efficiency. Forced induction needs less of this, and too much does result in wasted energy blowing by.

So much of what Dann wrote just doesn't apply in a FI situation - for example I can dial out 100% of my overlap and because it is FI it all still works because air is being forced in. It can breath and all is fine. There are benefits to having some overlap when FI but none of them relate to breathing.

You guys are confusing the crap out of the situation by introducing NA engineering requirements in a purely FI topic.

I'm out.

G

[wozzah1975]

You know what, I'm tired of debating with you two (cheers erupt from the forum members).

There are countless articles that recommend reducing cam overlap when supercharging, many include lengthy explanations similar to what I've provided.

absolutely,but this is assuming that you have started at the ideal n/a position in the first place

In addition, both DIYAutoTune and FlyinMiata both recommend you do this too. Both of those organizations I trust with them knowing forced inductions and MX5 engines.

once again, absolutely correct, and no doubt these people would be making direct comparisons with a correctly dialled N/a application as they should be with their experience

A common approach for FI cams is to have twice duration on the ex cam than the intake cam, with very little if any overlap. I can even find cam shops with product codes for cam shafts that prove that view.

correct more duration, but not twice the duration (think about that, twice the duration would mean the valve wouldn't close at all even if you doubled std duration figures @50 or advertised, they would both be over 360 deg), and correct again, less overlap but still directly related to where you want your power band, so not "no overlap", but less compared the equivelant n/a engine.

Finally, I'm convinced that both of you are thinking NA with respect to how all this works. Overlap is purely designed to support intake filling the cylinder chamber as a function of NA head design. It has nothing to do with exhaust efficiency. Forced induction needs less of this, and too much does result in wasted energy blowing by.

the theory factually works the same as n/a engine, but less exaggerated. It doesn't have 'nothing to do with exhaust efficiency", the margins are just not as critical, they still however apply. The graph on this link gives a good comparision to N/A vs S/charged vs Turbo port pressures, and gives good explanations about what changes should be made and why. http://www.hotrod.com/pitstop/hrdp_1011 ... ewall.html

So much of what Dann wrote just doesn't apply in a FI situation - for example I can dial out 100% of my overlap and because it is FI it all still works because air is being forced in. It can breath and all is fine. There are benefits to having some overlap when FI but none of them relate to breathing.

What Dann wrote does apply. Every theory that applies to an N/a engine also applies to an F/I engine, any reputable engine developement people will tell you this. settings change, but the theory is still essentially the same. You could dial 100% overlap out of an N/A engine as well, the end result will be just the same as an F/I, the difference would be just greater on the n/a application. Try it.

You guys are confusing the crap out of the situation by introducing NA engineering requirements in a purely FI topic.

The engineering requirements about making power are the same regardless of N/A or forced, there is nothing confusing about that at all.

I'm out.

G

Had you posted here and given any substance to your results, like a dyno graph of before and after i would be the first to commend you on your results, but the fact remains that all you have stated it has "3 pounds more boost and feels better", "i've read it works for others" and you're simply assuming you have made a change for the better. None of which means anything except an opinion based on no facts. Furthermore, I still stand by my previous statements that advising others trying this sort of change without correctly measuring what they have actually changed is the WRONG advice, and the correct advice is if you try these things to do it proplerly and base your results on fact, not opinion or what somebody else tells you. This applies to any aspect of modification, back it up with proof.

I highly doubt you have a clear understanding in the area of this topic based on the postings in this thread, and it's pretty clear both Dann and I have wasted time and energy trying to explain to you concepts, proven results and theories and forms of making accurate results, not only ours, but from people who have a vast understanding far superior to any of us reading this. Instead we have copped arrogance and personal attacks on postings that were civil, on topic and informative.

Happy to be out

Woz

[sailaholic]

Still a good thread! Assuming the explanations make sense to the person reading it. :-|

Sent from my GT-I9100 using Tapatalk 2

[gslender]

read the link wozza posted it covers most of my point and supports my thinking.

[NitroDann]

You question to me tells me the way you are thinking about this.

Without knowing what the intake resonance is like, how good exhaust scavenging is, and the RPM at which your question in posed the question is unanswerable.

I therefore conclude that you are thinking about a near stationary engine, and not considering the intake and exhaust to be part of the whole dynamic package.

You can have all of the overlap in the world, it just increases peak torque RPM and concentrates it over a narrower powerband.

You run smaller overlap in 2 equal engines only because the boosted one would have a powerband at 11,000 RPm due to the high VE, not because you want to make more power.

Dann

[gslender]

You run smaller overlap in 2 equal engines only because the boosted one would have a powerband at 11,000 RPm due to the high VE, not because you want to make more power.

...and there lies the point of my entire message. If you add FI to a NA car, you really should reduce the overlap unless you intend raising the max rpm to beyond what the engine can cope with. so unless you rev the engine to 11,000 RPM, the effect will be the same as adding more power. Even then, your point is a stupid argument because most turbo/sc would be out of puff at around the normal max rpm range and never reach the 11,000rpm target. So whilst technically you are right, you are also wrong. You waste power by not allowing the FI to be effective at a power band that is actually effective.

I guess if I had said more useable power then you would have agreed?

You guys are real hard work!

G