Yeah it's going to change it around a lot. I would expect that there is a lot of advance in that cam since the LSA is so high.

Bret

 

Part of the problem you guys are running into with the backpressure and manifold pressures of turbo setups is the way you're looking at it. It isn't a static case, it's a dynamic case. You just can't "pause" time and look at how the pressures relate to each other. You just can't say the pressure in the exhaust is "this" and the intake manifold pressure is "that" at "whatever" time, and conclude that the exhaust gasses will automatically revert into the chamber. A gas is a fluid, and fluids have the properties of mass and momentum. And for this reason, overlap is beneficial *to a point* on turbo setups. A particular turbo setup might be making 10psi of boost, with 15-17psi of backpressure at some high rpm. But some overlap doesn't mean that you'll get reversion. When the piston pushes with exhaust gases out of the cylinder during the period before the intake valve opens, they build momentum and "push" against the backpressure in the exhaust. And at an infentesimal amount of time there is no "backpressure" pushing the gases back into the port since the momentum of the exiting exhaust is canceling the forces of the backpressure and leaves a near pressureless area. Thus, when the fresh charge (at a lower psi) is being forced into the cylinder during this time you get the "sweeping" effect in the chamber you hear about. Even though there is more pressure in the exhaust, momentum gives the intake charge an opportunity to enter the cylinder for a small amount of time.

This kinda resembles ram tuning in reverse... remember that in NA setups you can still get more air into the chamber via the intake port even if the pressure in the cylinder is higher then the port due to the momentum of the entering gases pushing more into the cylinder until the valve closes ( i realize that this only occurs to a point... and you want the intake valve to close just at the point of reversion into the port). It's applicable to turbos too. You want this sweeping effect only to the point where the exhaust gases begin to reverse direction and push back into the chamber. You want this to be imminent when the exhaust valve closes to help force that little bit more exhaust out of the cylinder. You have a more "pure" intake charge than if there was no sweeping effect due to having no overlap.

There are also limits to this. The optimum point of most efficiency occurs only in a certain range of rpm and boost. And you have to make compromises. You must try to limit your time in the areas of least efficiency and make most of your run time in the areas of highest efficiency as possible. Whether you do this with stall converters, higher gears, ect. is up to you and how you race (dragstrip, autocross, etc). And as to just how much of a difference these effects make... who knows, too many variables to consider. But you can be rest assured that the most effecient turbo cams out there do have some overlap tailored to what their application is.

 

I don't mean to pick apart your post, but I'd like to clarify your wording.

>>"A gas is a fluid"?

Please elaborate.

>>"A particular turbo setup might be making 10psi of boost, with 15-17psi of
>>backpressure at some high rpm. But some overlap doesn't mean that you'll
>>get reversion."

From your post I can't quite gather if you are stating that:

reversion occurs after a point of engine RPM where intake pressure is constantly higher than exhaust pressure

or

reversion only occurs at points where the momentum of flow is not able
to overcome reflected pulses and atmospheric pressure.

Lastly,
>>"you can still get more air into the chamber via the intake port even if the
>>pressure in the cylinder is higher then the port due to the momentum of
>>the entering gases pushing more into the cylinder "

If the charge always moves from a high pressure area toward a lower
pressure area (trying to equalize), then it's safe to state that a higher pressure
in the clyinder will not allow a lower pressure from the intake port to enter.

An object of a certain mass that has momentum, also contains 'x' amount
of energy, or force.

If this forward moving mass (intake charge) has enough force to overcome a given pressure in the cylinder, can we not conclude that the forward moving
intake charge has a higher pressure than the cylinder?

IOW: veloicty of the gas is proportional to the pressure. The intake charge
will not enter the cylinder if the cylinder pressure is higher than the intake
port pressure.

The intake valve can hang open longer (IVC) and charge will continue to enter
the cylinder until the piston creates a higher pressure when reversing ABDC.

The amount of extra filling will vary and depends on the momentum and force of intake
charge which is dynamic throughout RPM.

 

you sure about that? I dont think its proportial (ie linear). Unless proportial doesn't mean linear.

isn't it to the power of (1/2) with some constant? Massflow anyway

 

I don't consider proportional to mean linear by any stretch.

From the quote you underlined, I do believe that a piston which accelerates
faster from TDC to BDC will create a lower pressure region in the cylinder
causing more air to enter. Couple that with a pressurized charge and the
VE% increases dramatically.

Also keeping in mind, the piston speed is not constant through the crank roation, nor is it constant as the engine winds up under acceleration (due to
dynamic engine load ).

If the mass/density of the charge is not constant, inertia will play a big role
in how much charge enters the cylinder I would imagine.

The torque plot of an engine is likely the best indication of when all of the pieces
are working to their best potential over a range of RPM.

I'm quoting most of this from a text, and trying to apply it to the best of my
ability. Obviously, most of us here don't have the test facilities to actually
gauge, or study these flow characteristics, so please correct me where I'm wrong.

 

Things can be proportional many ways: directly, inversely or by some factor or function (Sine, Cosine, etc.) or by a formula (D = 1/2 x a x t^2).

The piston accelerates from zero at TDC to max piston speed in about the first 75 degrees of crank rotation (SBC) then decelerates to zero at BDC. Intake charge velocity lags behind piston speed due to it's inertia as you suggested. Near power peak rpm in a properly cammed NA engine, intake velocity is still many hundreds of FPM when the piston is at BDC with zero velocity. Faster acceleration from TDC due to a shorter rod if stroke is constant, or longer stroke doesn't necessarily create a much stronger signal to increase VE dramatically. Forced induction can certainly increase VE dramatically, however.

PS varies continuously as the engine turns and is directly proportional (at any given crank angle) to rpm unless parts deflect substantially under "engine load".

Inertia always plays a big role in this especially NA. FI forces more volume of air/fuel into a given cylinder volume so density is increased, but the pressurized charge waiting at the intake valve for it to open still needs to overcome it's own inertia to get into the cylinder.

That's pretty much what everything is all about. Brake torque at any rpm, as measured by an engine dyno, is the sum total of everything that goes into producing indicated torque from combustion of the fuel/air less any parasitic losses like friction, pumping and inertia losses. The goal is to get the most torque at every rpm in the "working range" of a particular engine. One can tailor the shape of the torque curve to suit the application.

My $.02

 

getting off topic, but isn't proportional implying a scale of some sort- such as 3 or 4x bigger?

like when you say "their head is improportional to their body" or something.

3 entries found for proportional.
pro·por·tion·al ( P ) Pronunciation Key (pr-pôrsh-nl, -pr-)
adj.
Forming a relationship with other parts or quantities; being in proportion.
Properly related in size, degree, or other measurable characteristics; corresponding: Punishment ought to be proportional to the crime.
Mathematics. Having the same or a constant ratio.


I guess I can be a non-linear function, but i never took it that way. carry on!

 

As an engineering student, i've taken my fair share of fluids classes. Which all have dealt with gases and liquids. Some others here can further clarify on this than me i'm sure. But for time's sake, here's some reading to think about:

http://www.hyperdictionary.com/dictionary/fluid
http://www.efunda.com/formulae/fluids/overview.cfm

The second point is mostly what i was trying to emphasize, but with slight modification. The exhaust pulses in a turbo setup are not the same as in a NA or SC setup since the pre-turbine part of the exhaust is under pressure. These waves are much weaker due to the pressure. Kinda like how intake pulse wave tuning is much different in a forced induction setup from a NA setup (these waves are much more relavent and sought after in NA to make power). Reversion happens in the exhaust side (talking turbos here) when the momentum of the exiting gases can't overcome the pressure force in the exhaust manifold that is trying to push it's way back into the cylinder. Then there is a transition period where the gases must reverse direction to re-enter the cylinder. The pressures in the intake port, exhaust port, and the cylinder (which is basically just the combustion chamber since the piston is at the TDC area) are all different and changing rapidly as the piston moves and the pressures are trying to equalize. "Trying to equalize" emphasizes the momentum part of this process... the gas molecules are fighting their direction and velocity at any one instant to move into a different state and location (it's hard to seperate Momentum and Inertia, this is one reason why it can be so confusing). It is during this transition period where the overlap leaves the opportunity for the lower pressure intake charge to enter the cylinder and blow through to the exhaust port before the pressure in the exhaust has a chance to return to the cylinder. This is the sweeping effect.

Not a safe assumption, since you're dealing with moving objects with mass (air molecules). This is where the aggrivating and confusing subject of fluid dynamics comes into play. As the air fills the cylinder under the influence of vacuum, it "piles up" as the momentum of the intake charge carries it towards the piston and slows down. Then as the air molecules behind them rush in, they also hit and add to this piling effect. This piling effect represents to the pressure waves you hear about (this is true in both the cylinder and the ports). Also, the pressure in the cylinder is not uniform throughout... it varies as you look at different parts of the cylinder as the piston is moving and the inertia of the individual molecues causes them not to move in a uniform manner. i had the opportunity to look at some graphs and such at school modeling this fluid flow by the means of computational fluid dynamics (CFD). While i didn't understand most of what the visiting engineer was talking about, seeing those colored graphs with the vector fields helped to see how the air moves and the pressures differ. Very fascinating. And dynamics can be a really frustrating subject to understand... no matter what objects you're looking at.

Again, you are not dealing with a static case (no momentum in the system of moving parts/particles). You are dealing with fluids and their dynamic properties. There is momentum in the system since air has mass and when moving (velocity) it has momentum. This is a basic fundamental of ram tuning. Postive and negative pressure waves can travel up and down the port 3, 4 or more times between IVO events depending on rpm. The 1st wave carries more energy than the 2nd, the 2nd carries more energy than the 3rd, and so on. You want to tune the system to where the earliest positive wave possible hits the bowl area as the intake valve opens (again, you want this to be most effecient in the desired range of rpm). This helps to "kick start" the intake charge into moving into the cylinder quicker and faster when the valve opens since the air in that area behind the valve is now under pressure when the wave hits. During the intake event, when the air is traveling down the port, it gains momentum. It can't be stopped in an instant even if another force, like the pressure in the cylinder, opposes it. It is a large number of individual molecules, kept from "sticking" to one another (like in a solid) by the repulsive forces between them, moving as one large, loose mass. As the air is coming in, it rushes to fill those areas of less pressure in the cylinder. Also, keep in mind that the pressure throughout the cyliner is not constant. The pressure in the top of the cylinder is still lower than that on the top of the piston, even as it still travels back up the cylinder. If/when the pressures equalize towards the end of the intake event, the momentum of the intake charge moving down the port still further pushes more molecules into the cylinder just before the valve closes. All this only happens in a certain range of the powerband, which is why you tailor your intake design to work best in the range you plan to run in (like 6-9k rpm for NASCAR engines). It isn't a whole lot more air that is forced in, but it is enough to make a difference. It's hard to imagine these things since they occur in very small amounts of time.

I'm still a little rusty on this stuff as i'm still learning it, so any critique will be well taken.

 

Does anyone know the name of this book ? I think I have seen a post on this board before about this turbo cam or similar one that That David Vizard used in a turbo engine that made about the same power as above. So I think it may be the same cam.

Anyway , does anyone know the name of the book , and / or any more books with info from Vizard about turbo cams?

 

Yeah, it's "How to Build & Modify Chevrolet Small-Block V-8 Camshafts & Valvetrains". The relevant turbo cam info is on pages 60-63.

 

Fast Caddie,

Nice response.

You are confusing me however as I'm sure my posts indicate dynamic pressure
changes.

"Again, you are not dealing with a static case (no momentum in the system of moving parts/particles). You are dealing with fluids and their dynamic properties."

As long as the pistons and valves are moving, there is change in pressure
thoughtout the system.

When I'm speaking, I am refering to a moment in time...at least that is what
I hope to be portraying. Freezing, or taking a snap shot of pressure values
at different points in the engine.

IE: When the piston is at 5 degrees ATDC, the pressure at the intake valve
may be ~ 3 PSI; - 2 PSI in the chamber; -4 PSI at the exhaust port.

Again, much of this has been hashed from the text, "Scientific Design of
Exhaust and Intake Systems" and "Automotive Mechanics and Technology".

From the above scenario at that moment in time, intake charge will fill the
cylinder to a certain degree.

As the piston draws down futher (continues to accelerate) and creates even
lower pressure, there must be a further amount of intake charge to fill the 'void'.

I can appreciate that there are reflected waves, and at that very point in time
at the intake valve, there may be a pressure wave which has bounced off
the valve and heading toward the plenum, but there must be some amount
of forward moving pressure that can fill the low pressure area in the cylinder.

If there was a method to model the engine using eight syringes (as pistons and cylinders) and water (as charge), it may show us a weak representation
of how waves bounce around and the syringes fill up with water at different
rates over RPM.

I know we can't equate the flow properties of water with air, but I'm sure
using a visual such as water, and our imaginations, we would observe that
only a certain range of RPM would allow 100% (possibly > 100%) filling of
the syringe, and the less efficient RPM range would show a loss of water
in the syringe.

It would also be cool to see the waves crashing back and forth against the
valve changing their 'wavelength' as RPM changes and looking for that
initial relflection to be 'in phase' just as the intake valve opens so that the
water can force itself into the syringe.

 

I think we're on the same page as far as the moment in time, or "snapshot" while these events are occuring. The only point i'm trying to get across is that even with this snapshot, strickly looking at the pressure differences doesn't give you an accurate depiction of how the air is moving. It could be moving in one direction, but it could be in a transition period where it's decelerating and hasn't yet begun to move in the direction of lower pressure. All the particles have to decelerate in one direction, stop, and then accelerate in the other direction... which is what i meant by the transition period. Like the Old one said, there are lags in the system. Lags to accelerate and begin moving, lags to stop, and lags to respond to pressure differences(like more air entering the cylinder through the intake port even though the pressure in the cylinder might be higher than the port.. where reversion is imminent).

Using water in a slow speed system might give an idea of what's going on, but then again, water isn't classified as "compressible" like air is. Combined with the fact that since water molecules are much closer together, meaning their intermolecular forces are stronger, it won't give an absolute strategy to model air flow in a high speed system where water could almost act like sand or cement. That's why there have been MANY methods developed and years spent trying to test the theories behind all this air movement through an engine... it's utterly hard to make an exact model. Or even get cold, hard data to make precise emperical equations to give exact representative calculations. Hell, even trying to understand all this theory can be more than enough to make you pull you hair out. I'm half bald already like it is. lol

 

I guess we just disagree. I am not of the mentality that bigger is better with (any) blower cams.

excessive intake and overlap durration are never negligable. AAMOF, nothing is IMO. But thats just me, and race industry in general. It may be negligable on the street, but it can be done correctly, why not?

 

Boost.....

I guess excessive overlap to me is different from you. When we are talking about 30-40-50 degs of overlap that's excessive, blower cams on street cars never see that much, but -5, -10, 0 overlap I don't think is the right way to go either.

Bret

 

Thanks for the reply once again Fast Caddie.

I see where the confusion lays. Reciprocating pulse/air waves in the runnners would certainly have a tough
time changing direction and gathering new momentum.

On another note:

Has anybody sourced the valve timing for the LS1-GT7 camshaft?

I can't find it anywhere. Must be top secret