engineermike

I think the disconnect here is that the engine is not a fixed orifice, where if you enlarge the orifice (larger heads or cam) then the upstream pressure (boost) is less. An engine can better be described as an orifice (intake port and valve) leading into a fixed volume (cylinder with a closed exhaust valve). The object is to reduce the pressure drop across the orifice in order to achieve a greater pressure in the fixed volume. However, if you totally do away with the orifice (perfect cam and heads), then you will still have pressure in the upstream plenum and an equal amount of pressure in the cylinder.

John_D.

iTrader: (2)

Very true. On the other end of that spectrum, you can have a very restricted orifice, and a whole lot of pressure upstream, but less net power output from the engine.

That's where it gets confusing for most folks... how can it be, that I have more boost but less power... (or less boost but more power).

It all depends on how much air can get into that cylinder before the intake valve closes (and how little of it escapes through the exhaust valve). The amount of boost pressure is somewhat related to the total airflow, but it is also an inverse relationship to how well the intake tract has been optimized, which confounds the situation.

engineermike

Let me add just one more example. Let's say you have a supercharger spinning a set rpm.

If you have heads that flow 160 cfm, you might see 14 psi boost.

If you have heads that flow 220 cfm, then you might drop to 11 psi boost.

Now install heads that flow 280 cfm, and you only drop to 10 psi boost.

Install heads that flow 450 cfm, and you still see 9.5 psi boost.

At this point, the cylinder has reached the same pressure as the intake plenum because the port poses no restriction at all. Any further improvements in port flow yield no further drop in boost or power gain. In this case, boost is not a measure of restriction, but a measure of density of the charge in the cylinder. You can only drop the boost so much with gains in engine flow, and this is a GOOD thing because it means you have a charge in the cylinder thats at a pressure greater than atmospheric, a density greater than atmospheric, and thus a power greater than N/A - which is the whole idea isn't it?

Pro Stock John

iTrader: (34)

Vince there are several contributing factors as to why one combo will make the same power but with less boost compared to another similar combo:
-compression... the lower the compression the more you need to make back that power, so a bit more boost
-engine size... bigger engine can make more power with less boost... ie. a 400ci will make the same power as a 346ci but with less boost
-head flow, cylinder head flow... better flowing = more engine efficiency, less restriction, = less boost to make the same power but with lower iat's

VINCE

iTrader: (1)

You did not change your Avatar to the Bears

VINCE

iTrader: (1)

Got a ride in my small cammed TTi X car. Just NASTY!!!

obZidian

iTrader: (2)

ok, two things. Since when is vince going FI?! Just recently he had one of the baddest 408ci/nos apps... now FI? I guess....

Secondly, i love this thread. I have been after this information and have had only brief time researching this theory. If, by lowering restriction, you are able to decrease boost pressure, increase charge density, thus maintaining the same or increasing power productivity, how will a smaller duration cam, like vinces 225/219 115lsa cam achieve that if you are increasing the restriction with shorter durations and lift figures? I understand that the larger duration cam will "bleed" off boost, less restrictive. Also, the properties of the lsa... but this point eludes me.

By my rookie reasoning, the smaller cam will alow more charged air to be "used" and not bled straight through the exhaust valve. This, inturn, will generate more resistance or boost pressure, right?

engineermike

It doesn't. The smaller cam should show higher boost, as long as compressor speed stays the same.

Careful not to confuse turbochargers with superchargers. With a supercharger, a greater amount of camshaft overlap allows boost to go straight through the chamber and not be burnt. With a turbocharger, the opposite is more likely to happen. During overlap, the exhaust will backtrack upstream into the intake port.

Mike

52172

iTrader: (2)

what kind of power are you making vince with your new setup? Do you plan on swapping turbos in the near future? At what rpm are you hitting peak power? Looks like too much motor for the base turbo's? Anyway congratulations on making the move to TURBO.

obZidian

iTrader: (2)

ok, thanks mike. So shorter durations/less overlapp is what is looked for in a FI appl. so to decrease the amount of boost to bleed through or be backtracked. This shorter cam will have a better time "controlling" the boost produced by the turbine.

now, what are the basic requirements, for the exception of lsa, should we be looking for in a cam selection for turbo-equipped cars? Are splits recommended, more intake then exhaust? How about using a cam with shorter durations, like a 224/221 but with a larger lift area close to or over .600, estimations of course, to increase the amount of air being used per combustion cycle?

engineermike

That's not what I'm saying at all. Go to the track and check out the 8, 7, and 6 second FI cars and you'll quickly realize that they all have very big cams.

You'll get alot of different opinions on this and the fact remains anyone (with the exception of INTMD8) that's done alot of turbo cam testing isn't revealing their findings. I can tell you that Cam Motion pushes the reverse split cams on everyone, but I don't see where they work any better than a single pattern or even a conventional split. I'm using a 224/236 on my turbo and it works quite well.

Mike

obZidian

iTrader: (2)

hmmmm.... ok, im just trying to understand why people feel a smaller cam is the way to go when going from a n/a app. to a fi app.

For example, my current cam is somewhere in the 246/248 .613/.615 lsa114 straight-up. The general feel is that im being adviced to go into a smaller cam to better handle the boost.

408ci, s88, high hp, STREET/strip car.

2000 Tran Zam

iTrader: (2)

I think the idea behind that is that with the bigger cams, it creates more overlap, which bleeds out the psi that you need to make more power (cylinder pressure).

Im not sure exactly how duration, lift and lsa correlate to power, but i guess the best way I can describe the way I think of it is like this...

Lets say you have a straw, and at the end of the straw you feel (just a figurative number) 10 pounds of pressure.

When you run a bigger cam with more overlap, its like puttin a few holes in the straw (overlap bleeding boost into the exhuast).

So when you blow the same amount of pressure your may only have 8 pounds of pressure at the end of the straw, but maybe because it has those holes in it, it allows for the vacuum to build better (like poking a hole at the bottom of a milk jug when pouring out the liquid).

I could be totally wrong, and probably am, just trying to think outside the box.

brad8266

iTrader: (23)

The effects on boost pressure by relacing the cam is a no-brainer. A 220 cam will hold the valves closed for more time which allows more time for pressure to build up between the valves and the blower, hence higher boost.

Now take that exact same setup and swap the cam only to a 233 cam and the boost will go down because the valves are open more with the new cam, which keeps the pressure from building up.

brad8266

iTrader: (23)

Not only does overlap play a part but duration does also. Hold the vales open longer and there will be less time for pressure to build up. If out of 360 degrees you hold a valve open 220 degrees it has 140 degrees closed in whick boost will build. Now a 233 cam will have 127 degrees closed in which it will be able to build pressure.

Which one makes more PSI? The smaller duarion obviously.

You coulad also take 2 cams that have 5 degrees overlap and the cam with the most duration will still make less boost.

brad8266

iTrader: (23)

Go big cam with a big blower, then you will own all.

obZidian

iTrader: (2)

hmmm... interesting. Well, hows this.

In a FI combo, we are adding an extra amount of combustible air into the cylinder. If we have a cam that has to much overlap, a portion of that forced good air will be scavanged straight through the exhaust valve and out, therefore not being used. Now, mike just explained, if understood it correctly, that in a turbo app., that air has a tendency to backtrack into the intake port, therefore possibly being used again.

So, if are stuffing the motor with more air, and subsiquently more fuel, we need a cam event profile that will harness it appropriately. OR harness the increased cfm being stuffed by the blower/turbo. If we go by mikes model, boost, has two representative figures. One is restriction in the intake and a representation of the amount of charged, higher than atmospheric pressure in the cylinder. The more restrictive the intake, the higher the boost pressure if the turbo is pumping the same cfm constantly.

It would seem if you have a cam with larger overlap, you are compensating for a lack of intake port flow, thus increasing the amount of air being forced into the cylinder and lowering the overall boost pressure. Is this right?

I thnk its hopefully starting to make sense, please let me know if in wrong though. I understand, that picking a cam off the shelf isn't it. Alot of factors, like PSJ mentioned, are involved in trying to decide which cam is right for me. I want to understand cam events but its a bit difficult not seeing first hand.

obZidian

iTrader: (2)

brad8266: Hold a sec... your saying that a shorter duration will create more boost pressure, therefore more power productivity. Well, that seems to go against the idea that it isnt the boost pressure before the cylinder that is important as much as the boost/charged pressure in the cylinder. The headunit is still pumping the same CFM, so its not that it has less power headroom with less pressure buildup before the cylinder.... i feel that you would need MORE duration to charge the cylinder as much as possible since you are FORCING more air into to its throat. Thus the larger duration will have a charge in its cylinder that will have a larger volume of explosive property then the smaller duration with less charged pressure.

brad8266

iTrader: (23)

No, I did not say the smaller cam will make more boost AND POWER, I said that if you spin a blower at X RPM and then you compare 2 cams. The one with the smaller duration will make more boost, not necessarily more power. Boost and CFM are 2 different things. CFM measures the amount of air consumed my the engine, which is an indicator of overall power, while all boost measures is the pressure of the air which has nothing to do with power.

I agree with what you are saying, I think you just misunderstood me and assumed that I said the smaller cam would make more power and thats not what I said. Re read my post, I did not mention POWER once. All I was comparing was the effect that cam duration has on boost.

obZidian

iTrader: (2)

right, you never mentioned power but it seemed as if that is what you where implying. No worries.....