I learned today that a smaller cam will increase boost on the same setup if you used a bigger cam. I had no clue this was he case. I know bigger cams bleed off compression, but I did not know they decreased resistance in the engine as well. Now the bigger cam will make more peak power with less boost the little cam will improve low and mid range, but will show higher boost. Can someone more technical speak?

 

Well I can't speak more technical but it makes sense to me. The same principle applies to NA motors. The bigger cam will make more peak but the smaller cam will make more low end.

 

boost is a measure of how restrictive your engine is... if a smaller cam is showing more psi then the larger one your thinking about, then the smaller one is causing more restriction to air flow...

 

You actually can't measure by boost for something like this if the system is waste gated. You have to measure flow (CFM), not pressure (PSI).

 

you are correct, the turbo will just spin more to produce the same boost it was setup for, so a cam/heads swap on a turbo car could make a big power increase, but a blower car you need to change pulleys to see the real increase, power output is a good indicator of cfm

Vince you pretty much got it right on, more boost doesnt mean more power.

 

I went to a little bigger cam and lost about 2lbs. of boost. To get my boost back up i had to get a smaller pulley. My thing is which is better, more boost or a bigger cam?

 

And how did it run?

 

Well it seemed to run about the same seeing how i did not have it tuned yet. It breaks up really bad now in the upper rpm's.

 

That is a good question... Can someone answer this?

 

Boost is NOT just a measure of how restrictive the engine is. If your intake ports and cam posed no restriction whatsoever, you still want positive pressure in the intake manifold and the cylinder at BDC compression. Think of it this way, if you improve the intake port/cam, then it lowers the pressure in the intake and raises the pressure in the cylinder. In the ideal situation, the two pressures are equal, but still positive numbers (boost).

Mike

 

Hmm, well what does boost measure then?

 

Air pressure above atmospheric at the point of the gauge tap. With temperature you can factor air density. This has been discussed several times here before. The idea is to fill the cylinders with a higher density of oxygen. When you remove an exhaust restriction for instance you've made the space you need to fill larger which lowers the pressure if airflow remains the same.

 

Cams are a mechanical way to keeping valves open. Seems like a bigger cam would keep the valves open longer and provide less restriction. The potential downside of running a huge cam in a street car like Vince's is that there might be a downside in terms of reduced driveability, and potentially a loss in fuel economy. I wonder if Intmd8 would weigh in, I know he has some feelings about this.

 

Boost, with temperature, can be used to determine the density of the air in the intake. Obviously, you want the most dense charge possible entering the cylinder.

 

bigger cam as in lift should help power its the duration that will screw things up.

 

I agree, however I was trying to state it in simple terms... if your engine flows a certain CFM then when you go over that CFM you increase the PSI that PSI increase is a direct relative to the restrictions of your system... this is why when you add larger exhaust/intake whatevers it results in being able to move more CFM and lowers the PSI....

in simple terms PSI is reflective of how restrictive your engine is....

 

Keeping power the same while lowering boost (larger yet appropriate cam, high flow heads, etc) will keep IATs down, which should improve knock resistance at the same power level and type of gas. This is speculative, but makes sense to me. When I went to better aftermarket heads, I lost a couple psi but the power levels were exactly the same. This only applies to supercharged applications btw... turbos will produce the the same amount of boost due to the way their controllers work.

And easy example to see is a Sanden air conditioning compressor. They pump a fixed volume for every rotation. Period. If the high side line were disconnected, there wouldn't be any pressure built even though the same amount is being pumped. If the high side line were blocked, pressure would build until the compressor stopped turning, something explodes, or pressure bleeds past seals. In an "ideal" supercharger application, a fixed amount of air will be fed into the engine for every rotation of the supercharger shaft. If you increase the engine's ability to consume air at low pressures, then the air entering the engine (while exactly the same amount) will be at a lower pressure. There are all kinds of things that make supercharger applications non-ideal in this sense, but the principles are the same. All other things held constant, the engine will produce the same power in each case because it's based on the amount of air going through it. The important thing to consider is not the pressure (PSI), but the other factors that make the system non-ideal, because they do affect the resulting power.

A primary one is temperature. A supercharger with a higher pressure ratio will be producing air at a higher temperature. If your heads/cam package breathes better, the resulting intake temperatures will be lower for a given amount of air flow. This is good.

Another one is how well the cam is allowing the engine to remove all waste gas (exhaust) from the previous combustion cycle, yet contain fresh air from the intake for the next combustion cycle. This is the driving effect behind the fact that blower cams have traditionally longer duration than NA cams.

You should also consider a superchargers efficiency range - volume of air pumped based on pressure ratio and shaft speed. I haven't seen any real world analysis on this matter although I know it exists simply because the moving parts aren't ideal models. This may or may not make much difference in the scheme of things, someone else would know better. This would be affected by the type of supercharger of course.

If this didn't make sense let me know.

 

it didn't make sense..














j/k

 

Here's an easier one. The big inflatable toys that kids play inside. The fans push basically the same cfm under all conditions. If the toy has a small vent hole for the air to get out, it will inflate (build pressure). If you make a bigger hole for the air to get out, it will deflate (loses pressure).

The fan is blowing the same amount of air in both case.

(actually it may be moving a little more air with the less restriction - the bigger hole...)

We care about how much air is getting into the cylinders, not how much the air backs up between the valve and the blower/turbo.

(or back to the toy example, we care about how much air gets out of the vent hole, not how big the toy blows up.)

 

Interesting guys. I have a smaller cam on my current setup. It shows about 1.5 psi more than equiv setups with much bigger cams..