Pardon my french.

What is the purpose of cams for an FI application ? Why not just raise the boost ? Matter of fact what's the purpose of anything else on a turbo setup, like a better plenum ?

And a follow-up dumb question is... how can something like cams increase the stock boost of an APS setup from 8 to 12psi ? Someone explain this to a retard plz. Thx.

 

The short and sweet answer is more flow. Cams, plenums, etc increase the flow to the motor. PSI is not the measurement of air flow, CFM is. If you make it easier for the 6psi to enter and exit the engine, it will increase power output.

 

So, what exactly is psi a measure of, then ?

 

pounds per square inch of pressure between the turbo and the top-end of the engine...

and where do you get that cams increase the boost of a TT system from 8psi to 12psi? i'd think that cams that flow more would decrease the pressure?

 

^^Correct. think of the garden hose anology. turn the hose on with out a nozzle. now put a nozzle on. which flows more? which has the higher psi?

 

Yeah well that's what I heard at a local shop... still confuses me..

So, higher flow = less psi (resistance), yes ?

If so, what about stress put on an engine ? An engine with higher flow and turbo kit pushing 6psi producing 400whp will be under more stress than an engine with less flow and 6psi producing 380whp, correct ?

If so, does this mean shorter lifespan ?

 

maybe the guys at the shop said -- put cams on at boost to 8psi and you'll produce the power that a stock-cammed TT would produce at 12psi? just an idea

 

Read my post above again btw, I edited it.

Also, the guy said something like: "The APS kit boosts to 12psi with cams without any other changes". It's like... it magically starts running 12psi as the DEFAULT BOOST.

 

Well, flow into the combution chambers will increase with cams at the same psi and more air/fuel mixture makes more power but more power means more stress on internals.

also, with a boost controller, I don't understand how the psi would increase with cams.

 

i dunno about the statement in ^^^ post... i imagine you'd have to use a boost controller to get to 12psi, but then, i'm not TT quite yet, so i can't tell you from experience...

as for the engine stress statement... yeah, the more power an engine is producing the more inherent stress it is taking (mainly from torque causing flex in the conrod arms)

yes, and a higher flowing enginge will decrease the pressure read at the manifold (but in theory, if your turbo is big enough, it should be able to overcome any engine mods and eventually get to the point that the wastegate will begin venting)

 

This all comes down to physics and fluid dynamincs which can best be explained with Bernoulis principle. (yeah this is what makes planes fly and why I know about it). An increase in a fluids velocity (air is a fluid) is a decrease in it's dynamic pressure and the reverse is also true. An increase in pressure is a decrease in velocity. There is an optimum balance struck between velocity and pressure. For a given diameter of tubing there is an optimum flow rate for an optimum pressure. Disturb the balance and bad things begin to happen such as turbulence.

As the valve in a motor opens it must effectively and efficiently fill the cylinder volume before the intake valve closes. Increasing the pressure by turning up the boost will in turn increase the fluid velocity creating turbulence. Opening the intake valve more by increasing cam/valve lift, and keeping the intake valve open longer as with a cam of increasing duration will solve the problem of making more power without creating more problems as with increasing psi.

Why not put a big cam in in the first place? We'll a cam is a compromise. An auto manufacturer looks into factors that make a car a good daily driver at the expense of performance. Generally the higher the performance cam the more valve overlap it has (overlap is the time both intake and exhaust valves are open at the same time). Overlap helps the escaping exhaust gas scavenge the cylinder creating a vacuum pulling in the intake charge. The bad thing about that is that it also pulls out the gas air mixture screwing up emissions, and fuel economy. The cams in this car were never designed with turbocharging in mind. A different set of factors would go into designing a cam for forced induction than for an NA vehicle, as well as a cam for a daily driver and one for a performance car.

You can drive a screw in with a hammer... is it the right tool? No. Using a hammer is much easier. Using the right cam for the intended application is the same idea.

I hope this helps !!!

Respect
JET

 

CFM is not the best measurement of airflow, lbs/minute is, as CFM varies with temperature.

Increasing the lbs/min on the turbocharger(s) will increase the load moreso than adding partd such as an aftermarket upper intake plenum or aftermarket camshafts. It increases the power without putting as much stress on the internals, simply put.

BTW, adding cams should not increase the PSI, especially that much. He must have added cams, had it retuned, and maybe believed that the cams let the turbocharger(s) run more boost.

 

How is it possible to increase the overall power without the same amount of stress on the internals as upping the boost ? I still don't get it...

 

doesn't it just allow air to enter the engine easier or to maintain faster speed?

I mean what's the point of wanting a dual exhaust on a turbo setup? It's to free up the breathing so that the air can get out easier which means the air moves faster, which means more power.


with FI, you're cramming X amount of air in, but the engine still basically "sucks" air in too at the same time. If you make it easier for the engine to "suck" it in on top of forcing that X amount of FI air in, you make more power

 

Power is not completely relative to stress. You are increasing pressure on the internals by increasing the PSI on the turbocharger (all things being equal). If you increase any PSI through other parts, it is still not as much as increasing the turbocharger pressure as it will most likely flow more air.

They both increase stress, just one more than the other. They call it forced induction for a reason. N/A (or F/I-friendly N/A) mods are just unrestricting what the engine is already capable of.

 

so in other words, 2 cars both making 400rwhp

car 1 has all breathing mods and 6psi of boost

car 2 has no breathing mods and 9psi of boost


car 2 has more stress on the engine because it's cramming more air thru a thinner straw right?

car 2 will cost less though...

 

Huh...wait a minute...unless I'm wrong and please correct me if so

CFM= Cubic Feet per minute
lb/min= Pounds of air per minute Where is temperature a factor in determining which to use..I agree that in either case temperature must be a known factor to determine what the airflow is but lb/min is a derivative of CFM:

Mass Air Flow vs. Volume Air Flow conversion:
Mass Flow(lb/min) = 0.0756 x Volume Flow(cfm) IF the volume flow is measured at standard temperature and pressure (STP). Otherwise the Ideal Gas Law must be used to first convert the measured volume flow to STP

 

Ahh...I wonder why Peter@APS had said that before. I just accepted it to be true (as I do with other things ) and never thought it out.

You are correct, I was wrong.

 

Oh no don't take it personal I was curious myself as I learned it a different way. Hell I could be wrong and not know it but theory I was told seemed more sound. But when it comes to air flow when you are reading a compressor map its usually good to know lb/min as that is that common flow rate used for turbos...

 

It does make sense and I did not take it personal. I can't stand it when I realize something that was completely obvious if thought out...and I don't take the time to.