haven't seen a post about this yet so thought i should throw it out.

say we know that 10 psi on a small turbo will make less power than 10 psi on a big turbo(lets say T25 vs 16G). the question is: if the limits of the engine's internals is 10 psi on the T25, does that hold true for the 16G? what i see being different about the two is cfm. the reason why the 16G makes more that the T25 is because it will flow more air at a certain psi that the other.

if i'm not mistaken, boost pressure is taken at the intake manifold. doesn't that make it an indication of the efficency of the turbo and not the boost actually being introduced into the combustion chamber? CFM should really say something about the compression in the chamber, where as boost pressure on a gauge only tells you about the pressure in the manifold. how could reading pressure in the manifold tell you about the stress of your internals in the chamber?

soooooo, shouldn't we mark the limits of the internals by a cfm amount and not really a boost pressure?

 

Boost is a density measure, while CFM is a velocity measure. The velocity of the air doesn't excert any stress on the motor, it is simply a result of other occurences. The density of the air effects the motor two-fold... a denser air charge provides a greater resistive force under compression, as well as a higher expansive force under combustion. And of course heat is a significant byproduct of these forces. Velocity doesn't spawn any noticeable heat transfer from air molecules until mach speeds.

Additionally, boost pressure is a product of a closed, or nearly closed, system, thus it can only exist until the first restriction. As such, a measure at any point between the turbo outlet and the backs of the valves (whith closed ex. valve) is more or less accurate.

That's the way I understand it anyway.

 

measuring boost from the turbo outlet to the back of the intake valves doesn't seem like it really has too much an effect as to how much air is in the combustion chamber. if one turbo is flowing 200 cfm and another is flowing 300 cfm, which one is going to shove more air into the chamber if the intake valves are ticking 600 times a second? shouldn't we be looking to raise the density of air inside the chamber (by means of a more efficient higher flowing turbo) and be less worried about the pressure in the manifold?

but back to the original question, should we rate internals by cfm/effective compression, or by PSI of boost?

 

The pressure in the intake system has everything to do with how much air is in the combustion chamber. Just exhale with your mouth closed then open your mouth and you can see what I mean. The air is pressurized behind the valves until they open, then the air rushes into the cylinder until the pressure is equalized.

Now, regarding the turbo comparison... a larger turbo will only outflow the smaller one, in terms of volume, at higher RPMs (everything else being equal). But, this is because the turbo is actually rotating slower, it's exclusive of the motor itself. The smaller turbo eventually reaches a point where it can't overcome the drag of the rotating assembly and stops accelerating.

Anyway, its the explosiveness of the aircharge which damages pistons. The explosiveness is determined by how many air molecules are in the cylinder. And, the number of air molecules in the cylinder is determined by density, which exerts pressure, not velocity. So, a pistons structural integrity is dependent on pressure, not velocity (assuming proper lubrication).

So, yeah... I hope that makes more sense.

 

this is a bad example. the idea is too simple for the system. for one, the proportion of air displaced to the inlet area of the intake valve in the engine is way off proportion to the breathing-to-mouth example. second, if we're considering the rush after the intake valves open, how much pressure is relieved and how much pressure can the turbo replace after the rush? a larger cfm turbo will reduce the pressure loss after the opening of the intake valves and better continue the flow of air into the chamber. if the pressure in the manifold truly represented the pressure in the combustion chamber, the concept of volumetric efficiency would not exist.

this doesn't particularly explain "same psi, different horsepower." a 16G will spool maybe only 100-200 rpm later than a T25, but from there the 16G will make more horsepower at the same psi from then on. and i'm only talking 10-14 lbs of boost which is made around the 3k rpm mark. not crazy amounts of boost near their limits. it's true that a larger turbo will push more air rotating slower, but the argument is about cfm vs psi. i didn't particularly see any relation.

not to be pickin at your posts, but you're the only one else in this thread

 

Yeah, it'd be nice if someone else would join in. Even if they told me I was full of crap.

The way it looks to me, boost pressure cannot exist unless the turbo outflows the rest of the system. So, that would mean that motor restricts it's intake to the maximum volume possible. Volume maxes out, molecules start backing up increasing density, giving us boost. So, in the end, the only variable the internals see is the air density or pressure.

 

ur full of crap, lol, jp.

very informative post, thanks, however, i'd rather stick with a t-25ort-28, or a 20g at THE most, jus cause thats what rrm has, and they seem to know their ****. i have t-25 btw(not yet installed), but next yr. i plan to have a 20g on my baby.

sorry about the off topic, but i just had to chim in w/ my 2 cents. ask boe or hobie or rock, omarboy, or someone w/ extensive research in the F.I. area to help u guys out, they'll have ur answer in no time.

 

i would ask someone i know would know, but that would make one less good discussion on these boards. i'd rather people chime in on the thread and have a chance that people who want to learn something come across it.

 

kris, always starting the good topics....

 

Well, I am not all that experienced with this stuff but, generally speaking if the engine develops more power with a t-28 at 10 psi than with a t-25 at 10 psi, then there has to be an increased amount of stress on the engine therefore affirming rhyzin's belief that the limits of the engine should be a function of both cfm and psi.

What dosent make sense to me is that if the psi is measured at the IM how there would be difference in performance between the two turbos. Unless the power increase is only attributed to the larger turbos ability to more efficiently replace the pressurized air in the IM after the engine consumes it.

 

I love this thread.

Here is my 2 psi worth.

You are correct that a larger turbo will outflow a smaller one (i.e. more CFMs) but I dont think that we should measure the capacity of the internals based on neither CFMs nor boost pressure. Here is why. In a simple model, more CFMs equal more power equal greater probability for bang. But that is not always the case.

As we all know, the volume in our cylinders is always the same, regardless of RPM, boost pressure, atmospheric pressure or what you had for lunch.... it will always be equal to engine displacement. The thing that is altered is the density of the air-fuel mix, which is the amount of mass per unit volume. When air passes through the turbo, the volume is decreased and the density is increased (mass remaining the same) and hence more power can be made. However, if that same air charge is passed through an intercooler, the volume is decreased more, and the density is increased more. All this to say is that theoretically, you have a T25 flowing 300 CFMs at 10 psi, and a 16G flowing 400 CFMs, but you are running the T25 through a non-intercooled setup, and the 16G intercooled, you might reach destruction point first in the T25 even though the T25 flows less than the 16G.

So, I would have to partially agree with Gunshin's statements that when he says:

- even though I dont agree fully. Pressure to me, in this scenario, would be the pressure the pistons see, based on density, and not boost pressure.

All of that mumbo-jumbo to say, I dont think that engine tolerance should be measured in either CFMs or in boost pressure, because those are not the exclusive factors causing stress and failure.

 

Dude, Quez, it's Gunshin on here, not grasshopper.

Still, of all the variables involved, PSI would be the most logical to base a piston's tolerance on. Boost is the root of all the evils. If you flowed 1000cfm into a motor at atmosphere, I don't think the pistons would care, unless they were high compression pistons or the air was 300degrees. But, that all leads back to pressure one way or another.

Just like horsepower or octane or calories from fat or waist size, it's not the only factor governing the system, but as a generalization it's good enough that the least number of people will complain, and the most number of people can access it.

 

hrmmm, gotta make the most people happy. i love to hate that fact

sooooo, what if we made all conditions the same except for boost pressure? this makes non-ic/ic setups not an issue. how would either setup fare? which would reach detonation first and why? lets assume intake charge is rather cool.

 

Again a tough question......

Using those constants and variables you have there, it would still be a toss-up. Assuming same turbo, different pressures, obviously it makes sense that the higher pressure (psi) would be more prone to destruction, due to the higher forces exerted in the engine. Conversely, using two different turbos flowing different cfms, the one flowing more should in theory cause the pistons to fail quicker.

Btw, what precipitated this line of questions, rhyzin?

 

Wouldn't that be air temperature difference(air density)?