Del_Slowest

i've read all the posts that say you get more horsepower from a SC61 at 10 psi than having a T25 at 10 psi. Someone on here said however that the 10psi in each case has to do with the manifold pressure and not the actual cylinder wall pressure. so my question is

Does the SC61 at 10 psi put more pressure on the cylinder wall than a T25 at 10psi ?

EDIT- put the question in bold so ppl can see it better


Modified by Del_Slowest at 2:17 AM 1/9/2004

MaxSterling

i'm going to say yes !!! mmmwwwwuhhhhahahahahaaaaa you already know the answer, why even ask

_Endless_

t-25 is a small turbo esp for a b16 and is bigger than t-25. at 10psi, t-25 won't be as efficient as the sc61 hence it will heat up the air it puts out more than with the sc61, so warmer air equals less power. small turbos like t-25 are more efficient at low boost so if you're planning to boost high then go with a bigger turbo.

roydogg

There is no comparison between the two (Sc61 hands down). SC61 is alot bigger than the t25 fosho

Del_Slowest

i know about the the SC61 being a hell of a lot bigger than the T-25, thats why i'm using them as examples. What i want to know is if both of them are at 10psi then are the cylinder walls having the same pressure put one them in both cases

BTW- thanks for the reply MaxSterling, i didn't want to put my opinion in here b/c i thought it might sway some of the ppl that actually reply to me

HXMan

I wouldn't worry about if each is putting the same pressure on the cylinder walls...

The important thing to note is a SC61 will put considerably more air into each cylinder, resulting in more power.

Cylinder pressure during combustion is MUCH higher then any boost pressure you will run.

Arturbo

Pressure is a unit of messure. If you are only talking about that then yes. But you added cylinder pressure which is the result of combustion. You can have more cylinder pressure with a smaller turbo (bad tune) than you could with a large turbo that has a good tune... Am I making sense or am I confusing you more?

art

Del_Slowest

So having a T-25 at 10psi is less dangerous of causing detenation

Having the SC61 at 10psi is more dangerous of causing detenation

This is because the SC61 is putting a lot more air into the chamber than the T-25 is at the same psi level...

BTW- both have untuned basemaps....


Am i on the right track?

VaporTrail

yeah basically. It's about CFM. Look at your compressor maps, look at the cfm they are capable of at that pressure ratio. The SC61 or any T3 turbo larger is going to outflow it, and likely with better efficiency/air charge.

HXMan

In general terms I would say the larger turbo is more dangerous. Just because it flows so much air.

Del_Slowest

I've always been under the impression that it's better to have a big CFM flowing turbo so that you can have it set at a lower boost levels therefore lessining the chance of detenation. IF you have a small *** turbo like the T-25 you have to run higher boost levels and therefore i thought it was at a greater risk of detenation. Was i wrong in thinking this?

So should i be more concerned with the CFM that the turbo is pushing rather than the psi i am at when worring about detenation?


Edit- thanks a lot for replying to this post guys, i feel like such a noob dumbass right now but it's really helping a lot.

VaporTrail

no. There is nothing damaging about running a small turbo at high boost vs running a big turbo at small boost so long as your intercooler is sufficient and you are not running the small turbo out of it's efficiency range. If they happen to be flowing the same cfm and similar efficiency, there will be no difference in output, only a difference in the power curve.

This is the common misconception - boost is created in the intake manifold, from there, it's combustion pressure which you have to worry about, and cfm directly correlates to combustion pressure. Combustion pressure directly relates to engine output.

Del_Slowest

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by VaporTrail &raquo;</TD></TR><TR><TD CLASS="quote">
This is the common misconception - boost is created in the intake manifold, from there, it's combustion pressure which you have to worry about, and cfm directly correlates to combustion pressure. Combustion pressure directly relates to engine output.</TD></TR></TABLE>

Sweet, so CFM is what i need to worry about. PPL should starting talking about hwo much CFM a motor can hold instead of just saying the PSI level it can hold then. I need a CFM gauge now and ditch my boost gauge

HXMan

Lol....cfm gauge.

Del_Slowest

LOL, shhhhh....i'm serious. Now i understand why most of the smarter ppl on H-T tell you how much horsepower an engine can stand insead of saying how much psi it can hold

ion_four

Vapor trail touched on this, but to elaborate...any two turbos at the same pressure will be flowing the same CFM, if one was flowing less, it would be making less boost on that motor. The difference is the efficiency, meaning that after a certain pressure ratio (boost level) the smaller turbo will be much less efficient and will be pushing hotter and thus, less dense air. It will still be the same volume of air because the container, being your chargpipes, intake manifold and engine, has not changed in volume. I'm not exacrtly sure how this efficiency related to combustion pressures because you have more air with higher efficiency, but higher temps with lower efficiency...so, it could be a wash?!

It really bothers me that people always talk about larger turbos flowing more CFM and whatnot, because this is totally incorrect. They can just push colder, more dense, air at the same boost level. In fact, if one used a more efficient intercooler with a less efficient (smaller) turbo, one could achieve the EXACT same final product as the larger turbo and less efficient intercooler, but with a wider, more useable power band. Would you sacrifice, let's say, 5% efficiency for a 2000rpm lower spool-up? I sure as hell would on a street car, because it will be quicker.

If you don't believe me, explain why a compressor map is not just a series of points in a vertical line, as it would have to be if it flowed a particular CFM!

Erik, who is really, truly, bothered by everyone always wanting to have a damned huge turbo to run rather low boost levels to be a dyno queen...

Del_Slowest

I like you post, in fact i like it a lot. It makes a lot of sense actually, i never thought about the compressor maps and why they are egg shaped.

I have a question for ya though, if a SC61 is pushing colder and more dense air then the T-25 then it's actually putting more air into the combustion chamber because the air is more dense and the charge pipes can flow a lot more of it therefore making you put more fuel in. A T-25 and a SC61 will not have the same fuel maps because the SC61 is pushing a lot more air in becuase the air is colder and more dense. Am i correct in say all this? In conclusion the SC61 is more dangerous at 10psi than the T-25 at 10psi as long as the T-25 is still within it's efficiency range and not pushing super hot air.

Am i right in saying this?

HighPsiAwd

The SC61 puts much more power out at the same boost level. Psi doen't really mean much in the world of FI, CFM's do

Del_Slowest

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by HighPsiAwd &raquo;</TD></TR><TR><TD CLASS="quote">The SC61 puts much more power out at the same boost level. Psi doen't really mean much in the world of FI, CFM's do </TD></TR></TABLE>\

thats not really what i'm trying to know though, i just want to know which one has a higher change of detenation, the SC61 or T-25 at 10psi

blueturboteg

If they are both tuned well neither will be more likely to detonate. The one that will put more stress on your internals is the sc61(because you will be making more hp). But as far as detonation it's all in the tuning.

Muckman

iTrader: (2)

Didnt we just go over this topic last month?

10psi is 10psi, its also the same CFM. However a larger turbo will yeild more power at the same psi (assuming we're inside the efficiency range) mostly because there is less resistence from the exhaust housing (its bigger with a bigger turbo). Density also plays a role resulting from compression efficiency.

Thats what I learned. I hope Im on the right track.

VaporTrail

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Muckman &raquo;</TD></TR><TR><TD CLASS="quote">Didnt we just go over this topic last month?

10psi is 10psi, its also the same CFM. However a larger turbo will yeild more power at the same psi (assuming we're inside the efficiency range) mostly because there is less resistence from the exhaust housing (its bigger with a bigger turbo). Density also plays a role resulting from compression efficiency.

Thats what I learned. I hope Im on the right track.</TD></TR></TABLE>

This is incorrect. 10psi is not 10psi. Think about it...

Same size baseball bat, length, and thickness wise, one weighing 24 ounces and the other weighing 34 ounces. If I hit you in the head with the bat speed moving the same velocity, which will hurt more? Just kidding, that has to do more with mass and its ability to store energy, but the correlation is similar.

A better relation would be a basket ball. Take a normal sized basketball, inflate it to 20psi. Now, take one of those small miniature basketballs, and inflate IT to 20psi. Which basketball is carrying more air?

Just look at compressor maps. A T25 is a 25lb compressor, the SC61 is a 61lb compressor. True, the larger back side will lead to less exhaust back pressure, but the serious difference will come from the much larger compressor wheel able to compress more air into the same pressure. That's how 10psi is NOT 10psi.

And yes, that is why the serious tuners talk about HP and not about boost... besides if you knew how innaccurate most of your gauges were from a true lab calibrated sensor, you'd probably throw that data out the window anyway

Bailhatch

I beleive running a sc61 below it's efficency range will create excess heat just like running it above would (whatevert that is...like 35psi or something). Same for the T25 but smaller #s, i guess 17psi might be comparable....

kpt4321

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Muckman &raquo;</TD></TR><TR><TD CLASS="quote">Didnt we just go over this topic last month?</TD></TR></TABLE>

You mean, 3 times a week?

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">10psi is 10psi, its also the same CFM. However a larger turbo will yeild more power at the same psi (assuming we're inside the efficiency range) mostly because there is less resistence from the exhaust housing (its bigger with a bigger turbo). Density also plays a role resulting from compression efficiency.</TD></TR></TABLE>

You're correct, if you are talking about uncorrected CFM. At 10 psi, every turbo is exerting the same force on the cylinder walls, and every turbo is flowing the same UNCORRECTED CFM. Density means that the mass flow is different though.

A lot of the time, however people talk about corrected CFM, which is CFM at standard temperature and pressure, in which case the different turbos will have the same boost level but different CFM.

kpt4321

Measuring Turbo Flow

You need to be careful when you talk about turbo flow, or any flow really, in CFM. CFM is cubic feet per minute, which is a measure of the volume flow over time.

However, if you remember high school chemisty, you'll remember PV=nRT, which says that if you change the temperature of a gas, the amount of gas molecules that fit inside a certain volume will change.

What does this mean? If you have two 1 cubic foot containers full of air at 10 psi, but one is 100*F and one is 200*F, there will be more air molecules (and a greater MASS) of air in the 100*F container.

That means that, in the case of a motor, given constant displacement (volume) and pressure, cooler charge temps raise the mass flow and increase power.

So, when you are talking about flow rates, you can't just say "CFM," you need to list the conditions under which it is the volume flow rate. On the other hand, if you talk about mass flow, you don't have to worry about conditions because mass flow is constant no matter what the conditions are.

4 pounds of air is always the same number of molecules, no matter what the tempareaure and pressure are. 4 cubic feet of air could be 1 molecule or a billion, depending on temp and pressure.

Hope this helps. We really need a sticky about this ****.