• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

PSI vs. CFM and fueling upgrade

JohnMc said:
Now I vaguely want to hook a PSI gauge to my exhaust manifold and see how high the PSI gets when it's making 20 psi of boost.

Not enough to actually DO anything about it, mind you.
Click to expand...

linuxman51 said:
I haven't done this yet either, but it's on the list of things to add to racecar V....whatever version it is.
Click to expand...

I'd really like to see some different plots showing intake/cylinder/exhaust pressure versus rotation for a turbo engine and a few different operating points (idle, cruise, max boost).

Somewhere (maybe Heywood?) I remember seeing a plot that showed the expected blowdown pressure peak, followed by flat-ish presure for the rest of the exhaust stroke, but then a jump in pressure (to intake boosted level) at overlap. No reversion at all. If anyone knows where to find some example plots, that would be great.

Not to digress too far, but how do I account for an intercooler when using a compressor map to size a turbo? I know there will be some pressure drop across the intercooler, so the PR needs to be bumped up over what's desired on the boost gauge. For flow, the intercooler is going to drop temperature and condense the air so that the compressor needs to supply extra CFM to the intercooler to keep up with the engine CFM. Is there a standard way to scale compressor CFM for the estimated cooling?
 
You can set things up such that the pressure on the exhaust side is lower than the pressure on the intake side (then, with overlap, you might end up blowing some of the charge out, something the swedes occasionally talk about).

you could take before and after pressure readings on the intercooler to see what it's doing, it's been my experience though on a decently sized cooler, you don't start to see a significant pressure drop until you start closing in on it's max flow capacity (so that is to say, it depends on a number of factors, there's not a say... .01 multiplier or something to factor that in)

in real world terms, unless it's a really poor intercooler, it generally would just move the dots up a little and over a little.
 
Dirty Rick said:
...15psi is 15psi no matter what turbo is pushing it.
Click to expand...

You're right. 15PSI is 15PSI no matter the turbo used. HOWEVER, psi IS NOT A MEASURE OF MASS. Wiki does a very good job of getting down to the bottom of this in just a couple of sentences.

From Wiki said:
The pound per square inch or, more accurately, pound-force per square inch (symbol: lbf/in2;[1] abbreviation: psi) is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 N/m2.
Click to expand...

In other words, how much air is crammed into that square inch? Larger turbos tend to cram more air into a square inch than smaller turbos. And try to keep in mind that turbos do this by "compressing" that air. ;-)

linuxman51 said:
Threads like this continue to underline the fact that none of yall are ever actually going to catch me.
Click to expand...

:lol:

linuxman51 said:
We *know* that it is based on mass air flow
Click to expand...

Clearly this is where everyone get's it tossed! :lol:
 
Thanks for the intercooler info.

linuxman51 said:
You can set things up such that the pressure on the exhaust side is lower than the pressure on the intake side (then, with overlap, you might end up blowing some of the charge out, something the swedes occasionally talk about).
Click to expand...

There was a MS thread a couple years ago about a weird ridge in the VE map. I think that this was exactly what was happening, and when coupled with injection overlapping with the intake valve opening at higher RPMs, some of the injected charge went straight out the exhaust causing a need for extra fuel to get back to desired AFR.
 
linuxman51 said:
in all seriousness, where I was going with my comments and remarks was to make things obviously disproportionate enough to force a re-think. Part of the reason is I feel like some amount of effort needs to be exerted on the part of those inquiring, and the other part is cynical in nature--this very subject has been beaten to absolute death on this forum and others, and yet the insipid notion remains that the *only* reason bigger turbos exist is to run higher boost and/or cooler charge temps, and that that is the *only* reason they make more power. Its a discussion so old and worn out at this point I have a hard time not having fun at other people's expense. May not be the right way to deal with that, but horses and water and what not.

t.
Click to expand...
Thanks for teaching some anyway.
I feel your pain, when trying to explain other basic concepts like evolution and origin of life, I remember some very over simplified points you tried to inject that we're cringeworthy to me, but would have been answered with a moment's thought or bio 101.
It's hard to handle neophytes politely, I'm very guilty of that myself. I appreciate you sticking around enough in threads beneath your level so there really less of them.

I wasn't trying in any way to put Delta T as the only, not even a big factor in trbo choice.
I can see why you thought I might be though, hard to specify what you are talking about sometimes.

What got me thinking was this notion (not tours)that
"Bigger turbo flows more at same psi" go back and see the peppered in the early thread. I quoted you because I knew you'd know how to explain, because that just seemed to be a terrible weirdness in a bad way. I'm sure you agree that that is a bad phrase. The dynamic system can't be explained like that, when the cylinder gulps a set volume, a stable cfm in a way you can't say it flows more in cfm.
Wasn't accusing you of that stuff, was hoping to clear that up.
Problem I saw was higher flow, same psi, in a system that had constant flow limited by the rigid volume of the cylinder. Saying that turbo 'flows" more makes zero sense to me, nor you, I'm sure. Was primarily objecting to that, not trying at all to saying turbos are only different in temp.:)



I think we know that turbos and ICEs are hypercomplex, dynamic machines, with so many variables that narrowing down static shots are less valuable.
Thanks bearstronaut for pointing out that TLDR, I know your pain as well. Post a pic of some of the systems you work with, love big stainless industrial octopus things.

Next up, I have a built up 2.5L redblock waiting for a turbo choice, which since I'm not looking for one by temp change at 18psi:-P I will need help learning to pic a good match turbo for it.
I hope you guys help, despite the neophytism:-P
.
 
I guess a different way to say it is that the bigger turbo has to flow more air to make the same PSI, because the engine can use more of it at a given PSI with a bigger turbo on the exhaust side.
 
So how psi a stock can support depends on of CFM, T, volume, cats, intake valve size, humidity, and the price of rice in China?
 
apachechef said:
Thanks for teaching some anyway.
I feel your pain, when trying to explain other basic concepts like evolution and origin of life, I remember some very over simplified points you tried to inject that we're cringeworthy to me, but would have been answered with a moment's thought or bio 101.
It's hard to handle neophytes politely, I'm very guilty of that myself. I appreciate you sticking around enough in threads beneath your level so there really less of them.

I wasn't trying in any way to put Delta T as the only, not even a big factor in trbo choice.
I can see why you thought I might be though, hard to specify what you are talking about sometimes.

What got me thinking was this notion (not tours)that
"Bigger turbo flows more at same psi" go back and see the peppered in the early thread. I quoted you because I knew you'd know how to explain, because that just seemed to be a terrible weirdness in a bad way. I'm sure you agree that that is a bad phrase. The dynamic system can't be explained like that, when the cylinder gulps a set volume, a stable cfm in a way you can't say it flows more in cfm.
Wasn't accusing you of that stuff, was hoping to clear that up.
Problem I saw was higher flow, same psi, in a system that had constant flow limited by the rigid volume of the cylinder. Saying that turbo 'flows" more makes zero sense to me, nor you, I'm sure. Was primarily objecting to that, not trying at all to saying turbos are only different in temp.:)



I think we know that turbos and ICEs are hypercomplex, dynamic machines, with so many variables that narrowing down static shots are less valuable.
Thanks bearstronaut for pointing out that TLDR, I know your pain as well. Post a pic of some of the systems you work with, love big stainless industrial octopus things.

Next up, I have a built up 2.5L redblock waiting for a turbo choice, which since I'm not looking for one by temp change at 18psi:-P I will need help learning to pic a good match turbo for it.
I hope you guys help, despite the neophytism:-P
.
Click to expand...


At the base level, a bigger turbo does flow more at a given pressure ratio than a smaller turbo. This is clearly mapped out on compressor maps.. there's a range, and a surge line.. and if you size your stuff wrong and end up left of that line, reality changes (and not in a good way), but this is what gives us "ranges" for various turbos. As others have said above too, it's still somewhat of a "grey" area and very much application specific as to whether a given turbo in a given range will work better than another in the same range, etc. I think, again, in this specific thread, part of the problem is that a 13c, while villified, is not some order of magnitude smaller than a 15g. If you took max-effort setups for both, sure, 240-250 hp from the 15g is a reasonable upper limit (not a hard limit), whereas you might expect a max effort 13c to belt out 220. (cue the people who've done better with both lmao). In both cases however, I would posit that you've gone past what you should be doing, you need to step up again.

The problem with simply looking at swept volume in a more or less fixed point is that you're missing literally thousands of points all around it. Volumetric efficiency gets tossed around but in terms of what we're doing, changing the turbo setup on the car changes it's VE. Bigger housings breathe better, etc.
So.. at say, 15psi and 3000 rpms with either turbo you may see very little difference in performance, as the amount of air the system is moving now vs what its moving with the smaller turbo may or may not be exceeding the ability to exhaust this air. Where your differences start to come in to play, and where your fuel system starts to take it in the shorts is higher and higher (and 3k is really just an arbitrary number), as the speed and overall amount of air to make the same amount of torque goes up.

A somewhat common example of this can be seen across the pond with the "budget" 8v turbo setups a lot of swedes run around with.. big cam, springs, port the head, very large turbo, and a relocated powerband.. the 13c/15g car is effectively "out of steam" before this thing even gets going, and yet we're still talking about essentially the same engine in terms of swept volume (but not in terms of output obviously)

So, in the sense of how a given system performs, saying that a turbo flows a given amount makes complete sense to me. Remember, at the end of the day, power and fuel consumption are tied directly to airflow. Superchargers and turbochargers force more in. Talking about it in terms of flow is a lot like talking about it in terms of how much power it supports, be it CFM, lb/min, or just "hp". A lot of is somewhat lazy and knowing the platform at this point, and a fair amount of direct experience over the years lol.
 
hope some of it made sense. one thing im not good at is teaching lol. anyway, going forward, don't be afraid to ask questions even if they may be dumb. everyone's been there, and while we might have some fun at your expense its usually meant with good intentions.
 
Regular hotdogs and footlong hotdogs have the same PSI on their casing, but one has more meat.
 
To the meat store!

giphy.gif
 
You must log in or register to reply here.
Back
Top