Well I think you asked too many questions at once (whether you knew it or not) here.

Let me say back in my neophyte days when I'd spend all day reading stealth316.com and injesting just what Jeff wrote there I used to believe the myth that 10psi = 10psi = 10psi regardless of turbo. I've since changed that opinion.

First lets address the engine issue. While Jeff's assumptions may potentially be off, at least in theory, he is correct. A 3.0L engine will only displace xxxxcfm of air at xxxxrpm at xxxxpsi. This is 100% true.

It is also true that turbos are often rated with CFM ratings and their compressor charts can give us information about how they spool, build pressure, and how much air they are pushing around at a given pressure ratio.

Given those two pieces of data we could theoretically match a turbo to an engine demand and voila ... 15Gs are all we would ever need (according to Jeff and that math).

That's all good and well ... but it obviously doesn't translate to the real world all that well. Let's investigate the culprits.

#1. CFM ratings on turbos are in no way applicable to an engine environment. Remember when testing turbos they have the advantage of controlling the RPMS at which they spin the compressor shaft and they are using the flow chamber to measure pressure in. Depending on the volume/temperature of the testbed it could either be dead-on accurate to our vehicle's setup or off by a drastic margin. Basically this means we should take flow maps with a grain of salt. The flowmap only tells us how the turbo will perform given all the test environment variables. If our engine/intake system is drastically different in size and volume then the performance of the turbo in our intake system could be wildly different than the flowmap.

#2. The engine demand lines are entirely theoretical. In the real world our engines have to deal with so many different variables (backpressure, heat, oil, flamefront differences, water injection, etc ...) that I doubt we can expect our engine to use even 80% of the effective air coming into it ... meaning the demand lines ... while theoretically correct ... are the ideal. A real world engine probably effectively uses faaaar less air/fuel than the demand lines suggest. I'm not saying the engine lets in less air/fuel ... but that combustion effeciency is nowhere near 100% in the real world so therefore power output isn't what we expect.


Now onto turbos.

While the flowmaps are not necessarily accurate they are not useless. They are a wonderful way to compare turbos against each other since they are all tested under identical conditions in the flowmap ... so given that you know the nature of at least one of the turbos you have a flowmap for (9bs) you can extrapolate from that flowmap the behavior of any other turbos by comparing the two flowmaps. I'd say this is about the most accurate way to judge turbos potential performance besides actually putting them on your car. Using demand lines is misleading, using your own experience with you 9bs and comparing its flowmap to the turbo you are considering will be less so.

As for why different turbos make different power at equal psi ... well this comes from several things.

#1. Turbine restriction. Backpressure from a turbo can be likened to backpressure from a catalyst. Think of the backpressure from a TD04 equal to having a 10 foot long catalyst on that manifold whereas the backpressure from a pair of TD06's would be like having a 2 inch long catalyst. So typically with different turbos you get different turbine sizes or A/R ratios or whathave you and this changes how far each PSI goes on a given turbo.

#2. Compressor flow. Bigger turbos tend to flow more CFM's at lower pressure ratios ... so a DR650 at 10psi is flowing more CFM than a 9b at 10psi. Same manifold pressure but the air from the DR650 is moving MUCH faster so your engine can get more air. In the low RPMS this might not make a difference but as your engine demand climbs through the RPM range the larger turbo will soon distance itself in power output from the lower flowing turbo. The point is a 9b and a DR650 at 10psi probably make identical power at about 3,000rpms since they are both outflowing the engine's demands. However at 4,500rpms the 9b is not flowing anywhere near the engine's needed air while the DR650 is still pushing strong.

#3. Compressor efficiency. While I attribute the power differences between turbos to either #1 or #2 in 99% of cases ... compressor efficiency can sometimes play a (minor) role. Larger turbo are often times (but not always) more efficient than smaller ones in the boost ranges we typically seek to run. This can cause a minor difference in the output temperature of the intake charge. This difference is reduced quite a bit, however, by our intercoolers ... so take compressor efficiency with a grain of salt.

Ok ... I guess that's it.

 

That was more information that I ever could have hoped for Thanks man

 

[B However at 4,500rpms the 9b is not flowing anywhere near the engine's needed air while the DR650 is still pushing strong.
[/B]

I tend to lean towards agreeing with this, but is there any tests that have actually proven this? It makes sense, but I would just like to see the issue laid to rest with some proof... even from non 3/S applications.

 

The dyno tells no lies. Dyno a stock 9b at 1 Bar and then a 15G or a 13G at one Bar and you can see the difference for yourself. 1 Bar is 1 Bar however the turbo size can have a dramatic impact to the amount of HP that is produced at the wheels at any given boost level. That is why it is difficult for some to understand that is usually safe to run a 9B on the street at 1 Bar and with 91 octane, but that running a TD05 20G on the street at 1 Bar with 91 octane can create problems. However there are other characteristics that you should consider when choosing a turbo. I do believe that boost lag is overrated as a problem for larger turbo's, however it can be an issue if you are only interested in the ability to get from stop light to stop light. Be sure you understand your needs and desires before choosing a larger turbo. Regards Chuck

 

Since we are stepping into this arena let's boaden it a bit -

capable PSI (how much boost Could the turbo make- flaws and limit to the theory - Ray pushing 30psi on the 15G's, TD05's that aren't supposed to hit 20psi)
dyno's FROM each turbo (showing the boost, the power and the TQ- as well as the load - see next)
Load and how load changes the demands of the motor and therefore of the turbo (which helps explain that 3000RPM vs 4500RPM question too )
lastly, we could start a new topic I'm sure, but turbine interplay, and how upgrading the exhaust is more important AFTER the turbine than before - How a turbine limits the effectiveness of the motor and the turbo, how the turbine can be manipulated to decrease lag, increase flow (course i've not checked out the Turbo Clipping thead which might be a better place to check into this "hot side" of the debate

 

Chuck is absolutely right ... but we all know a 9b wont hold 15psi to redline ... but lets explorer this issue using some flow maps.

Lets compare a 9b to a 15G (since we have flowmaps for both).

At 10psi a 9b can efficiently (over 70%) make about 200CFM of air for a total of 400CFM of air between two turbos. http://www.stealth316.com/images/td04-09b-jlspecstock.gif

At 10psi a 15G can efficiently (over 70%) make about 325CFM of air for a total of 650CFM of air between the two turbos.
http://www.stealth316.com/images/td04-15g-jlspec.gif

That's the ideal case, according to the flow maps ... but as you can see even at 10psi there is a non-trivial difference between even such similar turbos. Now lets look at TD06-20G's

At 10psi a 20G can efficiently (over 70%) make about 550CFM of air for a total of 1100CFM of air between the two turbos.
http://www.stealth316.com/images/td06h-20g-jlspec.gif

Obviously, even at the same PSI ... the larger turbos push much more air (in these ideal cases). Still do remember those numbers above are very ideal and the real CFM of all of the above turbos will be much lower in an engine/intake tract environment ...

 

Since we are stepping into this arena let's boaden it a bit -

capable PSI (how much boost Could the turbo make- flaws and limit to the theory - Ray pushing 30psi on the 15G's, TD05's that aren't supposed to hit 20psi)
dyno's FROM each turbo (showing the boost, the power and the TQ- as well as the load - see next)
Load and how load changes the demands of the motor and therefore of the turbo (which helps explain that 3000RPM vs 4500RPM question too )
lastly, we could start a new topic I'm sure, but turbine interplay, and how upgrading the exhaust is more important AFTER the turbine than before - How a turbine limits the effectiveness of the motor and the turbo, how the turbine can be manipulated to decrease lag, increase flow (course i've not checked out the Turbo Clipping thead which might be a better place to check into this "hot side" of the debate

Um ... broadening it a bit is a BAD idea ... as we will diverge into too many topics ... lets keep this thread focused on what was asked and start a new one about something like "turbine resistence/backpressure vs thrust bearing RPMS" ... etc ...

 

alright... point conceded- new threads soon...

 

alright... point conceded- new threads soon...

My hope for this forum is for there to be few (less than half a dozen) threads about any individual topic just those threads each have tons of information in them and are always active ... making this place more of a focused resource rather than a divergent chaos like all the other forums are ... but then again I suppose this is somewhat myopic ... I don't know what gatecrasher has planned for the place.

 

Odin;
What I believe you are asking is why the difference in cfm's makes a larger difference when it comes to detonation and spark blowout.
Up to a given rpms, it is correct to assume that 10 lbs is 10lbs when comparing different sized turbos and cfm's, because of the fact that the engine has an air requirement to produce X amount of horsepower.
As far as PSI, 10 lbs is always 10 lbs and we use that as a base number. This is what confuses us by using pressure. In the back of our heads we also have to know the difference in CFM's is not equal. This is referred to as volume. At 10 psi, a 1/2" opening will flow far less than a 2" opening provided that 10 lbs is maintained at both outlets. Hence the difference in "Volume"
That is about as simple as I can make it in explaination.
As rpms increase, volume requirement is increased substancially. This is why you can see a 13g turbo stay even with a 15g turbo up to a given rpms. Once volume starts decreasing, pressure also decreases. This is the reason for higher traps because the volume can more easily be met by larger turbos.
Now, because it is directly related, engine management comes in heavily. The engine management has to add the correct amount of fuel to remain as close to stioch as possible. I won't expound on this too much, because it is not the subject at hand, but it has to be known that running too much pressure from larger turbos can hurt performance only because the associated hardware can't not effectively keep A/F ratios at the proper level so detonation occurs with a larger turbo volume. Blowout can also occur more readily because of that larger volume.
You can't just put more air through an engine to attain more HP. Our engine can adjust up to a point and only that far. From then on it's downhill unless you address management. This is the reason a larger MAS sensor or MAP sensors are used, but they alone can not achieve stoich. They have to again, be managed properly.

 

Now, because it is directly related, engine management comes in heavily. The engine management has to add the correct amount of fuel to remain as close to stioch as possible.

You can't just put more air through an engine to attain more HP. Our engine can adjust up to a point and only that far. From then on it's downhill unless you address management. This is the reason a larger MAS sensor or MAP sensors are used, but they alone can not achieve stoich. They have to again, be managed properly.

I agree with your points in general, but the references to "stoich" are misleading. If you run at stoich under boost your pistons will not survive. Stoich refers to a 14.7:1 air/fuel ratio, and we need to run more around 11.0:1 to 11.5:1 (or so) for power under load and provide enough cooling to the chamber so parts don't melt.

As far as the difference between turbos at the same boost pressure go, I don't think there's really a simple explanation of the interactions that cause various turbos to make more power at certain pressure levels.

Contributing factors are CFM (how much VOLUME of air can the compressor push at your pressure ratio with a given input on the turbine side), compressor efficiency (contributing to lower intake temperatures at a given pressure level, increasing or decreasing horsepower accordingly), and turbine restrictiveness. Each of these variables will contribute to the power output of the motor at a given set of circumstances.

Don't forget that boost pressure (psi) and CFM are not directly releated and are measuring two completely different things. PSI is air density (basically) and CFM is air volume. The motor can only ingest a certain volume of air into the cylinders per combustion cycle, but you can make it "effectively" more air by increasing the density (psi) of the air contained in that volume. Density is affected by the pressure ratio the turbo is supplying and the temperature of the air (lower is better - more dense).

I think the difference between turbos at the "same" PSI is mostly due to efficiency differences, and then in some cases turbine restriction differences (clipped, ported, 9B TD04 vs. "big" 15G TD04H, TD04s vs. TD05s, etc.).

 

Good discussion guys... subscribing.

 

I agree with your points in general, but the references to "stoich" are misleading. If you run at stoich under boost your pistons will not survive. Stoich refers to a 14.7:1 air/fuel ratio, and we need to run more around 11.0:1 to 11.5:1 (or so) for power under load and provide enough cooling to the chamber so parts don't melt.

To explain my definition as I use it anyway of Stoich. it is not a definate value of atmospheric pressure but rather
Chemically balanced mixtures (all reactants mutually consumed)—In the case of engine air/fuel mixtures, just enough air to theoretically burn all the fuel is considered stoich.

 

To explain my definition as I use it anyway of Stoich. it is not a definate value of atmospheric pressure but rather
Chemically balanced mixtures (all reactants mutually consumed)—In the case of engine air/fuel mixtures, just enough air to theoretically burn all the fuel is considered stoich.

Yes. And if you burn that type of mixture under boost things go BOOOOM. Too hot for the pistons while under load. Good for getting good gas mileage at a cruise though.

14.7 parts air by mass to 1 part gasoline by mass = stoichiometric for gasoline.

Set your narrowband O2 readings to be .5v under boost and you'll see what I mean.

 

mj, I think JRC is defining stoich in this case as "the right mixture of air and fuel". For example, stoich for partial throttle would be 14.7:1 while stoich for WOT would be 11.5:1.

Not saying this is correct but that is what I'm getting from it. He's using it as a general term, not as a specific quantity.

 

Ok, I'll chime in with more real-world info.... I really really enjoy the tech discussion, also!

I was able to run 17-18psi on my 9bs using 110 leaded, it would fall to 11psi by redline. Doing that, and with my exhaust dropped and truly running the car to the absolute maximum... I hit a 12.89 @ 105.

I then installed the 15g's and big injectors and ran a 12psi run... totally untuned, with hesitation due to blowout... granny shifting and with a 40lb tool bag and a 30lb subwoofer in the car and got a 12.80 @ 108... and also hit 110mph on another run.


Even more interesting is that I was only able to run 10psi on the 15g's with the stock fuel system. Higher than that and I leaned out and got knock, even hit fuel cut once. With 9bs I regularly hit 15psi with good o2 readings and no knock.

And I know this is subjective but 10psi on 15g's is MUCH MUCH MUCH stronger than 10psi on 9bs.

Actualy it's not that subjective... I lost a couple of times by a car length to a Z28 Camaro from a roll at 10psi on 9b's. Last year in Indy I was running 10psi on 15gs (stock fuel!) and pulled 2.5 carlengths on a Z28 Camaro from a roll. That's a 3.5 carlength difference so it's significant.

To answer the OQ, 10psi on 15g's puts out more HP than 10psi on 9b's.

 

Ok, someone correct me if I'm wrong, but this is the way I understand it.
When we talk about PSI we're talking about the pressure we are seeing at the plenum. This pressure isn't anywhere near the pressure the turbo is putting out. The pressure we see at the plenum is the amount of air the engine is NOT flowing, correct? The pressure builds up of all the air that is being restricted due to inefficiencies in the plenum, manifold, head, and finally the amount of air allready crammed into the cylinder. Ok, now that all that is out of the way:
I'm not sure if anyone directly responded to why 10psi != 10psi. Main reason being turbo efficiency and intercooler efficiency. The initial benefit of having a turbo operating at 80% efficiency opposed to 74% efficiency may not be immediatly aparant. But at 10psi you would see about 70* of difference in outlet temperature. That 70* is negligable at first because your intercooler sucks up that heat and drops it down to a manageable level. But the turbo consistantly putting out air thats 70* higher will heat soak the intercooler faster. This is where people will see gains with getting a front mount, larger mass, more heat sink capacity, more surface area for heat transfer.

As for next question CFM limits, I believe it ranges from 50cfm to about 400 cfm at redline on a stock NA assuming 100% VE. I'm pretty sure Jeff's site has some exact numbers on it somewhere? As for the TT engine, cfm increases based on psig. Different for any pressure level you want.

As for larger power from larger turbos. Like I said earlier I *THINK* that it has a lot to do with the efficiency, when you end up with a huge front mount and water/alchy/propane injection the only gain you'll be seeing from bigger turbos is the ability to run higher boost pressure.

Someone please slap some sence into me if any of that is wrong!

 

It sounds like we mostly all have a pretty good understanding of this, and put similar thoughts in different ways. The balance of factors is complex and at our "level of expertise" we can estimate but alot ends up trial and error, 'here are my turbos, now I have to work with them'
Well here's how I think about it anyway. This is sorta complex so let's break this into 3 main components:

1) Compressor and boost Pressure/Volume/Airmass
2) Turbine and "Backpressure"/Spoolup Driving Force
3) The Interaction of the Boost/Volume/Airmass _VS_ the Backpressure

1) Compressor and Boost Pressure/Volume/Airmass:

--The compressor makes boost pressure in PSI, which leads to airflow (FROM high pressure, TO low pressure) pushing air into the low pressure cylinder on intake stroke of the Otto cycle engine.
--The more air pressure (deltaP) the more airflow. This is easy! More boost=more flow. Less resistance to flow (better IC/piping flow, better manifold/head flow) leads to MORE flow at the SAME boost. Bonus!! (Still, "less resistance" is ultimately limited by the SIZE of the engine's cylinders and rpm it is turning at. Think of a curve going up fast, then levelling off as it reaches theoretical max point...diminishing returns to be had by more efficient piping etc).
--The COMPRESSOR MAP shows how efficient it is at MAKING that boost, and relates what it is physically able to make in PSI to the VOLUME/MASS of air at that point. Compressor wheels/housings can be designed for high volume/low pressure, low volume/high pressure, etc...
--Small compressors are EASY to spin, spool up faster, get you on boost faster, more lowend torque...but reach their max flow rates fast too, so are limited in topend HP. They also get OUT of their efficient compressor map islands at high flow, so at max they are beating up air alot/HEATING it, so though on top they can MAKE HIGH BOOST, the air is getting so HOT you are getting MORE PSI WITHOUT MORE AIR MASS! This is bad, makes for detonation, LESS hp...you have "gone over the hump" of that turbo's ability. See the TD04-9b at over (5500rpm+11psi) or so. 9b can do you proud at 16-18psi at 2500-5300rpm, but over that you get over the hump.

--Large compressors are HARDER to spin, spool up slower, but (with good design/housing) at mid/upper rpm regimes can make the SAME BOOST at HIGHER EFFICIENCY/FLOW than the small 9b ("flow/Airmass increase" is ONLY due to reduced temperature=denser air at same PSI/deltaP)...AND can make MORE PSI at HIGHER RPM regimes so can make more hp on top.

SO:
A Compressor makes PSI. PSI makes AIRFLOW. _WHAT/HOW MUCH_ you are flowing (AIR DENSITY/MASS) relates to TEMPERATURE of that air: Larger/moreEfficeint Compressor=Cooler=Denser=more O2 molecules/mass flowed at SAME PSI.
This comes into play mostly at topend psi/flows, where the limitations of small compressor are exceeded, while the larger compressor is still efficient there...though it will be LESS efficient at LOWER psi/flows. Our 9b's are GREAT for stock cars...NOT as good for modded/high hp targets.

2) Turbine and "backpressure"/spoolup driving force:

What DRIVES that compressor? TURBINE BACKPRESSURE = exhaust pressure released from the Otto engine on exhaust valve opening, escaping past/driving the turbine blades. MORE backpressure deltaP=Faster Spool=Great lowend.
LESS backpressure=SlowerSpool=Worse Lowend, BUT GAIN topend WHEN the BALANCE between backpressure and compressor gets BACK into its efficiency zone!

3) The Interaction of the Boost/Volume/Airmass _VS_ the backpressure:

MUCH of the reason "Bigger Turbos Make More Power" is that they have RAISED the engine RPM and BOOST LEVEL at which the maximum efficiencies of the COMPRESSOR and TURBINES happens, WITH RELATION to the ENGINE size, rpm, flow etc. These are CURVES, which intersect...where the COMPRESSOR PSI/FLOW curve and the TURBINE PSI/FLOW curves "come together" in MAJOR way determine how the engine acts, how much ultimate HP it can make, how fast it comes on boost!

-----------------
So, you can make a high hp turbo that has WAY big topend but bad spoolup because the turbines are too big for reasonable spoolup. It becomes a balancing act between what your backpressure is at max hp rpm, and what your compressor flow/efficiency is at max hp rpm. We must try to make good driving force (turbine/backpressure) while making good compressor flow.

Many turbos can work fine!...where do all YOUR curves intersect?
(Pics from chicks accepted in the Lounge ) We better not get into camshaft overlap, duration, etc but these systems all interact and each has its "curve"...the more you can get these curves to be at their PEAKS TOGETHER at the rpm you want, without losing "too much" spoolup, the better you are

Sure, bigger turbos make more topend. Sorta easy, if you target a 2 thousand rpm window and don't CARE about the rest (race/drag car)...the harder part is finding YOUR balance, especially those of us who street/roadcourse drive when losing spoolup/low/midrange really hurts.
Jack T.

 

Damn, and there you have it. Jack just summed up 300 pages on turbos into one very good post. Awesome!

I have one wish... that they could figure out a way to have a data cable that hooks up to Jack's brain and downloads everything to mine. Just make sure there's a check valve in it so that none of my stuff gets back to him.

 

subscribe :mitsu:

 

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pet peeve # 15

Jim Berry

 

TTT

Smart people keep talking. I am really interested in Chris Wood's comments. This is usually his conversational piece.

 

A Compressor makes PSI. PSI makes AIRFLOW. _WHAT/HOW MUCH_ you are flowing (AIR DENSITY/MASS) relates to TEMPERATURE of that air: Larger/moreEfficeint Compressor=Cooler=Denser=more O2 molecules/mass flowed at SAME PSI.

There's that intake 'temperature' thing again. So once again, cooler denser air means more O2. A bigger turbo can achieve that with less effort, no doubt. Plus, efficient intercooling can make a big difference in introducing cooler, denser air to your engine.

I don't mean to be defensive, but there are so many posts lately that dismiss the value of upgraded intercoolers (especially FMICs), labeling them as useless unless you're producing over 850 HP. It would seem that cooler air would produce more power at lower psi even at 400-500 HP.

 

There's that intake 'temperature' thing again. So once again, cooler denser air means more O2. A bigger turbo can achieve that with less effort, no doubt. Plus, efficient intercooling can make a big difference in introducing cooler, denser air to your engine.

I don't mean to be defensive, but there are so many posts lately that dismiss the value of upgraded intercoolers (especially FMICs), labeling them as useless unless you're producing over 850 HP. It would seem that cooler air would produce more power at lower psi even at 400-500 HP.

Front mounts are fairly useless, our stock intercoolers are pretty darn good, after a 1/4 pass I can grab onto them without gloves, unlike my friends WRX (I think it being mounted on top of the engine has a lot to do with that though). The only gripe I have with the stock intercoolers is at high airflow they will choke, but if you're on stock turbos stock intercoolers are more then enough. Unless you're a dyno queen, becuase I've taken my car through road courses and not felt a noticeable drop in power lap after lap. The reason people dyno with big gains after putting on a big front mount is because on a dyno the intercooler is a heat sink, not an intercooler, more mass means it cools the air for longer before it starts to heat soak.

 

Front mounts are fairly useless, our stock intercoolers are pretty darn good, after a 1/4 pass I can grab onto them without gloves, unlike my friends WRX (I think it being mounted on top of the engine has a lot to do with that though). The only gripe I have with the stock intercoolers is at high airflow they will choke, but if you're on stock turbos stock intercoolers are more then enough. Unless you're a dyno queen, becuase I've taken my car through road courses and not felt a noticeable drop in power lap after lap. The reason people dyno with big gains after putting on a big front mount is because on a dyno the intercooler is a heat sink, not an intercooler, more mass means it cools the air for longer before it starts to heat soak.

I agree with you on stock turbos, but relative to 'larger turbos and max CFM' (the subject of this entire thread), upgraded intercoolers are definitely not useless. Check out http://www.3si.org/forum/showthread.php?t=183568 so as not to hijack this thread and make it about intercooling, and see that Matt Monett (from building and racing experience), Jeff Lucius (from the engineering and scientific perspective, plus experience), Corkey Bell (from experience and intercooler engineering), and others make the point that upgraded intercooling means more power, all without ever going to a dyno.

Yours may be cool to the touch, and that's great. In the same thread, read that IPO couldn't possibly touch his after a few hard runs, thus his upgrade to better SMICs.

My comments here are in support of the post that Jack T. made about larger turbos making cooler, denser air. Upgraded intercoolers are superior to stock intercoolers in making cooler, denser air, and that means less detonation and more power at constant psi.