Turbo trim
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Joined: Apr 2003
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From: Cummins Technical Center, IN
Higher trim flows more for a given exducer-- but higher trims generally can't deliver the higher PRs very efficiently.
That's not something that tracks across turbo sizing. A larger turbo will generally be able to do more PR even with the same trim wheel because the larger diameter has higher tangential velocity.
Trim alone doesn't really tell you all that much, so you can't draw meaningful conclusions from it-- you can just note trends and generalizations. Wheel design elements like number of fins and such will trump trim imo.
As for plotting operating points, you have to know what you're shooting for and how it plays out on a given engine. This is where my big mama-jama spreadsheet helps.
I start with a HP goal. You want so many hp at so many RPM. Say we want 500hp at 2700rpm. After some calculations involving air fuel ratio, BSFC, volumetric efficiency, rpm, and so forth you come up with how many pounds of air at what PR you need to meet the goal you set.
For example, given the BSFC of a CTD at 2700rpm of .365, a volumetric efficiency of 95%, and air:fuel ratio of 22:1 (no smoke, decent EGT), we calculate that 500hp at 2700rpm will require 66.92 lb/min of air.
The amount of boost it takes to cram in that much air depends on intercooling efficiency, turbocharger efficiency, and atmospheric pressure and temp. For example, if the turbo is sucking in air at 90 degrees and 13.5psi (after air filter) and your intercooler is dropping temps by 320º, then it will take 40psi of boost to support 500hp at 2700 rpm.
The operating PR is (40psi of boost+13.5atmospheric)/13.5atmospheric= about 3.9:1
Now you change ANY of the conditions and things can vary a lot. Say your intercooler only drops temps to 150 in the hat, your turbo is only 65% efficient, and the inlet temp is 115 degrees. Now it will take 43psi to do what 40psi was doing before.
Anyway, I just do those calcs with a HP goal in mind based on the use of the truck. If towing is the game, then shoot for 300hp @ 2000rpm or so. Sledpulling, maybe 500hp at 3000rpm.
Based on the dyno sheets people have posted, you can build a power curve, then put that point in the spreadsheet and plot each point on the map.
WHERE the point falls on the map is important. If it's on the far right, you're pushing a small compressor to the limits, and maximizing spoolup. If it's on the far left, you're skirting the surge line with an oversized compressor (but you can get away with this by inducing intentional lag with a larger housing).
If it's in the middle, then you're in the "sweet spot", but that's not always where you want to be. You don't want your "sweet spot" to be in the middle of the map at peak HP unless you're building a dyno queen.
That's why you plot multiple operating points on the map.
Unfortunately, if you do this for a lot of points, you'll almost completely abandon the idea of a single turbo unless you're happy with very little low end power.
A single can make big power on these trucks, yes. But it can't make EFFICIENT power more than 470hp or so, no matter how big the compressor is. You run into the PR limitation more than anything else, and THAT is what drives the needs for twins-- PR. NOt flow, not spoolup--PR.
Justin
That's not something that tracks across turbo sizing. A larger turbo will generally be able to do more PR even with the same trim wheel because the larger diameter has higher tangential velocity.
Trim alone doesn't really tell you all that much, so you can't draw meaningful conclusions from it-- you can just note trends and generalizations. Wheel design elements like number of fins and such will trump trim imo.
As for plotting operating points, you have to know what you're shooting for and how it plays out on a given engine. This is where my big mama-jama spreadsheet helps.
I start with a HP goal. You want so many hp at so many RPM. Say we want 500hp at 2700rpm. After some calculations involving air fuel ratio, BSFC, volumetric efficiency, rpm, and so forth you come up with how many pounds of air at what PR you need to meet the goal you set.
For example, given the BSFC of a CTD at 2700rpm of .365, a volumetric efficiency of 95%, and air:fuel ratio of 22:1 (no smoke, decent EGT), we calculate that 500hp at 2700rpm will require 66.92 lb/min of air.
The amount of boost it takes to cram in that much air depends on intercooling efficiency, turbocharger efficiency, and atmospheric pressure and temp. For example, if the turbo is sucking in air at 90 degrees and 13.5psi (after air filter) and your intercooler is dropping temps by 320º, then it will take 40psi of boost to support 500hp at 2700 rpm.
The operating PR is (40psi of boost+13.5atmospheric)/13.5atmospheric= about 3.9:1
Now you change ANY of the conditions and things can vary a lot. Say your intercooler only drops temps to 150 in the hat, your turbo is only 65% efficient, and the inlet temp is 115 degrees. Now it will take 43psi to do what 40psi was doing before.
Anyway, I just do those calcs with a HP goal in mind based on the use of the truck. If towing is the game, then shoot for 300hp @ 2000rpm or so. Sledpulling, maybe 500hp at 3000rpm.
Based on the dyno sheets people have posted, you can build a power curve, then put that point in the spreadsheet and plot each point on the map.
WHERE the point falls on the map is important. If it's on the far right, you're pushing a small compressor to the limits, and maximizing spoolup. If it's on the far left, you're skirting the surge line with an oversized compressor (but you can get away with this by inducing intentional lag with a larger housing).
If it's in the middle, then you're in the "sweet spot", but that's not always where you want to be. You don't want your "sweet spot" to be in the middle of the map at peak HP unless you're building a dyno queen.
That's why you plot multiple operating points on the map.
Unfortunately, if you do this for a lot of points, you'll almost completely abandon the idea of a single turbo unless you're happy with very little low end power.
A single can make big power on these trucks, yes. But it can't make EFFICIENT power more than 470hp or so, no matter how big the compressor is. You run into the PR limitation more than anything else, and THAT is what drives the needs for twins-- PR. NOt flow, not spoolup--PR.
Justin
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TheMoose
3rd Generation Ram - Non Drivetrain - All Years
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Aug 6, 2003 09:48 AM
