427 engine (part 2) - RHS block

[Rich Z]

Just out of curiosity, what really happens when a turbo is run outside of it's efficiency range?

[Benjamin]

Originally Posted by Rich Z

Just out of curiosity, what really happens when a turbo is run outside of it's efficiency range?

The ratio, intake pressure to exhaust pressure, changes oddly and rapidly. In this situation, you are probably running in the 40-55% efficiency range, as these maps map out 65+%. in doing so the compressor is requiring more from the exhaust side for the given pressure / volume needed on the intake side.

On MOST turbos, I see the people over driving them on the big end, example, 12 valve Cummins diesel, with a stock Holset HX35 turbo, running 30psi you are probably in the 70-75% efficiency range. that is the sweet spot for that turbo, but to get more boost, say 40psi, this in turn increases the air volume since the pressure ratio increases, making the needed pattern to shift up and to the right on the map, and it gets out of the efficiency range and over spins the turbo (the numbers to the right of the map is the shaft speed in RPMs, as they normally operate at 75K-140K RPMs), leading to bearing or shaft failure and makes them go do far out of their efficiency range that the air temp goes through the room and you are making more heat than anything, so you may get 5-10psi more boost but see a 100+* increase in air temps and that in turn negates any gains you may get from the 5-10psi increase.

[Benjamin]

to show an accurate representation of what would be an ideal RPM range vs. volume and RPM for the 427 motor, we will assume the pressure ratio comes out to be (14.7 + 14.7) / 14.7 = 2.0, 14.7 PSI out of the turbo.

upper limit is 3205rpm
CFM = (7.0 x 3205 x 90 x 2.0) / 5660 = 713.5CFM or 50lb/min. To convert this value to lb/min take CFM and divide by 14.27.

For 1282RPM:
CFM = (7.0 x 1282 x 90 x 1.68) / 5660 = 285.4CFM or 20 lb/min.

using 25lb/min as the upper limit, and 10lb/min as the lower, for the turbo to be in peak efficiency for the highest amount of time, in relation to the volume of air your motor needs @ a 90% VE, you would have to have 14.7psi @ 1282RPM and have a max RPM of 3205RPM



that is ideally how a matched turbo should plot out in relation to flow and pressure for a given engine. you want as much of that center ellipse in the plotted area as you can get.

[Rich Z]

Originally Posted by Benjamin

to show an accurate representation of what would be an ideal RPM range vs. volume and RPM for the 427 motor, we will assume the pressure ratio comes out to be (14.7 + 14.7) / 14.7 = 2.0, 14.7 PSI out of the turbo.

upper limit is 3205rpm
CFM = (7.0 x 3205 x 90 x 2.0) / 5660 = 713.5CFM or 50lb/min. To convert this value to lb/min take CFM and divide by 14.27.

For 1282RPM:
CFM = (7.0 x 1282 x 90 x 1.68) / 5660 = 285.4CFM or 20 lb/min.

using 25lb/min as the upper limit, and 10lb/min as the lower, for the turbo to be in peak efficiency for the highest amount of time, in relation to the volume of air your motor needs @ a 90% VE, you would have to have 14.7psi @ 1282RPM and have a max RPM of 3205RPM

Attachment 5736

that is ideally how a matched turbo should plot out in relation to flow and pressure for a given engine. you want as much of that center ellipse in the plotted area as you can get.

Well, I know I said I would prefer the turbos to kick in sooner rather than later, but somehow I am doubtful that I will EVER see that much boost at 1282 rpm. Nor would I want the turbos to peak out at 3200 rpm, neither.

So maybe I am just missing something here.....

I've been pondering what you are saying about the engine's VE (volumetric efficiency) and I'm a bit confused. I've been working a LOT with the VE tables in EFILive, and they are TABLES, not a single value. There is one table from 15 to 105 kPa (B0101) and another for boost (A0009) starting at 105 kpa and going up to 285 kPa. I think with the 14 lb springs in the new wastegates, I will be limited to under 200 kPa, IF boost will even reach that high. I've actually been massaging A0009 tonight, first from the data from my last logging run, and then filling in cells to make sure they won't be leaning out the AFR on me when I do run with the new wastegates.

But in any event, it would appear to me that there will be a whole range of VE values for an engine in relation to a turbo in real life, not just one static value. And then there is the thought I had that doesn't forced induction itself change the VE values? Supposedly you can reach a VE of MORE than 100 percent, but so far I haven't seen anything like that in my own VE tables. The most I am seeing in A0009 at 105 kPa and above from data actually logged is around 85 to 90 percent.

You know, I have to wonder how many people really check those efficiency tables (much less even know how to utilize them) when looking to pick out turbos for their cars? I've read a fair number of threads about people selecting or upgrading their turbos, and pretty much it goes "that one looks bigger, so it should be what I need".

[Benjamin]

Originally Posted by Rich Z

Well, I know I said I would prefer the turbos to kick in sooner rather than later, but somehow I am doubtful that I will EVER see that much boost at 1282 rpm. Nor would I want the turbos to peak out at 3200 rpm, neither.

So maybe I am just missing something here.....

that was just a theoretical "at what RPM vs. Volume vs. Pressure would that turbo work in it's ideal efficiency range on your engine. that turbo is better suited for a smaller displacement engine than a 7L 427ci monster, err well 1/2 that a 3.5L 213ci engine. That turbo, the T-3 60 trim with a .63 A/R housing is optimal for a 2.3l motor. let me do a comparison on a 2.3L motor with 90%VE with 3000rpm lower limit and 6000RPM upper limit and 18psi as that is an ideal pressure easily obtainable and safe on a 2.3L turbo motor.

The pressure ratio comes out to be (14.7 + 18) / 14.7 = 2.22, 18 PSI out of the turbo.

upper limit is 6000rpm
CFM = (2.3 x 6000 x 90 x 2.22) / 5660 = 487.14CFM or 34.13lb/min. To convert this value to lb/min take CFM and divide by 14.27.

lower limit is 3000RPM:
CFM = (2.3 x 3000 x 90 x 2.22) / 5660 = 243.57CFM or 17 lb/min.

using 34lb/min as the upper limit, and 17lb/min as the lower, you can see this turbo would be well suited for use on a 2.3 engine. at a lower pressure ratio, maybe a 2.0 (14.7psi) it would likely be even more efficient than at 18psi.



now there are other variables that you can plot out IF you know your boost curve, like at what RPM you make 1PSI, what RPM you make 5PSI, and you can plot those marks up to your peak RPM and PSI readings to get a truer reading off of the map. this was just for generalization purposes, assuming you would be at PEAK boost at the lower RPM. I know 15-18 PSI can be made on a 2.3 with this turbo at 3000RPM. I have done it myself.

if you can provide me a map or list from 0PSI to 8PSI and the RPM ranges associated to the boost readings, I will plot them accordingly on the T-3 map for you to show the lb/min change across the board with RPM and boost varying.

[Benjamin]

Originally Posted by Rich Z

I've been pondering what you are saying about the engine's VE (volumetric efficiency) and I'm a bit confused. I've been working a LOT with the VE tables in EFILive, and they are TABLES, not a single value. There is one table from 15 to 105 kPa (B0101) and another for boost (A0009) starting at 105 kpa and going up to 285 kPa. I think with the 14 lb springs in the new wastegates, I will be limited to under 200 kPa, IF boost will even reach that high. I've actually been massaging A0009 tonight, first from the data from my last logging run, and then filling in cells to make sure they won't be leaning out the AFR on me when I do run with the new wastegates.

But in any event, it would appear to me that there will be a whole range of VE values for an engine in relation to a turbo in real life, not just one static value. And then there is the thought I had that doesn't forced induction itself change the VE values? Supposedly you can reach a VE of MORE than 100 percent, but so far I haven't seen anything like that in my own VE tables. The most I am seeing in A0009 at 105 kPa and above from data actually logged is around 85 to 90 percent.

You know, I have to wonder how many people really check those efficiency tables (much less even know how to utilize them) when looking to pick out turbos for their cars? I've read a fair number of threads about people selecting or upgrading their turbos, and pretty much it goes "that one looks bigger, so it should be what I need".

yes, there are times when it will be over 100%, though rare on a street car. if you use the VE readings in your computer, it will give you a better look at the volume the engine would need at a certain PSI reading. they vary throughout the RPM band, but you will see at whatever the peak RPM is for your TQ output the VE value should be 98+%.

there are always variables that will cause the mapping to change, air density, barometric pressure, temperature, altitude etc. if you factor all those things into the equation then you'll likely never get a true answer to what is needed.

[Rich Z]

Originally Posted by Benjamin

if you can provide me a map or list from 0PSI to 8PSI and the RPM ranges associated to the boost readings, I will plot them accordingly on the T-3 map for you to show the lb/min change across the board with RPM and boost varying.

Maybe I'm missing something, but it doesn't appear to be that simple to me to plot a single axis map relating engine speed (rpm) to boost (kPa). To give you a few illustrating examples of what I mean, I'm looking at the last log I captured and I see this:

• 4703 rpm, 48% throttle, 69.0 kPa
• 4706 rpm, 81% throttle, 88.0 kPa
• 4895 rpm, 97% throttle, 150 kPa
• 4920 rpm, 48% throttle, 71 kPa
• 5501 rpm, 55% throttle, 112 kPa
• 5531 rpm, 100% throttle, 144 kPa

All of the above are taken in the upslope of throttle engagement, and are not values taken during deceleration.

So engine speed is not the only determining factor of vacuum or boost in the intake manifold. And neither is throttle position by itself.

Heck, even using a 2 dimensional grid map that has engine speed on one axis and manifold vacuum/boost on the other will not show the same values at every intersectimg cell of those two axis while driving. While calibrating the VE tables, I have to take the AVERAGE of all the values produced for each cell, so obviously there is at least a third variable in a three dimensional grid necessary to properly plot what produces a set (and predictable) value of vacuum or boost.

[Benjamin]

Ideally, and this could be done on a dyno or "on a closed course with a professional driver", you would need to be in 4th gear, assuming it is the 1:1 ratio in the trans, and do a 1500-2000RPM start up to your max RPM, but this would need to be at WOT, 100% throttle as that is the constant needed for the above chart to work for me. with this info, you will know when boost starts, when it peaks and holds at max PSI. using this info I can plot the graph from an off-idle scenario and this will help understand the volume needed in relation to RPM and boost as the boost pressure will manipulate the volume numbers on the X axis.

[Benjamin]

Originally Posted by Rich Z

Maybe I'm missing something, but it doesn't appear to be that simple to me to plot a single axis map relating engine speed (rpm) to boost (kPa).

I will use the engine speed and boost for that specific engine speed to plot the flow, in lb/min or CFM, across the map at that specific pressure ratio. so the boost number will help equate the number on the Y axis, and the pressure ratio along with the rpm, VE% are used to calculate the flow, or volume needed/used and plot it on the X axis

[Rich Z]

Originally Posted by Benjamin

Ideally, and this could be done on a dyno or "on a closed course with a professional driver", you would need to be in 4th gear, assuming it is the 1:1 ratio in the trans, and do a 1500-2000RPM start up to your max RPM, but this would need to be at WOT, 100% throttle as that is the constant needed for the above chart to work for me. with this info, you will know when boost starts, when it peaks and holds at max PSI. using this info I can plot the graph from an off-idle scenario and this will help understand the volume needed in relation to RPM and boost as the boost pressure will manipulate the volume numbers on the X axis.

Ah, well, neither one of those appear to be likely anytime soon, if at all. I only know of two places close by (South Georgia Corvette and SS Performance) with dynos, and neither one of them are going to ever have my cars in their shops.

As for a "professional driver" and a closed course, not sure where I could find something like that nearby. I believe redline on the engine is 7000 rpm if I remember correctly, so not likely I could (nor would want to) do that on even the little traveled roads around here.

I appreciate all the insight you are giving me with this, but I'm not sure there is going to be any feasible way to give you the data needed to try to identify the best turbos for my setup. Actually, the way my luck runs, there probably doesn't exist such a beast anyway. Looking at my data logs, I can see incoming air temperature rising pretty rapidly while I am in boost, so I guess STAYING in boost for long periods of time wouldn't be a good idea for me to do. Not that I have been intending to do so anyway.......

So maybe by the time I wear out the turbos I've got and start looking around for replacements I'll have a better understanding of exactly what I need to be looking for. But for now, unless I find that there is a severe problem using what I've got, I guess I've got to be satisfied with them and what they can do.