Centrifugals and Screws, CompRatio vs. Boost and Detonation?????
10-31-2002, 02:54 PM
#1
Centrifugals and Screws, CompRatio vs. Boost and Detonation?????
Just thinking out loud here and would like to hear comments….
Regarding what would be a safe CR to run with a supercharger (SC), I’m thinking that much of that depends on the type of SC itself, i.e. screw, roots or centrifugal.
It appears, and I think most agree, that most general detonation generally occurs in the peak torque range of the motor when cylinder pressures are the highest. In most of our marine applications this rpm range is somewhere around the 3400-4200 rpm range.
I believe this thought can be verified on some production ECM timing curves setup for boost. For example, under full load on a motor that has peak torque around 3800 rpm they gradually increase timing from idle until it peaks around 3200 rpm then begin to pull it out as it approaches the 3800 mark and then gradually add it back in as it passes this point and cylinder pressures become less and thus allowing for more timing as you approach WOT.
That being said (and if accurate) I’m pondering the following.
With a roots or screw type SC on a BBC at full load you will be seeing full boost, say 5-7 lbs (under load) at these peak torque rpms so you would want to make sure your compression ratio is compatible; somewhere around 7.5:1-8.5:1.
With a centrifugal type supercharger (ProCharger, Vortech, etc…), when at peak torque rpm under full load, you will only be seeing about 3-4 lbs of the max 5-7 lbs you will see at WOT rpm when cylinder pressures are less. I would think this means a lesser chance of seeing detonation and possibly allow the use of slightly more CR.
This is not to say that one is better than the other but merely a different thought on setting up to achieve optimum results safely.
I’m thinking (and have been advised) that you should run slightly more static CR with a centrifugal, say 8.5:1-9.0:1. As discussed many times, the centrifugals lack bottom end punch. Lowering compression unnecessarily will only make this worse. Maybe you could run slightly more CR because you wouldn’t ever see full boost in the peak torque range. Also, if your fortunate enough to have the capacity, you will want to introduce more timing quicker in the lower rpms on a centrifugal setup to aid in bottom end punch where there is virtually no boost, and then pull it out as you approach the peak torque zone.
Just curious, am I on or off track with this? My interest, of course, is due to my project, which is a 9.2:1, 540ci, M3/504 PC, with aftermarket MPI and a F.A.S.T. ECU.
Of course I do run aluminum heads and boat primarily at 3600-6000 feet and will keep a larger pulley for the occasional jaunts down to, or near sea level.
Again, only thinking out loud...What can I say, it's getting cold and bordome is setting in already.
Later
Dave
Regarding what would be a safe CR to run with a supercharger (SC), I’m thinking that much of that depends on the type of SC itself, i.e. screw, roots or centrifugal.
It appears, and I think most agree, that most general detonation generally occurs in the peak torque range of the motor when cylinder pressures are the highest. In most of our marine applications this rpm range is somewhere around the 3400-4200 rpm range.
I believe this thought can be verified on some production ECM timing curves setup for boost. For example, under full load on a motor that has peak torque around 3800 rpm they gradually increase timing from idle until it peaks around 3200 rpm then begin to pull it out as it approaches the 3800 mark and then gradually add it back in as it passes this point and cylinder pressures become less and thus allowing for more timing as you approach WOT.
That being said (and if accurate) I’m pondering the following.
With a roots or screw type SC on a BBC at full load you will be seeing full boost, say 5-7 lbs (under load) at these peak torque rpms so you would want to make sure your compression ratio is compatible; somewhere around 7.5:1-8.5:1.
With a centrifugal type supercharger (ProCharger, Vortech, etc…), when at peak torque rpm under full load, you will only be seeing about 3-4 lbs of the max 5-7 lbs you will see at WOT rpm when cylinder pressures are less. I would think this means a lesser chance of seeing detonation and possibly allow the use of slightly more CR.
This is not to say that one is better than the other but merely a different thought on setting up to achieve optimum results safely.
I’m thinking (and have been advised) that you should run slightly more static CR with a centrifugal, say 8.5:1-9.0:1. As discussed many times, the centrifugals lack bottom end punch. Lowering compression unnecessarily will only make this worse. Maybe you could run slightly more CR because you wouldn’t ever see full boost in the peak torque range. Also, if your fortunate enough to have the capacity, you will want to introduce more timing quicker in the lower rpms on a centrifugal setup to aid in bottom end punch where there is virtually no boost, and then pull it out as you approach the peak torque zone.
Just curious, am I on or off track with this? My interest, of course, is due to my project, which is a 9.2:1, 540ci, M3/504 PC, with aftermarket MPI and a F.A.S.T. ECU.
Of course I do run aluminum heads and boat primarily at 3600-6000 feet and will keep a larger pulley for the occasional jaunts down to, or near sea level.
Again, only thinking out loud...What can I say, it's getting cold and bordome is setting in already.
Later
Dave
10-31-2002, 04:20 PM
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The advantage with the procharger is with less boost at low end you have less torque and your bravo drive is more likely to live. Your reasoning sounds good to me. With all the high tech stuff you are putting on engine you should know before the motor goes south
10-31-2002, 06:47 PM
#5
What cam? Your heads might be holding it back a bit on a 540 motor. Would like to see about 400/300 at .600". So what boat? Does it have a Stelling Ext Box? -into which an On-Board-Dyno could bolt into!
11-04-2002, 09:34 AM
#6
Hey Cobra Marty,
just to answer your questions; the cam is an UltraDyne 232/240 114 .600/.600. The motor was built for the high altitude (4000-6000 feet) duty that it is run at the majority of the time and I have always been told that large heads don't work to well when moving a lot of thin air. Velocity being the much more important factor here
Cam selection is also a factor with high altitude when you are trying to retain as much cylinder pressure as possible. Anyway, I had considered changing out the heads but they seem to work really well. As a good friend of mine pointed out, we don't think that we are going to be horsepower deprived
The boat is a 28 foot, 4800 lb, single step Nordic with a very high X dimension and naturally aspirated, runs 70 at 5000 feet and mid 70's at 1250 feet all on GPS.
Doesn't have an extension box and I'm not sure what the onboard dyno is. Could you maybe briefly explain?
Thanks,
Dave
just to answer your questions; the cam is an UltraDyne 232/240 114 .600/.600. The motor was built for the high altitude (4000-6000 feet) duty that it is run at the majority of the time and I have always been told that large heads don't work to well when moving a lot of thin air. Velocity being the much more important factor here
Cam selection is also a factor with high altitude when you are trying to retain as much cylinder pressure as possible. Anyway, I had considered changing out the heads but they seem to work really well. As a good friend of mine pointed out, we don't think that we are going to be horsepower deprived
The boat is a 28 foot, 4800 lb, single step Nordic with a very high X dimension and naturally aspirated, runs 70 at 5000 feet and mid 70's at 1250 feet all on GPS.
Doesn't have an extension box and I'm not sure what the onboard dyno is. Could you maybe briefly explain?
Thanks,
Dave
11-04-2002, 05:13 PM
#7
Hey Dave, The problem is dyno time cvosts about $500 and you have to pull the motor and use different exhaust, fuel and ignition system--all not what you run in the boat. Well I have a device the On-Board-Dyno the attaches to the drive shaft on a stellings box and it measures the actual real time Torque and rpm and calculates Hp all while you drive the boat. It can send a signal to a digital display, data logger or to custom 270 degree sweep guage with 0 to 1000HP or TQ. So any changes can be seen immediately while driving the boat. Look for more soon about this.
11-05-2002, 10:18 AM
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cobra marty
i think your on-board-dyno has to be one of the most innovative items i’ve seen generated via this board! it’s a great idea, i’ve been following it for almost a year now and i hope it all comes together for you.
question is, in an application like NordicHeat’s, are you incorporating provisions to link the data to to a laptop and have that info appear in real time, possibly transposed with his other data, available for recording and analyzing at a later date?
question is, in an application like NordicHeat’s, are you incorporating provisions to link the data to to a laptop and have that info appear in real time, possibly transposed with his other data, available for recording and analyzing at a later date?
11-05-2002, 10:54 AM
#9
Dean,
It's output is 0-5v and will go to special faced, white on white 2-5/8" 270degree sweep Nordskod guages, digital readout, or to a laptop or data logger which accepts 0-5v signal. The guages will be in real time while you drive as well as the laptop or data logger.
Right now it is for the stelling ext box for the coupler inside it and I hope next to get it on the trans or shaft of a VI drive.
It's all starting to come together now and I'll post soon. Marty.
It's output is 0-5v and will go to special faced, white on white 2-5/8" 270degree sweep Nordskod guages, digital readout, or to a laptop or data logger which accepts 0-5v signal. The guages will be in real time while you drive as well as the laptop or data logger.
Right now it is for the stelling ext box for the coupler inside it and I hope next to get it on the trans or shaft of a VI drive.
It's all starting to come together now and I'll post soon. Marty.
11-05-2002, 07:49 PM
#10
1975 X-18 Donzi, Arneson
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Dave,
I run a 434ci SBC, 10:1 cr, Procharger M3, AW324 IC, F.A.S.T. ECU, Wide band O2 sensor, 3.8 lbs boost @ 6300rpm's 738hp. 92 oct., run engine with cross-over and no thermostat, engine run's under 100 degrees, timing set max at 32 degrees. 12.2 A/F, 55lbs injectors @ 60psi. 1/2" (#10) fuel lines to pump, #8 lines to fuel rails.
The more cr you run, the less boost you can run, by keeping things cold less chance of detination. (Keep engine and intake air cool)
The more CR the better the low end power.
I was running my engine NA at 575hp and wanted more power so added the blower. If I was starting from scratch I would run less CR maybe 9:1 and increase boost.
It depends on the size and weight of the boat and the low end power needed.
I run a 434ci SBC, 10:1 cr, Procharger M3, AW324 IC, F.A.S.T. ECU, Wide band O2 sensor, 3.8 lbs boost @ 6300rpm's 738hp. 92 oct., run engine with cross-over and no thermostat, engine run's under 100 degrees, timing set max at 32 degrees. 12.2 A/F, 55lbs injectors @ 60psi. 1/2" (#10) fuel lines to pump, #8 lines to fuel rails.
The more cr you run, the less boost you can run, by keeping things cold less chance of detination. (Keep engine and intake air cool)
The more CR the better the low end power.
I was running my engine NA at 575hp and wanted more power so added the blower. If I was starting from scratch I would run less CR maybe 9:1 and increase boost.
It depends on the size and weight of the boat and the low end power needed.
Last edited by GEOO; 11-08-2002 at 12:05 PM.
