New Whipple 2.9L compressor, 175AX
10-19-2008, 09:20 PM
#22
The reason I was suggesting modifying the the plumbing in and out of the intercooler is to relocate the entire thing forward which would hopefully allow a 3.75" diameter cold air supply tube from the passenger side to sit on top of the intake manifold behind the intercooler.
I took into consideration that you said dropping the pulley diameter the last few increments did not make significant changes in boost. That should not be the case unless there were a restriction. It is my opinion that it needs, at minimum, a 3.75" diameter supply tubes from the filter to the mass air flow sensor and from the MAF to the throttle body. 4" would be even better. If the air pressure in the current cold air supply could be measured right before the throttle body during wide open throttle, I am sure you'd see vacuum there. There might be a pound of boost there to gain from a more freely-flowing induction system. To make room for it would require moving the intercooler forward. If the truck will run with the MAF disconnected you could experiment by removing the supply air crossover tube entirely with the TB inlet open to the engine compartment. If you get more boost it might be worth it to you to redesign the inlet air system. (Make sure there is no loose hood insulation near there or it will get sucked right in and ruin the compressor.)
I took into consideration that you said dropping the pulley diameter the last few increments did not make significant changes in boost. That should not be the case unless there were a restriction. It is my opinion that it needs, at minimum, a 3.75" diameter supply tubes from the filter to the mass air flow sensor and from the MAF to the throttle body. 4" would be even better. If the air pressure in the current cold air supply could be measured right before the throttle body during wide open throttle, I am sure you'd see vacuum there. There might be a pound of boost there to gain from a more freely-flowing induction system. To make room for it would require moving the intercooler forward. If the truck will run with the MAF disconnected you could experiment by removing the supply air crossover tube entirely with the TB inlet open to the engine compartment. If you get more boost it might be worth it to you to redesign the inlet air system. (Make sure there is no loose hood insulation near there or it will get sucked right in and ruin the compressor.)
Last edited by James B.; 10-19-2008 at 09:26 PM.
10-19-2008, 09:47 PM
#23
The reason I was suggesting modifying the the plumbing in and out of the intercooler is to relocate the entire thing forward which would hopefully allow a 3.75" diameter cold air supply tube from the passenger side to sit on top of the intake manifold behind the intercooler.
I took into consideration that you said dropping the pulley diameter the last few increments did not make significant changes in boost. That should not be the case unless there were a restriction. It is my opinion that it needs, at minimum, a 3.75" diameter supply tubes from the filter to the mass air flow sensor and from the MAF to the throttle body. 4" would be even better. If the air pressure in the current cold air supply could be measured right before the throttle body during wide open throttle, I am sure you'd see vacuum there. There might be a pound of boost there to gain from a more freely-flowing induction system. To make room for it would require moving the intercooler forward. If the truck will run with the MAF disconnected you could experiment by removing the supply air crossover tube entirely with the TB inlet open to the engine compartment. If you get more boost it might be worth it to you to redesign the inlet air system. (Make sure there is no loose hood insulation near there or it will get sucked right in and ruin the compressor.)
I took into consideration that you said dropping the pulley diameter the last few increments did not make significant changes in boost. That should not be the case unless there were a restriction. It is my opinion that it needs, at minimum, a 3.75" diameter supply tubes from the filter to the mass air flow sensor and from the MAF to the throttle body. 4" would be even better. If the air pressure in the current cold air supply could be measured right before the throttle body during wide open throttle, I am sure you'd see vacuum there. There might be a pound of boost there to gain from a more freely-flowing induction system. To make room for it would require moving the intercooler forward. If the truck will run with the MAF disconnected you could experiment by removing the supply air crossover tube entirely with the TB inlet open to the engine compartment. If you get more boost it might be worth it to you to redesign the inlet air system. (Make sure there is no loose hood insulation near there or it will get sucked right in and ruin the compressor.)
Here is a better picture of it without the intercooler installed.
Last edited by XLR8NSS; 10-21-2008 at 09:28 PM.
10-20-2008, 11:10 AM
#24
James (or anyone i guess),
Can you show and or explain to me the math behind why the 2.3L blower is out of its efficiency range trying to fill up the 454/496? Mike and Jeff at Whipple said they use that blower on the marine MAG 496 kits and get more boost out of them. They think there is something else going on and I would like to show them numbers that dispute that.
Can you show and or explain to me the math behind why the 2.3L blower is out of its efficiency range trying to fill up the 454/496? Mike and Jeff at Whipple said they use that blower on the marine MAG 496 kits and get more boost out of them. They think there is something else going on and I would like to show them numbers that dispute that.
10-20-2008, 05:26 PM
#26
You're MAFless, aren't you? If so, it sure would be easy to do James B's test. Just because your inlet is 4" doesn't mean it is not a restriction. That is a long straw you are sucking through. You know schoolbuses don't need mufflers because their exhaust pipe is so long, don't you? Just go to the auto parts store and get a Chinese "K&N" cone filter and jam it onto your throttle body to test. Don't oil it, to reduce its restriction.
10-20-2008, 06:30 PM
#27
You're MAFless, aren't you? If so, it sure would be easy to do James B's test. Just because your inlet is 4" doesn't mean it is not a restriction. That is a long straw you are sucking through. You know schoolbuses don't need mufflers because their exhaust pipe is so long, don't you? Just go to the auto parts store and get a Chinese "K&N" cone filter and jam it onto your throttle body to test. Don't oil it, to reduce its restriction.
10-21-2008, 01:54 AM
#29
Unfortunately Whipple doesn't post performance maps on their own Screws, but Lysholm does and those will be close enough:
I'm going to guess the crank pulley is the standard 7.5" diameter. The S/C pulley is 2.375", so 3.16x overdriven. I'm also going to guess you're shifting around 4800 RPM generally, maybe less or more under certain conditions.
Figuring about 4800RPM x 3.16 is over 15,000RPM input speed into the supercharger at about maximum engine speed.
These charts are based on pressure differential. That means difference between inlet and outlet. The tricky part here is that we do not know what either of these numbers actually are. Consider atmospheric pressure to be 14.5psi. If there is a restriction in the inlet tube it will reduce available "atmosphere" into the inlet. For example, at wide open throttle maybe there is only 13 psi of "atmospheric pressure" at the inlet. 1.5psi of loss here reduces output similarly, but multiplied by pressure ratio. Back to pressure ratio, we don't actually know what the outlet pressure is either because the intercooler is a restriction itself. You may have heard those intercooler are no longer manufactured. It's a highly parellel design (excellent) but the relatively small cross section and long cores make for some backpressure and loss across the heat exchanger. One of the guys that maxed out an 8100 Intercooled Whipple kit was Tom Byrne, he had a blue/gray 2500 Suburban everyone's probably seen a video of before. I can't recall exactly but I remember the pressure drop across the intercooler being about 2 psi. (He was also had one of the earliest kits and was plaqued by PCM "reduced-power" mode problems for a long time before tuning was as easy as it is today.)
If both the engine and the blower were 100% efficient air pumps, the engine would be pumping 248 cubic inches per revolution and the super charger would be moving 140 cubic inches per its revolution. It isn't simple math to determine how much boost you get out of this, you have to consult one of these charts and figure from there.
You're getting 6psi at best. With the intercooler pressure drop the blower outlet is probably at about 8psi boost (22.5psi atmosphere based on 13psi inlet pressure). That works out to a 1.7 pressure ratio at the flow you've got right now, approximately. That number can be checked against airflow, but what we really want to do is transpose the known numbers from the 2300 map to the 3300 map.
Take one paticular plot point from the 2300 map and move it to the 3300 map to see what you get. For example, take 1.7 P/R and 25 cubic meter/min and see that producing that takes about 43kw/hr (58HP) and increases the air temperature by 85C (185F) while requiring almost 12000 input RPM to the supercharger.
Plotting 1.7 and 25 against the 3.3L map requires only 8200 input RPM (3.3/8" pulley) and temperature increase is about 20F less and required input HP is 10 less. Less energy tunred into heat. At that plotpoint both graphs show a volumetric efficiency of about 93%, but clearly the 3.3 is doing the job better.
In simple terms, you want more boost. The 2.9L compressor pump 600cc more per revolution - 1.26x more. The 3.3L compressor pumps 1000cc more per revolution - 1.45x more. It's not as simple as it seems to get more boost from either one because there are factors creating diminishing return. For example, you won't be able to put the 2.3/8" pulley on the 2.9L compressor and expect the belt not to slip. It's pumping 1.26x more air, doing 1.26x more work, and I'm sure belt grip is already a problem. That means stepping up to a larger pulley just to get some traction. The 3.3L compressor will be doing even more work, 1.45x. As the compressor increases in size the pulley required to move the same amount of air as the smaller unit gets bigger (more traction) too. The smallest 6-rib pulley that will work on a 3.3L compressor is going to be about 3".
Belt Slippage could be another reason you didn't see a rise in boost with each pulley swap. Twin screw superchargers have a huge amount of inertial energy in them because the second rotor is overdriven by 66%. 15,000 input RPM is spinning the second rotor at 25,000. That much mass changing speed takes a lot of energy to overcome. It is my experience that the more overdriven a supercharger is the less responsive the engine becomes to throttle transitions. Inertial force is the same reason why big rims hurt quarter mile times.
There's a lot more to this discussion, but quite honestly I didn't think getting this much out so far would take so long and I've got to go to bed! I won't have time to post more until Tuesday night, so in the mean time look at airflow on these charts. Just for reference, my 383 with the same compressor you have and a 2.5" pulley only got me 8psi of boost, and that was with a more free-flowing intercooler. The 2.3 is too small!
I'm going to guess the crank pulley is the standard 7.5" diameter. The S/C pulley is 2.375", so 3.16x overdriven. I'm also going to guess you're shifting around 4800 RPM generally, maybe less or more under certain conditions.
Figuring about 4800RPM x 3.16 is over 15,000RPM input speed into the supercharger at about maximum engine speed.
These charts are based on pressure differential. That means difference between inlet and outlet. The tricky part here is that we do not know what either of these numbers actually are. Consider atmospheric pressure to be 14.5psi. If there is a restriction in the inlet tube it will reduce available "atmosphere" into the inlet. For example, at wide open throttle maybe there is only 13 psi of "atmospheric pressure" at the inlet. 1.5psi of loss here reduces output similarly, but multiplied by pressure ratio. Back to pressure ratio, we don't actually know what the outlet pressure is either because the intercooler is a restriction itself. You may have heard those intercooler are no longer manufactured. It's a highly parellel design (excellent) but the relatively small cross section and long cores make for some backpressure and loss across the heat exchanger. One of the guys that maxed out an 8100 Intercooled Whipple kit was Tom Byrne, he had a blue/gray 2500 Suburban everyone's probably seen a video of before. I can't recall exactly but I remember the pressure drop across the intercooler being about 2 psi. (He was also had one of the earliest kits and was plaqued by PCM "reduced-power" mode problems for a long time before tuning was as easy as it is today.)
If both the engine and the blower were 100% efficient air pumps, the engine would be pumping 248 cubic inches per revolution and the super charger would be moving 140 cubic inches per its revolution. It isn't simple math to determine how much boost you get out of this, you have to consult one of these charts and figure from there.
You're getting 6psi at best. With the intercooler pressure drop the blower outlet is probably at about 8psi boost (22.5psi atmosphere based on 13psi inlet pressure). That works out to a 1.7 pressure ratio at the flow you've got right now, approximately. That number can be checked against airflow, but what we really want to do is transpose the known numbers from the 2300 map to the 3300 map.
Take one paticular plot point from the 2300 map and move it to the 3300 map to see what you get. For example, take 1.7 P/R and 25 cubic meter/min and see that producing that takes about 43kw/hr (58HP) and increases the air temperature by 85C (185F) while requiring almost 12000 input RPM to the supercharger.
Plotting 1.7 and 25 against the 3.3L map requires only 8200 input RPM (3.3/8" pulley) and temperature increase is about 20F less and required input HP is 10 less. Less energy tunred into heat. At that plotpoint both graphs show a volumetric efficiency of about 93%, but clearly the 3.3 is doing the job better.
In simple terms, you want more boost. The 2.9L compressor pump 600cc more per revolution - 1.26x more. The 3.3L compressor pumps 1000cc more per revolution - 1.45x more. It's not as simple as it seems to get more boost from either one because there are factors creating diminishing return. For example, you won't be able to put the 2.3/8" pulley on the 2.9L compressor and expect the belt not to slip. It's pumping 1.26x more air, doing 1.26x more work, and I'm sure belt grip is already a problem. That means stepping up to a larger pulley just to get some traction. The 3.3L compressor will be doing even more work, 1.45x. As the compressor increases in size the pulley required to move the same amount of air as the smaller unit gets bigger (more traction) too. The smallest 6-rib pulley that will work on a 3.3L compressor is going to be about 3".
Belt Slippage could be another reason you didn't see a rise in boost with each pulley swap. Twin screw superchargers have a huge amount of inertial energy in them because the second rotor is overdriven by 66%. 15,000 input RPM is spinning the second rotor at 25,000. That much mass changing speed takes a lot of energy to overcome. It is my experience that the more overdriven a supercharger is the less responsive the engine becomes to throttle transitions. Inertial force is the same reason why big rims hurt quarter mile times.
There's a lot more to this discussion, but quite honestly I didn't think getting this much out so far would take so long and I've got to go to bed! I won't have time to post more until Tuesday night, so in the mean time look at airflow on these charts. Just for reference, my 383 with the same compressor you have and a 2.5" pulley only got me 8psi of boost, and that was with a more free-flowing intercooler. The 2.3 is too small!