Originally Posted by UselessPickles
You may think you are raining on my parade. But in reality, my parade has already gone by and the lingering crowds of people are watching as you stand in the middle of the street pissing fiercely into the wind.
I'm not saying that a centrifugal supercharger in general is not capable of producing big gains at low rpm, ever, on any engine, with any configuration. As you said, it's just a matter of the supercharger's gearbox ratio.
However, we are speaking specifically about a centrifugal supercharger that is known to produce a peak boost of about 7-8 psi on an engine that revs up to 6500 rpm.
The boost produced by a centrifugal supercharger is directly proportional to the square of the speed of the supercharger, and therefore directly proportional to the square of the speed of the engine.
Due to the fact that increasing engine speed reduces the amount of time available for air to flow in the intake valves, the gain in torque of a centrifugal supercharger ends up being approximately directly proportional to the speed of the engine (not the square of the speed of the engine, as the boost pressure is). This means an approximately linearly increasing gain in torque across the entire rpm range.
Again, that is approximately linearly increasing torque gains, starting with minimal gains at very low rpm, increasing fairly steadily to maximum gains at/near maximum engine speed (gains may taper off a bit near max engine speed due to the compressor falling out of its efficiency range).
Armed with this knowledge of how centrifugal superchargers work, and within the context of the entire RIPP torque curve across the rpm range, I can quite confidently say that the large bump/hump in the torque curve at low rpms is unexpected. See the following picture where I have circled the area in question, and drawn a line that approximates a reasonable expectation for the shape of the torque curve. I'm not saying the shape of the torque curve is impossible. It could be something about tuning, relative to the stock tune being intentionally de-tuned in that area for various emissions/efficiency reasons, etc.. I'm just saying just that it's unexpected, so I'd like to see whether other people have gotten similar results to see if this "hump" is consistent, or is this just a misleading side-effect of how this specific dyno run was performed.
The truth is most likely somewhere in between.I'm also a bit suspicious of the dip in the stock torque curve in that same area (I've seen other stock dyno charts without this dip). If we assume that dip in the stock torque curve is a fluke, then this is about what I would expect from the RIPP torque curve:
Not even wrong!
1) Centrifugal Superchargers have a gearbox so they can produce optimal compressor speeds no matter the speed of the crankshaft. So, let's say you have a 4.5:1 ratio gear box on the back end of this hypothetical CS unit-- that will give you 4.5 x times the input rpm. So let's say at 2000x4.5 gives you compressor speed of 9000rpm. This might create depending on the units size anything from 0.5-2.0psi in manifold pressure, but here is the big issue that you never seem to get this unit might be capable of putting out at >2.1psi 10-15lbs-minute of air mass-- which is what makes it possible to burn more fuel and more air at those low pressures.
2) Volumetric Efficiency is usually lower as well at low rpms in an engine and higher at higher rpms. So, let's say you have in our hypothetical engine at 14.7psi (standard one atmosphere at sea level) an air mass of 50lbs-minute being processes by each cylinder. If this is say 50% the engines maximum volumetric efficiency for Normal Aspirated induction. So, since the cylinder has 50% less air mass in it it is very easy to squeeze in 10-25 lbs-minute of air mass into the cylinder without really needing 20PSI to do so. So you can boost volumetric efficiency very easily at this point. So going to 65lbs-minute air mass is an increase of 15% in Volumetric Efficiency.
3) As the engine increases its efficiency as it increases RPM-- the amount of boost needed to reach higher and higher Volumetric efficiency ratings requires higher boost! So, to squeeze more air into the cylinder you have to use more PRESSURE or combine that with an increase in the size of the combustion chamber- i.e. reduce compression ratio usually via piston design.
4) Torque gains might not be minimal at low the end of the RPM spectrum... Again you can alter the horsepower and torque gains by changing the size of the unit and gearbox ratios to fit the needs.
5) The hump isn't what is odd-- it is the dip after it that is far more telling... The hump is pretty expected. The Dip, says that something in the engine radically changes and the super is no longer as efficient in supplying the required air-mass to maintain sufficient combustion to create the same levels of power. It could be that Vortec unit isn't able to keep pace with the rising Volumetric performance of the engine between 2400-2900rpm. It is boosting the performance over stock but the unit isn't keep up with the engine's own internal pressure. It could be related to only this unit. If you chart is right it isn't giving the supposedly expected 300ft-lbs of torque at the rear wheels either. That is odd too.
6) Like I said the units gearbox pretty much makes it impossible to use your squared proportional formula and get any meaningful data for generalization. What is really important is knowing how the gearboxes ratios affect the compressors speed for a given rpm and that has nothing to do with the square of the engine's rpm. The the impeller itself will be rotating at different velocities from the centripetal forces of the air being pressed against housing. Sure, you need to use the square of the radius impeller itself. But for just figuring out the actual impeller's increase in rpm over that of the input shaft-- that is simple multiplication. Ratio times input rpm!!!
8) Not all superchargers are created equal-- however, to determine what sort of Volumetric Efficiencies, Boost Ratings and Adiabatic Efficiency also known as Parasitic Efficiency...
9) Finally, I'm still trying to figure out what in the hell you are talking about with the valves. Is just puzzling it sounds like you're talking about the engine's VE but combining that with valve surge. Here is the thing like any device it has a small spectrum of efficiency where it reaches nearly maximum theoretical performance and then it falls off.
10) It's been a blast...
11) You do need more samples in your data set to make any general statement at all about these modifications and performance... That is a fact. Also you need better testing methods i.e. you first have to determine is your stock engine making advertized torque and hp at the crankshaft before you turbo it and then do it with the modification. Then get data on the drive train and then work it with the turbo unit replace it into the Jeep and do the same tests. Otherwise all you're doing is guessing. But I can tell you this for your 33% loss figures to be accurate would mean that you have to have 585HP at the Crankshaft for a 360ish WHP ratings... You're not getting 585Hp at the crankshaft without doing some serious modifications to the engine internally. NRE is getting those sort of numbers on pump gas, but, they are totally blue printing the engine to do that sort of thing! That is not the power you're getting from a bolt on modification and 93 pump gas. IT is more likily you're getting 90-120hp at most and only loosing 60-80hp.
Reply With Quote
Connect With Us