General Supercharger Thread

Correct. An easy way to think of it is MAF voltage = rpm ÷ pulley. More rpm and larger pulley is the same as less rpm and smaller pulley.

 

I would have to look at data logs of mine to see where I starting hitting 5v but it was more like 4-4500 rpm and up for me. Part of that may be because I have an AFE filter and you have a paper one still?

Either way with the MAF issues aside, I think a 75mm pulley is too much flow for a stock engine at sea level. @snivilous created a chart showing the different boost levels at sea level for each size magnuson pulley. The 2.1” on a magnuson moves a very similar amount of air for each engine Revolution as a Harrop with the 75mm pulley. Based on his chart you’re looking at 11.7ish psi at sea level. Even if you had the fuel to support safe AFRs there, I’m thinking that’s approaching that max safe horsepower range. You would probably need the RCF TB to take full advantage of it as well.

I’d love to see you do it though! Lol

 

wise words. I agree.

truth be told....I think this setup is presently ideal for me.
PLENTY of power
same MPG
everything works. (harness aside)



I'm sure this diamond will get smaller and smaller as time passes....

 

I’ll say this though, which I’m not sure if it would work in practice but maybe in theory. Using the 75mm pulley to get a quicker and more responsive low rpm boost would be awesome, if the stock TB actually did limit it enough to not move enough air and make enough power to blow the motor. That could make it worth while to try the 75mm for those benefits while leaving the stock TB. Use the Tb as a restriction for capacity control, similar to how I would love to have one of those smoothboost actuators for the bypass valve.

 

DO IT!

 

Gotta get a 75mm pulley and a 4” ball valve installed on the intake. Throttle it back til it maxes at 8psi

 

damn it..the one on my desk is only 3"'s

 

So far I’m loving this 75mm. With the maggy box and long tubes it looks like I’m maxing out at 9psi. Yesterday when I logged my highest maf voltage was like 4.75. We probably still have 1-2 more revisions with the tune but it’s looking pretty good so far. But even just normal driving habits it’s sick. You get into boost so much faster and easier.

 

so, that's a 4" inlet, right?

 

Ya the internal measurement isn’t a full half inch larger than the stock though. I thought it was but I’m hoping I’ll get away with using this box so I don’t have to get the pro speed one.

 

Guess I need to add the injectors to my list and figure out a intake 4.5" tube

 

Did you get a baseline dyno before supercharging by chance? I’d love to know how much power over stock that makes with Longtubes and the 75mm. What do you have for your fuel system upgrades?

 

I didn’t get one before supercharging. Just the one from when I had VFtuner and this other one when I switched to HPtuner with OTT guys. Once we finish tuning I’ll throw it on the dyno again so we can see the delta from the 80mm.

hopefully by next Saturday

oh but just stock fuel rails and 650 injectors

 

When I get the time to do the math and find the proper calculators, I’d like to determine what the ideal Venturi style restrictor size would be that I could use a 65mm pulley, but neck down the intake with it after the MAF so that it would flow the same at 6k rpm as an 80mm pulley. All the benefits of early peaky boost but limiting the top end. I was playing with some calculators to get a rough idea and I think it would be about 2.4-2.6” for the restrictor.

Also interesting, playing with the calculators (if I did it right) the air moving in a 4” tube on a Harrop with an 80mm pulley is going 140mph when the engine is at 6k rpm! At 6k rpm that’s also 1,087 CFM. Pretty wild!

 

I appreciate calcs like this.

it feeds my inner nerd.

 

This would probably be a cheaper/easier approach than that smooth boost kit huh? I emailed them to see if they had plans on doing one for harrop but haven’t got a response yet. I do like this idea… earlier boost>later boost lol

 

If it actually worked in principle as it does in theory of someone who isn’t an engineer…hell yeah lol. We are talking a $10 piece of 3D printed plastic.

1,087 CFM going through a 2.5” orifice though is moving 360mph. Is that even feasible?

I’m also not sure if these calculations are even close to accurate cause these calculators are figuring on a higher Inlet pressure vs a lower outlet pressure. Our supercharger is sucking the air so it doesn’t actually have a positive pressure at the inlet of the venturi, it’s actually in a vacuum.

 

ponder this...a variable circumference top pulley.

faster it spins, larger it gets.

starts @ 60mm...expands to 80mm



kinda like VTech power. but opposite.

 

more like a CVT transmission.

 

Well it's still positive pressure. The highest pressure you're getting on one side is 14.7psi, it doesn't matter how strong the "vacuum" is, the air can only rush in as fast as the atmosphere is pushing on it.

I've thought about that a lot, on the Magnuson it didn't seem feasible (or worth the effort) to make a variable geometry pulley. On the Harrop it might be more plausible due to the space to work with. I would do a segmented pulley with weights that get flung out as the rpm increases, similar to a UTV clutch, and then you could tune it in. Though that is not a simple gizmo or cheap.

 

damn straight....now your thinkin'

you could program that b1tch.

 

You’ve left me scratching my head, but I’m pretty sure that inside that 4” tube before the blower it is going to read a vacuum under boost. I’ve read about people having issues with their air filters collapsing under boost. Just like on the return of an Air Handler, dirty filters get sucked right out of the filter rack. When we take measurements with manometers, the returns read inches of mercury vacuum. Essentially, anything less than 29.92” of mercury (at sea level) is by definition a vacuum (in relation to the atmosphere).

 

Correct, but the concept of a vacuum where there is atmosphere is just less pressure relative to somewhere else. When you measure with a manometer you're seeing a "vacuum" relative to something else, ie just less pressure. Your statement

"I’m also not sure if these calculations are even close to accurate cause these calculators are figuring on a higher Inlet pressure vs a lower outlet pressure. Our supercharger is sucking the air so it doesn’t actually have a positive pressure at the inlet of the venturi, it’s actually in a vacuum."

which I think is flawed because the supercharger sucking is reducing pressure on one side, so you still have atmospheric on one side and just less pressure (relatively) on the supercharger side. The supercharger then takes all the air and compresses it into a smaller volume and makes boost (PV ~= T, so P ~= T/V). If you had a full vacuum on the supercharger side of the intake (pre-rotors) you'd have 14.7psi at the intake, and 0psi on the other side. Since the engine/supercharger isn't pulling a full vacuum, I'd suspect it's like 14.7psi -> 2psi or something which is just the "vacuum" the engine is pulling. Everything is still a positive pressure number though, just less pressure than the other.

Correct me if I'm wrong, I don't play with fluid dynamics often so good chance I'm wrong my whole point is you can never have negative pressure, your lowest pressure is 0psi (true vacuum), so for your quote of inlet vs outlet pressure I think you just have atmosphere on one side and atmosphere minus the vacuum of the engine which is ultimately just a lower outlet pressure.

EDIT: To expound on the manometer stuff, you are only reading a vacuum because you have positive pressure somewhere else. If you put the manometer in a vacuum it wouldn't measure anything because there's no positive pressure to push the mercury up.

 

What I'm trying to say above: If there wasn't positive pressure on the supercharger then there is no pressure trying to shove air into it.

EDIT: Btw I absolutely love conversations like this. This is why I'm on forums instead of facebook

 

My apologies to everyone for nerding out in this way. I understand what you’re saying and why you’re saying it. You’re saying it’s still a positive PSIA so you feel that’s still a positive pressure. But then what is a vacuum? Everything is RELATIVE. The reason you’re wrong to not call it a vacuum though is we (and our engines) don’t live in a world of 0 PSIA. It’s 14.7 PSIA and 0 PSIG. By your logic, when I evacuate an air conditioning circuit to 500 microns (an incredibly deep vacuum as atmosphere is 760,000 microns), prior to charging it it is still technically at .00967 PSIA. By your logic, even that’s not a vacuum then? If that’s the case; we shouldn’t even refer to vacuum as a principle in our world. If the pressure is anywhere below 14.7psia (at sea level) then it’s in a vacuum relative to the environment.

consequently if the supercharger is pulling air through the intake tube faster than it can be replenished by the atmospheric pressure (the intake tube is an orifice) the pressure in the tube drops to a vacuum relative to the atmosphere. It’s a vacuum lol. A container evacuated to 14 PSIA is still a vacuum in relation to an atmosphere of 14.7 PSIA.

 

Maybe my wording was wrong, I think we're saying the same thing. A vacuum is a relative difference in pressure if you look at PSIG, but if you look at PSIA then there's always positive pressure.

I agree you have a vacuum relative to the atmosphere. But going with your container analogy, you are still experiencing 14psi if you're in the container. The pressure in there is still positive, just if you open the door the outside air wants to rush in. So where you lose me is when you say "calculators are figuring on a higher Inlet pressure vs a lower outlet pressure. Our supercharger is sucking the air so it doesn’t actually have a positive pressure at the inlet of the venturi, it’s actually in a vacuum." The outlet is still pressurized, or if you're measuring in relative pressure then the outlet is at atmospheric minus the vacuum the engine is pulling, so your relative pressure difference is the same either way. It's not like the outlet value can be negative even though it has a vacuum on it, there is still positive pressure there.

 

I understand what you’re saying. A disagreement on semantics. You’re making the argument that the number Inputted into said calculator for the inlet/outlet pressure of the orifice will be a Positive number, and not a negative number so therefore it’s not a vacuum. I was arguing that just because that number is a positive number doesn’t mean it’s not a vacuum relative to the atmosphere. I don’t think there is such a thing as a negative pressure in PSIA, it only goes to zero. You can’t have less than zero atmosphere, can you?

Point being, the issue arises on what to put into the calculator for the Venturi orifice size for the values of inlet vs outlet pressure. There’s not really any good way of knowing other than trial and error with different size orifices and seeing what the resulting boost pressure is with my boost gauge. If we could figure it out, everyone with a magnuson could run the Sniv 2.0 pulley and get super instant peak boost! Us Harrop folk could run a 65-75mm pulley too without fear of overboosting.

 

Okay I think I'm following your predicament. The venturi calculates based on a pressure differential, so the real question is what is the pressure behind the venturi, and just because the supercharger is making boost doesn't mean you have a full vacuum behind the venturi. Does that sum it up correctly?

It would seem like you could tap the blower or make a plate with a location behind the venturi that you can pull pressure from, then just hook up your boost gauge and see what it reads. If you made a plate you have the chicken and the egg issue, but at least you'd start out with a data point to calibrate the equation with.

 

I would think the pressure before and after the venturi would remain constant and equal until such time as the mass air flow begins to cause a constriction.

thusly, lowering the flow. and I guess the pressure diff would be apparent as well.

 

I think that the technical definition of a vacuum is the absence off all matter. In the context of how it's discussed here, it'd be defined more like a pressure differential between two references.

Like with voltage, a potential difference. 1V would be in reference to ground. But not all grounds are the same. Some electronic devices specify a voltage above ground, but a specific ground point, not necessarily earth or chassis ground. The voltage is from a very specific reference ground that could be different in relation to the reference ground for a different circuit which could be above or below the ground for the first. The ground for your 4WD ECU may be different from the ground from the A/C ECU and there could be a potential different between those two grounds. Muddy enough?