alright, y'all

lately, i've been working some numbers trying to understand compressor maps, VE, CFM, etc., and how to use those to halfway-accurately have an idea of what to expect when changing pulley setups, possibly stroking my engine, or figuring out what i need for a specific target. that said, i'm asking the guys who actually know how to do this if i'm doing my math right:

for this scenario, i'm using the two pulley setups i've ran as a basis for my starting data:
first, it's a stock LS1 with 1 7/8 headers & a jackshaft TVS2300 - underdrive rear (3.0→3.49 for a .8596 reduction)
setup 1: stock balancer (7.5") and 3.3" pulley = 8.5psi up to 6200rpm & 480hp (rich tune)
setup 2: 2.9" pulley = 11.5psi up to 6200rpm & 540hp

now from what i've calculated (based on the TVS map below) is that it takes roughly 1.3 CFM per horsepower in my car (with the chart CFM vs boost level showing my engine VE to be about 65%), and my next goal is 700+hp, meaning i need... let's call it 950 CFM.



so with that as my going-in argument, here's the math i ran:
LS1 = 3.9 bore x 3.62 stroke = 346ci
TVS = 2.267L = 138ci (based on the 10000rpm airflow interpolation)
1728in³/m = 1CFM = 1.699m³/hr for airflow conversion
i want the power by 6500rpm

950CFM @ .54 blower VE = 1759CFM = 2989m³/hr needed
that interpolates to about 21,000 blower rpm... so how do i get that speed?
overdrive cogs = 36tooth→30tooth = 1.2 ratio... needing 17,500 jackshaft speed @ 6500rpm, or 2.692 crank→pulley ratio. using a 10%OD balancer [(7.5"π * 1.1)/π=8.25"] says i need a 3.065 snout pulley. Magnuson makes a 3.0, so i'll use that to get a blower speed of 21,450 @ 6500rpm
so applying those numbers to the chart, it looks like 3000m³/hr = 1766CFM, @ 54% VE = 953CFM
(346ci*3250 (½ rpm for 4 stroke))/1728in³/m=651 engine CFM, and @ 65%VE = 423CFM
953/423=2.095 pressure ratio (18.4psi)

so to summarize all those numbers, it looks like if i forge the LS1 and run a 10% overdrive balancer with a 3.0 on the snout & overdrive cogs on the back, i should theoretically be able to inefficiently spin the snot out of the blower to move enough air for 18psi on the top end (assuming i'm stupid enough to not make the engine more efficient with heads/cam work), and move enough air to make 700hp to the ground.
Disclaimer: this don't consider the timing pull from higher IATs or the extra parasitic loss from spinning the blower 2x what i am now

aside from this particular scenario being completely unrealistic, am i at least doing my math right?

 

Hey Zebra-
Unfortunately- It's just not that simple!
Your little disclaimer there should be more of a consideration- when I look at turbo compressor maps, i start with the target PR and target HP airflow (Manifold pressure required for a target airlfow= Lb/minx639.6x(460+ACT)/VEx.5rpmxCD) and trace out to the efficiency of the compressor to see if your in the ballpark.
The other big factor here is the engine VE: intake flow, cylinder head flow, cam specs and exhaust flow- as soon as any of these factors change- all your formulas go out the window.
And the best way to make BIG POWER on a SMALL blower (which is what I consider you are trying to do here) is not only to spin the blower faster, but you must increase the efficiency of that motor- and if you are going to forge it, then why not do a cam and heads?

From my dyno and tuning experience for blower compressors, the first thing you have to look at is rpm- what is the manufacture's reccomended max rpm? For the centrifgual blowers (Prochager etc) I find that you can run them up to and even a bit over their max rpm and you will always gain a fairly linear gain in power, but for the PD style blowers, I have found that the efficiency (or how much the rotors are heating up the air as it compresses it) drops off very quickly as you start spinning them faster and faster. The thing to remember about compressor maps is that they are generated sometimes by computer simulation or sometimes with laboratory collected data points and interpolated in between- and your LS1 motor under the hood driving down the track is not really a 'lab' condition.
Oh yeah- then there is the whole 'belt' and pulley slip issue too!

 

Looking at the efficiency islands, it's clear to see that the R2300 moves the most air using pressure ratios in the 1.6 to 2.0 range. Anything above and below that, the airflow diminishes. 18 psi is around a 2.22 pressure ratio at sea level and if you draw a line straight across and compare it to the different volumetric flows, you can see that air flow is always less.

I can double check your math, but on the surface it looks like you are calculating things correctly minus the fact that 3000 m³/hr doesn't even show up on the performance map.

You mention that you want the power by 6500rpm. Does your current setup make power at 6500 rpm or has it already started falling off? I'm asking because you list your peak dyno numbers at 6200rpm.

Are you wanting to try this or asking if you will make 700 rwhp?

 

thanks for the inputs, guys lots of good points there, so here are a few clarifications:

what are the "639.6" & "460+ACT" in your equation? i'm still learning here.

i already know this blower is small for my goals, but i've also seen it (in slightly different configurations) make the same or greater than my goal... even on bigger engines (surprisingly up to 434ci). it's just that it was the best PD option available when i bought it with no others looking like they had bright futures

i will definitely do cam & heads (considering doing that before the forge, but i don't wanna tear the engine apart twice) - i just ran the numbers as-is for easier math. i don't think there's any way on God's green earth stock 862s with a stock LS1 cam will make that kind of power in this setup.

max recommended TVS speed is 18000rpm (hence why the chart stops there & don't extend to the interpolated 3000m³/hr). above that speed, yes, it historically loses efficiency a bit faster.

i'm actually targeting 14-15psi for my build for that reason, which just requires the engine to just be that much more efficient (VE)

both dyno sheets are below. peak HP for both pulleys was at 6200, but torque starts falling after about 4300 (stock heads/cam), and the 6500 goal is because i don't want to sacrifice everyday usable power by setting it up to keep the very top within mechanical/stable/tuneable limits. keeping 7000+rpm on the chart means i'm losing blower speed & CFM at 2000-4000 where 90% of my driving occurs when compared to something that maxes out at 6000-6500. unfortunately, i understand horsepower is a very simple function: same torque @ higher rpm = more hp, but at that point, i'd rather have power under the curve than a peak number... but then again, when you're looking at 700s, you don't really need much more under the curve, so peak will suffice & be easier to drive. one last thing on rpm: i get that it's much easier to turn 7000 with a 346 compared to a 382/3, 402, or other longer-stroke engine.

i understand that just turning the blower up right now will only land me with fried pistons or holes in my block. i believe it's close-to-possible with an LS1 kit & displacement, but i don't think it'll be easy - i'd still like to try, though.

as promised, the 8.5psi sheet


and this one was actually just over 10psi because i swapped from a 3.3 to 2.9 on the dyno & hadn't got a shorter belt yet, so it slipped a bit. i picked up another pound of boost with the right belt.

 

639.6 is just a factor, which would be different if you are using standard or metric measurments. 460+ACT is another factor plus Air Charge Temperature- which you could figure can be anywhere from 150F to 250F depending on how efficient the intercooler setup is.
Are you thinking of using water-meth with this setup? Probably a good idea.

 

it'll be 100% meth, but only added to a pre-meth tune. don't want to blow the engine if, God forbid, the pump give out, line come loose, or tank run dry