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Supercharging a 944 - Why so hard?

 
Old 01-08-2018, 01:53 PM
  #196  
jderimig
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Originally Posted by mdnt08 View Post
Well in the end you should be able to produce more power by being able to run a higher ratio pulley set up (that will increase the air temp) and add back timing that will produce much more power......kind of a big difference.
Yes I understand that.

The reason I ask is that Corky Bell in his book Supercharged (Ch 5, page54-56) talks about the IC increasing charge density and thus HP (with a roots blower) over the same non-IC configuration. I can't get my head around this if you are getting 62CI of inlet air per rev regardless.
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Old 01-08-2018, 01:58 PM
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Originally Posted by jderimig View Post
Then all intercooler does (on a positive displacement blower) is to reduce air intake temps to prevent detonation? In other words, hypothetically, if I am not detonating without an IC, then adding the IC has no other effect?
that's all an IC does in any setup, takes the HOT HOT HOT air coming out of the supercharger (or turbocharger) and reduces that temperature as much as possible, making it cooler/denser as it goes into the cylinders.

the key is that cooling the air down so much lets you run more aggressive spark timing which makes the difference in power.
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Old 01-08-2018, 02:03 PM
  #198  
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Originally Posted by jderimig View Post
Yes I understand that.

The reason I ask is that Corky Bell in his book Supercharged (Ch 5, page54-56) talks about the IC increasing charge density and thus HP (with a roots blower) over the same non-IC configuration. I can't get my head around this if you are getting 62CI of inlet air per rev regardless.


the 62 CI is a measurement of volume, eventual pressure factors in density that is affected by the temperature of the air.
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Old 01-08-2018, 02:14 PM
  #199  
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Originally Posted by jderimig View Post
Yes I understand that.

The reason I ask is that Corky Bell in his book Supercharged (Ch 5, page54-56) talks about the IC increasing charge density and thus HP (with a roots blower) over the same non-IC configuration. I can't get my head around this if you are getting 62CI of inlet air per rev regardless.
the blower is stuffing 62ci of air thru per rev. doesn't care what temperature the inlet air is - volume is independent of temperature.

the air is only compressed into boost once it's on the outlet side of the blower, being forced into a "room" (manifold) with all the air that has come thru the blower previously and not been ingested by the engine.
if the air has been intercooled before entering that room, mass X of air will take up less space (volume) than it would have if it weren't intercooled. mass per volume = density.

so if the room is a given size, and the intercooled air takes up less volume for a given mass, there's now more room for the blower to pack in even more air.
blower doesn't have to work as hard to blow this extra air into the room = less parasitic drive losses = more hp at the flywheel end instead of being used up at the crank pulley end.

the difference in parasitic loss isn't a whole lot though - as i said before the key is the ability to run more aggressive spark timing.
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Old 01-08-2018, 02:42 PM
  #200  
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Corky's calculations show with IC you can achieve the same power at a lower pressure ratio than an non-IC system. This is just based on the amount of air entering the cylinders. Does not take into consideration tuning advantages that can be exploited with cooler air charges. He also shows an IC system requires less SC CFM than an non-IC system. Again this is without tuning exploits.

I am not arguing with you, I am just pointing out Bell's inconsistencies with (everyone) here is saying....

I will see if I can post a snippet of his calculations if you don't have access to Bell's book. Has anyone here read this book and comment on these calcs in Ch 5?
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Old 01-08-2018, 03:12 PM
  #201  
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ive read that book a few times.

one thing that can be confusing is that in a piston engine with boost (either turbo or SC) CFM into the blower does not equal CFM out.
the engine will only ever ingest it's own displacement VOLUME, regardless of temperature, density or pressure.
when the intake valve opens (on a 2.5L 944) a given cylinder is going to pull in ~38 ci worth of air volume.
the CFM difference between what the blower will pull in per rev, versus what the engine will consume per rev, makes the pressure ratio.
a 500hp, 5.0L SC engine is pulling 750cfm at the blower inlet but only 450cfm is going thru the manifold, just much denser cfm's.

because the SC makes the charge air so hot, it is not very dense (low mass per unit volume).
38ci at 100*C is still 38ci, but has much less air mass in it than 38ci at 40*C.
more air mass + corresponding fuel mass = horsepower.

increasing the supercharger output pressure (drive ratio/pressure ratio) is one way of increasing the density of the air in the manifold - simply forcing even more air into the same size container, at an even higher temperature, will still pack more density (therefore mass) into a given volume.
however it is easier (less supercharger work) to get the air denser for more power by intercooling it, dropping the temperature which naturally makes the air molecules stick closer together (denser) to get more mass into the same 38ci cylinder.
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Old 01-08-2018, 03:26 PM
  #202  
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Originally Posted by V2Rocket View Post
because the SC makes the charge air so hot, it is not very dense (low mass per unit volume).
38ci at 100*C is still 38ci, but has much less air mass in it than 38ci at 40*C.
more air mass + corresponding fuel mass = horsepower.
Yes but conservation of mass still has to apply... the mass flow rate at SC input has to be equal to the mass flow rate through the engine....And as I understand it HP is directly proportional to air mass flow rate. (All other tuning being equal).

Again the genesis of my original question... Is SC 62 CI per revolution based on the inlet conditions, outlet or something in between? If its the last 2 then I can understand how IC can affect mass flow rate.

When I get home from work I will post the sections I am questioning and see if you agree.
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Old 01-08-2018, 03:58 PM
  #203  
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are you looking at the equations like these?
http://www.lextreme.com/icvsnic.htm

i believe what happens is that not all of the air actually makes it out of the blower case because it's being pushed back by the compressed air already in the manifold.
so the rotors scoop up 62ci of air at the entrance but only push out into the manifold say 2/3 of that air.
the rest gets carried back around by the rotors which then displaces new air that would've come in.

higher pressures/temperatures exacerbate the "blow back" because the rotors don't have a very good seal between lobes (like a screw compressor does)
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Old 01-08-2018, 04:18 PM
  #204  
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https://hydemotorworks.com/2015/10/1...gain-formulas/

Another write up of the power potential to gain.
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Old 01-08-2018, 04:36 PM
  #205  
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Originally Posted by V2Rocket View Post
are you looking at the equations like these?
That site is blocked by my company's firewall.


Originally Posted by V2Rocket View Post
i believe what happens is that not all of the air actually makes it out of the blower case because it's being pushed back by the compressed air already in the manifold.
Then its not positive displacement. That effect should be accounted for in the VE spec for the blower. I can see cetri's having this effect.
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Old 01-08-2018, 04:37 PM
  #206  
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Not to throw everyone off...but you could just go with a standalone and do a proven air to air IC with https://www.ebay.com/itm/HX35-HX35W-...4383.l4275.c10

The main reason I went with a roots SC is...Hawaii's SCCA tracks are in a small area. I wanted the most immediate and low rpm boost possible for the tight course.
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Old 01-08-2018, 04:42 PM
  #207  
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I will try and make a post showing Corky's formula chain without violating copyright laws.

His chain is HP based not "boost" based. Mass flow rate is the objective. (in his calc chain). "Boost" is an outcome of increased mass flow rate and the engine cfm.
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Old 01-08-2018, 08:08 PM
  #208  
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Originally Posted by jderimig View Post
Then its not positive displacement.
that term's a little misleading but still generally correct.
remember that a Roots supercharger is an ancient device originally designed for blowing air into steel furnaces.
in the 1850's when it was invented it was not really a pressure-creating device, just a simple air mover "blower" that might've been barely above atmospheric output pressure.

the first major, widespread use of the roots blower in ICE design was the detroit diesel industrial (later automotive) engines from the 1930s.
2-stroke diesels can't aspirate from the crankcase like a gas engine so they used a roots blower on top to blow air into the chamber thru ports (no intake valves). even this was barely above atmospheric pressure - just enough to get roughly atmospheric pressure into the cylinder and account for flow losses caused by the complicated path the air had to take to get there.

in these simple air-moving applications they are truly "positive displacement" moving a fixed amount of air per revolution. there's no pressure on the outlet side of the rotors pushing back.

when the roots blower got borrowed for hotrodding purposes in the '40s and onwards was the first time this design was really making "boost".
the design has been heavily refined in the last 150+ years, adding more lobes to rotors and twisting them a little bit (GMC/DD), then a little more (1990s Eaton), then adding another lobe and twisting it further (Eaton TVS) to minimize the effect of "blowback".
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Old 01-08-2018, 08:20 PM
  #209  
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Here is a summary of Corky Bell's calculation chain.

Desired Power = stock power x pressure ratio x density ratio x VE ratio x drive power efficiency.
For his example he assumed: Stock engine: 302 CID, 220hp at 5500rpm, VE 80%, Objective: 320hp @5500 rpm
He assumes a VE ratio of 115% and a 90% drive efficiency for his sample calculation.

Solving the top equation for pressure ratio:
Intercooled 85% efficiency: density ratio = 0.97, pressure ratio = 1.45 or 6.6psi boost
Non-intercooled, temperature gain 180F: density ratio drops to 0.75, solving for pressure ratio = 1.87 or 12.8psi

For blower CFM required. Basic engine airflow rate = (cid x rpm x 0.5 x VE)/1728 = 384 cfm (assuming stock engine assumptions above)
Blower CFM required = stock engine CFM x Pressure ratio
Non-intercooled: 384 cfm x 1.87 = 718 cfm
Intercooled: 384 cfm x 1.45 = 557 cfm

So what is wrong with Corky's model?

Last edited by jderimig; 01-08-2018 at 08:47 PM.
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Old 01-08-2018, 08:50 PM
  #210  
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nothing's wrong with it, it's just not covering the reasoning behind why IC vs non-IC need different pressure levels to get the same air mass into the cylinders.
it does seem to be modeled around achieving an arbitrary temperature in the cylinder though.

you might find this interesting:

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1126&context=icec
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