RETed

Ideal Gas Law!
PV=nRT
If pressure goes up, temperature has to go up to...if everything else is held constant.



-Ted

ddub

Superchargers generally do not need them AS MUCH as a turbo, from what I understand. But with any kind of boost your temperatures are going to rise the more psi you run, so an intercooler would be a good idea.

Technically you don't need an intercooler on a turbo either! That doesn't mean you shouldn't have one. I've seen drag cars with turbo's that only run 1/4 at a time at the strip run with no intercooling, so what?

ddub

How many times do you have to say "who cares what they think"? We are also trying to HELP him get more out of his investment, or are you not even reading those posts? There are people on this board that know more about tuning rotaries and running boosted applications. I am not one of these, I only have basic suggestions based on what I've read and what is logical, but others that have posted in this thread are trying to help, not just put him down.

RX7FROMCAL

ok , i wanna say im thankfull for all the feed back in here good and bad, if i wasnt interested in what people had to say i wouldnt be here, so beelave me i take it all in . and ya i wish it was pushing more , but the bottom line thats what it has, so with some other tweaking of some things it will go up. thanks for every ones thoughts on the sc. as i was going down he road tonight and slammed it there was a big difference from the old stock way, it threw my head back in the seat i never got that before it was tuned, so alls good. thanks everyone . gil

pontious

Rather than try an deal with moles of air (well nitrogen, since air is not an Ideal Gas), and univeral gas constant I think it would be easier to look at our friend:

The Combined Gas Law:
(p1 * v1) / t1 = (p2 * v2) /t2

and v1 = v2

so:

T2 = (p2 x t1) / p1

p1 = 14.7
p2 = 24.7 psi (10psi of boost)
t1 = 70 degrees F

T2 = (24.7 * 70 ) / 14.7

T2 = 114 Degrees F

Maybe this is enough of a temp increase to justify a intercooler. I don't actually know. That is what I was asking in the first place.

I still would think that the exhaust temps would have a MUCh larger effect on a turbo than the Temp change do to the pressure changes...

Evil Aviator

While the heat absorbed through a turbine wheel and housing is a factor in turbochargers, the heating that causes detonation mostly comes from the compression of the air. Physics states that air will heat when compressed, and the less efficiently it is compressed, the more heat is produced. All of this "compressor efficiency" stuff you hear about on the internet is directly related to how much heat is produced during the compression of the air. A roots blower is extremely inefficient, and therefore creates more heat when the air is compressed. Not only does this increase the chance of detonation at a given boost level, but it also reduces performance because the air is hotter and less dense.

This link explains it with some nice graphics. However, it references a more efficient centrifugal compressor (used on turbochargers or centrifugal superchargers), so a blower would add about 40deg rather than the 24deg shown, but you can still get the main idea:
http://www.procharger.com/intercooled.shtml

The main reasons that you rarely see an intercooler on a blower are:
1) Unlike a turbocharger, the Roots type blower compresses the air in the intake manifold tract rather than in the blower itself. This makes it difficult to add an intercooler because the size and shape of the intake tract is critical, and adding to its length or volume will ruin the engine's performance.
2) Most of the drag cars that use a blower are running on alcohol and/or have nitrous injection, both of which significantly reduce detonation.
3) An intercooler would add a lot of money to a blower kit.
4) Most of the blower kit market is aimed at those who do not want to run a lot of boost.

Roots blowers need intercooling more than a centrifugal turbocharger, assuming the same boost level. Centrifugal and Lysholm superchargers are more efficient than the Roots blowers.

pontious

I think their numbers for the first column are wrong. A 70 start temp and 10 PSI boost sees only a 44 degree temp increase. They are saying closer to 70 degrees for 8 PSI.

The 24 degrees do to mechancal inefficiency may or may not be correct. It is really more complicated than I am sure they would want you to believe. Some of the heat from the SC is radiated out to the engine bay (then some of that is returned via heat soak. and some is blown out by the finger chopper), some of it woudl of course directly impact the air in the SC.

20 degrees from heat soak. Sure, if the car is not moving you will see a lot more of this. If air is being constantly pulled through the system the time it is in the intake I HIGHLY doubt its temp will increase 24 Degree in what a half a second?


I am not arguing that the temps wil be higher, I have already proven they will be. I guess if you want TOP performance you would cool ANY air that goes in the car.

pontious

Well that is part of it, mostly I would bet it is mechanical efficiency that they are referring too. That thing is spinning on a bearing or a fluid, this causes heat loss do to friction.


What we need is a damn Mag-Lev bearing.

NZConvertible

Please guys, say whatever you want about supercharging, but do not call it new! There is nothing new about it, and that includes on rotaries.

Evil Aviator

You need to use absolute temperature in your equation.

snub disphenoid

Well, i'm not considering doing that right now, but I would be interested in seeing how well the motor could do without that big, snaking intake manifold and a larger, single plate throttle body.

ddub

That's true. I've always wondered about that intake mani and how it could've been done better as well.

Superchargers have their upsides and downsides, just like turbo. I'm really still trying to decide which I'd be better able to do and would want to more, but so far I'm leaning towards turbo. We'll see

snub disphenoid

After my experiences with the Camden, I'd have to say go turbo. Maybe the dyno trip on Friday will change my opinion, but that's what I think. I mean, think about it. What's the fastest street driven supercharged RX7 you've seen? You're really not going to get anywhere past high 12s, at the best. If you go turbo, though, there's always room to improve.

So if you're searching for only modest power with a flat, useable torque curve(and you don't mind your car sounding like an airplane), go for a supercharger. If you want a jump in power, with the ability to make more and more jumps, go turbo. Also go turbo if you don't have money to throw around for further upgrades. That's just what I think.

SonicRaT

iTrader: (6)

348rwhp whipple is the fastest I've seen (helped build it too), owner lives just outside of Syracuse, NY, damn thing is a garage queen though.

RX7FROMCAL

one thing i can say , this cars slams me back when i nail it, omg

pontious

Yup, My mistake, it was too late and that IMPORTANT fact slipped my mind. (It has been 8 years or so since I even though of this stuff
Anyway, Converting 70 deg F to Kelvins and re-solving to T2:

T1 = 70 deg F = 21.1 deg C = 294 deg K
T2 = (24.7psi x 294 deg k) / 14.7psi
T2 = 494k = 221.33 deg C
T2 = 430 deg F

That means an increase in temp of 360 Degrees. at 10 PSI.

These numbers are pretty misleading though, as the volume is not constant becasue the air is actually flowing through the SC and the engine. I think to really do these calculations you would need to whip out a Thermo Dynamics book...

Joshshift

i work here at atkins rotary and i thought i would post this dyno sheet of one of our supercharged motors, with a carburetor no less.

boo hooo its so expensive, cry me a river. if its too expensive for you then just go buy your junkyard turbo kit, and stop whining about it here. 267hp at the flywheel isnt bad.

RETed

Why bother?
99.44% of the people in here won't understand, and it'll probably take PAGES of proof to show your work...

Increase of boost means increase of intake temps - not good enough for you?


-Ted

RETed

Yeah, except a stock turbo can do that...
You're right...booohooo


-Ted

Evil Aviator

Take your car to an autocross and then you will realize the advantage of the supercharger. Drag racing and dyno queens generally favor turbocharging.

Those are some impressive BSFC numbers! (I thought I would mention that because I doubt anybody else here is going to appreciate it).

1987RX7guy

iTrader: (3)

That sheet is awesome! Its also interesting to see the EGT and AFR numbers


Dunno if you included the info with the chart Joshshift but if you can reveal this info what PSI was it running? I might be looking at it wrong but is the second to last column the boost figure?

Joshshift

im not sure if that is the boost column, but after we configured this efi kit correctly the boost gauge was reading 12-13psi in our test car. these hp numbers would greatly improve with some type of intercooler, possibly some type of water injection.

RETed

Lesse...NA secondary port in the side housings are SIGNIFICANTLY bigger.
NA ECU runs more aggressive ignition timing due to it only has to worry about "0" on the pressure sensor.
That means everything points to better flow and more aggressive ignition timing, unless you slapping on an MSD BTM or equivalent to chop the ignition timing down?


-Ted


(Edit...Remove flames)

Atkins Dan

you cant be serious?

you actually think that ports on Turbo are smaller than ports on an N/A take a look again there dude?



Dan Atkins

RETed

From 86-88 FSM scan from http://www.iluvmyrx7.com/

Turbo primary - 32ATDC to 50ABDC
Turbo secondary - 32ATDC to 50ABDC

NA primary - 32ATDC to 40ABDC
NA secondary - 32ATDC to 30ABDC
NA 6-port - 45ATDC to 80ABDC

The NA secondary + 6-port is effectively - 32ATDC to 80ABDC.
Compare that to the turbo secondary, the NA intake port closes 30-degrees later.


-Ted