NoSeeUm

The CE for a reciprocating compressor is pretty close to 100%, so DR = PR. For a Centrifical (or Axial) compressor you have to consider CE, so DR < PR.

Jim

HOHN

Great post Jim.

Density is the better way to look at it, because we don't really care about the temp or pressure per se-- they are just means towards the end of increasing mass flow. Density tells us how well we've done, because it points towards mass, and mass is king.

The spreadsheet I made does all these calcs for me. I calc compressor outlet temps for both top and bottom turbos. I can compare the effects of intercooling to those of aftercooling, and examine the effects on other factors.

I have my spreadsheet set up a little differently than some may be doing theirs. Mine is comparative. I have the "theoretical" portion which is how much air I THINK I need based on hp goals, VE, RPM, AFR, etc. Then there's the "actual" portion of the spreadsheet which I submit that actual values for comparison: compressor inlet temp, compressor inlet pressure, operating PR of the turbo(s) (which I can control with a WG), CAC temp drop, etc.

Then I get a comparative ratio of the "theoretical" to "Actual". If it's greater than 1, then I have more air than I've calculated that I need. If it's less than 1, then it's a little less air than I've calculated that I need.

So I have this setup for every 100 engine rpm from 1500 to 3000, and I can build a power curve and examine the airflow requirements.

It's also setup to do calcs for a twins setup-- things like "if I run big charger at 2.5PR instead of 2.0PR, what happens?" I have it set this way because I want to be able to match turbine and compressor map data to the other data and KNOW that I'm picking the turbo correctly for the application, and knowing where the turbo is operating within it's range.

Anyone want the spreadsheet, just PM me your email addy and I'll dump it to ya to fiddle with.

HOHN

This is why the compressor maps are useful. The CE is right on the map, so you can plot your operating conditions and know the CE for that circumstance.

Most centrifugal compressors are over 70% in their operating range, but can dip to 65% on the very fringes. In the middle of the map, they are typically 75%-78%--- which is pretty good.

Larger centrifugal compressors are generally more efficient.

Lil Dog

CE is not quite that high with Recip compression. Ratios under 4, the CE can be up to 80-90%. Sometimes VE and CE get confused. None the less I see how CE is the determining factor in cent compression. Thanks.

NoSeeUm

Not completely true.

Hp = ( Q * p ) / ( 229 *u )
Q = Flow Rate in CFM
p = Pressure in psi (change in pressure --> Pout - Pin)
u = Efficiency coefficient (CE from the compressor map)

Increasing flow and / or pressure increases the Hp requirement. The temperature change only effects the relative density. But that is only important if, for example, you are using the air for combustion.

I had thought reciprecating compressors operated pretty close to isentropic compression with the temperature rise very close to ideal. Thanks, I had a misconception there.

LOL Books, what are those?

Jim

Mike Holmen

dog, have we talked you into hot rodding your diesel up abit more yet? You need to try out one of these BB garretts. Sweet turbo bud. It still shocks me how fast it lights up. Today I was driving back from town, usually I kick my od out and floor it. Smoke levels with my old HX 40 would cloud out Deer Foot if someone would cause me to double foot the throttle, today it just lightly hazed the road, and spin up to 40psi in nothing flat, my old 40 would still be grasping to break 10psi. I even barked it a few times on the weekend. Its still spinning nice. On the weekend I pulled a 60psi run, and it supprized me how fast I got 60psi. Even done some street light wars vs a few cars, no vettes or vipers yet but its still earily spring.

Lil Dog

You aren't totally off, I am dealing with methane gas mixtures so the theory isn't totally on par with air. I use a computerized analyser to collect the P-V data to determine throughput and power.


Mike,

I might have to look at one of those puppies. This S300 is gasping for air at high RPM levels, now more than ever when I did the GSK. Anything over say 2700 Rpm and 40 psi boost and I am pedaling for 1300°F with lots of throttle left. The exhaust side is just too small for these injectors. It spools great though..I am a little leery on twins just yet.

Mike Holmen

Thats ok dog, twins aren't for everyone. It might be cheaper route than buy a garrett, just put a BTH-3B under that S300 you got. Boost problem fixed, need more fuel problem coming. Next meet let me take you out for a ride. I'll just haze the tires abit, though I have to warn ya. Do you do the PV curves for the recip's and time traces? I haven't looked at that stuff in years. All I work on is centrifigal compressors and my program doesn't do mass flow for 100lbs/min to well.

Looking at the earlier air flow equations, I didn't see any adjustment for the different compressiblity that occurs at 200 deg's F and 30 psi. I found a curve on the internet, the number for about 0.065 from standard Air density at sea level is 0.076lb/ft3 constant you use to converter mass flow lbs/min to actual flow cfm.

My neighbor came over to laugh about my welds that I put on the down pipe. I thought that it looks pretty cool.

HOHN

Mike, I did a little searching on compressibility and found this:

The compressibility of air at 100psi is .997 (Z-factor). So, at 100psi, we're still well over 99% of what an ideal gas would behave as.

To me, that means we can completely disregard any variance in compressibility. Our gauges aren't accurate enough nor our pressure high enough for compressibility variance to matter enough.

I'm pretty sure my boost guage isn't accurate to less than .5%.

The effect on our calculations as far as centrifugal compression goes are in my mind therefore neglible, and basic compressor efficiency is more than adequate.

Am I missing something? My calcs aren't carried out to 3 decimals, so I can't see how compressibility matters as I figure it.

Justin

Captain

any comparison on that garrett BB vs a II SPS62/14? Mine runs hot and spools way to slow. I'd love to get my hands on something alot faster spooling. Its so slow the truck wont even spin the tires without dumping the clutch.

Lil Dog

Captain,

I think running hot is a 12V thing... With the extra fuel you have a 64 may be more the ticket with that same 14 backend on a 71 turbine??? Anyhow, I am on the other end of the curve, turbine too small. Spools fast but runs out of air up in the RPM band.

Those compressibility numbers look similar to the numbers I see on Methane mixtures up to 2000 psi. Those usually float around from 0.996 to 0.998. That is the argument I get from theoretical engineering that density affects HP, it doesn't, its area under the curve. It does affect mass flow. So in effect the PR's aren't affected with the change in density, but mass flow is.. I hope this is making sense. Yes Mike, its time/phased pressure data..The Analyser

HOHN

Bingo! The PRs stay the same, it's just that you get less mass for the same PR because of the reduced density.

jh

HOHN

A Garrett might be a good option for you. They have a stage 2 charger that has a slightly tighter turbine housing that will spool EVEN FASTER than what Mike has experienced. Mike's turbo is a .99 A/R, while the stg 2 is a .89.

Then again, with a manual trans you could probably get a way with the larger of the two housings and have plenty of spoolup and a little more air to boot.

I'm inclined toward the larger of the two myself, and I value spoolup more than a lot of other guys here seem to.

(Hence, I still run the stock charger).

jmo

Captain

It sounds like spool up is greatly improved over the SPS62, but what about EGT's? I'd like to not get any hotter although I guess I'd consider twins if I could pair it with something cheap like a s400

HOHN

There are tons of EGT variables that make things hard to predict.

If you go to a better spooling turbo, you can typically expect lower EGT when the turbo is in its operating range, but higher once you push the limits.

These Garretts are on the map at PRs over 4 (barely), so you can theoretically run up to 42 psi and still be on the map (if barely there).

The S300 maps I've seen don't go up that high-- they top out around 3.8PR which means they should be limits to lower boost levels-- 36psi or so.

Since these BB chargers spool so well, I'd say using one as a top charger with even a super-cheap bottom like an HT3b would still be a killer twins setup without tons of $$.

The top charger is crucial in overall compound charger response, so that's your biggest return on investment for a fast-spooling BB turbo.

JMO