tomcat

Just a word about BSFC numbers on blower engines. BSFC is a measure of how efficiently an engine turns fuel into HP. It is defined as the lb/hr of fuel divided by the observed HP. The catch for blower engines is that, although the observed HP does not include the HP that came off the front of the crank to drive the blower, the fuel flow does include the blower drive HP.

So if you observed 500 HP with a fuel flow of 300 lb/hr, you calculate 0.6 BSFC. This has led people to believe that supercharged engines need to be a lot richer. But you should add the estimated blower drive HP and recalculate the BSFC. For example, if the blower drive used up 100 HP, the total HP was really 600 HP and the BSFC was 0.5. This BSFC is equivalent to an A/F ratio of 12:1 and you can't say that is rich. I'm not saying all blowers use this much HP, just keeping the example simple.

The rule of thumb of "0.5 BSFC" is derived from experience with relatively mild engines with moderate compression and typical combustion efficiency. On such an engine it takes about 60 lb/min of air to make 600 HP. At an A/F ratio of 12:1 that much air would require 5 lb/min of fuel, or 300 lb/hr of fuel. Again, the BSFC = 300/600 = 0.5

So why are some engines showing less than 0.5 BSFC? Because they are moe efficient! Chamber designs allowing higher compression, more timing and producing better combustion efficiency make more power with the air they take in than the mild engine described for the "rule of thumb".

All this assumes you are not adding excess fuel as "coolant" to control detonation. Which you shouldn't have to do if your boost, octane level, blower efficiency and intercooler are appropriate.

Hi Smitty!

SB

Tomcat - great info !!!!
(Sorry Smitty for getting off the outdrive experiment thing for a second)
So, on easier words for old guys like me, if you where to size up injectors for a S/C engine you would need to do it for whatever net hp result you think you will have and add in the hp it takes to drive the supercharger.

So, for easy math, 1000 flywheel hp + 100hp loss to drive the SC would = 1100hp for fuel needed. Since, said another way, the air + fuel is feeding 1100hp and the SC is taking 100hp away to drive for a net flywheel result of 1000hp.

Thanks again Tomcat !!!!

I learn a new thing everyday from other people even though the answer is usually right in front of me. Doh ! Gotta love the forums !

tomcat

Hi SB:

Glad to help; I knew I was posting for some reason.

The only fly in the ointment is estimating blower drive HP. It goes up as you increase the amount of pressure and flow. A 1000 HP engine could be making 1200 HP and paying 200 HP to the blower. If I was sizing injectors that's what I would estimate anyway, just to be on the safe side.

When I really want to know I spin the (centrifugal) blower on a mule engine. The mule engine isn't supercharged, but by running it on a dyno with and without the blower drive belt, the difference in observed HP is what you lost to spin the blower. I just use a throttle body on the outlet of the blower to duplicate a given boost level. With that pressure and the blower RPM you can get the flowrate from the compressor map. Repeating this experiment at different RPM gives you a real compressor HP curve.

I have also found the estimated compressor HP predicted by Engine Analyzer Pro to be accurate enough for design purposes.

Tom

articfriends

Tomcat,have you spun a centrifugal on a mule motor on a dyno to see what they really take to spin,I'd love to hear what the true parasitic loss actually is at about 10 psi boost,Smitty

articfriends

So SB,you could really put some theory into injector size if you figure you want to make 1000hp,add in the fact the blower uses 200hp to spin THEN add in the fact the blower manifold boost pressure lessens the actual output of the injectore even though you have increased fuel pressure to it,you would need some big mo-fo's ,Smitty

articfriends

Tyler doesn't worry much about bsfc#'s,only actual air fuel ratio's on his wide band 02sensors and egt temp's. I could see in my case why he wouldn't care as I did my own combination/engine package and built it myself. I would think (because I know the bsfc IS a good measurement of effieciency) that he would be very concerned as it would show how effiecient what he's building actually is. He did bend over backwards to help me and worked very hard on making my motor run the best it could on the dyno so I will probably use him again when I freshen the motor but maybe I will prompt him to hook up fuel flow meter. I think the issue lies in the fact you have to measure both supply flow minus return flow on these efi motors to get a true bsfc,Smitty

articfriends

Borrowed a 28 pitch 5blade maximus this week from a fellow oso'er,tried it fri night back to back on the river against my 30 pitch labbed w/ext box w/1" spacer. Incredible cruise but slower on top end by 1 mph and lagged accelerating above 5000rpm's,also tq steered to port even wide open which boat has never done. Drained oil from lower yesterday,no metal from lower gears yet since newest treatment, even after some very long passes at hardy pond last week plus the testing fri and a couple of weekends of boating too. Pulled lower from upper,removed spacer and studs this morning putting lower back up to 1" raised position to try with 28 5 blade before returning it. Took boat out today,pulled fairly well with 5 blade even with heavy load in boat except still takes 10-20 seconds to go from 5000 to 5700. I made a half dozen high speed passes ,was heading in and leaned on it one more time,brought it up to 5000 fairly easy,started leaning on it with throttle to the wood and sheared something off in the drive . I have put 5 or 6 sets of gears in lower after they started breaking thru the hardness but haven't blown up or sheared any gears or shafts in the past 3 seasons. Either something was fatiguing or the 5 blade just pushed something past its limit,we'll see in the next few days,I think a 5 blade is just too hard on my drive with the kind of torque I'm making,Smitty

articfriends

For the 1st time I can remember the oil (75/140 royal purple)I drained out yesterday smelled a little burnt too,it had less than 10 hours on it but they were some hard ones,Smitty

SB

I'd be willing to bet the 5 blade helped reduce some life.

rmbuilder

Smitty,
Did any of the treated components fail or was it collateral pieces? I have additional info on the short run gear-sets we spoke of.
Bob