articfriends

iTrader: (1)

I wil be at brower all weekend,look for a red 06 duramax 4dr pulling a 272 baja named "never enough_____?" or if out on the water flag me down,Smitty

SB

iTrader: (1)

I'm not sure if you accounted for the following:

Don't forget to subtract boost psi (intake psi) from fuel psi to figure out actual psi at injector .

Example: 75psi fuel measured at rail minus 10psi boost = 65psi actual at injector.

Whitey

See ya there!
Dwight

articfriends

iTrader: (1)

I've never accounted for that but you bring up a very interesting point and something for us all to think about factoring in. Kurt will be watching his a.f.r and egt's on the dyno when making his pulls plus adjusting his fuel tables so he probably won't melt motor down if injectors turn out to be too small but it would suck if he had to rip motor apart and change them half way thru his dyno session,Smitty

articfriends

iTrader: (1)

I ran my numbers thru the calculator,unfortunately I don't have bsfc from the dyno because Crockett doesn't measure it when doing EFI motors.
I'm using 63lb injectors at 75 psi under boost,they become 82.7lb injectors from 75psi and 77lb injectors if pressure is changed signifigantly from the 10 psi of boost I'm running to 65psi.
82.7lb inj will support1058hp at.5bsfc,882hp at .6bsfc 77lb inj will support 985hp at .5bsfc,821hp at .6bsfc
My motor made 944hp/962ft lbs tq with a correction factor under 2% so either my bsfc is between .5-.55 or dyno was incorrect. I do know that Crockett checked the calibration after we made 900 plus ft lbs in the first pull to 4800 rpm's because he was in disbelief,Smitty

SB

iTrader: (1)

I believe the .6 BSFC's used really aren't accurate anymore. Remember how people thought near .5 was normal for NA BBC ? Well, today we are in the lower .400's due to better components and better understanding of stuff. Oh, and this is with carb's too .

Now, add into the equation much more effecient centrifugal SC's with big intercoolers and the .6 BSFC's (typical roots , carb's, older engine parts) for the most part dissappear.

Also, old thinking said 'add way more fuel then you need' to keep detonation at a minimum.

Anyway - as you can see with your math, a difference in injector needed at .5BSFC is much different tha at .6BSFC.

Those equations are to give you a baseline to go from when deciding what injector size you'll need when planning an engine build.

Last important notes:

1) You need to know what psi the manufacturer uses when rating each certain injector

2) You need to know how accurate / close to those ratings your injectors actually are. Some manufacturers/models of injectors are a lot 'looser tolerance' than others.

3) The dyno or vehicle w/measuring equipment will ultimately tell you what's happening.

SB

iTrader: (1)

Smitty - did he atleast use the fuel flow meter ? If not, I guess this explains why no BSFC's.

cstraub

Smitty,
You may want to check these guys out:
www.metalax.com

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

iTrader: (1)

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 !