Quote:
Originally Posted by 68 Silverbird
X2! I'm trying out my plate this weekend.I've read to go up 68 fuel jets,or 30% more diameter and pull 1/2 degree additional timing per 100hp on E85 vs gas.This info is from Heath on raceone85.com.

Ok,I can accept that one may need 30% more fuel mass/volume for E85,that's a fairly commonly percentage I've seen thrown around on that topic.
But the 30% more jet dia. comment is almost certainly wrong.
If it's using like 40 lbs/hr of fuel,and you then needed 30% more fuel mass than that,I can see that no problemo,if my math is right that would be like 52 lbs/hr of the "new" fuel.
Seeing as 12 is 30% of 40,then 40 + 12 = 52.
But the 30% more jet dia.,,,that has gotta be wrong...
First thing first,you cant really look at orifices in terms of diameter.
To quote the guys @ Kinsler fuel injection,the flow of an orifice can
not be interploated by the ratio of the diameter.
IE: an .080" orifice does not flow twice that of an .040" orifice.
As I understand this,one has to go off of the orifice
area.
To convert the orifice diameter to the orifice area use this formula:
(radius x radius) x 3.14159 = area
Or just use this online calculator:
onlineconversion.com/shape_area_circle
So the area of the .080" orifice is .00527" sq/in
And the area of the .040" orifice is .001257 sq/in
That's a ratio of 3.999.
So the area of the .080" dia jet is nearly four times that of the .040" jet.
Now I'd buy he meant 30% more jet area maybe,just not 30% more jet diameter.
If using a .040" orifice above as an example,that would mean adding .000371 sq/in of area to that .001257 sq/in of the .040" orifice.
And that would indeed end up @ an orifice of appx. .045" to .046" or so (yep that's the 68 jet sizes mentioned).
But adding 30% to the jet diameter,that would end up being a bit more than that.
30% of .040" is like .012" (see above ^^^^),so adding 30% more jet diameter would net something like a .052" orifice as SOP.
That would increase the area of said orifice a bit more than 30%.
That would increase the orifice area to like .002123".
If my math is correct,that would be more like 41% more orifice area.
So I'm fairly sure he meant 30% more jet area,not 30% more jet diameter.
As for the lbs/hr that change makes,you'd almost hafta flow the orifices to be 100% sure that's how much more fuel mass your actually getting for that sorta jet change,just as Ron (TINTIGR) said.
I've seen some reliable flow numbers given for some larger jets/orifices (holleys),and some smaller (MFI) ones too,but actual flow numbers for jets/orifices in the .040" to .055" neighborhood are kinda hard to find for some odd reason.
And this all assumes your working with a set fuel pressure too.
Change the systems fuel pressure,and you'll change the lbs/hr as well.
That's a whole other can of worms...
Though once you know the actual flow of an orifice for a given fuel pressure,then figuring out the lbs/hr changes from a given fuel pressure change is a pretty easy thing to do as well.
It's knowng the orifice's actual lbs/hr flow @ a given pressure that's the real trick there.
Then all that's left is to figure out the nitrous side lb/hr numbers.
HTH
Bret P.