New to MFI

[jlesliel]

Been around racing all my life but never MFI. Bought an FED back in February. It's a 406 SBC, 14:1 comp with a 3 piece Hilborn and DSR pump on alcohol. Keep in mind the last car I made a pass in was 20 years ago in a door car that went 1.48 60ft. So being cautious I backed out the pedal adjustment screw all the way which gave me 2/3 to 3/4 at the butterfly. Also being cautious I put a 3000 chip in the low side and shifted at 6000. First time at the track it went 1.16 60ft, 5.33 and 5.28 @129. I was really impressed.

 Then came COVID19. The weekend before the tracks closed I was at a Funny Car Chaos race in Lufkin TX. I witnessed Dennis Piranio testing a high speed bypass at the track. That got me to thinking about my car. With COVID19 putting a halt to everything, it gave me even more time to think until I couldn't stand it anymore. I had to test my high speed bypass. Testing it I slowly raised the air pressure and @ 50lbs it released.

 Fast forward to this past weekend. I have a pressure gage on the pump so I had my grandson record it while I brought the rpm's up against the converter. Converter stalls 5400 and just as the rpm passes 5000 the pressure hits 50lbs, flickers and drops to about 48lbs. In my mind I'm thinking that as soon as I let off the brake I'm hitting the high speed going down the track. Am I wrong?

[Spud Miller]

  Hi! Sounds like you're having fun.

 Yes, the pressure setting for the high-speed is related to RPM, but most folks have their systems nozzled to give them higher pressure than they used to. People used to nozzle everything big and see maybe 60 PSI at the top end. Now, smaller nozzles and higher pressure mean you might achieve 100 or 120 PSI at the finish line. That 50 PSI high-speed check valve is open right off the starting line, yikes! Some would tell you not to use a pill in it and as you can see, that check valve open at the starting line and no pill could really hurt stuff.

 You could send me a questionnaire off my web site and I could help you figure a good starting point for your high-speed. Or, you use my free web calculator and do it yourself. It would give you an idea of what kind of fuel pressure you're making at the top end (@8000 RPM). From there you could easily get the pressure for any RPM during the pass.

Calculators (to do it yourself): http://www.fuelinjectionent.com/tuneup.php

Questionnaires (if you need my help): http://www.fuelinjectionent.com/questionnaires.php

Spud

[jlesliel]

Thank you Spud for your response. I registered to use the calculator and took a look at the questionaire. I can kinda fake my way through the calculator but on the questionaire I don't really know all the details of the motor, (head flow, valve size, pump size, porting) who knows, it might not even be a 406? I'm just trusting the seller. Guess I should have had stuff flowed during Covid lock down oh well.

I had thought about blocking the high speed since it's 1/8 mile and not 1/4 but since learning the high speed is open when it leaves I don't have the pills to support that. Currently running .110 main and a .045 in the high speed so if my math is correct that's equal to a .155 pill. guess i need to spend some more money. ha!

Thanks again 

[Spud Miller]

 I would say a high-speed lean out isn't worth messing with most times in an 1/8th mile situation. You could turn the check valve around so it is disabled, remove all the stuff or put a blank pill in it.

When you add pills together, you have to add the areas not the diameters...the area of the two combined and then converted back to an equivalent diameter. In this case:  .110 + .045 = .119

 Have fun!

 Spud

[jlesliel]

Thanks again Spud. i have read about area of a jet instead of diameter but it didn't click till you said it.

[jlesliel]

Hi Spud and thanks again for all your input. When I bought the car it came with a box of stuff AN fittings, leakdown tester, odds and ends. At the bottom was a spare chute, didn't go any further. Finally took the chute out and found the pump tag underneath it reads 2.82 gm @1800rpm. Now I have a little something to go on.

[George]

We ran a Hilborn stack injected ,methanol, 355 CID SBC in a FED for a few years. Almost all 1/8 mile. I wouldn't be concerned with a high speed by-pass until you are happy with every other aspect of tuning. We ran a best of 5.07 @138 . Combo weight was #1460.

[jlesliel]

Cool! nice pass. I'm not really concerned about high speed bypass just wasn't sure about my findings on the gage. thank you also for your input.

[dreracecar]

Would you even get into the HS on an 1/8m pass that it would make any difference?

[jlesliel]

I've been waiting for others input but I'll reply. I see no need for a high speed for instance, in my car it's geared 4.30 so i'm crossing the 1/8 @130 so rpm is low, It's still torquing it's way down the track. IMO

I do have another question though. I always raced @ 3600 elevation unless I went to Dallas or OKC. Home tracks gave me a density altitude of say 7000 ft. in the heat. Here in dallas area DA is say 3700 ft. I was carbureted on gas and had to jet up when going to those tracks. With that being said, now that I have pump flow numbers and can use the FIE calculator, there is no input for density altitude. I know I'm old school so I'm asking, is DA no longer a factor?

[AF150]

I am old school also. The density altitude is THE major factor. When using the FIE calculator, you can set the DA to anything you want just by changing the barometric pressure input. Leave everything else alone and see what main jet is reccomended for a range of DAs from say 2000' to 10000'.(last week in Denver we ran with the VDRA and in the afternoon the DA was 9200') These jet vs. DA solutions should fall in a straight line because the pump output is linear. Make sure your pump and nozzle combination will keep the system pressure up around 80 to 100 psi. DA is available on line for most race tracks. This approach should get you in the ball park...fine tune as required.

MB

[jlesliel]

Thanks for that info, I'll play with calc some and see. It does make sense tho.

Thanks again!

[jlesliel]

I am old school also. The density altitude is THE major factor. When using the FIE calculator, you can set the DA to anything you want just by changing the barometric pressure input. Leave everything else alone and see what main jet is reccomended for a range of DAs from say 2000' to 10000'.(last week in Denver we ran with the VDRA and in the afternoon the DA was 9200') These jet vs. DA solutions should fall in a straight line because the pump output is linear. Make sure your pump and nozzle combination will keep the system pressure up around 80 to 100 psi. DA is available on line for most race tracks. This approach should get you in the ball park...fine tune as required.

MB

So, when you say keep the pressure up around 80 to 100 PSI, what RPM? My car is geared 4.30 so I'm crossing the 1/8 @ about 5500. I currently have #18 nozzles (.0325) and as I play with the calculator it makes me consider #14 nozzles (.0287). 

[AF150]

The calculator solution shows the system pressure at 8000 RPM.

[jlesliel]

oh, Ok. I wasn't sure if it was calc RPM or actual RPM