running out of breath in the 1/4 mile

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diymirage

diymirage

HP@idle > hondaHP@redline
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hey guys
yesterday I got to take my duster to the strip for the first time
drove the car out there (about an hours drive) made a few passes and drove her back home

altogether, I'm pleased but I did have an issue making the actually passes

about the time the car hit 3rd gear (727 auto) she would stumble and fall on her face, the after a little while she would recover again

time slip is pretty telling, especially if you compare the 1/8 MPH to the 1/4 MPH

My best slip looked like this
Reaction -.0365
60 ft. 1.9853
330 f . 5.5913
1/8 et 8.7194
1/8 MPH 74.78
1000 et 11.8284
1000 MPH 72.96
1/4 et 14.9149
1/4 MPH 71.88

I actually lost 2 MPH between the 1/8 and the 1/4

the car is a 71 duster, with a MRL build 408, dynoed at 423 HP and 470 foot pounds
Comp Cams XE268H cam with a preform 750 (holley style carb)
I'm running a stock mechanical fuel pump, with a steel line (whatever stock size is, 3/8th maybe?) and a steel fuel filter going into a braided line to the carb

it looks like I'm emptying the bowls quicker then I can refill them
would this be a float issue, a bottle neck between the pump and the carb or do you guys think the fuel pump is just not up to the job?
 
Most mechanical fuel pumps will run 12's and 13's all day long. Unless it's struggling to pull fuel from the tank, or some other restriction, no reason to run out that fast. Looks to be running out before the 1/8, according to your speed.
 
the line from the tank to the fuel pump is the stock line that was in place when Ma Mopar build the duster, with a slant 6 in it
 
diymirage said:
the line from the tank to the fuel pump is the stock line that was in place when Ma Mopar build the duster, with a slant 6 in it
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I ran the factory slant six fuel line from Ma Mopar in a Dart from the tank to a 360 that went 12.9's without fuel delivery problems. I did install the Holley 80 gph mechanical fuel pump, though I didn't see any affects from the Holley fuel pump to the parts store Carter pump by how the car ran.
 
I had a similar problem recently. My Dart was running out of steam at the top of 2nd gear. Found out the fuel sending inlet was 2" off the bottom of the gas tank.(Fuel sending unit is replacement junk) With 1/2 a tank of gas, the fuel was moving to the back of the tank and the inlet was sucking air.
 
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Yep, almost assuredly a fuel delivery problem...
 
Running out of fuel. Start at the tank and work your way forward. /6 used 1/4" fuel line. v-8 used 5/16" fuel line. Hemi's used 3/8" line All had matching pick-up's in tank. I would start by replacing the pickup and line to 3/8". 1/2" would be even better. All the way to the carb. Install a fuel pressure regulator and guage as close to the carb as possible. You also need a tach to determine your rpm @ the 1/8th and 1/4. A 408 may need more fuel. After that perform a jet loop. Keep going richer until the car slows down then go back one size. You have some tuning work to do, good luck
 
I don't know if a 15 second pass is pinning you to the seat, but if you suspect the fuel is uncovering the pickup, just get most of the fuel out of it. Then just jack up the front end about a foot and a half and see if the car will continue to run after a while. You didn't complain about first or second gear much, so it would seem to be getting fuel. If it goes into third and then recovers, why is that. Maybe the gear is too steep although I doubt it with the hp you are claiming. That means the problem could be the ignition not firing well when it gets into a heavy lug position. Just guessing though.
 
thanks for the replies guys

we had one guy say the /6 line is enough to get into the 12.s and another claiming this line is the restriction
any other thoughts on this?

(ill post a pic of my fuel filter set up later so you guys can eyeball that too)

any more thoughts on float level?


SPACECOWBOY said:
Do you have a fuel pressure guage?
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no I don't, though I'm thinking I need to put it on the list now

Bad Sport said:
Ima thinking the snowball is about to grow. :p
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I lost it with the new software, but at some point I added the phrase "roll, snowball, roll" to my siggy

cannucky said:
What gear you running ?
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3.23s

1969reddart said:
I had a similar problem recently. My Dart was running out of steam at the top of 2nd gear. Found out the fuel sending inlet was 2" off the bottom of the gas tank.(Fuel sending unit is replacement junk) With 1/2 a tank of gas, the fuel was moving to the back of the tank and the inlet was sucking air.
Click to expand...

the P.. said he had a new sending unit installed, that would have been maybe 2,3 years ago
but I have no idea what is was, as far as OEM, Chinese or whatever

magnumdart said:
I don't know if a 15 second pass is pinning you to the seat, but if you suspect the fuel is uncovering the pickup, just get most of the fuel out of it. Then just jack up the front end about a foot and a half and see if the car will continue to run after a while. You didn't complain about first or second gear much, so it would seem to be getting fuel. If it goes into third and then recovers, why is that. Maybe the gear is too steep although I doubt it with the hp you are claiming. That means the problem could be the ignition not firing well when it gets into a heavy lug position. Just guessing though.
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it did not fall right at the 2-3 shift, but around that point
sometimes at the top of third, sometimes at the bottom of third
 
were you coasting from the 1/8 to the end. the times and mph posted are screwy
 
As for the /6 fuel line, it makes NO sense to try to get by with a small line for what you are doing. The pump has a long pull from the tank as it is and a smaller line makes for more restriction. You don't want to do more upgrades and keep running up against any fuel issues; it impacts your jetting work and everything. 5/16" line minimum is pretty much standard practice; heck I used to run 5/16" in a 1.9L 130 HP 4 banger! And you have a 408?!? 3/8" line man! IMHO, 1/2" is overkill here.

No real silver bullets in this; it's one of those situations where you just gotta start trying this and that. As for the pickup, look at the sender/pickup as installed in the tank; if it looks like it is installed crooked, it's a good bet that the pickup is up from the bottom a bit.
 
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Let's say I replace the fuel sending unit and the line leading up to the pump with 3/8 line, will I need to replace the pump to accept the?

And how hard is it to replace the sending unit?
 
I had a fuel pump test done when my engine was on the dyno. The stock pump I was running could not provide enough fuel for my engine. It started loosing pressure @ 3400rpm and by 5k, it only had about 2 lbs of pressure.

My engine only made 380hp and 420tq. standard stroke 360, same cam as you, same size carb.

After recommendations from this site to stay with the stock 5/16" line. I simply installed a Carter Street/Strip pump that has more than enough fuel delivery for my engine. I shouldnt have to worry any longer.
 
Get a Carter HP4601 and put a regulator and return line on it. Regulate it to 6 PSI. Goodbye to fuel problems.

If all you have is a POS auto parts store replacement stock pump and you're trying to race with it, here's your sign. As Bill Engvall would say.
 
RustyRatRod said:
Get a Carter HP4601 and put a regulator and return line on it. Regulate it to 6 PSI. Goodbye to fuel problems.

If all you have is a POS auto parts store replacement stock pump and you're trying to race with it, here's your sign. As Bill Engvall would say.
Click to expand...

so would this pump be able to suck enough fuel through the stock line, or does this pump still require the 3/8th line to feed the beast?
 
5/16 should be fine. But of course, going to 3/8 would not be overkill, plus that would handle purt near any future upgrades.
 
Bodyperson said:
Vent line plugged? Happened more than once
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mhhh that's an interesting point
I think the vent line used to go into a three nipple breather, but when I swapped over to the V8 I may have just tucked it away and vented it to the atmosphere
 
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