95' Junker Drag Truck

Big Blue24 said:
I wanted to also provide a little analysis on my top-end defueling issue....I still have one.

My stock mechanical pump makes 64 psi at idle. My stock mechanical plus the Holley Black Knock-off pusher pump makes 85 psi at idle. When i turn on my third lift pump, the Walbro 392, it will instantly make 115 psi at idle.

The triple lift pump setup will maintain 80 psi through the traps at 119 MPH.

During the long burnout contest at 4200 rpm, the triple pump setup was maintaining 65psi, so I think the stock mechanical pump struggles at high RPM and the Walbro picks up the slack.

I have a new probable culprit for the defueling and it's something very basic, I think I need to tighten up the 5.5K governor springs. I'll have then adjusted tighter before my next outing and will hopefully be able to get better 3200+ RPM fueling.
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This is just a thought. Do you think your problem is coming from your 175 hp pump? I was told that the cam profile in the pump causes it to start defueling by like 3900 and anything past 4k it drops drastically.
 
I thought the mech lift pump used spring pressure to pump not the cam. So no matter how fresh your mech pump is you still need another spring to help the pump return to the cam fast enough when the cam is twisting 6-8k (2:1 the crank rpm).
 
Gasoline Sucks said:
I thought the mech lift pump used spring pressure to pump not the cam. So no matter how fresh your mech pump is you still need another spring to help the pump return to the cam fast enough when the cam is twisting 6-8k (2:1 the crank rpm).
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Cam turns half engine speed...
 
Are you suggesting the cam lobe gives the return position and the spring does the pumping?
 
The spring just returns it so it isn't stuck in one position once the cam lobe makes one revolution. you got it backwards. That's like saying the valve spring is what actually opens the valve DOH


Sent from my iPhone during class
 
Then I do have it ass backwards but point being a stronger spring helps.

Im used to being wrong so let me have it!
 
Actually, Gasoline Sucks is correct, the spring does the pumping, the cam lobe simply resets the spring. If you dismantle a stock mechanical lift pump, you'll see that the spring is 100% responsible for the fuel pressure and the cam lobe/push rod simply reset the piston/spring assembly. At high RPM, I'm almost certain the pushrod is floating and not maintaining constant contact with the fuel pump lobe.

I do think a simply aftermarket spring could really enhance the high pressure flow of the stock lift pump.
 
24vriviera said:
Nicely done sir!
As the walmart man asked, u coming to ennis next month?
I want to race you!
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I'd need a fuel sponsor to travel that far, unless you can guarantee I'll finish first in Super Street and Super Diesel :hehe:

Btp 2332 said:
Congrats on the new personal best!
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Thanks! I really wanted to open it up and see what it would do but I thought I needed to run at least one qualifying run slower than the index to qualify so on my second and final "test" pass, I made sure it ran slow.


NickTF said:
pinion brake pinion brake pinion brake........:bangLOL
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I'm not yet at the level of needing a pinion brake or transbrake. I can hold 10 psi on the factory rear drum brakes and I couldn't regularly hook 5 psi until recently. When it starts pushing through the brakes, I'll research brake options. I honestly think it would be cheaper and easier to convert to disk brakes on the rear in lieu of a pinion brake.

92dak_r6 said:
This is just a thought. Do you think your problem is coming from your 175 hp pump? I was told that the cam profile in the pump causes it to start defueling by like 3900 and anything past 4k it drops drastically.
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I too have heard that 3900-4000 rpm is where they start to lay over. My pump, however, is laying over at 3000-3200 rpm. And really cutting back fuel at 3500 rpm. I'm going to play with the governor springs so more and if that doesn't work, I'll be forced to upgrade to a different pump.

straight 6 roar said:
Great runs man I enjoyed seeing your truck race finally. My buddies loved the ghettoness/ET ratio.
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Thanks. I too love seeing the $/ET ratio, it's fun to create inexpensive solutions to attain maximum performance.
 
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Big Blue24 said:
I'm not yet at the level of needing a pinion brake or transbrake. I can hold 10 psi on the factory rear drum brakes and I couldn't regularly hook 5 psi until recently. When it starts pushing through the brakes, I'll research brake options. I honestly think it would be cheaper and easier to convert to disk brakes on the rear in lieu of a pinion brake.
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Is this the first time you've put an actual drag racing slick on the truck? Pushing through the brakes is not my worry but actually spinning tires once a certain pre-launch boost is obtained. I would imagine your 12v is similar to mine and many others. If you can launch and hook a 20psi + launch this truck may see a 10 second pass and you will likely find a few mph given you will be in the meat of the power band even faster. As I mentioned before it will take some suspension work, adjustments, etc. but the first 60' is your biggest weakness and the most important part of a good quarter pass. Good luck either way. :Cheer:
 
We switched from drums to disk brakes because we were pushing through the brakes. It helped a lot. I still want to try a pinion brake to see if we can leave a little harder.

We also run a big single instead of twins which might make a difference on how your truck compares.
 
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Food for thought, Rona did his recent 1200hp+ run with just the stock lift pump supplying his 13mm.
 
NickTF said:
Is this the first time you've put an actual drag racing slick on the truck? Pushing through the brakes is not my worry but actually spinning tires once a certain pre-launch boost is obtained. I would imagine your 12v is similar to mine and many others. If you can launch and hook a 20psi + launch this truck may see a 10 second pass and you will likely find a few mph given you will be in the meat of the power band even faster. As I mentioned before it will take some suspension work, adjustments, etc. but the first 60' is your biggest weakness and the most important part of a good quarter pass. Good luck either way. :Cheer:
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Yes, this was the first time a real drag slick has been on the Junker Drag Truck. I was blown away by how hard it hooked. The Nascar roadrace slicks felt like they made a nice improvement over street tires, these slicks made a huge improvement over the Nascar slicks.

The biggest seat of the pants difference was down the track on the 1-2, 2-3, and 3-4 gear shifts. I didn't even know the Nascar slicks were spinning a little on these gear shifts till I strapped on these true drag slicks. Every gear shift whacks me in the back so hard, I was amazed that the axles, u-joints, torque converter, and home-built tranny could handle the abuse.

The launch was a little better but I never left hard enough to notice the 100% improvement like I could feel on the gear shifts.

I can't fathom how the aluminum planetary forward gear is still in-tact. The aluminum planetary must be stronger than the common failure pictures would lead you to believe.
 
Idk my aluminum planetary broke at half throttle merging onto the highway LOL I'm glad you got the slicks on her and had a good time!
 
RJM06590 said:
Food for thought, Rona did his recent 1200hp+ run with just the stock lift pump supplying his 13mm.
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I did read about Ron's truck making big HP without his booster lift pump.


All I know, is this little 160 pump is hungry for fuel pressure and the more I feed it, the harder it fuels. You have to remember, I have the factory lift system intact and and the pusher pump and front feed pump on separate toggle switches. There is an honest 75-80 HP gain flipping on the pusher pump, especially in the 2600+ rpm range. The front feed Walbro pump doesn't make a noticeable increase in power till about 3200 rpm.


To give a little perspective, on the 11.73 pass, it was running 97.5 MPH in the 1/8th. That kind of trap speed will normally net 126-128 MPH at the 1/4 mile. This truck is defueling, boost falls, smoke clears from a light haze to no visible emissions, and acceleration rate drops dramatically around 1000' out.

126-128 MPH at 5600 lbs calculated to 750-800 HP average to the wheels. Now the actual 119 MPH trap speed calcs to just the mid 640 HP range but I know that the truck is defueling on the second half of the track and falsifying the calculated wheel HP.
 
Seems like we are both having problems half track. Your just a full second faster! I took my truck out and just matted it in first and held it there an 3300 is where if falls off real fast. I've been looking at mechanical belt driven setups to solve my issue for good.


Sent from my iPhone during class
 
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