Technical > Spud Miller's Cave

Fuel pressure seems to drop before engine RPM just after launch???

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GlennLever:
We went and did some test and tune today.

The symptom we have is the engine comes up to RPM on the trans brake, I release the trans brake and the dragster launches, and then falls on its face. It then picks’s itself up and has a pretty normal run. I hit my shift point at 6300 RPM, RPMs drop off as you would expect and then climbs again.

What is pictured below and in the attached files (not sure which is easiest for you to view) is a graph produced in the data logger.

It appears to show a drastic drop in fuel pressure before the slump in engine RPM. The first graph shows the complete run. The second graph is just the area between the blue vertical lines to zoom in on the troubled area.



This is the zoom in on the trouble area.
When the engine RPM reaches 4756 the fuel pressure is at 31.92 pounds.
When the engine RPM reaches 4810 the fuel pressure has dropped to 3.37 pounds.
When the engine RPM drops to 3527 the fuel pressure recovered to 16.13 pounds and continues to climb.

The red vertical lines are for reference between the two graphs of fuel pressure and engine RPM.



If we lean the engine out (not changing the fuel main bypass, but add air by decreasing the top pulley the problem is not as pronounced. We are blowing fuel out the number 8 cylinder.

This graph represents an 8.904 ET run in the ¼ mile
We lean it out and ran an 8.409

We have check compression, valve lash, put in a new MSD box, New coil, removed the primmer, check timing, increased timing to 30 degrees, check the inside of the distributor cap, remove and cut open the torque converter (no problems found)

George:
High speed ?

GlennLever:

--- Quote from: George on September 23, 2013, 05:48:35 AM ---High speed ?

--- End quote ---

I only have an idle and a pump saver. We did check the idle, but not the pump saver. I'll do that today

George:
I'm far from and expert and don't have much data on my deal but it looks like something could be opening when fuel pump speed/pressure comes up. I know since we have introduced our injected engine to 10-15% nitro the system pressure has risen to the point we are too rich on the big end (1/8 mile) and next time out I'll be playing with a high speed bypass.

Good luck and safe travels. George

masracingtd1167:
Glen are you leaving at full throttle with a chip?

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