Footbraking Enderle hat SBC

[Curly1]

I have some questions and I think I am on the right track but since it is Winter and I can not verify it on the track stuck with bench racing.

On my Enderle hat injection works great on the trans brake with 4400 RPM chip in two step. My problem was in footbraking it for No Box classes.
I need to raise the RPM up around 2400-2800 to get good reaction times but when I do it will stumble and flat fall on its face. SBC 760 Hp Enderle hat with .038 nozzles and 60 PSI fuel pressure at finish line. What I did was lowered nozzles size to .034  and it should raise finish line fuel pressure to around 100 PSI. This is with Enderle 80-A1 pump flowed at 6.9.

I also added a small shim to the idle return but did not want to do too much there.  My data logger is reading zero at low RPM.

I believe what was happening was the fuel pressure was too low at the the hit causing the stumble. I believe the smaller nozzles will put out same peak power but at higher pressure and better fuel control at the hit with less stumble.

1. Is my thinking right there?

2. Is there anything else to do to help low RPM stumble?

3. What would be right size nozzles for foot braking a 760 HP SBC?  Not hard to get it tuned perfect on a dyno or at the finish line but a little trickier for foot braking.

[dreracecar]

Never used a trans brake on our blown Chevy, always the hand brake and never a stumble. One thing
 You should try is a heavier flex plate, the flywheel inertia gives a little boost and not let the RPM drop at the hit

[BK]

It sound like your idle pressure is too low. At idle you should be anywhere from 7-10 lbs for good throttle response. Nozzle adjustment is really for down track tuning. Idle adjustment and throttle response are done in the poppit and at the barrel valve.

[Curly1]

Let me add a few more things. It idles and takes gas (alcohol) real good and crisp. Also I have worked with barrel valve some fattening it up and leaning it down and nothing seemed to help at the higher RPM of 2400-2800.  Idle is good and high RPM is very good the problem is right at 2400-2800 launch. It even seems to run well from dead idle but can not race like that because reaction times are slow. Raise RPM up reaction times are good but creates a stumble. I did not have much test time to experiment with this at end of season and will not get much testing before season starts. We expect to get two test days weather providing. Note my converter stalls at 6300 so that is not much of an issue.

So You think I am on the right track with the smaller nozzles to increase pressure and need to add more pressure to idle return?

[Spud Miller]

 With a hat, if you hold it at partial throttle for a second or two (like you are to hit your launch RPM), it will go lean. The butterflies let in way more air than the spool lets in fuel. So, when it's launch time, you end up too lean and get a stumble.

 To help with it, you can change the angle of your arms to make the barrel valve arm quicker in relation to the butterfly arm. Stand the barrel valve arm up more vertical, the butterfly arm more horizontal (or both) and experiment with that. I bet you'll see improvement.

 You could also use an enrichment valve or something to make things richer until you launch. Not sure how much that would help however at such a low RPM.
 
Spud

[Curly1]

I checked the idle bypass and it has a very small thin spring and you can easily blow through it. I believe it needs to be around 8-10 PSI? As it was probably closer to 1 or 0 PSI opening.

I think I am going to increase it a little at a time and adjust barrel valve as needed for the changes and hope that fixes the issue.

[Spud Miller]

 A blown setup should have an idle check valve in the 10 PSI range. A tunnel ram setup should be 1-3 PSI. It wouldn't hurt to boost it a few PSI and see if that helps the situation. Just know that when you do, your barrel valve leak down percentage will need to decrease to have the same idle fuel mixture as before. In other words, if you just boost the check valve pressure, your idle will be RICHER.

 Spud