Myth Busting Fuel Delivery

//Myth Busting Fuel Delivery

Myth Busting Fuel Delivery

Attached are some more responses from e mails that cover the bulk of the questions we receive.  I hope these help and feel free to Email or call if you have any further questions.

QYou built me a trick pair of 4150 Holleys for my nostalgia super stocker last year.  Towards the end of the season the engine was hard to start and it seemed like the floats were too high but after checking them out they were fine.  I guess this is too open ended of a question to troubleshoot over email but I wanted to use you as a resource.  I started to look into the fuel pressure and regulator for answers.  Some times the fuel pressure does change from too low to too high (every two races).  I am using a BG400 pump and a Holley 4-port regulator.  Should you have a return line out of the regulator back to the gas tank?  Should I swap out pumps or regulators? 


A: The carburetors are probably fine.  Your point of focus should obviously be the varying fuel pressure.  This will cause the complaint.  Inspect your fuel pump, pump relay, or regulator for proper operation. ROCK SOLID fuel pressure is vital.  A bypass system will give you less pressure drop under a load, but a dead head system can still be consistent. Although it never hurts to add a restricted return bleed line on a dead head system.

Most times its a low side regulator issue.  But varying high side pressure or even a bad relay will often cause inconsistent fuel pressures (oftentimes paired relay systems will act up if one regulator cuts in and out) .  In a return system be sure the return line is larger than the forward feed line.  Be sure the bypass line goes back to the top of the cell opposite the inlet side and low fuel levels in your cell are a bad thing.


QIf aerosol or tube type boosters are so good, why aren't they used in NHRA Pro Stock.

A: You answered your own question.  Against my judgement we were hired to experiment with them in NHRA Pro Stock.  I told the customer what we would run into and to be ready as here’s what to expect and I was correct.  With permission I am free to release these findings as this information is now well known in the affected community.  Here is a summary of our testing results: During dry testing the resistance ratios looked similar to current designs and acceptable to test with, however in a wet/live environment at low air speeds and during shift recovery fuel atomization was poor as fuel would just drip off the tube.  The air would then striate and detonation was a big problem (under statement).  With good nozzle air velocity the tube does a good job atomizing (it wasn’t amazing…but it worked so let’s test it).  We found on the track when your rpms are low like during launch or shift recovery, the tube delivered fuel is thick and raw and just rolls off the end like a low flowing garden hose.   During launch and shift recovery the engine displayed air to fuel ratios numbers from the 02’s like the car was very lean, but the fuel turbine numbers and shear numbers were showing that the consumed quantities were dead on.  That information confirms the poor atomization, distribution, misfire and detonation suspicions I had concerning the concept.  The tube only atomizes toward the inboard side of its body so the far side air is clean.  During low air velocity the fuel would enter the runner as a long thin waterfall and tended not to mix for the duration.  If a particular cylinder had a pulse issue it may steal a section of that clean air and would ALWAYS be lean no matter what you jet that cylinder.  As that cylinder would steal the air from that particular section of the intake that was clean.

The tube concept would work fine on a single carb on alky with a lot of air speed and a fuel that requires very little atomization no matter the air velocity.
But honestly if it was a great idea and head and shoulders above all others.. we’d all be using it.  It was just one of the many gimmicks that was thought to
have some merit but in our testing… did not.

The advantage that a booster has over the tube (face it, the aerosol name is a marketing tool) is that it develops an ever expanding fuel cone that impacts more and more air as it travels down the venturi.  This not only increases adiabatic expansion but ensures more even cylinder to cylinder air to fuel ratios.  Also modern booster technology offers on average better low speed signal which offers the ability to install a larger carburetor which reduces air friction.  This increases cylinder fill and horsepower.

QI bought a dominator from you a few years back.  Can you explain why its important to calibrate a carburetor for a specific application.  I would like
to be able to more clearly explain the specific modifications and benefits to my friends.  

A: Like port sizes in a cylinder head, metering port sizes are just as critical to have the proper velocity for shift recovery and the proper emulsion package for
a smooth fuel curve.  If for example the carburetor is calibrated for a horsepower level that is 75-100 hp or more below what you are using it on.  The emulsion package will max out early and the carb will go rich up high or worse yet, if its a high emulsion package (which offers better power) it’ll go lean up high.  That’s typically why high emulsion packages are not offered on “out of the box” designs.

Factory designs are not calibrated or tweaked after assembly to fill the voids in the fuel curve from the emulsion package.  Since all ports don’t offer equal
flow at all flow rates, the emulsions are tweaked to fill the gaps and improve the fuel curve in those ranges.  You may for example be lean 2-3 main jets
from 5500 rpm to 5800 rpm and then too rich between 6400 and 6800 rpm.  So you see why its critical not only to performance, but predictable consistency to have a unit flowed and the fuel ports sized for its
intended fuel and air flow range.

Thanks for reading.

By |2005-03-04T00:24:48+00:00March 4th, 2005|Uncategorized|Comments Off on Myth Busting Fuel Delivery