One of our customers recently inquired about idle lope in his funnycar dragster. ‘What can I do to minimize idle surge on our funny car? It’s difficult to stage at the starting line. It has a Bruno (torque converter), Lenco transmission, Enderle Bird Catcher fuel injection hat, Enderle K type barrel valve with port lines on a 14-71 retro blower on methanol. The hat butterfly clearance is 0.007.’ from G. R.
Response — background
Idle surging or lope is most common in mechanical fuel injection with Roots blowers. When I first encountered it in a racer’s blown FI drag car, he bragged about how radical his setup was with all of the camshaft lope. Then I saw that his car really did not run that well.
Idle lope from a radical camshaft is at a higher frequency than idle lope from a fuel system.
Through the years, I saw many blown FI engines with dramatic idle lope. I recently saw several tractor pullers with multiple FI engines with dramatic idle lope.
Idle lope in cackle fest and exhibition engines may be impressive. On the race track, especially in drag racing, tractor pulling, or exhibition wheel-standers, it is hard to stage and drive. Here are some examples.
Some drag racing cars:
>> over-stage from idle surges.
>> drop out of the second stage on the starting line when the lope RPM takes a dive.
>> stall after a burnout or backing up.
Some cars with torque converters are very difficult to stop against the RPM surges.
Some racing boats:
>> surge too fast in no-wake zones.
>> stall during the RPM drop before the engine gets warm enough.
Many with idle lope learn to tolerate it and adjust the engine speed variation to a best-fit range: midway between stalling and surging through the brakes (or wake speed limit for boats). Frequent boo-boo’s occur in excessively cool or hot weather, when jetting is changed, or when blower overdrive is changed!
Some of the causes of surging or lope & solutions
When an engine first starts, the RPM tends to go up as the air leaks through the cracked throttle blades. In a mechanical fuel injection setup, fuel also goes up with RPM, however it is clipped by an idle bypass poppet when (and if) idle fuel pressure is reached*. Some setups lope at idle. That may increase against the load of a torque converter in a car or a propeller in a boat. With that in mind, here are some idle lope contributors and solutions:
(1) FUEL PRESSURE TOO LOW: Overall fuel pressure is too low at low RPM from a large nozzle and main bypass combination. Several suppliers of blown alcohol fuel injection systems routinely set up jetting for less than 100 psi fuel pressure at 8,000 RPM. This common setup has very low fuel pressure at idle speed that may not be enough to hold the idle poppet open.
>> We ran a higher pressure system to maintain an idle fuel pressure above our idle poppet setting.
(2) IDLE VACUUM CONFLICTS: Idle vacuum in the FI hat can pull fuel from the nozzles in a Roots or screw blown application. This can distort fuel distribution if idle fuel pressure is low. The vacuum will surge from each lobe of the blower rotors, 6 total per revolution, uncovering and dropping the air pressure in the hat in surges. Vacuum in the hat can measure 9 inches of HG. That is almost 5 psi of vacuum pulling on nozzles at some low pressure from the low RPM.
(3) IDLE POPPET PRESSURE LOW: Several suppliers of blown alcohol FI systems routinely set up idle poppet pressure in the 5 to 6 psi range.
>> We ran 12 psi in most of our setups.
>> Idle speed was steady in our combination of higher system pressure & higher idle pressure.
(4) PRESSURE POPPET CONFLICTS: Some setups also have additional bypass pathways such as a remote main bypass or pump loop next to the fuel pump output. These are usually equipped with low pressure poppets to assist starting. A value of 5 to 10 psi seems to be common in these setups. These pathways may conflict and pop on-and-off as the engine searches for an idle speed.
>> We ran our main bypass in the barrel valve, away from the fuel pump output.
>> We sized our fuel pumps in the various setups so we did not have to run a pump loop.
(5) PORT NOZZLE IDLE PROBLEM: Manifold vacuum affects port nozzles (if so equipped). If the engine is idling on port nozzles, fuel pressure in the port nozzle distribution block can be dropped by vacuum pulses from individual cylinders that rob fuel from other cylinders. Idle fuel distribution is distorted.
>> More idle fuel pressure is needed to reduce this problem.
(6) PORT NOZZLE SIZE PROBLEM: Some setups run large port nozzles. They are harder to pressurize at idle pressures.
>> We ran port nozzles on the small side to load the fuel system at idle.
(7) PORT NOZZLE PRESSURE POPPET CONFLICT: The port nozzle distribution block is usually equipped with a port nozzle poppet. If an idle pressure is run that is close to the pressure value in the port nozzle poppet, it may load and unload causing fuel surging.
>> When we ran port nozzles, we ran a low pressure port nozzle poppet and high idle pressure.
>> Some racers set the port nozzle poppet to a higher pressure such as 30 psi. This is done so that the engine does not idle on the port nozzles. In that case, it is idling on the hat nozzles only. That would eliminate any contribution to lope from the port nozzles. However, the fuel distribution at idle is then determined by the blower outlet opening. Delta openings put all the fuel to the front, chilling the front cylinders, unless the blower is centered on the manifold. The engine speed where the port nozzles are enabled may become a setup issue for the staging and launch method. Port nozzles may tip-in too soon or too late depending on the launch method. A change in the main bypass for air density changes will change the RPM where the port nozzles tip-in. I watched more than one racer through the years cause as many new problems with the addition of port nozzles as were eliminated. It is a tuner’s big leap in a blown engine without prior experience or a good understanding.
Our Blown Nitro Racing on a Budget book has an entire chapter on port nozzles. Several port nozzle issues are covered in our Fuel Injection Racing Secrets book as well.
(8) THROTTLE BLADE/SHAFT DEFLECTION: Some setups were reported to deflect the large throttle blades and shafts, common to blown FI hats. Deflection would oscillate that would further contribute to the lope.
>> We carefully set throttle idle stops on all sides of our throttle shafts to properly seat all throttle blades in the idle position to help avoid this problem.
(9) THROTTLE BLADE PRESSURE OFFSET: Some racers drill holes on either side of a throttle blade for a fixed idle air flow with throttle blades that are shut off. Holes on one side of the throttle blade can cause an offset to the air pressure distribution on both sides of the throttle blade. We experimented with offsetting the throttle blades in a large hat assembly to create idle air flow on one side only. That did not work and made the throttle action very troublesome.
>> In all others, we did not drill holes in either side of our throttle blades.
>> We did not offset the blade from center for fear of unbalancing the throttle blade air loading at idle or part throttle.
*Some fuel systems use the idle bypass for a secondary bypass function. This is common in a circle track or sprint car MFI setup. It is usually set at a blow-off pressure that is a lot higher than idle fuel pressure. For more information about idle characteristics with the secondary bypass, contact your fuel injection supplier or manufacturer.
“A worn fuel pump may have a lower pressure at low idle speeds. That pressure may be near the idle poppet setting, resulting in poppet bounce. Changing to a fresh fuel pump could reduce the lope in that case …” Fuel Injection Racing Secrets, Chapter 13, Idle Lope, p. 114.
Further Idle Information
- Fuel Injection Racing Secrets, idle lope pp. 10, 59, 89, 114; added idle info throughout
- 5000 Horsepower on Methanol, idle info pp. 77, 141-142, 163, 165, 169, 245
- Blown Nitro Racing on a Budget, idle info throughout.
- High Horsepower Tuning for Mechanical Fuel Injection, idle info throughout.