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.
Rons Fuel Injection has provided us with air flow data for their throttle body setups. We analyzed the power potential of these simple and very popular fuel injection systems. Racing engine size capabilities were determined at the flow CFM numbers provided by Rons Fuel Injection. For 100% air density, horsepower capabilities were also determined.
Rons mechanical fuel injection throttle body model: Toilets
3-3/4″ = 1240 cfm*: can feed 595 cubic inch engine @ 8,000 RPM with 1,071 HP at 1.8 HP / ci
4″ = 1435 cfm: can feed 689 cubic engine @ 8,000 RPM with 1,240 HP at 1.8 HP / ci
4.1″ = 1800 cfm: can feed 864 cubic inch engine @ 8,000 RPM with 1,555 HP at 1.8 HP / ci
Rons mechanical fuel injection throttle body model: Terminators
1-7/8″ = 1150 cfm: can feed 552 cubic inch engine @ 8,000 RPM with 993 HP at 1.8 HP / ci
2-1/8″ = 1475 cfm: can feed 708 cubic inch engine @ 8,000 RPM with 1,274 HP at 1.8 HP / ci
2-1/2″ = 2100 cfm: can feed 1,008 cubic inch engine @ 8,000 RPM with 1,814 HP at 1.8 HP / ci
Rons mechanical fuel injection throttle body model: T2’s
2-1/8″ = 1880 cfm: can feed 902 cubic inch engine @ 8,000 with 1,624 HP at 1.8 HP / ci
2.400″ = 2300 cfm: can feed 1,104 cubic inch engine @ 8,000 with 1,987 HP at 1.8 HP / ci.
The engine sizes that can be supported by these various throttle body assemblies are quite high. Reasonable horsepower per cubic inch potentials are shown that are also quite high from these very simple fuel systems. Achieving these high horsepower numbers would depend on high compression and top-of-the-line cylinder heads, intake manifold, & exhaust manifolds.
Flexibility in Application
The beauty of mechanical fuel injection is that smaller engines can also be fed by these throttle bodies without bogging. Mechanical fuel injection pumps fuel into each intake port at high pressure when the throttle is slammed open. Fuel pressures often higher than those in EFI can produce more horsepower from better vaporization. Furthermore, air is not interrupted by venturies or boosters necessary in the carburetor alternative, providing even more horsepower. Stand back!
Response from Rons Fuel Injection*
‘Good morning Bob, yes these numbers are very similar to the calculations I’ve figured for engines cfm usage. We do typically try to “oversize” the throttle body for motors though so that there is no restriction and since mechanical fuel injection does not rely on a vacuum signal to pull fuel, it isn’t an issue just as long as the motor can get as much air as it wants. For example, our normal standard we use for new customers is as follows:
- 1240 cfm toilets for up to about 383 ci
- 1435 cfm toilets up to 502 ci
- 1800 cfm toilets for up to 572 ci.
The Terminators are typically paired at:
- 1150 cfm Terminators up to 383 ci
- 1475 cfm Terminators up to 496 ci
- 2100 cfm Terminators up to 632 ci.
Also this depends on the heads on the motor, how well they flow, and their combustion chambers. As you have shown, the engine is not pulling air into all cylinders at once. So the cfm calculations you came up with show how large of an engine that the throttle bodies can actually support, but it does complicate tuning a bit (regarding the larger sizes). Looks great and thanks for adding us to the news letter.’
*Thanks to Alan Greszler, Rons Fuel Injection.
Math for These Ratings
The engine size and HP additions were determined using the following math:
- step (1) CFM x 12 x 12 x 12 = cubic inches per minute
- step (2) cubic inches per minute / 8,000 RPM = cubic inches per revolution
- step (3) cubic inches per revolution x 2 = cubic inches of 4 cycle engine (normally aspirated)
- step (4) cubic inches of 4 cycle engine / 0.9 = adjusted ci @ 90% VE typical at horsepower peak
- step (5) adjusted cubic inches at 90% VE x 1.8 = horsepower assuming 1.8 HP / cubic inch.
Further Rons Fuel Injection Information
- Fuel Injection Racing Secrets, pp. cover, 3, 22, 129, 139, 177, 185
- 5000 Horsepower on Methanol, pp. 7, 32, 37-38, 40-43, 57-59, 244, 257, 286
- Jetting for Racing Mechanical Fuel Injection for Normally Aspirated Small Block V-8 Racing Engines, pp. 9, 32, 35, 44-46, 63-63
- Jetting for Racing Mechanical Fuel Injection for Normally Aspirated Big Block V-8 Racing Engines, pp. 9-10, 13, 14, 17, 49, 54-55, 74-75, 77-78
- Blown Nitro Racing on a Budget, pp. 36, 158
- High Horsepower Tuning for Mechanical Fuel Injection, pp. 3, 16, 24, 25, 38, 43, 51, 52, 53, 64, 90, 101, 105, 147, 149.
We attended the July 7th race with about 30 winged FI sprint cars and several other carburetor fed racers and late models all competing in front of a couple thousand spectators; thanks to Dani Noceti and staff for a great evening; many couples and families with children all enjoying the competition; very safe and clean environment; the entire staff was friendly and all enjoying themselves; reasonable parking facilities; well equipped food services; the licorice was great; I really enjoyed those powerful circle racecars; final heat of 30 laps ran after 11:00 for cool and comfortable entertainment.
Most of the MFI cars were Kinsler or Engler fuel injection. Several crew chiefs said flat lining fuel pressures were maintained over 100 psi for the high speed bypass opening point. Most had the conventional fuel pump location behind the motor with a suction pickup from the rear mounted tank. I was impressed with the punch coming out of the turns. There was only a momentary let up approaching the turns, then back on the throttle around the turns. Stockton dirt track is a little over 1/3 mile. Full throttle most of the time. I was impressed with how orderly the drivers and push cars all worked together to coordinate the starting positions. Only a couple red flags for a flip. The safety staff was johnny-on-the-spot getting to car rotations or the occasional flip. The Stockton dirt track has a large flat run-off for the racecars so no outside guard rails to collide with. There were numerous high dollar semi rigs in the pits filled to the brim with competitors. I spoke with 28 of the crew chiefs all of whom were friendly and open about the various tune-ups.
Stockton 99 Speedway; July 3; tune & test
Stockton 99 Speedway; July 4; Ron Strmiska Memorial Late Models, Bombers, B4 Cylinder, Mini Cups, & Fireworks
Stockton Dirt Track; July 7; Salute to Leroy Van Conett, Elk Grove Ford 360 Sprint Car Challenge (racing mechanical fuel injection), Hunt Magneto Sprint, & B-Modifieds
Let us know if you will be racing.
On Sun, Jun 10, 2018 at 12:42 PM, Chris Murphy wrote:
Hi Bob, I have Don Geradot’s Yellow ’78 Corvette Funny Car… John Dunn built a 540″ Fuel Hemi that he converted for alcohol: JP-1 Block, Veney fuel heads… SSI 14-71 Blower, Enderle Hat, CS-1 3 speed Lenco, & Crowerglide 3 disc pedal clutch. I have Pastor Bill Huff, who is very well known and respected as my driver. He has 12 years experience driving Alcohol Funny Cars and never as much put a scratch on one. We hope to be capable of dipping into the 6.0 range? If you would like, You can visit our FB page, Pure Madness Racing to see what we are doing. I have a great team assembled, and hopefully we will have fun on the Nostalgia circuit. Thank you for your wonderful books you write and the information you share. You help me more than you may realize! Thank You Again Bob!
Dani Noceti & staff at Stockton 99 speedway did another great job organizing this event. All of these racecars on pavement are fast. The turn-around of the various classes was very quick. The racer participants kept their cars reliable with few show interruptions. I was impressed with all of the fuel injection cars storming out of the turns. There were two cars with six 2 barrel carbs on ram manifolds; very impressive. A lot of families were present enjoying the racing vibrations. Thanks to the staff for all their effort conducting on a great show.
Tom Manning Memorial race: Gun Slingers Wing Sprint Car Series, Midgets, Vintage Midgets, Ca Hard Top, Legends & others. We’ll be there taking photos and studying the setups. Let us know if you are racing, and we’ll look you up.
Rons Fuel Injection provided MFI throttle body flow data so that we could analyze the power potential of these low cost, simple fuel injection systems. Racing engine sizes and horsepower potentials were determined at these flow cfm numbers for 100% air density, and are included.
Rons FI MFI Toilets
3-3/4″ = 1240cfm: can feed 595 cubic inch engine @ 8,000 RPM with 1,071 HP at 1.8 HP / ci
4″ = 1435cfm: can feeds 689 cubic engine @ 8,000 RPM with 1,240 HP at 1.8 HP / ci
4.1″ = 1800cfm: can feeds 864 cubic inch engine @ 8,000 RPM with 1,555 HP at 1.8 HP / ci
Rons FI MFI Terminators
1-7/8″ = 1150cfm: can feed 552 cubic inch engine @ 8,000 RPM with 993 HP at 1.8 HP / ci
2-1/8″ = 1475cfm: can feed 708 cubic inch engine @ 8,000 RPM with 1,274 HP at 1.8 HP / ci
2-1/2″ = 2100cfm: can feed 1,008 cubic inch engine @ 8,000 RPM with 1,814 HP at 1.8 HP / ci
Rons FI MFI T2’s
2-1/8″ = 1880cfm: can feed 902 cubic inch engine @ 8,000 with 1,624 HP at 1.8 HP / ci
2.400″ = 2300cfm: can feed 1,008 cubic inch engine @ 8,000 with 1,814 HP at 1.8 HP / ci.
The engine sizes that can be supported by these various throttle body assemblies are quite high. The horsepower potentials are reasonable and also quite high from these very simple fuel systems. The beauty of mechanical fuel injection is that smaller engines can also be fed by these throttle bodies without bogging. Fuel injection pumps fuel into each intake port at high pressure when the throttle is slammed open. Further more, air is not interrupted by venturies or boosters. Stand back!
Thanks to Alan Greszler, Rons Fuel Injection.
Note: The engine size and HP additions were determined using the following math:
step (1) cfm x 12 x 12 x 12 = cubic inches per minute
step (2) cubic inches per minute / 8,000 RPM = cubic inches per revolution
step (3) cubic inches per revolution x 2 = cubic inches of 4 cycle engine (normally aspirated)
step (4) cubic inches of 4 cycle engine / 0.9 = adjusted cubic inches @ 90% VE
step (5) adjusted cubic inches at 90% VE x 1.8 = horsepower assuming 1.8 HP / cubic inch.