Fuel setup change by going to a taller rear gear?

pump speed

You might have to run it somewhat richer...not a bunch though. 2 main reasons is it will lug and work the motor a bit more and with the lower rpms your pump speed will be down.

Richen it up and come back at it. how big of a main jet you running now? How lean do the plugs look lifting early?

 

No change required
Your pump is geared to the motor and pump output is related to engine speed and if the engine rpm is down then the fuel requirement is less. The AFR in that motor doesn't change because you changed that rear gear. If the AFR is correct now then all is well. If it is a clutch car then the time the clutch locks up in first gear might change so if you have any fuel controls with times related to engine loading when the clutch locks up then those times may need slight adjustment. Also if you had been changing the timing to maintain traction in first gear then you may néed to adjust that to keep your driveshaft speed up.

 

lug

Assuming you had a main jet that was pretty close - meaning if you leaned it much more it would start trying to hurt itself - and you go from a gear that lets the motor rev to a rear gear that keeps the motor tugged down, it's going to want a little more fuel or slightly less timing, take your pick.

If you are working the motor harder, it will need more fuel. As I said, not a bunch. If you have it plenty rich, probably won't need to mess with it.

That's my opinion...

 

Will, you and I are going to have to sit down and have a beer and a good discussion on this. A motor is as dumb as a box of rocks. It doesn't know anything that you do outside that combustion chamber. To make maximum horsepower you need a specific air to fuel ratio and to maintain that exact AFR through the changing rpms for the whole run. If you used a wide band O2 sensor and recorded it you would show a flat AFR curve if the motor was tuned correctly. To have the requirement to change the amount of fuel would mean that you changed the amount of air going into the motor by changing the density altitude or the amount of boost or something else similar. The gear change does not change the amount of air going into the combustion chamber. In order to make more power you do have to burn more fuel but you have to burn it at the correct AFR or you will lose power. If you change the main jet and get a performance increase then the AFR somewhere in the motor's varying rpm range was not correct to start with and you corrected it by chance.

 

JW, I would like to think that I could explain this without getting too complicated because every blown alcohol racer and tuner should understand Air Fuel Ratio to get a better tuneup. There are a lot of factors in what is really happening during that short time we make that run. Just having the final EGTs at 1050* doesn't mean the fuel curve is optimized for the rest of the run. Also one has to understand that two different tuners can be in violent agreement but sound different because of different terminology.

 

What kind of car and tranny? What do you weigh? The question is if you have enough power to pull a taller gear. Changing tire diameter and rear gear will probably require tranny ratio changes. What is the motor rpm doing in high gear now? Does it flat line or climb? If it flat lines then no boost increase so no power and all is lost. Are you running 1/8 or 1/4?

 

work/load

Why is it that if the chutes come out at half track and the driver never lifts, it usually hurts the engine?

Why is it so hard on an engine if you leave in high gear and never lift?

Why is it my pick up in high school would ping/knock if I tried to leave in second or third gear?

The answer to those three questions is load.

Why is it when you have a car right on the ragged edge on the main jet and you short shift it, it is prone to burn a piston? Pump speed and load are major contributors (assuming the motor still pulls down and doesn't blow through the clutch).

I think it is safe to assume that if a car were to make a run at the exact same throttle position as stage, it would burn itself up. No load vs. load.

Why is it the A/F ratio to make peak hp on an inertia wheel dyno pull will not live on the track?

The target air fuel mixture is in proportion to how much timing and compression you have. How much timing and compression is usually tailored to how much air the supercharger can get in there and how good the heads and intake can flow it in there.

I will have to disagree that load has no bearing on the fuel or timing required. Your two choices are either retard the timing or add fuel. When you don't have timing management, it will probably be faster to richen the main jet a bit and work your way back down.

All other things being equal, I disagree that you could go from a 4:30 gear to a 2.90 gear with the same jet and timing. While the original change discussed is a much smaller change, the same principle applies, in a smaller proportion.

 

Will, you are mixing ignition timing up with AFR/fuel.

The pickup truck pinging at low RPM under hard acceleration is caused by detonation because the load is too great at where it is on the timing curve and not fuel. You can eliminate that knocking or pinging by either retarding the ignition or increasing the octane of the fuel but not by adding or taking away fuel. Same thing on leaving in high gear and never lifting you are going to detonate because that internal cylinder pressure is going to get too high for the ignition timing being used and how slow the engine RPM is going to change. As far as short shifting and taking out a piston I have never seen that happen but remember as a engine increases RPM the more air is being sucked in and the more air is being pushed in by the blower so it needs more fuel at the top of each gear. Normally we only stay at the top of each gear for a short period of time so when they lean out at the top it is quick and can be tolerated. Again as I said before this is because the fuel curve required for the changes in engine RPM and Boost is not correct and NOT because of the load. Don't understand the throttle position VS stage and the run. Don't know what you mean but I will take a guess. If you are at part throttle on a Barrell Valve it is going to run lean and so if you run part throttle it is going to burn up the motor. This has nothing to do with load. It is because the part open BV has made the AFR too lean.

The target AFR has nothing to do with the compression and timing. Again you are mixing up timing with AFR. An increase in compression can change the amount of air coming into the cylinder and therefore requires a different amount of fuel to adjust the AFR to the exact number.This is not load. Timing adjustment is for the speed of the flame front in the chamber so that peak burn is accomplisehed at the correct time. If this occurs too early you get detonation. You can cover an incorrect ignition timing setting by adding more fuel which people do all the time BUT to make max power you should be at the correct AFR and then adjust the ignition timing to match it. This is not loading.

The correct AFR has to be maintained during the entire time of the run and as the engine RPM goes up and down and the boost changes and the ram air into the the injector hat increase the boost as the car's velocity increases and the density altitude changes and the temperature of the engine and intake manifold air changes. Just adjusting the cadium burn on the plugs at the end of the run does not accomplish this.

 

ere are special lab quality O2 sensors that will survive in alcohol and go to a 2.50 AFR but they are expensive and my wife would shoot me if knew how :confused much. There are other ways to obtain AFR also but that is a secret.

 

LOL. Sorry I would have to shoot you if I told you. There are somethings you have to keep secret. I have learned so much from it and I feel it is a big advantage for me in tuning.

 

But that Racepak Boost/Fuel doesn't take into account changes in engine rpm. It will show leanout correctly when a bypass opens but is inverse during rpm increases as the motor increases rpm in each gear.

 

Ditto here on that also.