Pure Adrenaline

iTrader: (2)

This is more obvious at the drag strip, but I've noticed that there's a bit of a dead spot in the lower RPM range in my car, and possibly in all our cars when you floor it off the line. I always thought it was a lack of low end power, but I have come to a new theory.

When you initially floor the car, the throttle body opens up completely to let air in at its maximum airflow capacity. However, due to the lack of throttle-by-wire, there's an instant moment where the engine runs lean because of the sudden amount of air coming in. The computer, overseeing this, dumps more fuel to even out the mixture (around 2-3k RPM range) which causes the engine to run rich/richer in that range. Then the AFR levels out, picks up and goes. At least that's my theory. I didn't think of this when I ran a wideband on my car last time, so I'm gonna have to hook it up again to see what's happening. Maybe it's just my car.

Or am I wrong on the theory? The way I understand it is that the O2 sensor watches the AFR and adjusts fuel delivery accordingly, meaning adjustments are made post-combustion therefore a delay. But if the PCM adjusts fuel delivery based on the throttle position (pre-combustion), then my theory is obviously going out the window.

Does anyone know what I'm talking about? You floor it off the line, tires spin (err, one ), the engine bogs for an instant, then picks up. This all happens in a matter of a second or two at the most. Any ideas?? I'm just tinkering around with theories/ideas to better my 60' time. Notice how I'm blaming the car, not the driver's lack of skills.

fsttyms1

Sounds like a good theory. Ive noticed it on mine before and wondered

JimmyTLS

You may be correct in your theory all depending on when and how fuel adjustment is made. I guess its really so instantaneous (1-2 seconds) that it really is negligible. Although, I don't think that it runs rich at any point. If there is a sudden increase in air, the computer will adjust it accordingly and not hold all injectors wide open and dump a tremendous amount of fuel. What I am trying to say is, it should go from a small period of lean to stoich.

I think the lack of power down low has to do with the cams and the VTEC system. I mean, down low, the intake valves will only open to a certain extent allowing only a certain amount of air to actually get in. So even with the throttle body wide open, the valves are still only allowing a specific maximum amount of air make it to the combustion chamber. Only until the secondary vtec lobe and pin for the rocker arms engage will the valves provide a higher lift and duration will more air make it to the combustion chamber and produce more power. Sorry but I am not too good with putting my thought process into words...anyone understand what I mean?

ou sig

I notice it also...but I never noticed that when I drove my old truck with a V8, or my friends GT Mustang, or any other car with a V8 - therefore I really think its just a lack of Torque down low more than anything. The computer adjusts so quickly that I doubt you would be able to tell a compensation by it.

sbuswell

sounds like not enough low end torque, my car does the same. on a launch it's .5-1 full second then boom almost like a turbo the powerband opens right up. TL's definitely lack in the low end torque department, but I would figure with pure's setup low end torque would be pretty good. I hope mine improves when I get my crank pulley put on next week.

Pure Adrenaline

iTrader: (2)

It's cool, I undestand exactly what you mean and I agree.

I guess I should say that I'm talking about delayed compensation rather than over-compensation. To elaborate, let's say at 1,000 RPM, just rolling off the line under WOT, there's a lean condition due to the sudden increase of air coming in. The PCM monitors this via the O2 sensor and adds more fuel. But by then, the engine will be reaching a higher RPM, say 1,500 RPM, at which point there would be no rich/lean condition otherwise caused by the PCM compensating for the lean condition that happened at 1,000 RPM.

And excellent description of valve timing and air intake in the second paragraph. But based on what I was going on... at 1,000 RPM, the intake valves' lift and duration would only allow a certain amount of air coming in, which would be considerably less than, say, at 6,000 RPM in VTEC mode with higher lift and longer duration. But at 1,000 RPM (by the way, I keep using this because my car idles at around 800 RPM and when I'm just coming off the line, the bogging happens at 1,000 RPM on to about 2,000 RPM) there's still maximum amount of air going in allowed by the valves, but not enough fuel being delivered at that instant moment. So basically, regardless of what the maximum air intake is allowed by the valves, there's still a shortage of fuel at that instant because of the sudden air coming in. Then going back to what I said in the previous paragraph, the O2 sensor notices it, and the PCM adds more fuel (delayed compensation) causing a rich condition as the revs climb. Is this making any sense? I'm confusing myself.

Or perhaps I'm just reading too much into it??

ou sig

I really think you are - its more a function of the torque available (or lack thereof in our case) to initially turn the wheels and get the 3500 (roughly) piece of metal moving.

though all of the thoughts are very interesting and probably have some basis

Pure Adrenaline

iTrader: (2)

I don't know, it's just really bugging me. It goes slower than a turtle in that range.

fsttyms1

partially correct. the valvs are a set durration. they arent infinitly adjustable. down low or up to vtec engaugement they open the same amount. then when the vtec lobe gets engauged you have the changed duration. I think it ultimatly comes down to a AFR, tuning and overall design of compononts

JimmyTLS

Well I mean there is the variable of wheel spin. For that split second off the line, your wheels break traction and spin then grab traction and you move. Real brief physics lesson on motion.

When your tires make contact with the ground...force from the tire is directed --> while the force exerted from the ground on your tire in the form of friction is directed <--. Well when your tires are spinning, there will be less force --> because force is not exerted totally in --> direction. Instead, it is directed in various other directions and not focused for one direction. Therefore, friction will also not be totally in this direction <--. (Remember, every action has an equal and opposite reaction). Well when your tires finally get the traction and the force is now totally pushing you -->, the force from the ground is now <-- so that extra force is what makes you feel sluggish. Once your car gets to a higher speed, momentum now comes into the equation and gives you more force --> while an equivalent amount of force is directed <--.

Hmm, by the end of this lesson, I just thought of something. In that RPM range, you're coming right off the line correct? In that event, the initial burst of acceleration -->, will also yield a pull <--. That is why you pulled into the back of your seat during initial acceleration. That can account for the initial "turtle" feel. Its the initial jerk backwards.

Also, back to the wheel spin thing. As we all know, when wheel spin occurs, bogging is a possibility where the wheels spin and the car revs into a higher powerband but when traction is attained, revs drop and more force is needed to compensate for the extra friction gained when traction is attained. Therefore, the revs will drop lower in the powerband. So lets say you're taking off from the line, start at 800rpms, launch, tires spin ever so slightly, even unnoticeable, goes to 1200rpms then drops back to 1000rpm, this very slight amount of bogging (unnoticeable bogging) can account for the sluggish feel off the line.

Also, why are you in that range while at the strip. Power brake and get your revs up, then off the line you should never be under 1500rpms. Then all your problems are solved.

Did I confuse anyone else because by the end of that, I lost myself.

JimmyTLS

Thats what I meant. The valves only allow a certain amount of airflow into the engine until the secondary lobe engages and the pin locks in the rocker arms will the lift and duration increase.

fsttyms1

just by floring it im above 1500 rpm. the torque converter allows it to be there offthe line. it just seems like a small dead spot around 2000-2500ish

ou sig

all of these things that we have said are true, but I also think it has to do with the fact that this car isnt made to be real fast and the gearing could also be a culprit. It seems like most VTEC cars are pretty boring below the VTEC point - maybe to save gas mileage during normal driving? who knows, but Im sure if we had a real computer upgrade/chip available for our cars, we could free up some nice power.

JimmyTLS

Oh yea, that sluggish feel, at 2000-2500 rpms. Well from all the dyno sheets I've seen over these past few years of our cars, it seems that the tuning and the computer is the culprit. With the base maps, the computer limits the injectors to a specific amount of fuel. After that, the computer may tell the injectors to allow more fuel to a certain RPM range and so on. There's no way around UNLESS, like ou sig stated, get a piggy back ecu or chip and retune that area in the powerband. Until that's done, guess we just have to live with it.