a/f ratio concerns

[norbs]

I have concluded that, the a/f ratio requirements on the dyno changes compared to the a/f ratio on the track. I would use 11.5 -6 on the track and about 11.8-11.9 on the dyno. I have raced my car and actually felt it surge from lack of fuel at 11.9 on the track, but it was fine on the dyno. I would asssume there is more load at the track then on the dyno. Anyone else run into the same problems?

 

[TwinTurboIroc]

Originally posted by norbs
I would asssume there is more load at the track then on the dyno. Anyone else run into the same problems?

exactly

 

[bobc455]

More "load" at the track? Define "Load".

I'm not trying to be a wise-ass, but this term is kicked around all the time, and this is a potentially very interesting topic.

To me, if you're at WOT, then the load is at max.

Some people think that there is a higher "load" in third gear than in second, but this concept still baffles me.

To me, WOT = WFO in every way, except perhaps when you rev it up in neutral.

Could there be some other change, like humidity or engine bay temp etc.?

-Bob Cunningham
bobc@gnttype.org

 

[V6Power87]

There is more load in third gear vs. second because of the extra wind resistance, also you are pulling against a taller gear...... makes the engine work more.

I'm sure there is more...... anyone?

 

[norbs]

Ever try and ride your 10 speed bike and start off in 10th gear= alot of load, same thing with a transmission, the lower the ratio 1:1 the more load on the engine. Most knock occurs in 3rd gear, and last bit of the track usually, which is the highest load area.

 

[ijames]

At the track the load on the engine is the weight of the car, coupled through the tire height, axle ratio, transmission ratio, and converter slippage, plus wind and rolling friction. The effect of this load is to determine how fast the engine can accelerate. In first gear you go from 4000 to 5500 rpm in less than a second, while in third it takes 4-5 seconds for a 12 sec car. I've only seen one dyno session, with a 14 sec car, and it needed maybe 5-10 sec to go from 45 to 120 mph on the roller compared to at least 15-20 sec on the track. So the evidence is that things happen much quicker on the dyno, with the same tires and gearing, so the "load" must be less. With the stock ecm I see higher O2 readings in first gear even with higher boost, compared to third gear, so somehow this lower load translates into needing less fuel (and I'd say it's universally accepted that dyno tuning will be lean on the track). Why this is, I don't "know" but I have a theory. The faster an engine revs, the farther behind the actual airflow through the ports and intake will lag the demand at the intake valve, because of the inertia of the air through the intake plumbing. The airflow entering the air filter is what the engine wanted several hundred rpm ago, but now it wants more. The lower the load, the faster the engine revs, and the lower the effective (dynamic) volumetric efficiency of the engine. Only when the load is exactly matched to the engine output so the rpms aren't changing (like a waterbrake dyno) do you actually see the "real" VE. Anyway, that's my theory.

 

[bobc455]

Carl-

The point about scavenging is agreed- all cam designs consider this phenomenon. However I can't believe that scavenging in third gear is any different than scavenging in second gear (at same RPM).

I could believe that over the course of the 1/4 mile the engine builds up more heat (at 12 seconds of WOT instead of 4 seconds) so that may create a difference in higher gears.

Lets assume that you are accelerating through second gear. The engine could be acceleratinge about 1000 RPM per second. At 4000 RPM, the crank is rotating at 66 revs/second. If we do a bit of math, that means for each revolution of the crankshaft, the engine increases RPM by 15 RPM. That means that for each cylinder firing, the engine is increasing RPM by 2. What I am saying is that from a piston's perspective, the engine could be 4000 RPM on one revolution, and 4016 on the next.

Jump to third gear now. The engine will jump from 4000 RPM to about 4010 RPM the next time that piston fires (excluding aerodynamics, which are fighting acceleration more in third gear than in second).

From the engines perspective, those are essentially the same! I would even argue that the engine sees negligible change in operation conditions from on revolution to the next.

I still don't buy for a second that the "Load" (from the engine's point of view" is any different in second gear than it is in third gear than it is in OD. The pistons are still trying to accelerate the flywheel, and there isn't much of a change in the conditions that the engine sees (again, ignoring heat buildup which is not insignificant).

When you gun the throttle in neutral, the engine RPM's can vary significantly from one revolution to the next.

I still say that WFO = WFO!

I also can't see the acceleration of the air being a significant issue at WOT (it would be when you change throttle position, but not after the engine has settled at WOT for a few revolutions) (the "dynamic VE").

Please demonstrate that I'm wrong, because I would like to learn more. I feel like I'm missing something.

-Bob Cunningham
bobc@gnttype.org

(I know that some tuners will have more timing in first gear than in higher gears, but I would like to understand why- it doesn't make sense to me)

 

[norbs]

Very interesting reading, but i dont agree. The engine sees the most load in the top of 3rd gear, usually where most headgaskets go also, knock etc, can you explain that?

 

[RickWI]

OK think of it this way. Wide open throttle on the dyno (NA car for example) will show zero vacuum at WFO from say 3000 - 6500. There is no more load you can put on a motor than this. In fact on the dyno you can load a motor way harder than you can on the street or strip as it is easy to open the waterbrake and just drag that scker down from 6500 to zero in no time and just stall it out. Unless I bolt on a 2000 HP motor (what the waterbrake is max designed for) I can put more instant or steady state load on a motor on the dyno than it will typically see on the road or track basically.

Since vacuum is an indiation of load what is dif between zero vacuum at a similar RPM in any gear, nothing. Now it is true that the torque multiplication from the tranny will differ as you move higher through the gears but from the motor perspective nothing changes. If it made 500 lb/ft at 5000 rpm on the dyno that is what it will make no matter what gear you are in. How could it not. How it is multiplied after the flywheel does not change what is occuring before the flywheel. Now on the dyno as was mentioned the dwell time through this range is short, by design I might add. So the total amount of time, or dwell under load is less. That is why I make a couple of pullls on the dyno with a motor to build up some heat in them, especially with aluminium heads and then make a banzai run/pull.

So what is dif on the track or street between first, second and third? The amount of time the motor is running through the same fully loaded part of the RPM band due to less torque due to lower torque multiplication due to higher gear ratios in the tranny. Wind resistance and weight also increase the dwell time but the fact remains the motor is producing the same torque at the same rpm in each gear. Only the torqe mutiplication is changing and thus the amout of time the motor is at those points increases.

The dwell time is much longer. The motor sees the same max load only for a longer period of time. More heat rejection. maybe a bit leaner running therefore a bit more prone to detonation, and maybe the need to retard timing a bit or richen mixture. When you data log a dyno run you can see the increase in oil temp, water temps and sometimes drops in oil pressure.

 

[TurboJim]

I kinda disagree with you Rick. You're not taking torque multiplication into account. What does it matter? Well I dyno cars every day (emissions inspection, not horsepoer rating) and if you can probe to me a Ford Escort has 200+ ft lb or torque at 1/4 throttle without the torque multiplication of 1st, 2nd gears, then I'd say you have a HOT escort.

Ok, you're talking horsepower, but the point here is through gear reduction you CAN change the output measured. Thats why dyno pulls are made usually in 3rd ( 1:1 ratio).

Whats this got to do with anything? Maybe nothing, but if you think about inertia, an object at rest, wants to stay at rest, and an object in motion tends to stay in motion, you will realize, theres more load trying to get something moving, than there is from sustaining it in motion, even if you're accelerating, the object is already in motion, so its easier. How many driveshafts you see break mid track vs at launch thru 60'? Tell you somehting?

Thats how I look at it. Maybe I'm wrong.

 

[10SV6]

I'm not going to get into the debate about why, but yeah, when I take a track tune to the dyno I'm rich, take a dyno tune to the track, too lean.

Normally aspirated I haven't had a problem, only been a problem with the turbo cars. I haven't had any experience with a supercharged car yet.

 

[RickWI]

Jim - I know what you are saying, but if the motor makes 400 hp or 400 lb/ft of torque on the dyno it ain't gona make any more on the track or on a chassis dyno either. Sure if you gear way down you multiply torque but the motor ain't got no more to give than what it has at the flywheel.

How many times on the street or track do you load a motor to a point where it beings to pull the rpm down while you are running at let's say 6500 WFO. In other words face your car into a solid building and concrete the rear tires into the ground so they can't turn. Push in the clutch and start the car. Let it warm up and rev it to 6500. Now let out the clutch slowly and keep feeding the gas to it to keep it at 6500. If the clutch is strong enough it will continue to pull the motor down even though you are at full throttle with the clutch almost fuly engaged (slipping like mad of course. The dyno does this with water.

 

[ijames]

I could believe that over the course of the 1/4 mile the engine builds up more heat (at 12 seconds of WOT instead of 4 seconds) so that may create a difference in higher gears.

Sorry to post and then not respond but it's Sunday . Yes, everything is getting more and more heatsoaked the longer you stay at wot, so it's hotter on the track than they dyno. By the way, the dyno's we started out talking about here are chassis dynos that you spin up, not waterbrake dynos. My theory says that a waterbrake dyno operated so that the rpm was constant or rising very slowly should give the same results as the track. I just think, without doing any math to back it up, that changing the rate at which the engine is allowed to rev up is more important. It's just a theory, put out for discussion. Does everyone agree that the engine will rev faster in first gear than in third, on the track, and that on a chassis dyno it will rev about as fast in third as it will on the track in second? You are going to make the same hp on the dyno as on the track (rich/lean tuning aside; otherwise we just keep going in circles ). Also, if the engine revs faster then it just makes basic sense to me that the load must be less. The torque multiplication of the lower gears is why the load (weight of car plus rolling friction plus air resistance) appears lower to the engine in first gear than in third, and with the same gearing the faster rev rate on the dyno says to me that it is less of a load than the car is on the track.

Lets assume that you are accelerating through second gear. The engine could be acceleratinge about 1000 RPM per second. At 4000 RPM, the crank is rotating at 66 revs/second. If we do a bit of math, that means for each revolution of the crankshaft, the engine increases RPM by 15 RPM. That means that for each cylinder firing, the engine is increasing RPM by 2. What I am saying is that from a piston's perspective, the engine could be 4000 RPM on one revolution, and 4016 on the next.

Jump to third gear now. The engine will jump from 4000 RPM to about 4010 RPM the next time that piston fires (excluding aerodynamics, which are fighting acceleration more in third gear than in second).

And that means that the airflow requirement has increased faster in second gear than in third. True, it's small, 4015/4010 or 0.1% faster.

I still don't buy for a second that the "Load" (from the engine's point of view" is any different in second gear than it is in third gear than it is in OD. The pistons are still trying to accelerate the flywheel, and there isn't much of a change in the conditions that the engine sees (again, ignoring heat buildup which is not insignificant).

Yes, but the flywheel is connected to the transmission which is connected to the hipbone, er, to the tires and road . Think of the load as trying to stall the engine. Just to keep the rpms constant takes some power to equal the load, and to accelerate the engine takes even more power. The faster the acceleration the more power. The lighter the load for given power the faster the acceleration.

I also can't see the acceleration of the air being a significant issue at WOT (it would be when you change throttle position, but not after the engine has settled at WOT for a few revolutions) (the "dynamic VE").

Except for resonance and forced induction the VE is always less than one so the engine is always slightly starved for air. I just think that if it already slightly starved and you increase the rpms it will be more starved - it couldn't get quite as much as it wanted before and now it wants even more. Just a feeling, without any math to back it up (which is embarrassing for a scientist but like I said, it's Sunday ).

 

[RickWI]

If the engine accelerates faster then it just makes sense there is less load??? I don't think this is true. BUT I think there is a variable here. What is load? When I think of load I think of it from the standpoint of torque (or horsepower doesn't matter really) produced by the motor. So are you saying in first gear there is less torque being produced by the motor than in 2nd or third? That I would think not true.

 

[bobc455]

One attempted at a definition of load so far is how fast the engine (not the car) will accelerate at WOT. To me that's the same as torque, eh?

Another definition of load is trying to stall the engine- I don't understand. Reading what you're saying, I think you're getting flywheel torque mixed up with torque seen at the output of the tranny.

So what difference will this "load" make? Will you see more cylinder pressure? More airflow? Less fuel?

The engine is still just trying to turn the flywheel.

Norbs, I think that most headgaskets blow in 3rd gear because of heat buildup.

TurboJim, dyno pulls are usually made in 3rd gear because it is a direct 1:1 coupling- in 2nd or OD you have additional frictional losses in the transmission from the gears moving against each other. I do my dyno pulls in 2nd gear- my HP/tqe numbers would be slightly higher in 3rd, no question (but I'd be doing like 160 MPH and my tires aren't made for that).

Ijames, I think if you do some quick math, I think you'll realize that the acceleration rate is insigificant. Remember how quickly these engines spin- you have a piston firing ever 0.01 second at 4000 RPM (even more for an 8-cyl), the rate of change is really, really small from the engine's perspective.

I still say that "load" is just another word for torque. And everyone else has basically agreed- it is how hard the engine tries to spin the flywheel. This rate will be different in different gears, but that will not make a difference in what happens inside the engine.

On a chassis dyno, there are a few things different than real world- for example, the engine comparment isn't being blasted with cold air, so the intake air can be warmer, humidty, etc. This could explain a different A/F ratio.

-Bob Cunningham
bobc@gnttype.org

 

[TurboJim]

Originally posted by bobc455

On a chassis dyno, there are a few things different than real world- for example, the engine comparment isn't being blasted with cold air, so the intake air can be warmer, humidty, etc. This could explain a different A/F ratio.

-Bob Cunningham
bobc@gnttype.org

Dont discount aerodynamic drag.....these cars are bricks on wheels! The faster they go, the more drag = more power needed to overcome that. I think it takes ~15-19hp to sustain 55mph on the dyno....I bet its higher on the road..

 

[bobc455]

TurboJim,

That means that the car will accelerate slower.

That doesn't mean that the engine is producing less torque.

-Bob Cunningham
bobc@gnttype.org

 

[TurboJim]

Right, but my point is, he'd need to make more power against the wind resistance, therefore, he'd need to burn more fuel.

 

[bobc455]

Why would he need to produce more power?

Are you claiming that an engine will make less power (torque) in 2nd gear than in third gear?

You could claim that a higher % of the available torque is being used to overcome aerodynamics in third gear. But to the Flywheel, it just pushes as hard as it can- either it's trying to overcome inertia (to accelerate), aerodynamics, drivetrain friction, etc... You may vary the portion of where the torque gets distributed, but not how much is produced by the engine.

-Bob Cunningham
bobc@gnttype.org

 

[Craig Smith]

Wow. It doesn't get any heavier than this. Anyway, here is my opinion:

I support the idea that an optimized dyno tuneup is leaner than an optimized track tuneup. It seems to me that in order to reach that optimized level on the race track, if it takes more fuel to do it, that means either the engine is making more power OR it means that the engine is running at a higher BSFC at the track than on the dyno. I don't really see how it could make more power, but with all the effects that a race track will produce that a dyno won't, I could certainly be talked into the idea that power isn't transferred to the ground as efficiently on the track as it is on the dyno. That extra fuel HAS to be used for something.