I've been reading Doug Roe, and in the book he has said that for an economy car, you want part throttle to be rich and WOT to be lean. I'm confused by this. Wouldn't you want the opposite?

WOT is not for economy, so it should be calibrated for power no matter what, in my opinion. But part throttle should be either stoichiometric or possibly lean to conserve fuel (I would think).

He took a 1972 Blazer and made part throttle rich and WOT lean (on the primary side) and claimed a gain of 3 mpg. I don't understand this. "A slightly lean mixture generally gives crisp response." What?

Then he said that for a race car, you want part throttle to be lean, because a rich car would be "sluggish coming out of the hole." Why?
What happened to rich is for power and lean is for economy?
Can someone explain some of this to me please?
Thanks,
Homer

 

Lean is mean! More complete burn instead of being rich which is just pushing out watested gas and probably a slower burn.

 

Ok, that makes sense. But if lean gives you a better burn, why isn't it optimal for economy as well?

And how lean is the best lean for power? 15:1? 15.5:1?

 

Wait, I'm still confused. If lean is mean and rich is weak and economical, why did Roe write this?

"Twelve to one is generally a good maximum power mixture because the horsepower will be good and the richer mixture aids combustion-chamber cooling to help avoid detonation and preignition." p. 8

"Maximum economy requires an excess of air to ensure that all of the fuel is consumed." p. 9

The above quotes seem to contradict "lean is mean." And if lean really is mean, isn't the power system designed perfectly backwards?

On the power system: "When the engine is called upon to produce power in excess of normal cruising requirements, the carburetor has to provide a richer mixture." Which is what the power system does, it richens the mixture when vacuum drops (load increases).

Thanks for reading this. I'm really getting confused here. I don't want to mod my car to produce the opposite of the effect that I want.
Homer

 

You don't ever want a car lean. Lean bad. Lean not only costs power and efficiency, but causes knock which can lead to damage. You can knock at cruise too...just like any other circumstance. I'm gonna go out on a limb here and say Dougie Roe here is an idiot.

At WOT..12.5:1 is a good all round number to shoot for. At all other conditions being ideal, 14.7:1 is where you want it for cruise. Less won't really help you on gas mileage. It'll make the engine less efficient, making you use more pedal to do whatever it is you're trying to do. Essentially making you use a higher load at a lower efficiency to get the same job done.

 

Where is the part about the Blazer?

 

I have always read (on multipple occasions) the same thing that Jester says.
No where have I ever heard what Roe is talking about.

 

The Blazer is on p. 244 (of my edition). It's the chapter Economy-Driveability & General Performance. He does mention that the car began with a part throttle lean spot (which would decrease fuel economy and power) so richening it up a little would be in order. But the things he had said about economy, and later power, left me baffled. If you can't find it, let me know.

So Roe was wrong. Ok. Now everything makes sense. (Except now I'm wondering what else he's lied about).

He's also said that the two 1/8 inch tubes that feed the air valve (like an accelerator pump) should be drilled with four holes, like the 1970 Cobrajet carburetor has. The purpose of this is "To create a smoother transition when the secondaries are opened."

I don't understand how the mod would do this. Does this mod really work, or should I just stop listening to Roe altogether?

P.S. He also says that if the carb is properly sized to the engine, you don't have to rejet for exhaust mods. This isnt' true, is it?

 

Which book are you reading? Mine doesn't even have that chapter, and it only has 176 pages.

 

Rochester Carburetors By Doug Roe & Bill Fisher H.P. Books

ISBN 0-812656-10-7
Copyright 1973

Mine has 296 pages. Has a picture of a non cc quadrajet on the cover with all throttle blades, air valves and chokes open. Background is a green/blue psychadelic swirl.

"Quadrajet, Two Barrels, Monojet, 4GC"

 

Oh ok, mine is copyright 1981 and also covers computer-controlled.

This is what I think (Disclaimer: I don't claim to be all-knowing and I could be wrong)-
Exhaust modifications allow more mixture to enter the engine, not just air. You would change the jets to change the mixture. The shape and size of a jet determines how much fuel comes out per unit volume of air at a certain point in time. If air flow is increased, it simply draws the fuel from the fuel bowl out at a different rate. I think it is true, and the mixture isn't affected.

Here is my reasoning-
The exhaust modifications increase volumetric efficiency by decreasing backpressure (already assuming that the carb is properly sized for the engine). Using the equation ((cid x rpm)/3456) x Volumetric Efficiency = cfm, you can see that air flow increases as VE increases. As you know, cfm is a measurement of volume per unit time. In our case, the volume would be constant, as the shape of the carburetor did not change. Therefore it must be time that is changed. The air to fuel ratio compares the mass of the air in proportion to the mass of fuel given a specific unit of volume. The Density of the mixture, m/v, is independent of what exhaust the car has and can be assumed constant. We already know that the volume of space where the air and fuel mixes does not change, so the mass must remain constant. Therefore neither the mass of the air nor the mass of the fuel is affected by changing exhaust, and the A/F ratio stays constant.

EDIT: I wanted to go ahead and include the direct quote from my book: "Headers will usually reduce exhaust back pressure so that the engine's volumetric efficiency is increased (it breathes easier). The main effects of headers will be seen at WOT and high rpm. In most instances, no jet changes will be needed if the carburetor has been correctly sized for the engine displacement and rpm. The carburetor will automatically compensate for any increased airflow by increasing the fuel flow in the correct proportion. The mixture ratio will not be affected."

 

I'd thought of that too. Increased airflow just means more air/fuel charge.

But if that's true, why would you ever need to rejet for a mod? More airflow is pretty much all that most mods do.

 

I think right there you found the fallacy to your own thinking. For that matter, why would you ever need to tune anything? Wouldn't every engine just pull more or less air/fuel?

Doesn't work that way. A mod, for example headers and an open exaust, will change the airflow characteristics of your motor, and therefore require some retunign to compensate. Now it may be a small enough change that in practice no jet changes are required...but it doesn't magically re adjust itself.

 

...And I've even already experienced this.

When I gutted my cat, I developed a pretty serious WOT bog, particularly in higher RPMs. I think it doesn't bog as bad when I flip my aircleaner lid upside down, but my butt may be lying to me. Is it possible that the hotter air from the radiator makes the mixture less lean?

I was arguing with my buddy about it the other day, who said that my whole car is designed as a unit and changing the exhaust like that does a few things. #1 he said it would reduce the efficiency of the motor, so less power and less economy. #2 I need a different cam, because the cam is designed for the exhaust as it was. #3 The best thing to do to get the most power would be to get another cat.

I think he's full of it, really. I don't buy that argument that a hollowed cat is bad for airflow because it gets bigger then smaller. Um, a normal cat does that too! Personally, I think tuning the carb will fix it.

Is he right about the cam though? I'm not going to buy a new cam just because I gutted my cat, but If I made a drastic change to my exhaust, is a new cam really a good idea?

 

i have a stupid question...how do you figure your a/f ratio?

 

You don't figure it, you have to measure it. There's several ways to do it. For the best way, go see the tpi board. The fuelie guys have it down to a science. Basically, you have to buy a wide band O2 sensor for a Honda, a digital A/F guage, and hook it up.

Then there's the cheapie way, not even sure it works. Since you probably have a regular O2 sensor in your car (or 2), you can just hook up a voltmeter to that somehow (don't remember the details or where I read it)