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Thread: Reduced mileage when using 52mm TB

  1. #21
    Supporting Member Turbo Mopar Contributor mopar-tech's Avatar
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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by acannell View Post
    The same way that engine wear effects mileage. You have to push the gas pedal harder to get the same speed. Inefficiency. So the idea being tossed around is that a bigger TB hurts atomization and therefore power. So more gas pedal would fix that. And therefore worse mileage. Thats the theory anyway.
    Wrong concept.

    The larger TB hurts the velocity of the air passing through it if not the optimum size.

    Think of water in a pipe. A large pipe that steps down size will INCREASE the pressure at the transition while a smaller pipe opening into a larger area will drop pressure.


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  2. #22
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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by acannell View Post
    Reduced atomization caused by a bigger TB cant be compensated for by opening the throttle more on the bigger TB. Or thats the idea here. I am not saying I agree with it.

    Another way to look at it is: forget about the big TB, just have a stock TB. But now make it so the injector spray pattern is much worse and has less atomization. Fuel economy will go down. Opening the throttle more will not fix it.
    Quote Originally Posted by contraption22 View Post
    Yeah, I am not on board with the idea that the throttle body size effects atomization either.
    Above, I am simply stating what I feel the point of the very old comments are. I am not serious about the 1 piece intake or the theory.

    With TB's, I run modified throttle cams that are progressive. My only real experience with going to an unmodified larger throttle body is loss of low opening control. Years later I made my daily's 58mm TB so slow to react (how I set up the throttle cam) that it feels dang laggy.

    The complication I see changing TB's is that OEM computers use TPS input where my megasquirt setup in one car doesn't have to.

    A progressive throttle can give you back the same control and ability to drive like a smaller TB but you are getting more air at lower throttle openings so its not perfect. A straight TB swap with no modifications is the least balanced approach. Not that it really matters.....I do like the improved control off idle that a progressive setup gives you. No more tap the throttle and hit 2500 rpms type stuff.

    -----------------
    On a side note, if you wanted to make your car feel peppier, you could do the opposite progression with the throttle cam and have it open extremely fast off idle and open very slowly near WOT. I think a lot of drivers mistake throttle movement for engine performance.
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  3. #23
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    Re: Reduced mileage when using 52mm TB

    tps , map , o2 yada yada ..

    with an ecu that has "learning" wouldn't this be a moot point - IF you remembered to disconect the battery before starting the throttle body swap , thus causeing the ecu to relearn it's parameters when started again with the new TB installed (?)

    also , I don't see port velosity changeing one iota as the port size or shape remains unchanged and the throttle body is a heck of a long way from the port

    as for a seat of the pants feel , there shouldn't be much beyond a slightly different sence in throttle responce - initally (on an otherwise stock application)

  4. #24
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    Re: Reduced mileage when using 52mm TB

    I've run 52's for years on various cars, never noticed any mileage or driveability issues.
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    Super Moderator Turbo Mopar Staff contraption22's Avatar
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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by mopar-tech View Post
    Wrong concept.

    The larger TB hurts the velocity of the air passing through it if not the optimum size.


    But how does that effect anything in the intake port, injector, or combustion chamber? Once through the TB, the air still has to pass through the plenum and the runner.

    And even if it were to effect it that far down the flow path, I don't see how throttle body size has any effect on part throttle fueling, other than requiring less throttle angle for the same air flow.
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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by Ondonti View Post
    Above, I am simply stating what I feel the point of the very old comments are. I am not serious about the 1 piece intake or the theory.

    With TB's, I run modified throttle cams that are progressive. My only real experience with going to an unmodified larger throttle body is loss of low opening control. Years later I made my daily's 58mm TB so slow to react (how I set up the throttle cam) that it feels dang laggy.

    The complication I see changing TB's is that OEM computers use TPS input where my megasquirt setup in one car doesn't have to.

    A progressive throttle can give you back the same control and ability to drive like a smaller TB but you are getting more air at lower throttle openings so its not perfect. A straight TB swap with no modifications is the least balanced approach. Not that it really matters.....I do like the improved control off idle that a progressive setup gives you. No more tap the throttle and hit 2500 rpms type stuff.

    -----------------
    On a side note, if you wanted to make your car feel peppier, you could do the opposite progression with the throttle cam and have it open extremely fast off idle and open very slowly near WOT. I think a lot of drivers mistake throttle movement for engine performance.
    I had wanted to work on a progressive throttle cam for my engine as I will be using a 75mm TB. I shelved the idea for now in the interest of getting the car running first to see if it's needed. I noticed an issue with my old DOHC neon, which I had swapped from the MTX TB to an ATX TB which had a larger ID. The autos also had a "faster" throttle cam for throttle response, which made for some jerkiness when installed on the MTX car.
    Last edited by contraption22; 05-06-2015 at 09:47 AM.
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  7. #27
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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by acannell
    The same way that engine wear effects mileage. You have to push the gas pedal harder to get the same speed. Inefficiency. So the idea being tossed around is that a bigger TB hurts atomization and therefore power. So more gas pedal would fix that. And therefore worse mileage. Thats the theory anyway.
    Quote Originally Posted by mopar-tech View Post
    Wrong concept.

    The larger TB hurts the velocity of the air passing through it if not the optimum size.

    Think of water in a pipe. A large pipe that steps down size will INCREASE the pressure at the transition while a smaller pipe opening into a larger area will drop pressure.
    The concept at hand is that a bigger TB reduces the velocity of air through it, which hurts atomization (presumably as that reduced velocity affects the area where fuel is being injected), and therefore power, which more gas pedal fixes, which hurts mileage.

    Quote Originally Posted by Ondonti View Post
    I think a 1 piece intake manifold has all the restriction you need :P
    What the term port velocity comes down to is restriction. More restriction = higher velocity of airflow and in theory, better fuel atomization and thus more complete fuel burn that leads to improved fuel economy.
    I dont agree with this and it turns out Ondonti wasn't really being serious about it. So now I'm explaining an abandoned idea that I don't agree with for the second time..

    And actually, the fluid in a stepped down section of pipe like a TB decreases in pressure as its velocity increases.

    http://en.wikipedia.org/wiki/Venturi_effect

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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by Dr. Johny Dodge View Post
    also , I don't see port velosity changeing one iota as the port size or shape remains unchanged and the throttle body is a heck of a long way from the port
    Yep. The intake charge re-expands in the plenum once it gets past the TB, so any increase in velocity gained by a small TB is lost.

  9. #29
    Supporting Member Turbo Mopar Contributor mopar-tech's Avatar
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    Re: Reduced mileage when using 52mm TB

    FYI- If Mopar felt they could have gotten away with running the 52 mm throttle body on the SOHC cars they would have done so as a cost savings. Mopar was very big into consolidating parts to save money, hell the bean counters even convinced the engineers that the hood vents were not necessary as a cost savings and reducing the oil fill on the turbo cars from 5 quarts to 4.5 quarts in 1988.

    There was a specific reason for keeping the 46 mm throttle body, it could have been the cost of retooling the intake molds or the perception of "laggy turbo chryslers" which they were crucified in the press for on a regular basis.

    I noted on my car *with its setup* that there was a slight loss of throttle response with the larger throttle body. You can debate the reasons why but it was there and Y.Y.M.V. depending on your combination.

    - - - Updated - - -

    Quote Originally Posted by acannell View Post
    Yep. The intake charge re-expands in the plenum once it gets past the TB, so any increase in velocity gained by a small TB is lost.
    The discussion were having is for mainly when the engine is in vacuum.


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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by acannell
    Yep. The intake charge re-expands in the plenum once it gets past the TB, so any increase in velocity gained by a small TB is lost.
    Quote Originally Posted by mopar-tech View Post
    The discussion were having is for mainly when the engine is in vacuum.
    An air mass moving through a restriction then into a larger volume like a plenum will still experience the venturi effect. It must increase in velocity through the restriction, and it must slow down when it expands in the larger volume. The overall pressures relative to atmospheric don't matter.

    A practical example would be carburetors, which operate at below atmospheric pressure.

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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by acannell View Post
    An air mass moving through a restriction then into a larger volume like a plenum will still experience the venturi effect. It must increase in velocity through the restriction, and it must slow down when it expands in the larger volume. The overall pressures relative to atmospheric don't matter.

    A practical example would be carburetors, which operate at below atmospheric pressure.
    Yes, but we aren't talking carbs, they need the venturi effect to work, our cars don't. The injectors do all the atomization followed by the intake valve sucking the atomized mixture in. Even if the air was dead, the intake still sucks the atomized fuel in. The only way things could go south is if by a total fluke, you bolted on a bigger TB and an injector shitt the bed.

    If you're talking an HSC Topaz/Tempo, then things might be different as they were designed to swirl the air to promote better cylinder filling and fuel mixing, but our heads/ports aren't.
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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by turbovanmanČ View Post
    Yes, but we aren't talking carbs, they need the venturi effect to work, our cars don't. The injectors do all the atomization followed by the intake valve sucking th atomized mixture in. Even if the air was dead, the intake still sucks the atomized fuel in. The only way things could go south is if by a total fluke, you bolted on a bigger TB and an injector shitt the bed.
    I'm not agreeing with the idea that a bigger TB hurts anything. The discussion has drifted off into the underlying principles surrounding an idea nobody supports and was just a joke to begin with.

  13. #33
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    Re: Reduced mileage when using 52mm TB

    Despite air velocity and everything, the worse fuel mileage comment makes sense to me. At any given throttle angle the bigger throttle body let's more air in because the opening is bigger. The computer senses this via the map sensor and speed/density programming and adds more fuel to adjust. The same throttle position with a smaller throttle body uses less fuel because less air comes in...

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    Super Moderator Turbo Mopar Staff contraption22's Avatar
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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by RattFink View Post
    Despite air velocity and everything, the worse fuel mileage comment makes sense to me. At any given throttle angle the bigger throttle body let's more air in because the opening is bigger. The computer senses this via the map sensor and speed/density programming and adds more fuel to adjust. The same throttle position with a smaller throttle body uses less fuel because less air comes in...
    But you'd need less throttle angle to allow the same amount of air. So at a given load, you'd have to press the accelerator pedal slightly less. The engine is still pulling the same amount of air, and metered the correct amount of fuel, to do the same work.

    Think about this. If I change the size of my fuel line from 5/16 to 3/8, will that hurt my mpg?
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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by RattFink View Post
    Despite air velocity and everything, the worse fuel mileage comment makes sense to me. At any given throttle angle the bigger throttle body let's more air in because the opening is bigger. The computer senses this via the map sensor and speed/density programming and adds more fuel to adjust. The same throttle position with a smaller throttle body uses less fuel because less air comes in...
    You're forgetting that the driver closes the loop. Fuel economy only makes sense when comparing equal driving. So the driver would reduce throttle blade angle on a bigger TB to get the same speed/acceleration between the tests. Fuel economy isnt compared by setting equal throttle blade angles and seeing how much fuel the computer sends between tests.

    - - - Updated - - -

    Quote Originally Posted by contraption22 View Post
    But you'd need less throttle angle to allow the same amount of air. The engine is still pulling the same amount of air, and metered the correct amount of fuel, to do the same work.
    Exactly.

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    Re: Reduced mileage when using 52mm TB

    To me, the only way it will effect mpg is if you go WOT from every intersection, then wide open, there will be less restriction, leading to MAP reading more air, then adding more fuel and thus getting a tad more power. That's how a plate with a half inch hole in it across your TB would give you good mpg, it doesn't really "do" anything low down, it just restricts the max fuel and air you can use.
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    Re: Reduced mileage when using 52mm TB

    Ok so driving style plays a big roll in fuel economy, BUT like Road Warrior said, if 2 exact cars with different throttle bodies were driven at constant WOT the one with the bigger throttle body would get worse fuel mileage. So we could word it like this: A bigger throttle body will increase power most noticeably at higher rpm, but leaves potential for worse fuel mileage depending on driving style. Port velocity is largely untested and theoretical at this point. Chrysler probably didn't offer a bigger throttle body for one of two reasons, money or emissions as those seem to drive the auto industry in general.

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    Re: Reduced mileage when using 52mm TB

    I think anything that helps make more power will hurt fuel economy at WOT. That's kind of the idea. Get more fuel and air in the engine.

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    Re: Reduced mileage when using 52mm TB

    I've never noticed a difference in mileage with different size TBs. The best mileage I ever got out of my shadow was with a 2.5" U bend coming straight off the turbo and going straight into the TB. It was a 2.5 car and could routinely return 42mpg on the freeway at about 60mph.

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    Re: Reduced mileage when using 52mm TB

    Quote Originally Posted by RattFink View Post
    Port velocity is largely untested and theoretical at this point.
    Harry Westlake, lots of good reading there.


    Chrysler probably didn't offer a bigger throttle body for one of two reasons, money or emissions as those seem to drive the auto industry in general.
    As I already pointed out, Chrysler would have dumped the 46 mm TB if they could as reduction of part numbers was always an ongoing program in cost savings.

    Thats also why we ended up with the dumb coil on the t-stat housing.


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