Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can it hurt an engine's performance by having to big of a throttle body?

  1. Jan 18, 2007 #1
    It is always a big debate on mustang sites on whether a big X throttle body is not needed or that it will not hurt anything as far as the performance goes.

    Some say, "too much air can't hurt."

    Some say, "too much air can hurt."...

    Which one is it?
     
  2. jcsd
  3. Jan 18, 2007 #2

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    Well, if it's too big, you can lose the ability to control inlet manifold pressure well enough, and therefore engine control. But performance shouldn't be an issue provided you can get enough fuel in too.
     
  4. Jan 18, 2007 #3

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    I'll add to that.

    Think of improvements to inlet systems as "bottleneck removing". The idea is to keep reworking restrictive flow paths until you have as low an inlet depression as possible. That way, your volumetric efficiency (essentially a measure of how good your intake/exhaust manifolds are at getting air in, and combustion products out) can approach 100%.

    Obviously your throttle is a key component of your inlet tract, but it's there for a reason. At part load, you intentionally limit the amount of torque the engine produces by closing the throttle, and reducing the airflow into the engine. If you weren't able to do this, your engine would 'run away' with no load applied, and soon turn itself into a million tiny little pieces scattered over a very wide area!
     
  5. Jan 18, 2007 #4
    For example a 302 cubic inch engine flows about 520cfm at 6,000 rpm, and yet some say to get the 75mm TB that flows 924cfm from a particular company. Why do they "need" that extra 400cfm?

    I had read somewhere about the effects of "too much air" could be that their can be to much pulses of air that bounces around too long, etc...etc...I wish I could recall all of it.

    Thanks for the replies, further thoughts?

    A bigger blade would cause more "touchy" fluctation of the engine right? aka: touchier gas pedal?
     
  6. Jan 18, 2007 #5

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    Are you saying that a standard engine will breathe 520cfm at high idle (I take it this is a NA engine), and that with the uprated throttle body the same engine will flow 924cfm? I don't understand. If that's the case, and the rest of the engine is actually up for the extra airflow of the 75mm throttle body, then the standard arrangement sounds pretty horrendously inefficient.

    Your comment about "pulses" is likely concerned either with pressure pulse effects in the inlet/exhaust tracts (which aren't directly caused by the actual amount of air going into the engine), or possibly lean misfire or detonation, caused by having too rich a mixture?

    I suppose, superficially, that yes, a bigger throttle plate would result in a more sensitive throttle pedal, but I would imagine there are limiting issues requiring sufficient flow velocity over the venturi to ensure adequate carburettion (for a carburetted engine, of course).
     
  7. Jan 18, 2007 #6
    I'm sorry brewnog, I should have clarified.

    A 320 cubic inch engine with no restrictions has the capability to flow 520cfm at 6,000 rpm. When people are modifying these engines, a bigger throttle body is "standard". This particular throttle body has the potential to flow 924cfm but the engine can only take in 520cfm. Doesn't seem like it is the best matching of components to me???

    To counter the cfm comments on how the throttle body doesn't use but half of it's capabilities, I always hear, "well it will not hurt to have to big of a throttle body"...so what I am asking is, can to much air into the engine effect the performance at part throttle or at wide open throttle? What about velocity, how is that affected?

    Yeah, an N/A engine.
     
  8. Jan 18, 2007 #7

    turbo

    User Avatar
    Gold Member

    I'm not a fuel-injection mechanic by any means, but I have a lot of carburetor hours under my belt (Harley-Davidsons), and it's a common problem to see a bike with an unmodified engine with an S&S Super G (large-bore carb) instead of the more appropriate S&S Super E. The Super E is well-matched to the flow requirements of the normal Harley engine. You would have to have a stroker or some other high-displacement engine before you would need a higher-flow carb, but some bike owners (and even some bike mechanics!) think that the bigger carb delivers more power. Some of the problems caused by over-sized carbs are inadequate fuel atomization, inadequate mixing, incomplete "burns" (smoking, backfiring), poor idling, and touchy throttle response.

    To extrapolate to throttle bodies - it seems to me that the throttle body should be closely matched to the air requirements of the engine. Look at it this way - if you've got a throttle body that delivers 100% of the peak air requirements of your engine when the throttle plate is fully open, you have control of the air throughout 100% of the throttle position range. If you go to an oversized TB that delivers 100% of the air that your engine can consume while the throttle plate is only 60% open, you have given up usable throttle-control range for no advantage.
     
  9. Jan 18, 2007 #8

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    Yep, I suppose these are all issues which would go with having too low a flow velocity over the venturi in the carburettor. It's not a direct result of having a large throttle, but if your airflow isn't adequate for correct mixing of the fuel in the carburettor itself then you'll have problems.

    This seems to me to be the case; there's no point putting a massive throttle body in the tract if the upper limit on its effectiveness is 60% say. You'll just upset the flow in the tract at wide open throttle. Whether or not the 'uprated' throttle is better than the standard; well, I suppose experiment will tell!
     
  10. Jan 19, 2007 #9
    The majority of DIY engine mods do more for the profits of modding suppliers than the vehicle.
    Putting bigger throat carbs on gives an impression of more power because you go from idle to full flow slightly quicker, but as the engine can't really respond you just dump a lot of the fuel. Also fuel mixing in the inlet manifold is less efficient if the air flows are not matched.
    If too much air was not a problem why would manufacturers have a different carb set-up for every vehicle?
    As brewnog said it is about remioving restriction, and the biggest restrictions are typically in the exhaust, then the valves and only then the carbs. But swapping carbs is a relatively cheap mod so is often done 1st.
     
  11. Jan 19, 2007 #10
    It depends on how you look at it. DIY engine mods can be a complete waste of money depending on how many mods are done. Engine components typically have to work together to form a 'tuned' system or probably stated more correctly, a system where all the components compliment each other in a positive manner.
    -
    Some questions need to be answered before starting to mess with an engine:

    What RPM range do we want to produce the power and how much power are we willing to sacrafice elsewhere in the RPM range to obtain this?
    How much money are we willing to spend?
    How far are we willing to dig into the engine?
    Is fuel economy a huge issue?

    As for the comment about the engine not being able to respond and dumping fuel; if the mixture remains correct, the engine will only take in the amount of fuel that it is able. Also, air flows are typically NOT matched perfectly in an engine. If I'm not mistaken, a dual plane intake manifold intentionally does this to widen the peak horsepower range.
    -
    Personally, I would never mess with an intake without serious consideration to the camshaft. If you're going to do that, might as well go through the valves, so you end up pulling the heads. After that it may be wise to rering depending on what the bore looks like and at that point go through the bottom end. So, what I said earlier, how much money? Where does it end?
    -
    None of this has even come close to gearing to compliment a certain horsepower range or any suspension mods to help for traction, trans-brakes, high stall convertors, etc. In other words, dumping all kinds of money into an engine can be a complete waste if you can't get the power through the tires onto the ground.
     
  12. Jan 19, 2007 #11
    Note that throttle bodies are typically flow rated where the restriction causes 28" H2O (about 1 psi) of pressure differential across the throttle body. You'll notice on the Accufab website where you got that data that the same 75mm TB flows only 390 cfm with 5" of pressure drop across it. On a N/A engine, that bit of restriction won't cause you much HP loss since you are unlikely to flow enough air to really see the restriction, never mind that the tuned port manifolds tend to somewhat compensate for a slight restriction, but it also won't hurt anything - since fuel injected engines don't suffer from the issues that carbureted engines with over sized carbs do (as mentioned above).

    On my positive displacement supercharged engine, every psi of drop in front of the SC inlet is good for about 15 HP loss, so I use an 85 mm TB (flows 1322 cfm @ 28" H2O, but only 558 cfm @ 5" H2O).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?