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Thread: Actual effect restrictor plays in power

  1. #1
    When it comes to airflow through the restricter, when does the restricter start playing a role in limiting power. Is it throughout the entire powerband or does it mainly effect ultimate hp?

    Thanks
    Steven

  2. #2
    When does the restrictor start playing a role? At idle. Seriously. Has anyone ever heard a formula car that idled smoothly below 2000 rpm? Especially given that the engines used, in their factory configuration, idle smoothly around 1000-1500 rpm. It's also a lot harder to get them to start nicely. The restrictor has noticeable effects across the entire operating range of the engine.
    Dr. Adam Witthauer
    Iowa State University 2002-2013 alum

    Mad Scientist, Gonzo Racewerks Unincorporated, Intl.

  3. #3
    Thanks, wasnt sure. Anyone have a dyno comparing and engine with and without a restricter with the same kind of intake manifold and setup?

  4. #4
    Originally posted by Adambomb:
    When does the restrictor start playing a role? At idle. Seriously. Has anyone ever heard a formula car that idled smoothly below 2000 rpm? Especially given that the engines used, in their factory configuration, idle smoothly around 1000-1500 rpm. It's also a lot harder to get them to start nicely. The restrictor has noticeable effects across the entire operating range of the engine.
    I think you will find that the restrictor has absolutely nothing to do with engines starting and idling. It's the large plenum volume that causes that. When your engine goes to start your throttle is restricting almost all airflow anyway, in this instance the relatively large 20mm hole is going to be negligible, yeah???. I took our plenum off the engine once and ran the restrictor straight into the injector housing. I've never seen an engine start so easily and idle so low, down to about 850rpm on a KTM 525... hard too believe but true. This is coming from an engine that is a pig to start and cant idle below 3000rpm with the plenum installed. With this setup however you could hold WOT and the engine could not rev past 9K. I would be interested to hear from someone who has removed the restrictor from their engine but left their plenum installed. I believe that engine hunting and high idle would still be the result. I believe that a large plenum makes it difficult to maintain low pressure on your inlet port at low engine speeds.

  5. #5
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    Originally posted by Adambomb:
    When does the restrictor start playing a role? At idle. Seriously. Has anyone ever heard a formula car that idled smoothly below 2000 rpm? Especially given that the engines used, in their factory configuration, idle smoothly around 1000-1500 rpm. It's also a lot harder to get them to start nicely. The restrictor has noticeable effects across the entire operating range of the engine.
    I don't agree with this.

    We have a 600 cc engine that idles beautifully, starts easily, and idles at the same rpm as our base engine.

    I'll say the same thing that I said in an earlier post: if your engine doesn't start easily and idle well, it's your fault, not the restrictor's. Most teams focus on 100% throttle tuning. Where you set your idle (mass flow rate at 0% throttle) is up to you, and how your car idles at that position is also up to you. Most ECUs come with temperature and throttle-based enrichment - add a little fuel and remove some timing during cold start, and your engine will start on the first crank

    Do you think your restrictor really cuts off mass flow rate at idle rpm? If so, why not just open the throttle a bit more and tune properly? You'll still get the same mass flow rate as a stock engine, just with a wee bit more throttle actuation.

    P.S. I once got to listen to our car idle next to U.W.A. at a test event. Sounded like two content kittens purring away...

  6. #6
    I agree with both of the last two posts. A large intake manifold makes tuning more difficult at low rpm but certainly not impossible. But I think there is more to it than just tuning (although this is probably the most important part). A very important factor to consider is fuel preparation. If your injector is spraying fuel onto the runner walls, dribbling, operating at a very low duty cycle, pointing at a bad angle or spraying at the wrong time you will more than likely have big issues with starting and low speed running.

    The other issue with high performance motorcycle engines and low speed running is cam timing. At low speeds fuel and air can often be forced out of the cylinder at the start of the compression stroke and back into the plenum causing irregular fueling and hunting. In a stock engine the carb/throttle bodies are quite close to the valves and generally have a flat sliding plate as well/instead of a butterfly. So at idle and low throttle openings there is effectively a barrier reflecting the pressure waves which helps to minimise the effect I talked about earlier.

    The restrictor has an effect on engine performance once there starts to be a descent pressure drop across it. At low engine speeds it will have a very small effect and become increasingly more annoying as engine speed increases. So yes it will effect your whole torque curve at WOT but to varying degrees. Remember that for an NA engine, maximum torque at any speed will be when the pressure inside the manifold is atmospheric (ignoring pulse tuning effects). When you throttle an engine it decreases the pressure inside the intake manifold thus decresing power.

    The magnitude of the effect of your restricter at any point will be proportional to the mass or volume flow rate of air through it.
    UoA 07'-?
    www.fsae.co.nz

  7. #7
    Originally posted by Jimmy01:

    Remember that for an NA engine, maximum torque at any speed will be when the pressure inside the manifold is atmospheric (ignoring pulse tuning effects). When you throttle an engine it decreases the pressure inside the intake manifold thus decresing power.
    I believe that the answer to the original post is in the above statement. Could we not say that for a given engine at WOT at any particular operating speed, that when the manifold air pressure is lower in a restricted intake than that of an intake with no restrictor that the restrictor “starts playing a role in limiting power”. Depending on the design of said engine and manifold combination, the magnitude of and operating speed at which these variations occur will be affected.

    Getting an engine to start and idle is all about correctly managing the manifold pressure, fuel and ignition. IMO I would say that this is easier to achieve when you have a small plenum and multiple cylinders. I would also go as far to say that it is easier to make more power with a smaller intake when you have multiple cylinders.

  8. #8
    The starting and idling is all related to tuning. If we are running something thats not tuned well, it wont start for shit. But we spend alot of time with starting compensations and the car starts right up. The biggest thing we see with the restrictor is limiting of RPM. We do not make power above 12700 rpm. However we are making more than stock torque from 6-10k rpm. This is from a direct comparison of a stock F4i with stock airbox/ecu and the same engine with our duel plenum restricted intake, which isnt fully tuned in.
    Branden Wittenauer
    2008-current
    Engine team Leader
    The Ohio State University

  9. #9
    D'oh! OK, I stand corrected, and take back everything I said. Actually, I read the first couple replies right before I went to bed, and couldn't get to sleep until 5am (and subsequently slept through my first class of the day). The "big light bulb" went off. Honestly I'd say I've underestimated the importance of plenum volume; now that I think of it, I've never even played with plenum volume independent of other parameters, and attributed low-rpm unhappiness with primarily cam timing and intake port size. Naturally this didn't explain how a sportbike could idle nicely at 1000 rpm and still pull like a raped ape to 15 grand; this also explains why bikes use ITBs or individual carbs (honestly that confused me, seemed uselessly complex). I know plenums can work magic up top, but never realized how much they hurt down low. I never gave it too much consideration because FSAE plenums typically aren’t any bigger than the airbox on the bike they came out of, but now I see the key factor is that the near-port throttling masks the big plenum down low, while at WOT you still get to keep your Helmholtz system and all its pulse-charging goodness.

    One example of this is with my beloved ’88 Ninja 600. The first thing I noticed when I got it (while cleaning and giving some much needed TLC to the ancient carbs) was how gawd-awful the airbox was on it. The air had to change directions about 7 times and pass through all these tiny nooks and crannies before entering the airbox through these tiny little triangles no bigger than your thumb before it got to the plenum. Any sane person would have just put individual filters on the carbs, tossed in a jet kit, and called it a day. But NOOO, I wanted to keep the plenum. So I hacked all the stock filter accommodations off the airbox and grafted on the biggest K&N filter I could cram in there. It was “effective”…so much so that it would no longer start or idle without massive amounts of ether. And that’s where I originally developed my theory that the restrictor messes up your idle. Ended up having to turn the idle fuel screws out a whopping 1.5 turns to get it to start and run perfectly; although in hindsight the fact that I could completely restore factory idle quality and startability with basic tuning pretty much debunks that theory.

    But that system definitely worked well up top…so well, in fact, that I put in a Stage 3 jet kit and then proceeded to burn the valves. Ended up having to go something like 4 main jet sizes up from the Stage 3 kit to get it to run perfectly, and it may well respond to even further jetting (I just haven’t had the desire to torture myself by wrestling those damned carbs out again…another reason to go with individual filters!).

    +1 to Jimmy.

    But it does make perfect sense, a lot of the bad attributes of a big-plenum are the same as having a bad vacuum leak (and interestingly enough, I’ve found that vacuum leaks are slightly trickier to diagnose on FSAE cars, mainly because they don’t pull much vacuum anyway, and the net result is it just acts “slightly crappier than usual”), and from a dynamic standpoint, pretty much the same thing is going on with low vacuum. And while low vacuum can play real hell with a carb, I thought FI was fairly insensitive and could effectively compensate for it…not the case I see.

    However, I will say that in all of our cars, idle quality and starting could only be improved so far with just tuning. With a lot of tweaking we managed to get our idle from 3000 to 2000, and it only requires a moderate amount of cussing and only a slight magic touch to start when cold. (although E85 may play some role in that too)

    I have also found a new level of appreciation for UAS Graz’s multi-volume intake plenum.
    Dr. Adam Witthauer
    Iowa State University 2002-2013 alum

    Mad Scientist, Gonzo Racewerks Unincorporated, Intl.

  10. #10
    To put it simply, the restrictor deteriorates the hydraulic losses after the throttle valve (which is not the #1 issue with venturi designs and a lot of CFD work) all over the range.


    The #1 issue with it is of course choking the flow. When the incoming air reaches Ma=1 the flow chokes and you simply cannot add any more to the cylinder. No more air, no more fuel --> no more power.

    After all that's the point of its introduction. Limiting power to about 100BHP and as a result no killed 20yo and no lawsuits

    I think under the critical RPM at which choking occues, there is no significant effect on performance, taking that you have a good intake system designed.
    You see, I think Ferrari is a scale down version of God, while a Porsche 911 is a jumped-up VW Beetle

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