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Thread: Loss of HP after exhaust and BBK

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    Charter Platinum len3.8's Avatar
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    Question Loss of HP after exhaust and BBK

    I was watching a show, forgot which one, In that show they had taken an RSX and added an aftermarket exhaust. The exhaust was to free up some bake pressure. They had also put on a big brake kit, with larger rotors. After dynoing the unit with the BBK and exhaust the car was down 12hp... After going back and forth from stock exhaust to aftermarket, and not being able to figure it out, they then called a tech... He said the loss of HP/power was due to the Rotors being so much larger, the inertia of the rotors were having an adverse effect on the hp....
    I understand the sprung/unsprung weight issues.

    I had never heard the loss explained like this, is this a front wheel drive thing?
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    Charter Platinum satan_srv's Avatar
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    do you have the graph? The BBK if it was responsible for the loss would show across the board losses...the exhaust would be refined to ranges...its hard to say without a graph.
    Joel
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    Charter Patron KGP's Avatar
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    I saw the show too. It's not a front wheel drive thing at all. It's just that because the mass of the rotor is farther away from center, it requires more effort to turn the wheels than the OEM rotor, even though OEM weighed more. So, more effort results in a lower dyno reading. Golfers, ever hear the term swing weight?

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    Charter Platinum satan_srv's Avatar
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    right, it;s the same reason 19 inch rims will degrade performance. Even if you move from a 16" to a 19" rim and the weights are the same (unlikely) you will still lose power due to the weight being further from the center of the rim.
    Joel
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    Registered User JOPRIMO's Avatar
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    (excuse these dumb questions - im a HP noob)

    Aren't the BBK's and rotors heavier? Does the added weight all around reduce HP?
    95 Red/Blk (Sold 6/11)
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    Charter Hero docjohn's Avatar
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    Actually because of design and metalurgy the big brakes are often lighter overall than the stock components.

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    Registered User ncdogdoc's Avatar
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    I cannot honestly believe that larger rotors (which weigh less if I understand) would create a 12 hp loss. That is like 7-8%. The inertia effect is true, but it cannot be that great. Even a lighter flywheel will not affect hp that much, and engine/driveline changes (before driveshafts) are much, much more productive/degrading than anything at the wheels. I would be looking at Dyno error or operator error, or ecu/gas/air temps. Sorry, just don't buy it.
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    Charter Platinum len3.8's Avatar
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    Originally posted by satan_srv
    right, it;s the same reason 19 inch rims will degrade performance. Even if you move from a 16" to a 19" rim and the weights are the same (unlikely) you will still lose power due to the weight being further from the center of the rim.
    Are you losing the power or the ability to accelerate as quickly?
    I think the power is still there, just the rolling diameter is larger
    Right or wrong?
    LENN9S5X-T (The T is for Tenacious!)
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    Registered User td2k_nsx's Avatar
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    They lost power but were running faster lap times if u watched the show. The big brakes helped with faster lap times but they lost some power. The weight being out further takes longer to rev. Similiar to holding a pole. Hold a lighter one at the end and a heavier one in the middle and see which one is harder to swing.

    good ex. of power not being everything and how important weight can be and even how everything works as a system and u can not just factor more hp = more car

    I recently noticed this racing karts too i slowed down significantly on one segement and really improved my lap times by 2 seconds. I have a tendency to over drive and lose time... so now driving it like a granny... I am faster. it is not as fun though
    // l e e //

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    Originally posted by len3.8
    Are you losing the power or the ability to accelerate as quickly?
    I think the power is still there, just the rolling diameter is larger
    Right or wrong?
    Flywheel HP would be the same.

    Measured HP @ the wheels is different.

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    Charter Platinum len3.8's Avatar
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    Deuce that is what I was thinking.

    TD2k, I watched, the were faster due to a combination of a few things. Driver, suspension, and being able to brake later. I am aware of those factors, I go to the track a lot. The hp makes a difference if I am able to brake later and exit with more torque.

    I was just wondering where there HP loses were coming from.
    There summations led them to call out the inertia issues with the larger brake rotors, just hadn't heard it put that particular way.
    I had experienced the larger wheels, due to having 18/19 and then running 15/16 on the track. The car was much quicker on acceleration with the 15/16.
    Thanks all.
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    Flywheel Weights

    I my small amount of spare time, I also ride MX. I have a KX500 set up for desert and sand. To tame the bike for trail / track use, I put on flywheel weights. The few ounces of weight added to the rotating mass of the flywheel dramatically tames the hit and makes the bike much easier to ride on hard packed surfaces.

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    Charter Platinum satan_srv's Avatar
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    Originally posted by len3.8
    Are you losing the power or the ability to accelerate as quickly?
    I think the power is still there, just the rolling diameter is larger
    Right or wrong?
    like others have said you're still losing power to the wheels.
    Joel
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    Charter Silver nsxtasy's Avatar
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    Originally posted by ncdogdoc
    I cannot honestly believe that larger rotors (which weigh less if I understand) would create a 12 hp loss. That is like 7-8%. The inertia effect is true, but it cannot be that great. Even a lighter flywheel will not affect hp that much, and engine/driveline changes (before driveshafts) are much, much more productive/degrading than anything at the wheels. I would be looking at Dyno error or operator error, or ecu/gas/air temps. Sorry, just don't buy it.
    I agree. Note what Gary is saying - that the theory may be correct, but it's hard to believe that the practical effect would be as great as reported. I don't buy it, either.

    Originally posted by satan_srv
    right, it;s the same reason 19 inch rims will degrade performance. Even if you move from a 16" to a 19" rim and the weights are the same (unlikely) you will still lose power due to the weight being further from the center of the rim.
    Originally posted by len3.8
    Are you losing the power or the ability to accelerate as quickly?
    I think the power is still there, just the rolling diameter is larger
    Right or wrong?
    There are several factors that change when moving from smaller wheels to larger wheels, and these are getting confused here:

    - weight
    - distribution of weight
    - rolling diameter

    First, let's remember that what we're talking about is actually the wheel and tire, not just the wheel.

    Weight is a performance factor, but that's not what we're talking about.

    Rolling diameter is a factor - it's often referred to as "effective gearing" because changes have the same effect as changing gear ratios. Changing the size of the wheels and tires may or may not change the outer (rolling) diameter of the tires, depending on the size chosen. (Usually, you try to choose a tire size that doesn't affect the diameter.) Anyway, this too is a tangent, because rolling diameter is not what we're talking about here, either.

    The distribution of the weight is what we're talking about here. You can have two different wheel/tire assemblies, with the same weight and the same rolling diameter. However, let's say one uses a 16" wheel, and the other uses a 19" wheel (which would be called a "plus 3" fitment). The statement above assumes (AFAIK correctly, but I'm not sure) that the 19" wheel/tire assembly would have its weight distributed further out towards the tire tread, whereas the distribution of the weight of the 16" wheel/tire assembly would be concentrated closer to the middle. If that is indeed the case, then the 19" assembly would require more force to accelerate, due to its greater "moment of inertia" (that's the physics term for the force being distributed towards the edges vs in the middle), thereby resulting in a greater difference between crank hp and wheel hp (although again, like Gary, I question how much of a difference we're talking about).

    This concept is very familiar to all airplane pilots on the board. Before taking off, you perform a "weight and balance" calculation. You calculate the weight to make sure that the airplane is not overweight. You then calculate the balance, using the moment of inertia to calculate the moment arm of each component (the empty airplane as well as each of its contents that vary - the fuel, the front seat passengers, the rear seat passengers, the baggage, etc). You make sure that both calculated results - the weight and the balance - are within the performance envelope of the airplane. It is even required by law that the information needed to calculate the weight and balance is stored inside the airplane, to permit this calculation.

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    Registered User Jimbo's Avatar
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    Something else to consider is the exhaust.

    A less restrictive exhaust is not always better. There is an optimum size and configuration.

    Hot air is less dense than cooler air. A over-large exhaust contains a greater percentage of cooler, denser air which has to be pumped out of the exhaust.

    Light, hot air is more easily pumped out, this also creates scavenging effects too.

    There are many examples out there where someone just increased the diametral size of the exhaust and lost power.

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    Registered User Dr.Honda's Avatar
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    Thumbs up

    Another point is that the RSX- S apparently leaves the factory pretty well tuned. BTW, Sports Car Revolution is a pretty cool show if you haven't seen it yet!

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    Charter Hero docjohn's Avatar
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    when a figure skater wants to spin faster they bring the arms closer to their body if they want to slow rotation they extend legs and arms away from the central mass thus slowing rotation (assuming all other things are held constant) Car wheels are bound to similar physics,now in the real world quantifing that is the job of our brainiac engineers!Andy......

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    Charter Platinum Sig's Avatar
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    Originally posted by ncdogdoc
    I cannot honestly believe that larger rotors (which weigh less if I understand) would create a 12 hp loss. That is like 7-8%.
    It certainly does seem like a lot, but this effect could be larger in this case because of the normal torque output characteristics of Honda engines. I didn't see the show, so I don't know if the losses were across the rpm range or just on the top-end. Intuitively, I would have thought the top-end would stay in tact and you would lose hp down low and through the midrange. The free-flowing exhuast will only further cripple the cars ability to make troque down low by reducing the back pressure. Theoretically, the car should get it back on the big end.

    So, if a low-torque car further weakens it's torque production in the low to mid rpms with a new exhaust while simultaneously increasing the inertial mass of wheel/brake assembly, then it doesn't seem that far off base that hp could drop by 12.
    Last edited by Sig; 03-30-2004 at 10:20.

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    Charter Silver nsxtasy's Avatar
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    The show - apparently "Sports Car revolution" on Speed Channel - is also being discussed on another board, only there the figure they attribute to the rotors and wheels/tires is 7 hp, not 12 hp. This is on a Mugen-sponsored RSX Type S.

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    Registered User NSX2play's Avatar
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    Ah, Sports Car Revolution - a show featuring a genuinely funny Canadian (is that redundant?), Tom Hnatiw.

    I watched the episode in question for a second time a few days ago and the power loss was indeed 12 hp. While inherent dyno error as well as different test conditions could attribute for some of the loss, there is no doubt that a big brake kit combined with big wheels and tires will account for a loss in rear wheel horsepower. The increase in the rotational moment of inertia from the new brakes and tires is significant. Even though the brakes and the tires and wheels may be lighter the distribution of the weight farther from the axis of rotation is farther resulting in greater rotational inertia. This is Mechanical Engineering 101. And it is one of the reasons that Honda brought the NSX to market with what are now considered tiny 15" front and 16" rear wheels.

    As for the gains, I recall that they regained about 7hp with the Mugen cat-back exhaust system and about 5 hp with the cold air inlet system. So the net usable power loss was zero with a significant increase in handling and braking (modulation and fade resistance).

    It sure is great to get no BS facts from a show that still manages to entertain.
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    Did anyone happen to catch the type of dyno used?

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    Registered User ncdogdoc's Avatar
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    Originally posted by NSX Maven
    Ah, Sports Car Revolution - a show featuring a genuinely funny Canadian (is that redundant?), Tom Hnatiw.

    ......... there is no doubt that a big brake kit combined with big wheels and tires will account for a loss in rear wheel horsepower. The increase in the rotational moment of inertia from the new brakes and tires is significant. Even though the brakes and the tires and wheels may be lighter the distribution of the weight farther from the axis of rotation is farther resulting in greater rotational inertia..........
    I have yet to see the show in question. I saw the one where they "got it all back". I do not disagree with your statement I highlighted above, but the original post makes no mention of larger wheels and tires, which would make sense for the loss involved. My statement is that there is no way that "just" rotors could rob 12hp from even a low torque motor. Maybe I will finally catch the rerun later this spring.
    Gary Yates
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