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piston dwell

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    Default piston dwell

    im interested in increasing combustion efficency by incressing the pistons "dwell" time at tdc if im thinking correctly if we "hold" the piston at tdc "longer" the combustion pressures are going to go up and there will be a more complete "burn" increasing efficiency, and if im right then having the piston "dwell" at bdc then you will have a more complete intake stroke reducing reversion, im aware that cam choice and intake runner length also have a huge roll to play in that aspect....... the main point im tring to make is if there is a point where to long of a rod can have negative effects on the "attitude" of the engine if anyone has a clue of what im talking about id like to hear your thoughts

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    gn7
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    Quote Originally Posted by bowtiebro View Post
    im interested in increasing combustion efficency by incressing the pistons "dwell" time at tdc if im thinking correctly if we "hold" the piston at tdc "longer" the combustion pressures are going to go up and there will be a more complete "burn" increasing efficiency, and if im right then having the piston "dwell" at bdc then you will have a more complete intake stroke reducing reversion, im aware that cam choice and intake runner length also have a huge roll to play in that aspect....... the main point im tring to make is if there is a point where to long of a rod can have negative effects on the "attitude" of the engine if anyone has a clue of what im talking about id like to hear your thoughts
    Oh yeah, this could get interesting.

    My two cents.
    How do you plan to have the piston dwell longer at the top and the bottom? Or are you kicking around the idea of one over the other?



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    its that theory that the longer the rod you have the longer the piston "dwells" at tdc and bdc but on the other hand in mid stroke the piston speeds up so the piston is "slowing down" at tdc and bdc and mid stroke is faster the most stress on your wrist pin is when the piston changes direction, ie the 400 sbc had a very short rod ratio and was limited in stock form to around 5500rpm but building a long rod 6" rod 400 allows you to run up to 7krpm of course this is depending on rod and piston quality

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    gn7
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    Quote Originally Posted by bowtiebro View Post
    its that theory that the longer the rod you have the longer the piston "dwells" at tdc and bdc but on the other hand in mid stroke the piston speeds up so the piston is "slowing down" at tdc and bdc and mid stroke is faster the most stress on your wrist pin is when the piston changes direction, ie the 400 sbc had a very short rod ratio and was limited in stock form to around 5500rpm but building a long rod 6" rod 400 allows you to run up to 7krpm of course this is depending on rod and piston quality
    Long rod ratios dwell longer @ TDC and less @ BDC
    Rod length does effect piston speed at mid stroke. Long rods are a little slower on the top half of the stroke on average, short rods on the bottom half, on average.
    Its a can of worms.
    Single biggest gain at the level 99% here run build their engines, the reduction is friction way out ways any gain they will see from combustion and intake gains. And maybe, just maybe, with the wrong head and exhaust system, lose more than the gained with a long rod.

    The SINGLE biggest reason for running a longer rod in a BBC is the ability to PROPERLY internal balance a 4.00 or longer crank, or because of piston selection is better. Second would be friction reduction. Some combustion gain, or intake gain is a very very distant 3rd reason.

    How much worse is a stock rod length 496 BBC ratio than a stock rod length 400? Is the stock rod length 496 limited to 5500?
    Are you SURE the limit of the 400 is due to the rod length?



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    Last edited by gn7; 05-05-2012 at 10:55 PM.

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    Dwell time @ TDC isn't necessarily a good thing, especially during the combustion process. Don't believe me? Put a piston at TDC and push down on it and see what happens.
    The dwell time @ TDC dictates how much cylinder pressure is actually useful.
    Short rods have faster pos. and neg. piston acceleration rates, (better initial cyl. filling) but like GN said, more friction and cyl. side loading.
    For short duration quick accelerating engines, the shorter rods can make more power and typically over a wider rpm range. When the rpm starts to climb, piston acceleration rates can exceed desirable limits and can be "slowed" with a longer rod. For endurance engines where more upper rpm torque is desired and longevioty is a factor, longer rods can be a benefit. In either case, the cyl heads and valve timing need to be taken into consideration...both long and short rod (relative to stroke) designs have their requirements.
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    Quote Originally Posted by steelcomp View Post
    Dwell time @ TDC isn't necessarily a good thing, especially during the combustion process. Don't believe me? Put a piston at TDC and push down on it and see what happens.
    The dwell time @ TDC dictates how much cylinder pressure is actually useful.
    Short rods have faster pos. and neg. piston acceleration rates, (better initial cyl. filling) but like GN said, more friction and cyl. side loading.
    For short duration quick accelerating engines, the shorter rods can make more power and typically over a wider rpm range. When the rpm starts to climb, piston acceleration rates can exceed desirable limits and can be "slowed" with a longer rod. For endurance engines where more upper rpm torque is desired and longevioty is a factor, longer rods can be a benefit. In either case, the cyl heads and valve timing need to be taken into consideration...both long and short rod (relative to stroke) designs have their requirements.
    Exactly. Longer rods can make for a little narrower of a tuning window. A longer rod engine will be into detonation a little sooner than a shorter rod version of the same engine. Your only going to see this if you run on the ragged edge.
    Short rods kill the pistons & cylinder walls, so you have to pick a happy medium.
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    Quote Originally Posted by bowtiebro View Post
    its that theory that the longer the rod you have the longer the piston "dwells" at tdc and bdc but on the other hand in mid stroke the piston speeds up so the piston is "slowing down" at tdc and bdc and mid stroke is faster the most stress on your wrist pin is when the piston changes direction, ie the 400 sbc had a very short rod ratio and was limited in stock form to around 5500rpm but building a long rod 6" rod 400 allows you to run up to 7krpm of course this is depending on rod and piston quality
    This subject has been beat to death ever since someone decided to divide rod length and stroke. Don't try to re-invent the wheel...By changing rod ratio all you do in reality is move the torque curve slightly and depending on which way you go with rod length is slightly add or subtract piston friction but so slight it might only be found on a dyno. Long rod/high rod/stroke ratios is fine on diesel engines but consider the rpm range where they operate it is an "apple/orange" compairison to a high rpm gasoline engine. Especially when you consider the dynamics of diesel fuel, fuel injection and it's timing, long injection cycles, fuel lag and overall engine life a diesel benefits more than a typical automotive gasoline engine for a long rod, long TDC dwell. BTW, a longer rod decreases BDC dwell, just the opposite of what it does at TDC. Detroit Diesel 71 series 2 stroke uses a relativly short rod so that piston dwell is increased at BDC to allow for a longer intake cycle, a longer exhaust cycle, more complete and cleaner intake charge. But you can always subscribe to the Smokey Yunick philosophy of engine building...."Use as long a rod as you can fit in the block."
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    Quote Originally Posted by 058 View Post
    This subject has been beat to death ever since someone decided to divide rod length and stroke. Don't try to re-invent the wheel...By changing rod ratio all you do in reality is move the torque curve slightly and depending on which way you go with rod length is slightly add or subtract piston friction but so slight it might only be found on a dyno. Long rod/high rod/stroke ratios is fine on diesel engines but consider the rpm range where they operate it is an "apple/orange" compairison to a high rpm gasoline engine. Especially when you consider the dynamics of diesel fuel, fuel injection and it's timing, long injection cycles, fuel lag and overall engine life a diesel benefits more than a typical automotive gasoline engine for a long rod, long TDC dwell. BTW, a longer rod decreases BDC dwell, just the opposite of what it does at TDC. Detroit Diesel 71 series 2 stroke uses a relativly short rod so that piston dwell is increased at BDC to allow for a longer intake cycle, a longer exhaust cycle, more complete and cleaner intake charge. But you can always subscribe to the Smokey Yunick philosophy of engine building...."Use as long a rod as you can fit in the block."
    LOL...Smokey... Because of this thread I re-read an 18 page discussion I've got archived that was written by Larry Widmere, and in it he discusses rod ratio quite a bit. He mentioned that he and Smokey used to sit around and debate this for hours. Trust me...if these two couldn't come to a consensus, there isn't one.
    "Ideal" rod ratio based on geometry is about 1.73-1.75:1, for all dynamics considered. If you want to accentuate one dynamic or the other, change the ratio. If you're stuck with a particular ratio, understanding the dynamics can help with decisions on building the rest of the engine.
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    Quote Originally Posted by 058 View Post
    This subject has been beat to death ever since someone decided to divide rod length and stroke. Don't try to re-invent the wheel...By changing rod ratio all you do in reality is move the torque curve slightly and depending on which way you go with rod length is slightly add or subtract piston friction but so slight it might only be found on a dyno. Long rod/high rod/stroke ratios is fine on diesel engines but consider the rpm range where they operate it is an "apple/orange" compairison to a high rpm gasoline engine. Especially when you consider the dynamics of diesel fuel, fuel injection and it's timing, long injection cycles, fuel lag and overall engine life a diesel benefits more than a typical automotive gasoline engine for a long rod, long TDC dwell. BTW, a longer rod decreases BDC dwell, just the opposite of what it does at TDC. Detroit Diesel 71 series 2 stroke uses a relativly short rod so that piston dwell is increased at BDC to allow for a longer intake cycle, a longer exhaust cycle, more complete and cleaner intake charge. But you can always subscribe to the Smokey Yunick philosophy of engine building...."Use as long a rod as you can fit in the block."
    This is exactly what im getting at im a diesel tech and very familiar with how a diesel operates, what im tring to do is find the best of both worlds torque is a main priority in any engine i build torque is acceleration, add rpm to that torque and you will have horsepower, fuel quality is a huge factor in my thoughts on a long rod motor but if you can hit the sweet spot with cylinder pressure, you should increase combustion efficiency and inturn make clean power im not tring to go green at all but an average engine only uses 1/3 of its heat energy and that is not enough if you ask me

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    THERE IS NO MAGIC BULLET IN ROD RATIO
    There are trade offs on top of trade offs.
    Do own a dyno and have 100s of hours to dedicate to this endeavor? There are people who do own dynos and the time and have done this, and there is no clear cut most correct ratio.
    99% of Smokey's reason for the long rod idea was friction and piston weight. 99% of my decision for choosing a rod is friction and piston weight. In a BBC with a stroke 4.00 or longer the longer rod allows for a more favorable counterweight for balancing. Nothing else abotu the rod length really interests me that much.

    For every reason you can think of for running a long rod, I can give one for not running one in a pump gas recreational engine. You're not building a 10,500 rpm pro stock engine are you.

    You spend the next 6 days thinking about rod ratios, and i will spend the next 6 hours on cylinder wall prep, and i will make power from the time than you will.

    There are better places to spend you time and money to find power than in the rod ratio.
    If rod ratio was all that in power production, the Ford 385 and the Mopar wedge would own racing. They don't. Because rod ratio is small bit player in the power game.



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    change the crank (longer stroke) win win

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    Quote Originally Posted by Reservoir Dog View Post
    change the crank (longer stroke) win win
    Usually makes the rod ratio lower, and yet, almost always shows a significant gain in power. I will almost always trade stroke for rod ratio.
    Even in a SBC with the short ass rod in the 400 over a 350 with any rod.



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    And dont forget offset wrist pins .

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    Quote Originally Posted by steelcomp View Post
    "ideal" rod ratio based on geometry is about 1.73-1.75:1, for all dynamics considered. .

    427 fe......?

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