Performance Boats Forum banner

1 - 20 of 109 Posts

·
Registered
Joined
·
196 Posts
Discussion Starter #1
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
 

·
Premium Member
Joined
·
25,975 Posts
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?



100% free webcam site! | Awesome chicks and it is absolutely free! | Watch free live sex cam - easy as 1-2-3
 

·
Registered
Joined
·
196 Posts
Discussion Starter #3
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
 

·
Premium Member
Joined
·
25,975 Posts
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?



100% free webcam site! | Awesome chicks and it is absolutely free! | Watch free live sex cam - easy as 1-2-3
 

·
steelcomp was here
Joined
·
26,515 Posts
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.
 

·
Registered
Joined
·
6,278 Posts
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.
 

·
Registered
Joined
·
4,115 Posts
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."
 

·
steelcomp was here
Joined
·
26,515 Posts
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.
 

·
Registered
Joined
·
196 Posts
Discussion Starter #9
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
 

·
Premium Member
Joined
·
25,975 Posts
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.



100% free webcam site! | Awesome chicks and it is absolutely free! | Watch free live sex cam - easy as 1-2-3
 

·
Premium Member
Joined
·
25,975 Posts

·
Resident Ford Nut
Joined
·
10,075 Posts

·
Registered
Joined
·
196 Posts
Discussion Starter #16
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.
This has turned into a cubic inch thing instead of a rod thing im not going to deny the fact that a cubic inch increase will make more power but if we touch on what actualy happens, for instance we will just use round numbers for this, if you had a 400 inch motor and it made 400hp. At 5500 and all we did to it was stroke it same heads cam carb intake timing settings everything the same the larger cubic inch engine would make the "same" power but at a lower rpm now if we retained the same rod ratio as the smaller engine would it in turn move the torque curve up or down in the rpm range this is what im tring to discuss not how much timber we swing
 

·
Premium Member
Joined
·
25,975 Posts
If you change the stroke, you are almost certain to gain power. even with all the other stuff remaining the same. unless it was ALL miraculouly maxed out in the smaller engine. Look at all the cam, manifold and carb on a L88 427 and LS7 454.
The Z28 302 has basically the same cam, manifold and carb as a the 375 horse 327. About the only thing changed was the stroke. AND THE ROD RATIO........NEGATIVELY.

Think you could make anywhere near that much change with just a rod change.

Again, you haven't even put up a single detail about a proposed engine. Some combos will go backwards with a longer rod.

You started this thread with the idea that a longer rod would dwell longer both top and bottom. It doesn't. Have you given any thought to the negatives of a quicker piston thru the bottom of the stroke, and the imact that could have on the exhaust stroke and the pumping loses?
You still seem convinced that there is something to the rod thing. If so, why do virtually all the top ProStock teams run shorter than standard BBC blocks? Imagine the rod they could stuff in a standard deck block.

Wasn't this about making power? Of ALL the varibles in a engine, the rod offers the least power for the effort. An oil pan will kill it in the HP gain department.
If all you're interested in is moving the torque band, screw with your LSA or the LC. You see way more change.
Given my choice, AGAN, I would take less rod ratio and a longer stroke EVERYTIME.



100% free webcam site! | Awesome chicks and it is absolutely free! | Watch free live sex cam - easy as 1-2-3
 

·
Registered
Joined
·
196 Posts
Discussion Starter #18
Im not over looking any of the said points im just simply looking for pros and cons of both, its about maximizing your specific combination and learning im tring to weigh the pros and cons im not tring to gain 1000hp from a rod change just simply tring to get a productive conversation started and the 2 engine examples you used were highly under rated from the factory 302 was rated 290 but was closer to 390 and the l88 427 was def no 430hp engine
 

·
Premium Member
Joined
·
25,975 Posts
If you think the 302 chevy put out more power (390?) than a 375 HP 327, you're dreaming. Sadly, they never installed the cam in a 350, or we may have seen even more HP gain, all from the same basic package with nothing more than stroke, AND A LOWER ROD RATIO!

There are volumes written on the subject. Google it till you find something that you agree with, or backs up your beliefs on the subject. GOOD LUCK.

LINK: Rods ratio I
LINK Rod ratios II



100% free webcam site! | Awesome chicks and it is absolutely free! | Watch free live sex cam - easy as 1-2-3
 

·
Registered
Joined
·
4,318 Posts
If you think the 302 chevy put out more power (390?) than a 375 HP 327, you're dreaming. Sadly, they never installed the cam in a 350, or we may have seen even more HP gain, all from the same basic package with nothing more than stroke, AND A LOWER ROD RATIO!

There are volumes written on the subject. Google it till you find something that you agree with, or backs up your beliefs on the subject. GOOD LUCK.

LINK: Rods ratio I
LINK Rod ratios II
What cam came in a '70 LT-1?
sorry for thread jak:no:
 
1 - 20 of 109 Posts
Top