I was wondering what could be gained from going to a longer rod. I know it is like 5 - 8 HP maybe due to a slight gain in piston velocity, and a better rod pistion ratio, but what else is affected by a longer or shorter rod, and why waste the time, $$$, and effort?
decreasing the angle What about the dwell and both top and bottom dead centers, can we play games with cam at these positions or not?
cmon input......cmon team. I am looking for advice from the guys that are wrapped up in this type of content. I only know what I read.
Can you picture a can with worms in it? That you just opened? :eek2: Anyhow, I haven't done dyno tests to prove one way or the other, but I have read some very interesting information from several very experienced individuals. Most recently I kept up with a thread on another forum that involved the main R&D guy for Reher-Morrison's Pro Stock program. His name is Darin Morgan. In a nutshell, he explained that they don't dwell on rod length with their motors. They basically decide on the stroke, design the piston for the best ring package and wrist pin placement and then connect the dots with the connecting rod. In the past several years, the trend has been toward "low-deck" blocks in order to achieve a more favorable intake port angle and a narrower intake manifold (sheet metal, 2x4). Obviously, when you lower the deck, you shorten the rod. Now, Mr. Morgan did say that their research applies to unlimited, high RPM, Comp and Pro Stock style motors. He readily admits he doesn't spend any time researching lower HP combo's. I have heard some interesting theories on long rod vs. short rod from others such as Smokey Yunick. But, remember, those guys were testing 30 years ago with old style heads, pistons, cranks, etc. With newer technology materials, some variable have changed. The long rod believers talk about increased dwell time, and higher piston acceleration away from TDC. Short rod motors have less dwell, but slower acceleration away from TDC. Piston acceleration away from TDC will effect cylinder filling. A smaller, high velocity intake port may work better with a long rod. Bottom line is, IT'S THE COMBINATION. What works on a 8000 RPM SBC may not apply to a 4500 RPM Buick, but then again, it might. Test YOUR combination. I'll stop now. I'm sure there are others who can shed more light on this subject. Feel free to dispute what I said. This is an open discussion.
Don't forget one downside with longer dwell time... there is higher acceleration and deceleration... and thus higher piston/pin/rod bolt loading.
ok that is very interesting about load on parts inconjunction with OLD technology. I think the area of the landing is decreased also, causeing bad ring sealing , or for rings to lose their seal over time? Was increasing piston velocity away from TDC the thinking behind all this. does it help with a better cleaner mix in the Cumbustion chamber?
The main reason I see a gain with a longer rod is the reduction of reciprocating weight. With the longer rod the pin can be moved up in the piston and reduce the weight of the piston. A 7" rod is not much heavier than a 6.6" rod but the pin is .4" higher in the piston allowing the piston to be shorter and reducing weight and a shorter piston has less drag than a taller one. The buick has a deck that is too tall for the stroke and rod used from the factory requiring the pin to be too low in the piston. The low pin placement causes the piston to try to tip requiring a tall piston to try to hold it straight. I am working on a 7" rod motor that is nearly identical to a 6.600" rod motor I just got done with that should make more usable power and rev faster using the same induction set up. Reciprocating weight is also a factor in engine strength, force is equal to recip. weightxRPM causing a heavy motor to break at a lower power level and RPM than a lighter one. I think the girdle is used often as a band aid for a heavy rotating assembly and high RPM to reach a power goal. There are some 700HP motors out there without a girdle staying together well and 550HP motors that are dropping the mains on the ground. There are also 550HP motors that outrun other 550HP motors, don't let dyno numbers rule, the track is the only thing that matters when comparing engines. Trap speed will tell the tale of actual HP and E.T. will tell the true tale of the engines output including acceleration rate of the engine which you wont find on a dyno sheet but is very important to E.T. My opinion, if you have the oportunity to use a longer rod, use as long of a rod as will fit the application and expect some gain from it also switch to internal balance and use the lightest parts you can without sacrificing strength. This is for race application, the extra cost of this is not feasable for street engines. I am not trying to knock the use of a dyno and think they are a valuable source of information, but it ends there and picks up when the engine proves its self at the track. There are powerful engines from the dyno that never seem to reach expectations and engines that never see a dyno that prove themselves at the track.
Good points Dennis. As you said, making the rod longer weighs less than making the piston taller. That brings up an interesting point about rotating weight. A misconception I hear alot is that "a lighter rotating assembly makes more HP." That is not true. In a steady state RPM, a lighter rotating assy will make the same HP as a heavy assy, the difference comes in the torque required to accelerate that mass. In drag racing we are accelerating the engine. At Daytona, the Winston Cup cars run at a fairly steady RPM. You will see a difference in the weight of the rotating assy. I have seen some dyno results from a Pontiac 455 Super Stock motor. The owner made some changes that made more HP on the dyno, but went to the track and nothing improved. After thinking about it, he looked at his in-car data recorder and figured the acceleration rate. In first gear the motor was accelerating at 1100 RPM/sec, second gear was around 700 RPM/sec, and third was around 300 RPM/sec. I don't remember the exact numbers, but I know the 1100 RPM/sec is correct. As you probably know, most dyno's accelerate at either 300 or 600 RPM/sec. On a lightweight motor you may not see much difference, but on a Pontiac (or Buick) with an 80 lb crank, you will see major changes. A heavy motor likes to stay at a constant RPM - it's Newton's law. Good discussion. Have fun.
Jeff I agree that less rotating weight doesn't increase horsepower. I reread my post to see what it said, stated makes more usable power and increases acceleration rate, what I meant by this is that with the same horsepower and torque the motor with the lighter assembly will out perform the one with heavier parts in an acceleration test. For a sustained speed a heavier assembly can be a blessing because once in motion it is easier for it to stay in motion. I will still say that a lighter assembly is easier on parts and may be able to obtain a higher power output before block strength becomes an issue. I was using only an acceleration example as in drag race not in sustained speed as in street or circle track use. The heavy flexplates or flywheels can be helpful in holding power during shift changes and should cause no strength issues because this is rotating weight and the strength issue comes into play with reciprocating weight. If properly balanced the rotating weight no matter the diameter could spin to whatever RPM you wanted without placing an adverse load on parts, but reciprocating weight places a load on parts that multiplies by RPM. The reduction in reciprocating weight may save the need for a girdle to a higher point removing extra weight from the front of the car where you don't want it to be. And I agree that an increase in meassurable power on the dyno doesn't always mean that you will see an acceleration gain from that power, if making more horsepower it should show up in MPH (unless higher speed would require spinning past effective RPM range) but not necessarily in E.T.
Dennis, I didn't mean to sound like I was disagreeing with you. I was just building on top of the foundation you laid. I see your point about less reciprocating weight not stressing the block, but I still think I would put a girdle on a Buick block for making HP. But it is an interesting point. So, the weight of the assembly may not stress the block, but HP is made by mean effective pressure (MEP) pushing down on the piston. That force is also trying to push the crank out the bottom of the block. Just a point for discussion. Have fun.
one race motor compared to another race motor Dennis, any Idea how much weight you're going to save on your new pistons with your 7" rod combo? I know the good JE's are about 640 grams, what do yours wiegh? Is that enough to offset the longer Rod? I understand that by shaving your piston, it will take off more weight than a skinny rod, but arn't the rods made of a material that is more dense? I did some calculations with some data of one race motor with good equiptment compared to another figureing 4340 rods, and the good Aluminum Pistons. If my calculation is correct you will save about 45 grams total on your entire rod piston swap, or just under 6 grams per rod/piston. I think in addition to the little weight savings, and the little increased piston velocity the drag loss will help dramtically. Some is better than none. The deck being to tall for a buick is a good point, Any ideas as wy it is so high?
I didn't take offence to it, I was trying to clarify what I tried to say and may have missed with the first post. I agree that there will be a point of failure to a buick block no matter what the reciprocating weight is. I also feel that the point of failure is at a higher output level with a lighter assembly. Most of the mild performance 455's that I have torn down show significant main cap walk, this cap walk can be held to a minimum by pining the main caps. There was a recent post about the stock windage tray being torn apart on a 455 race motor, this was from cap walk without a doubt. I feel that the main web failure is more of a distortion from caps walking than actually being pushed out the bottom by excessive force in a straight line. Even with a girdle mounted on the block cap walk can be a problem, the girdle just doesn,t allow as much movement of the cap. There are several racers that have started to pin the caps to the girdle to solve this problem. A better block would be nice to have but is cost prohibitive to me, I like to race with engines that cost less complete than a new block will cost bare.
Josh, My new piston and rod package is very light. The 7" crower rod is a custom lightweight at only 785 grams compared to a standard eagle rod 6.800 length at 835 grams. The piston is also light, I don't remember the weight but will weigh next week, it is substantially lighter than the JE because of less height above the pin where most of the piston weight is. I have no idea why the block is so tall, I would have rather had a shorter deck with the extra weight in the bottom of the block for strength, but I am talking about something the block was not designed to do. This engine has stood the test of time and works very well for its intended purpose, it just doesn't fare too well when you push its limits.
those are light rods Yeah that makes sense, those are light. So not only can you race folks, you can race them to 6000 RPM.:laugh: What did that cost you for the rod and piston setup? Josh
I got them second hand but new. The rods were originally $1400 and the pistons close to $800. I don't have that much in them.
hot rod Longer rods: Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque. Piston speed: TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft's rotation.
Interesting conversation... "Panic"...Who said velocity? Acceleration and velocity aint da same thing. Velocity or speed is linear (stated as distance/time). Acceleration is not (stated as distance/time squared). No offence ment
