More hp, less torque.

[Joe G.]

MM,



From reading the same article it appears that the author is getting the length of the rod confused with the stroke. The complete quote is: "The longer the connecting rod the more force (torque) can be applied to the crankshaft. Note from the below engine specification sheets that the diesel stroke is 4. 72 inches compared to the gasoline engine stroke of 3. 58 inches. " The second sentence doesn't make a lot of sense if it is intended to support the length of the rod statement. You can have a long rod with a short stroke and vice versa. However, a short rod with a long stroke gets into some leverage angle problems. The biggest factor in leverage as shown by the author's example of the ratchet is the length of the stroke which is twice the distance from the center of the crank to the center of the rod journal or (expressed another way) the distance that the top of the piston travels from top to bottom in the cylinder. This number and the diameter of the cylinder determine the displacement for one cylinder (pi * r2 * h). That times the number of cylinders determins the displacement of the engine. Comparing the bore and stroke of an eight cylinder engine vs a six cylinder engine is not really a good comparison. The length of the rod is really a measurment of how far the piston is from the crank (roughly). Within limits this measurement does not have a lot to do with torque. A long rod vs a very long rod would produce about the same amount of lever advantage on the same crank shaft. Increase the distance from the center of the crank to the center of the rod journal and there will always be an increase in torque. This is what the author of that article is demonstating with his ratchet example not the length of the rod which in the case of a ratchet is the length of your arm.

 

[Swamp Donkey]

Re: Thanks Strick-9,

This is what I can gather from what I have heard so far. With a custom grind fuel plate that allows for less fuel at lower rpm's and more at higher rpm's would be a big part of the problem solved. I think this is why a lot of the big hp guys would probably have a custom ground fuel plate. The TST plates are great, but I believe they are designed for max torque at 1600 - 2000, not max hp at 3000.



Also anything a person can do to make these breath better in the higher rpm's would also be benificial. Head porting, larger turbo intake and exhaust manifolds... . Now we are getting somewhere I hope! As for the governor springs that Strick-9 was mentioning..... way outa my league to comment on.



Of course everything I say could be all wrong, just trying to learn.

 

[Strick-9]

Swamp Donkey,



There is a reason most fuel plates add alot of torque low in the RPM range and don't add much on the top end. Simply because they can get away with it. I'm just guessing here, but I'll bet nearly all camplates are designed with EGT's being the limiting factor. Our engines can tolerate substantial increases in power in the 1600-2300 RPM range before EGT's get out of hand. Drivetrains are in danger before EGT trouble occurs. On the other end of the spectrum, at high RPM's, EGT's quickly get out of hand with mild torque increases.



There seem to be two common paths of performance increases.

You can either have tons of low rpm power and deal with drivetrain breakage, or you can have a high revving, high horsepower truck and fight excessive EGT's.



At least we get to pick our enemy.



There seems to be a market, however small it may be, for camplates with less of a torque spike on the low end and less high rpm defueling.



I know Piers is working on this, but I don't know if anyone else is.



-Chris

 

[Dennis Garrett]

(I just posted this message about HP/Torque readings on a dyno in KC, on the 2nd Gen. board by mistake, but it might get moved to the 12 valve section later. ) Anyway, I have a 97 2500 w/auto, and it just hit 132,000 miles. Right after I bought it new, I installed the TST 230HP kit. Today there was a small TDR get-to-gether at a hotrod shop in KC. I had my first chance to put it on the dyno, & was surprised at the numbers it spit out. They tested it twice, and at 2300 rpm, it showed 278. 8 HP, and 646. 1 max. torque, at the wheels! This was done in locked-up "D". If you figure about 10% mechanical loss from the flywheel to the tires, this thing could be hitting over 300HP! I have a K&N filter, a hollow cat, and have slid the plate forward about 1/16", but no other performance mods. Also using straight #2 fuel with no additives. After my test, they hooked up a new Ford F-350 with the Powerstroke & auto. It hit 275 hp, but only about 450 torque. It was 95. 6 deg. in the shop, it might have done better if it had been cooler. Seems like a lot from a 230 HP plate, you should have seen the smoke.

 

[Missouri Mule]

Joe,

You sure he isn't using the nut,rachet, hand, arm (elbow grease =power) as an anology to a crankhaft/short rod and long rod/ piston/explosion( = lots more power and no elbow grease)



As I recall if you change to different size rods you change the crankshaft to accomidate them or vice versa. Of course this the expensive way.



As far as long rod and very long rod hot having much of torque difference. I will be more than happy to use a very long breaker bar/rachet (2 ft or longer) to break loose (lets say head bolts) you can use 6 inch to 12 inch breaker bar/rachet.

 

[Joe G.]

MM,



The comparison with the long connecting rod to the long breaker bar doesn't work. Here's why. The leverage length is from the center of rotation of the crank to the center of the rod journal. That's what compares to your long breaker bar. The longer it is from the crank center to the center of the rod journal the more leverage is applied to the crank. On the other hand, the longer connecting rod corresponds to the length of your arm to the breaker bar. That doesn't make any difference in the amount of force you can make with the breaker bar. A short armed guy can pull just as hard as a long armed guy on the same breaker bar if they have the same strength.



Using a different length rod with a change in crank throw is because the piston will go higher in the bore with a longer throw. To correct this you can replace either the rod or the piston.



When I first read the article I noticed that it didn't make any sense, but I decided to not respond to the mag since the reply would not happen for six months.

 

[Missouri Mule]

Joe,

That is what I was getting at you can change the crankshaft to get your desired torque or horse power. Rods would have to be changed to match crankshaft. This would be a costly solution but one would not have to worry about burning piston up because of over fueling.



to any one,

Actually in more horse power, less torque one would go to crank shaft that would need the use of shorter connecting rods.



Because the longer the connecting rods ( with corresponding crankshaft) the more torque the engine has. The down fall is the longer the rods (with corresponding crank) are the lower the max allowed RPMs are. Something like our diesels but not has bad as luxury liner. Also the with long the rods if one would go over max RPM bad thing, can happen in the engine( such as a twisted rod... ) Aren't you glad you have a Cummins with those huge thick rods.

The smaller the rod (with correspondig shaft)no low end torque but more availabe Horse power because engine has more rpms to play with.

Oh yes BTW long rods = long/longer stroke, short rods= short/shorter strokes. All based on the crank shaft that is being used.

I verified all the above with my mecahnic who specialises on big equipment and diesels. But has vast knowledge

with all engines.

Joe

Here is a comparison between V-8s from1988

Ford 460=7. 5L hp=245@4000, tq= 380@ 2800, B&S= 4. 36x3. 85, comp= 8. 5 This engine is a work horse, I had one.

Ford 444=7. 3?or 6. 9 hp=180@ 3300, tq=345@ 1400 B&S= 4. 11x4. 18, comp= 21. 5 Guess which one is the diesel. Its a work horse too.

There was a 400 before the diesel, the figures look like a diesel but the compression ratio is that of a gas engine. hp=153@3500

tq=296@1600, B@S= 4. 00x4. 00, comp=8. ,2 barrel carb.

 

[Joe G.]

Bill,



In your examples where is the length of the rods? It's not mentioned and it's not relevant.



The stroke is the indicator of how much torque an engine will produce for a particular displacement. In fact, I've never seen the length of the rods mentioned in any specification about an engine's power. The bore and stroke are almost always mentioned. The length of the rods is NEVER mentioned. Of COURSE, rods or pistons have to be changed if the stroke of the crank is changed. The length of the rods will depend on how high the deck is, piston used, and stroke. If you have a piston that travels higher in the bore because of an increase in stroke then the rod would may have to be shortened or the piston would travel too high. That can happen if you put a real stroker crank in an engine. So the rod would have to be shortened or pistons would be needed that had the wrist pin closer to the piston top. In that case, in order to increase torque you would end up with shorter pistons and/or rods. If the higher travel is within limits and the compression ratio is ok, then you can still use the old rods and pistons. If you could only change one thing in an engine to increase the torque it would be stroke. Increaseing the length of the rods would do nothing for torque. That was exactly the case with Ford and Mercury flat head V8 engines. The Mercury had 1/4" more stroke so it developed more torque in the same block with the same pistons. One of the first things done to increase power in a Ford flathead was to put a Merc crank in it. You could get away with this because the increase in piston travel height was only 1/8" which increased compression but was still ok. (We usually got a Merc engine instead of fooling around with the Ford if we could find one cheap. )



So, in the same engine if the length of the rods was increased with the same pistons the stroke would have to be shorter so there would be less torque. If the length of the rods was reduced with the same pistons the stroke would have to be longer so there would be more torque. There certainly is a correspondense between crankshaft and rod length. The piston must be in the proper place in the cylinder bore and rod length does that.



There is a limit to all this of course. That limit involves the angle of the rod in relation to the bore as the crankshaft rotates.

 

[Missouri Mule]

I agree with most of what your saying Joe. But I believe I stated in earlier post one could change the crank to get desired results or something to that effect.

In effect rod journals that are closer into crank bearing journals( short stroke) should produce less torque(at lower rpm). Usually these journals use a shorter connecting rod.



This type of set up should have max torgue in a higher rpm range and a higher max horespower because the crank shaft spins around faster.



Where if one wanted more low end torque journals farther away from crank bearing journals(long stroke) would be the way to go (more leaverage). This type of crank usually uses long rods so the pistons don't bottom out on crank and so piston can reach top dead center. The longer the rods are the lower your max RPMs.



Pistons mite need to be changed out also.

Unless there is a lot of room down in the block I would think that there would be only about an inch maybe a little more to play around with.



What I disagreed with that short rods were used with a long stroke or crank shaft with long stoke.



I do think rods(long) play some part in torque but than again maybe not.



Of course your right they don't mention length of rods.

The only comparison I have seen between rods is a photo of the Cummins 5. 9 , 6. 9 or 7. 3 Power stroke and 6. 2 chevy of course the latter two are V-8s. The B5. 9 was giant compared to the other two not only in length but in thickness.

But I would think in V-8 mite have different length any way because of the angel.

But to compare stroke between straight 6's and V-8's I have no problem with it.

There is only one formula to figure length of stroke? But correct me if I'm wrong.

 

[Joe G.]

Shorter rods may be have to be used. If you start with the piston top even with the deck. Then if you replace the crankshaft with one with a longer stroke the piston will go higher as well as lower. Then if the piston top is not supposed to go higher than the deck a shorter connecting rod will be necessary. If the stroke is 1/2 inch longer than before the piston will go up 1/4 inch and down 1/4 inch more. If you get interference with the crankshaft a longer rod won't fix the problem because that will just make the piston stick out of the block more.



Using the breaker bar example, the nut to be turned corresponds with the crankshaft. The breaker bar corresponds with the crank rod journal and your arm corresponds with the connecting rod. The length of your arm has no affect on the amount of torque you can apply to the nut. The length of the breaker bar certainly does. This corresponds to half of the stroke.

 

[Eric_77]

Joe we are the same page. The rod length has minimal effect on the torque/hp of a motor; the crank IS the major player.



Missouri Mule

The rod just moves the rotating force into a linear force the stroke puts the limit on the RPM as the linear speed of the piston is determining point of critical RPM.



Length of the ROD is determined by the stroke and what the deck height/desired compression ratio is. If you have a short stroke motor and do not modify the deck height then you must use a LONG rod to achieve your same compression ratio. Think of it this way squish (distance between piston and head) 1/4" Stroke 4" rod 8" If stroke is reduced to 3. 5" you would need a 8. 25" (half the stroke difference) rod to maintain 1/4" of squish.



If you have a long stroke motor you must then put a short rod in it or the piston will interfere with head/valves or result in too much compression. If you have 1/4 squish and stroke is rasied to 4. 5" then 8" rod must be shortened to 7. 75" or piston would be 1/4" into head.



Pistons with altered pin bosses can effectivley change the length of the rod.





Torque is effected by the leverage applied to the crank and the short Rod will apply torque at more of an angle than long ROD which is less of an angle. Think of halfway betwwen TDC adn BDC There is a triangle formed if you the rod is the hypotenuse the horizontal axis is 1/2 the stroke and the vertical axis is the distance from center line of crank to piston pin. The shorter this vetical axis is the greater the angle of rod to horizontal (crank plane).