Rod ratio vs reversion
 

I have been doing a lot of reading on rod ratios and to be honest its enough to make you dizzy. my machinist suggested I use a longer rod on my build to help with piston side loading. In my reading I found it changes the torque curve and piston speeds and dwells at different points in the cycle, unless I'm not reading things correctly a longer rod might help fend off water reversion?.

longer rod, slower in changing direction (at top at bottom) and faster in the middle.. but overlap is still overlap and that pulse in the exhaust wave (similar to intake wave) is where the risk of reversion comes from... closing the exhaust valve a bit sooner will help with reversion but will hurt the HP IMO PUT A BLOWER ON IT!!! cause ya can't suck and blow at the same time!!!

Don't worry about it. Total cubes, CR, heads, cam, exhaust and induction are what affect your power and where you make it.

I had a girlfriend that can prove you wrong on that one. ;)

OP - it sounds like you're just purging your brain of info overload, but I've never heard an engine builder bother to take stroke into consideration about reversion as of all of the components to consider on that issue that aspect is minimal.

The longer rod will increase dwell time at TDC and BDC, and will reduce reversion affects. Cam intake and exhaust overlap will indicate the potential for reversion but without piston movement the flow just doesn't reverse on its own accord. The longer dwell allows the exhaust to flow a bit further down the exit before piston movement on the down stroke begins to reverse the flow on the exhaust valve closing event.

In a nut shell a longer rod/stroke ration can dampen reversion.

Would you agree though within the usable stroke/cylinder length like in a 9.8 deck block that a 6.385 rod with a piston pin height of lets say 1.270 would be better for longevity, ie ring seal than any benefits of running like a 6.535 rod with 1.120 pin height? I see in drag racing guys will use these 1.080 to 1.120 pin heights but I assume they are going in motor thats ran 100th the time for re-ring vs a offshore boat? Thanks, Smitty

a drag car only lives is spurts never really put to the endurance test like the life of a marine engine... 1.270 is ok but 1.120 NEVER... at least you'd never see my name attached to it... I'm sure others will also speak to this as well

if you wanted to build a killer and reliable 522... 4.560 bore w a 6.535 rod and a 4.0" stroke will give you a good compression ht of 1.255 putting it .010 in the hole if all is std

would be higher RPM friendly

The longer rod will increase piston dwell time at TDC and will shorten dwell at BDC..

Short-rod should have the shortest dwell at TDC and the longest dwell at BDC.

Exactly I would never use that short compression height to try to get a even longer rod in

Not saying it was right, but I ran the 1.120 for 10 years, my brother's on 11 years.

I don't believe that is correct. The dwell will be the same at either end (TDC or BDC). It is a circular/trigonometric function with angular rates.

I don't think anyone asked but what is the combo your thinking or referring to?

I will address it this way. The longevity using a 1.120 pin height and 6.535 rod would be from minimizing piston rocking / skirt wear during max rod angle deflection as opposed to a shorter rod and taller pin height. Ring seal benefits do come into play though. Would I jam the longest rod in the bore to maximize dwell? No, but there might be an optimum I'd target.

For a drag car reversion isn't an issue, and tears downs are likely to occur more often (depending on builder motivation and finances), so the combination chosen is likely the one that yields max HP / results.

I'm not sure there is an absolute correct answer, but I prefer minimizing rod angles so that the rod spends more time going up and down the bore rather that pushing sideways at the extremes.

even the so call experts couldn't agree,,

i think most builders of marine engines use proven combination,s that last the longest rather than reinventing the wheel and spending stupid money for little or no gain,jmo.if you want to build a 800 inch engine that changes everything as you do whatever it takes to fit it in the block and keep the rings from coming out the bottom of the bore.

Darin Morgans take on the topic

"Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero diff-erence in average power and a zero difference in the shape of the horse-power curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length

Super Chevy or hot rod did a 377" comparison. Power was identical.

So don't build a stroked big block, the rod ratio is bad.

Its been a while since I study up on this(15+yrs) but if I remember right longer rods are more proun to detonation or pre-ignition then shorter rods. Keep that in mind as well.

Yes, and that is why alot of the builders in Engine Masters go with shorter rods if they can. More compression, less chance for detonation, and a little more torque. Remember, one off builds mainly for best average power from 3k-6500,

In another thread, I mentioned how longer stroke smaller bore, needs more camshaft, than shorter stroke, larger bore, even though they may net the same cubic inch. In otherwords, say we are comparing a 4.560 bore x 4.375 stroke to a 4.5 stroke, 4.5 bore. Both measure out to about 572ci .

It has to do with piston speed. . At 6000 engine RPM, the 4.375 stroke, will have a piston speed of 4375FPM. The 4.5 stroke, will have a piston speed of 4500 FPM.

As Darin morgans' quote mentioned, regard the rod length, there could be a situation where the cam timing may need to be changed, as a result of the rod length. its all relative to what the piston is doing. Valve timing, igntion timing, intake flow, head flow, and so on. The piston movement is what sets the rules kinda.

Then there is the whole piston speed concept in regards to reliability. If you're building a long stroke BBC, and want to turn some RPM, you want light pistons, very good rods, rod bolts, main caps, piston pins, and so on. Why? again because of piston speed. Forget rod length, that has very very little to do with this. Stroke, and bore (big piston) is what can limit your reliability/rpm potential. This is why short stroke engines have always done better with high rpm.

Piston speeds at 6000 RPM with a BBC
3.76=3760
4.00=4000
4.25=4250
4.375=4375
4.50=4500
4.625=4625

ADK61 mentioned a 522ci, 4 inch stroke combo. Good example. You can rev that 522ci, to around 7000 RPM, and have about the same piston speed, as a 4.625 stroke engine at 6000rpm. Switch to a 3.76 stroke, around now 7400 RPM for same piston speed.

Camshaft wise, they would both theoretically need the same cam. The cam that will support a 604ci turning 6000rpm duration wise, is the same cam, that will support a 522ci turning 7000RPM.

A 454 turning 6000rpm, can get away with alot less high doller parts in the bottom end, than a 632ci turning 6000rpm, before chit breaks. For years, guys used to run the old GM dimple rods, and heavy trw pistons, and rev the chit out of them in 427's and 454's. Stock blocks, ductile caps, sometimes even 2 bolt mains, etc. Strap in a 4.5 stroke crank, some heavy pistons, stock rods, in a 2 bolt production block, and try spinning that to 7000 plus. The oil pan will likely get reshaped in a hurry. Kinda why I don't believe in HP ratings for connecting rods, or even cranks. Theres way more too it.

Joe the piston speeds you mention are average piston speeds that are based only off of stroke length and RPM. Rod/stroke ratio details the acceleration profile the piston actually sees from tdc to bdc at a given RPM.

To cut to the chase if your builder is well established take his advice. I don't think you listed what your combo is??? That might help as others here may have same build with either good or bad results and would likely chime in.

If this is in regards to your 489 it's a no brainier. 4.250/6.385.

Agree, but what kind of numbers are we talking about? Sure, you can compare a F1 car R/S ratio, to a big block chevy, and see substantial changes in piston velocity, but what about, a 1.53 R/S vs say, a 1.49, or 1.46 R/S ratio in a big block chevy?

At peak piston speed, say in a 4.5 stroke, with a 6.535, at 6000rpm, is 7,478 FT PM. Change to a 6.700 rod, and peak piston speed, goes down to 7459. Now change that to a 4.375 stroke, with 6.535 rod. Peak piston speed 7250 with a 6.535 rod, and 7232 ft per min with the 6.700 rod. The 1/8" of stroke changed peak piston speed, by 228 FT per minute. The rod ratio change, changed it by 19 FT per min.

Three variables affect piston movement: Crank Angle, Stroke, and Rod Length.

Four variables affect piston velocity (and acceleration): angular velocity, Stroke, and Rod Length.

The mean speed of the piston is determined by RPM and stroke.

Mean Piston Speed = 2* stroke * RPM/12 (ft/minute)

This number is simply the calculated average speed of the piston. If the piston(s) were moving in 1 direction (or in a perfect circle) at one speed this would be that speed (circular velocity). This simple calculation can be helpful in quickly comparing average piston speeds and the affects of stroke on piston velocity. Unfortunately this calculation is often misused by the uniformed and used as proof that Rod Length (Rod Stroke Ratio) does not affect piston speed. We know this is not the case.

To understand the motion of a piston it is helpful to visualize the actually problem. Below we have a diagram of a piston, rod, and crank.

P is the y axis position of the piston (blue line)
L is the rod length (black line)
R is ½ the engine stroke (yellow)
q is the crank angle
a is rod angularity




Figure 1: Piston, Rod, & Crank Diagram.



We know the stroke and rod length so for any given crank angle we can determine the piston position using the law of cosines.

P2+R2-2 RP cos(q) = L2 (Law of Cosines)

So P = R cos(q) + (L2 - R2 sin (q)2) 1/2

To find velocity and acceleration of the piston we need to convert the crank angle (q) to angular velocity (w) and time (t).

q = wt
w = 2p(RPM) (radians/minute)

for simplicity sake RPM = U

w = 2pU (radians/minute)

This gives us an equation for position as:

P(t) = R cos(2pUt) + (L2 - R2 sin (2pUt )2) 1/2

To determine the position at q degrees (p) insert a time of 60s/((U)(360/q))

To determine velocity and acceleration we need to derive the position formula.

V= d/(dt)(Position)
A= d/(dt)(Velocity)


V(t) = -R sin(2pu)pu - 2 R2sin(2pU)cos(2pU)pU
(L2 - R2 sin (2pUt )2) 1/2

A(t) = -4 sin(2pu)p2u2 - 4 R4sin(2pU)2cos(2pU)2p2U2 - 4 R2cos(2pU)p2U2 + 4 R2sin(2pU)p2U2
(L2-R2sin(2pUt)2) 3/2 (L2-R2sin(2pUt)2)1/2 (L2-R2sin(2pUt)2) 1/2



OK, so what does this calculus mean?

It means BOTH stroke and rod length affect velocity and acceleration of the piston.

If you increase stroke, you increase piston velocity and acceleration.
If you reduce rod length, you increase piston velocity and acceleration

Rod to bore angles.

4.5 stroke

6.700 = 19.622*
6.535 = 20.139*
6.385 = 20.633*

4.375 Stroke

6.700 = 19.056*
6.535 = 19.556*
6.385 = 20.385*

4" stroke.
6.135 =19.026* (stock 454/502)
6.385 = 18.254
6.535 = 17.821
6.700 = 17.368

From Flatlander racing, The piston rod diagram didn't paste over.

wtf.... lol

heres another article on this topic. That math is over my head to try and do comparions. Pipemax helps make it easier. It be cool to be able to simulate from graphs based on the math results

http://www.epi-eng.com/piston_engine...ion_basics.htm

I think the reality of it, for us boaters, is pick a rod that fits in the box. We aren't building engines that are competing to the tenths. Heck, the majority of us here, are so undercammed and undervalved (except guys like 14apache lol), chasing rod length is over the top, from power production standpoint. I mean, guys are building 598ci NA engines, with .630 lift hydraulic cams, and 2.25 or 2.3 valves. So far from what the engine actually wants/needs from an airflow demand standpoint, a change in piston acceleration from rod length, is almost microscopic in the grand scheme of things. When you look at what the top engine guys are running for valve lift, and valve diameter, that are making big power on motor, its a world away from what most here are running. none the less, pretty interesting topic.

Ya and that's why my sh!t is in buckets.... LOL

lol..but how fast do they push that 41 Apache to?

Pump gas engines without trying different props 110.7 Think it needed a little les pitch. It had a bunch of trim in it. Without a lot of trim good water 105.

Props 17x31 15*rake 5blade For got to say NA engines

Thats humping. Prob wouldn't happen with some 310cc heads and 651 crane cams. :riding:

Boat ran around 106 with 351 brodix ovals that head will haul the mail and still have bottom end. Head hunter 383mc hurts the bottom to plane off. But they run like a bear.

I'd much rather have good head vs having a longer rod, :)

A long rod you get more enjoyment from the small end to the big end. Lol