With a ratio of 1.40 +or- the maximum piston velocity occurs at approximately 110 degrees.With a ratio of 1.50 + or - the maximum piston velocity occurs at approximately 105 degrees.Therefore,is one or the other more prone to detonation?And,is the other one more prone to preignition?

CcanDo,
PPV on the return side (BDC-TDC) does not occur @ 180* from PPV on the takeaway (TDC-BDC).

Example:
If PPV occurs @ 72* ATDC, PPV on the return will not occur @ 252*(108* BTDC/72* ABDC) of crankshaft rotation. The piston velocity trace (green line) shown below illustrates that well. If you have specific rod length/stroke combinations you want velocity mapped let me know. Below are notes regarding the detonation issues you mentioned with credit to the author Jere Stahl.

Short Rod is slower at BDC range and faster at TDC range.

Long Rod is faster at BDC range and slower at TDC range.

I. LONG ROD

A. Intake Stroke -- will draw harder on cyl head from 90-o ATDC to BDC.

B. Compression Stroke -- Piston travels from BDC to 90-o BTDC faster than short rod. Goes slower from 90-o BTDC to TDC--may change ign timing requirement versus short rod as piston spends more time at top. However; if flame travel were too fast, detonation could occur. Is it possible the long rod could have more cyl pressure at ie. 30-o ATDC but less crankpin force at 70-o ATDC. Does a long rod produce more efficient combustion at high RPM--measure CO, CO2? Find out!!

C. Power Stroke -- Piston is further down in bore for any given rod/crank pin angle and thus, at any crank angle from 20 to 75 ATDC less force is exerted on the crank pin than a shorter rod. However, the piston will be higher in the bore for any given crank angle from 90-o BTDC to 90-o ATDC and thus cylinder pressure could be higher. Long rod will spend less time from 90-o ATDC to BDC--allows less time for exhaust to escape on power stroke and will force more exhaust out from BDC to 90-o BTDC. Could have more pumping loss! Could be if exhaust port is poor, a long rod will help peak power.

D. Exhaust Stroke -- see above.

II. Short Rod

A. Intake Stroke -- Short rod spends less time near TDC and will suck harder on the cyl head from 10-o ATDC to 90-o ATDC the early part of the stroke, but will not suck as hard from 90-o to BDC as a long rod. Will require a better cyl head than long rod to produce same peak HP. Short rod may work better for a IR or Tuned runner system that would probably have more inertia cyl filling than a short runner system as piston passes BDC. Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod.

B. Compression Stroke -- Piston moves slower from BDC to 90-o BTDC; faster from 90-o BTDC to TDC than long rod. Thus, with same ign timing short rod will create less cyl compression for any given crank angle from 90-o BTDC to 90-o ATDC except at TDC. As piston comes down, it will have moved further; thus, from a "time" standpoint, the short rod may be less prone to detonation and may permit higher comp ratios. Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closes--ie. may permit longer intake lobe and/or later intake closing than a long rod.

C. Power Stroke -- Short rod exerts more force to the crank pin at any crank angle that counts ie.--20-o ATDC to 70-o ATDC. Also side loads cyl walls more than long rod. Will probably be more critical of piston design and cyl wall rigidity.

D. Exhaust Stroke -- Stroke starts anywhere from 80-o to 110-o BBDC in race engines due to exhaust valve opening. Permits earlier exhaust opening due to cyl pressure/force being delivered to crank pin sooner with short rod. Requires a better exhaust port as it will not pump like a long rod. Short rod has less pumping loss ABDC up to 90-o BTDC and has more pumping loss from 90-o BTDC as it approaches TDC, and may cause more reversion.

III. NOTES

A. Rod Length Changes -- Appears a length change of 2-1/2% is necessary to perceive a change was made. For R & D purposes it appears a 5% change should be made. Perhaps any change should be 2 to 3%--ie. Ignition timing, header tube area, pipe length, cam shaft valve event area, cyl head flow change, etc.

B. Short Rod in Power Stroke -- Piston is higher in the bore when Rod-Crank angle is at 90-o even though at any given crank angle the piston is further down. Thus, at any given "time" on the power stroke between a rod to crank pin angle of 10o and ie. 90-o, the short rod will generate a greater force on the crank pin which will be in the 70-o to 75-o ATDC range for most engines we are concerned with.

Ok, Bob I tried to follow along...

But I guess the simple question is:
For a marine engine that will run between 3000 and 5500rpm, what is the best rod length for a 4" stroke?
And for 4.25" stroke?

For most of us that would build a stroker (4.25" stroke) out of a 454 or 502, which rod length should be used? stock length or 0.25" longer?
Thanks,

Ok Bob,
Can you map my rod stroke combo please. I am interested in which way to go with timing on my motor. I have run it at different settings and have not had the opportunity to test for best results. Started with 34* locked in and lowered to 30*.. At this point I have 4.25" crank and 8" rods.. 1.88 ratio I believe. So what would you start running the timing at on this combo??

Thanks
Dick

DUDE, You left them! Great info!!!!

Nice work rmbuilder,In support of your response, the following is intended to profile the basic difference in long and short rod to crank ratios.I'll use my engines with known values and Mr Gadgets engines with some assumed values.Both will have 4.536 bore,1125.45cc/cyl,0 deck,15 deck cc,129.278 total piston sq.in. and compr Ratio of 8.680.Mr Gadgets RPM is shown is 6250; whereas mine is shown at 6000, so that the load forces are similar.NOTE:Zero Degrees is BDC,Mr Gadgets;assumed bob wt.=2650 gr.,Max PV occurs at 105*/7194.88 FPM/Acceleration -52.13 g's,compression at BDC 1731.44 g's/10115.41 load lbs.,Tension at TDC -2983.97 g's/17432.94 load lbs. MINE:bob wt.=2394 gr.,Max PV occurs at 110*/7924 FPM/Acceleration-136.13 g's,compression at BDC 1557.90 g's/8222.31 load lbs.,Tension at TDC -3299.09 g's/17411.96 load lbs. Therefore;how might both be tuned with 5 lbs. boost.Perceptibly, Mr. Gadgets will accelerate rapidly to maximum horsepower.Personally,I'm hoping for GOOD idle torque,dock manners and reasonable, durable horsepower.Presently,we have 1000 HP at 6000.

"DUDE, You left them! Great info!!!!"

You have that correct.. you left me.. LOL!

Addendum to my last post:I failed to list the crank and rod sizes for my motors.The crank is 4.75 Bryant,Rods are 6.625 oliver billet,pins are DLC casidum,pistons are JE w/ coated skirts and ceramic domes,oiling is 5 stage dry sump w/10 inches vac.,and .040 piston sprayers.Mr Gadgets lists his crank at 4.25 and rods at 8.00,for a ratio of 1.882.The C/R ratio on mine is 1.395.

Stroking a motor usually will make it wear more side loading the pistons. usually stroker motors are good for 200 hours. Especially true of 572's. I had a 540 built, Tall deck instead or a 572 I was talked out of it. I have seen the type of unusual wear that occurs in a stroker motor. Sure various rod lengths / piston pin heights come into play and can help, but it dosent solve the problem. Also have seen pistons that have too small of a margin from the top ring land to the top of the piston. this is not good. Especially if you ever experience detonation from bad/ wrong fuel. It happens ! There is really no free lunch. It's wiser to buy a taller, better block and start from there and build a big motor instead of trying to to make a smaller motor bigger inside. I bought a new bowtie 10.2 deck block. After all the machine work, I could have had a Dart for less money. So anyway you can make a 502 a 540, a 454 a 496, etc. But they will never outlast a stock stroke motor built from equal quality parts and workmanship. BBB

Any tuning comments,criticism or suggestions would definitely be appreciated. Redundantly, my objective is to design a user friendly motor,with good dock manners,reasonable power and durability,for pleasure boat application.Nothing is more frustrating than trying to start a hot, super tuned, big horsepower,long rod motor that won't start.Then ,when it does start we "clean it out",put in gear and it dies for lack of IDLE TORQUE.Finally,we're off,but because low gears and props are required to get going in the first place,now higher RPM's are needed to go somewhere.While the long stroke motor may live longer in the bottom end, the higher RPM will shorten life in the top end.Personally,the simpler the better,taller gears,bigger prop,lower RPM,more IDLE TORQUE.Granted,for full blown race I'll vote for the long rod.That is a totally different application where acceleration miliseconds make the difference between winning or losing.