Crank to Rod ratio
 

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?

Re: Crank to Rod ratio
 

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.

Re: Crank to Rod ratio
 

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,

Re: Crank to Rod ratio
 

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

Re: Crank to Rod ratio
 

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

Re: Crank to Rod ratio
 

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.

Re: Crank to Rod ratio
 

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

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

Re: Crank to Rod ratio
 

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.

Re: Crank to Rod ratio
 

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

Re: Crank to Rod ratio
 

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.

Re: Crank to Rod ratio
 

My motor is NA, 565", 4.605" bore, 11.625" Deck, 10.1 cr.. merlin block, sheet metal intake, with 2x1000 cfm throttle bodies, EFI. I have my MSD locked out at 34*.. It starts great 99% of the time.. and doesnt kill that often. If I idle it down under 600rpm.. then it will. New computer for the EFI this year.. and it will be tied to a WB O2 sensor.. and I will be able to control the MSD 6A box with it.. Pretty excited about that.. Built it myself.. I presently have a Cutler system that tunes itself.. works, but not as good as I think it should... Some day I may take it to 650", but that costs and I am not sure what kind of an animal I would end up with..

Dick

Re: Crank to Rod ratio
 

Mr Gadets,I just ran some numbers on your combination using a crane #13R000190 cam w/solid roller lifters.I'm impressed,you do have IDLE TORQUE,especially around 1200 RPM.Are you using 18* or Big Chief heads,wall thickness and diameter push rods,belt drive,lifter bore bushings,lifter valley spray bar,35#flywheel ? Do you feel your sheet metal intake is super cooling the airflow, there by rejecting heat soak at engine shut down?

Re: Crank to Rod ratio
 

I am running an Ultradyne cam... 263/268 .686" lift.. solid roller.. Dart 320 heads with wedges, intake flow is about 380cfm.. not sure on the exhaust..
Thick wall 3/8" push rods... bushed lifter bores, steel flywheel.. not sure how heavy.. Regular old timing chain.. no spray bars. EFI doesnt seem to have a problem with the heat soak..

hope this helps..

Re: Crank to Rod ratio
 

The cam you pick, and how it's installed is going to affect the idle and possibly the startup of your motors. There are so many cam grinds out there. You should find people that built similar motors and see what cams they used and make a decision that will satisfy your application. I know that Comp cams is very friendly and may give you advise over the phone. Also I believe Msd has a device that you can back off timing when you start up and can bring it up after motor is running, saving wear and tear on your starter. Also if the motor runs cool enough, heat soak will be of minimal concern. Usually 20 degrees above normal operating temperature. Hope this helps < BBB

Re: Crank to Rod ratio
 

Big Block Billy,Those are good points,I agree,comp is good people,in fact they confirmed using 7/16 x .120 wall tapered push rods because of the harmonics they see in thinner wall while testing on their Spintron.The heavier push rod also serves to hold the lifter wheel on the cam a little bit better.Comp also sees further reduction in harmonics with a belt drive.I believe Daimer Elgin may have been the originator of the cam mentioned above.However,installed are,basically, merc 1075 cams,only opened up a little because of the cubic inch and 110*max PV.Thermal heat rejection (heat soak) is a lengthy subject.Shortened,Mr Gadgets has a sheet metal intake.A sheet metal intake can be designed with applied Bernelle's principle (Venturi) to cool the charge similar to an inter cooler.Therefore,subject to various influences, Heat Soak may be applicable.

Re: Crank to Rod ratio
 

Very interesting stuff indeed. If all of the top persons in whatever field they are in, be it doctors, engineers, motorbuilders, etc... got together as a group of thier kind, all of them would have learned something. No one has all of the knowlege there is except for God. I really love this site because it really helps people expand in knowledge if they really want to learn more. And most people on here want to help, which benefits all who listen ...BBB

Re: Crank to Rod ratio
 

Also the machine shop I use dosen't trust a cam card, in fact every motor they build, the cam goes on a cam doctor and is thoroughly checked and then used or rejected. They don't go by advertised duration , or .050 lift figures, they check each lobe and know what it really is, not what it is supposed to be ! Maybe it was Juan's or Leroy's or Tony Baloney's first day grinding a cam and it made it out the door !!! BBB

Re: Crank to Rod ratio
 

Ccando,
The heat of vaporization is a natural process that takes place when a liquid changes to a gas. Deceiving at first, by definition, it would be assumed that the transformation of liquid to a gas is creating heat. But quite the contrary. While the composition of the liquid is being changed to a gas energy is consumed in the form of heat. The area where the transformation takes place gives up heat to the transformation process and this heat is carried away by the newly formed gas. This is the law of physics that creates cooler charge air temperatures in carbureted engines when compared to dry-flow port fuel-injection systems. The conversion of the liquid fuel to a gas in the carburetor and intake manifold plenum pulls the heat from the manifold runners, effectively creating a substantial drop in charge temperatures. Credit: evans
Here are some links addressing Bernoulli’s principal as it relates to flow rates, cross sectional area, pressure, and velocity.
http://home.earthlink.net/~mmc1919/venturi.html
http://theory.uwinnipeg.ca/mod_tech/node68.html
Before I map this, I have some data that does not jibe with yours. On Dicks engine I calculate: (4.25” / 8”)
103.9*/76.1* PPV @ 7200.5 FPM @ 6250 RPM (within tolerance)
Yours I calc (4.75” / 6.625)
107.3*/72.7* PPV @ 7939.6 FPM @ 6000 RPM (the PPV is < 2.7*)
Bob

Re: Crank to Rod ratio
 

This is all taking me back to college days. Thought I had forgotten all that.

Re: Crank to Rod ratio
 

Rmbuilder,once again,you have provided a good amount of specific knowledge. As referenced about the aircraft wing,as velocity increases,pressure decreases AND temperature also decreases.Those three components are most influenced by parasitic drag. Parasitic drag is the little missed burr on the inside of the manifold or otherwise fancy port job on the heads. Incidentally,Rmbuilder's post also provides insight about the deck contour of some of the new boat hulls. Hydrodynamics does most of the work until the airfoil can reach takeoff speed,then the boat breaks loose and runs. Rmbuilder,I'm still looking for opinion on DETONATION vs. PRE IGNITION . I do agree with your numbers, but rounded, there remains a 5*spread in the two C/R Ratios.

Re: Crank to Rod ratio
 

CcanDo
Looking strictly at the mechanical parameters of the two short block designs, your 4.75"/6.250" long stroke/short rod (short relative to Dicks 4.250"/8.00") will be less prone (given both engines having the identical 4.536" bore) to detonation. The primary reason behind this (again from the short block dimensions) is the reduction of dwell time @ TDC. The rapid approach and take away from TDC (given equal bore size) will minimize dwell time and the risk of a secondary flame front promoting detonation. That being said, there is also a new set of parameters related to where to begin the burn that must be considered from engine to engine. Combustion chamber design and piston shape (flame propagation and efficiency of the burn), mechanical compression ratio, maximizing peak cylinder pressure at the optimum crankshaft angle, fuel quality, IVC angle, and spark lead will all contribute to overall efficiency and detonation resistance.

Are you using coatings, thermal/friction, in either of these builds?

The graphic below illustrates the piston travel per degree of rotation for both engines. The red trace is CcanDo.
Bob

Re: Crank to Rod ratio
 

Bob,I'm very impressed,you appear very thorough.I'll bet you build a good motor. Where I'm coming from is,I'm wondering if there isn't two separate designs,one for V hull and the other for a cat.Several V hulls seem to be under load all the way to red line.Whereas, several cats need torque to get broke loose and then with a reasonably flat torque curve coupled to taller gears and or props be able to make a power reduction.Subject to Torque to weight ratio,hull design, props,gearing and etc. one might expect to run at competitive poker run speeds at reduced RPM by using the bigger crank, lower C/R ratio...........Maybe a little more dock manners,little easier to tune and easier on valve train plus,prudent red line looks like 6350RPM.(However,Scoggins-Dickey,Lubbock,Tx.,has a 632cu.in.w/14.5:1 in their catalog redlined at 7000.They reportedly sold 70 ea. last year,with no known failures.They are using the same C/R ratio that we are) I'm in total agreement with you about which engine should detonate first,all other things being equal. Yes,my motors have coated pistons,Teflon skirts and ceramic 15cc inverted domes.We are also using DLC casidium pins,piston sprayers and a 5 stage dry sump pulling 10 inch's of vacuum.

Re: Crank to Rod ratio
 

Bob,
I am running a thermo coating on the 4.600" flat top pistons, along with some slippery stuff on the skirts. Been a while since I did the coating, cant remember what they called it.
I am running Dart 320's and the chambers have been modified by Jim V. Running a 10:1 compression ratio.

I understand most of what you are saying... but what is the IVC angle?
Sounds like I may have made a bad choice with my rod ratio, not that I am going to change it tomorrow, but for the future I have been thinking about making a change if this motor doesnt work out.
So what are my choices, go to a short rod?? Or should I just increase the crank throw till I hit the rings to make the rod look small?? I can run a 5" crank with these rods and just get into the oil ring. I could shorten the rod and go with more crank throw.. I think it is suppose to handle 5.4".. But I am not sure what kind of an beast I would be producing. I am limited to a Bravo drive setup, and dont want to make an instant drive breaker out of it.

Sure wish I understood all the theory behind all this motor stuff!!
Thanks
Dick

Re: Crank to Rod ratio
 

RROCKPIRATE,In my opinion;First,study the application,(we have a tendency to ignore the drives rated torque and horsepower rating and etc..)Second,conclude your objective(user friendly,durability,residual value,maintenance and etc.)Third,establish a realistic budget......Then,one is better prepared to start working with the various motor build combinations.......However,as I believe RmBuilder eluded to the fact,the cam (valve train) is THE HEART OF THE MOTOR. Therefore,prudently, one might design around the HEART!

Re: Crank to Rod ratio
 

Looks like the trend these days is to go 565 or 557 CI.

I have 580's, they were 572's but now 30 over. When we pulled the motor appart the piston skirts were pretty bad so we had to scrap the pistons and get new ones. I would assume this is from a bad rod/stroke ratio. The rods were 6.70" Oliver rods which forced me to order custom pistons and take 5 wks. to get so I ordered 4 spares. There's something to be said to building a motor with readily available parts.

Re: Crank to Rod ratio
 

Could the damage to the pistons be from detination, rather than rod to stroke ratio.

Re: Crank to Rod ratio
 

In my case no. the sides of the piston skirts were scuffed from side load on the pistons.

we tried finding a piston with the deepest skirt to try and help things out.

Re: Crank to Rod ratio
 

Just curious, I had to replace 2 pistons last season, my machine shop said I was detinating the motor. Excessive heat caused the skirt to expand, which scuffed the piston and embedded aluminum in the cylinder wall.

Re: Crank to Rod ratio
 

I dont have much of a problem with skirt scuffing until I take a few lobes off of the cam and the metal gets between piston and cylinder wall. If I could make it through a season with a cam.. I could tell if the long rods are helping... *S*
My buddy is running 588" and he sees some scuffing but that is after several hundred hours of use..

Re: Crank to Rod ratio
 

Ttt

Re: Crank to Rod ratio
 

MrGadets,Have you considered big bore Jessel lifters and bushings? Also some feel a cam belt and the biggest,thick wall push rods help protect the valve train significantly.......I know, just a few minutes and a few bucks.....But, you already have a very expensive, high RPM motor.....It has been said a super tall deck block is a challenge when getting the lifter bore geometry perfect.

Re: Crank to Rod ratio
 

CcanDo,
I have had the lifter bores bushed and installed .904 flat tappet lifters from Joe Schubeck. Problem was the cams could not hold up to the hard surface on the lifters. This last year I ran Joe's Roller X lifters, but after 20hrs I had oil psi issues.. started to fall off by 5psi. So I did a tear down and found metal in the mains.. I didnt clean things good enough apparently. So this year, starting fresh.
But lack of money has promted me to not buy that new set of 18* Oval port Darts, Jessel belt, and T&D rocker system and replace the Hogan sheet metal intake. Of course with those heads I need dished pistons and I might as well toss in that 5" crank while I am at it.. All those pieces are on the dream sheet!! It just comes down to money and time as always.. :rolleyes:

Dick