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