For a forced induction application, let's discuss bore & stroke limitations.

Bore...how big is too big? Bigger bores increase displacement, and can allow increased cylinder head airflow. But, too big could get too thin between the cylinders, resulting in increased chance of head gasket failure. A smaller bore could allow higher boost. But, a larger bore could match the power on less boost.

Stroke...rod ratio needs to be sufficient as not to cause undue wear to cylinder walls and pistons (more of an issue for street cars than race cars). And, pin and ring placement must be positioned to be able to handle the boost. Assuming enough clearance (rod bolts, cam, block), at what point does too much stroke become a liability? Consider standard and tall deck options.

Please post your opinions.

 

If you're discussing stock LSx blocks, there is a limit to bore size before the
sleeves need re-work. I read that number a while ago, but can't locate it
for you.

4.160" comes to mind, but don't quote me yet.

From what the pros say, don't worry much about rod ratio as a 'limitation'.
Select your combination based on what the piston requires. The wear/n/tear
theory is likely one of the lesser problems. Side loading on the major thrust
side for a street engine probably wont amount to much difference between
rod lengths over the life of the engine.

 

Rod length directly effects the piston velocity profile

 

I will try to stay away from 'non-advanced' speculation...

Measure center to center on two adjacent bores, and that is the physical limit on how large of a bore you can make. Although in the real world that will never work, it gives you an idea of the absolute maximum that the block will do.

As far as cylinder sleeve strength, you will have to use a little calculus, but here goes anyway.

You are interested in finding the maximum 'hoop stress' of the cylinder wall. Since the sleeve wall is thicker up top (to allow for more pressure), and thinner towards the center (because the combustion pressure will be much lower) You have to figure hoop stress for every part of the sleeve, and determine the maximum stress value.

hoop stress = 'cyl_pressure * radius of bore' / 'thickness of bore'

Hoop stress will probably occur somewhere near the center of the sleeve, just so you have a place to start calculating.

Use the maximum hoop stress in relation with the modulus of the material of the sleeve to determine if the bore will inelastically deform (go out of round), or crack. As long as you stay within the range of elastic deflection, your cylinder sleeve will be fine.

Apply that boost, N/A, nitrous, the method above will work for all situations.

I have a piston speed calculator somewhere on this board, and using the same idea, you can calculate speed of the rods as well.

Use the rod/piston speed, along with cylinder pressure, area of the piston, wrist pin diameter, rod bolt diameter, rod length, rod cross sectional area, weights of each piece, and modulus' of each component, to figure out the stresses on each component to see which one will fail first. I think I left out a few, but using excel/vba it is possible to figure out which component is going to fail first.

I made a 1/4 mile simulator this week, and the remade a piston speed calculator a few weeks ago, and I actually made a calculator that simulates stresses on each of the components that I listed above a couple years back (that I might remake here within the next couple weeks). I know it can be done.

By using a few equations you can figure out that a longer rod ratio is usually better, larger wrist pins are better, larger cross sectional diameter is better, lighter weight is better, stronger parts are better... all common sense stuff. To see if it will break, either you can use some calculus and simulate it first (which will give you a 90-95% accurate result, because of minor imperfections in the components), or throw the parts together and see if they break.

 

At what point does a rod ratio become so short as to cause excessive cylinder & piston wear for a 99% street driven car? (Think about the issues with stock 400 SBC motors...many blocks were worn out below 100k miles.)

How short can you make the piston skirt before it becomes unstable?

How far below the sleeve can the piston skirt go before it causes issues?

Would a 4" bore with higher boost be better than a 4.125" bore that can handle lesser boost?


Looking at building a Warhawk tall deck block...possible bores between 4.000" and 4.125"...strokes between 4.250" and 4.500" (which is believed to to be "clearanceable").

Moderate boost levels (10# - 15# on pump gas, 20# - 25# on race gas...will also use alky).

Moderate RPM...shift points no higher than 7500rpm.

Keep the comments coming...

 

as short as a standard LS1 piston, pre "piston slap" modified

 

"At what point does a rod ratio become so short as to cause excessive cylinder & piston wear for a 99% street driven car?"

The answer to this question is when the lateral force from the piston on the sleeve causes enough friction force to eat at the sleeve faster than you want it to. How long do you need it to last? 20k miles? 5k miles? 100k miles? Theres more than just rod ratio envolved here. Lubrication, rpm, tolerances, power output, and piston/sleeve hardness gradient are just as important if not MORE important. Pushing 25# of boost is going to cause more damage than switching to a 6.3" rod from a 6.2". I hope that makes sense.

Why would you want to use a short rod ratio anyway? I'm not sure why youre asking the question.

 

I've read that "more" bore increases the potential for detonation (on www.turbomustangs.com).

For conventional deck blocks, we are limited to around a 6.100 rod otherwise our range of FI pistons becomes limited due to pin height and less piston crown.

 

Sorta..

Within the fairly narrow r/s ratios you can fit into most engines (say 1.4 to 2.1 r/s) the rod length has only a small effect on piston velocity. Mean piston speed is rpm x stroke / 6. Rod length isn't a factor.

Rod length only enters into max piston speed and piston g calculations. There is only a few % difference in max piston speed and "velocity profile" between short and long rods in the same stroke engine. Rod length can move the angle of crank rotation where max piston speed occurs +/- a couple of degrees.

 

Everybody "wants" long rods. But, in reality, in order to increase stroke, while maintaining acceptable pin placement, you don't have a choice...the rod has to become shorter. The question of when that becomes an issue is not an easy one to answer...that's why I posted it here in the advanced section.

 

Or you keep the stock stroke and go with a longer rod and a stroked piston.

I have done this many times with great results.

To help answer the question of how do you know your limits with bore and stroke, just use a simple rule.


Rule 1) Use only as much bore as needed to clean up the cylinder. You want room for error and a rebuild and strength.

rule 2) Less stroke. Dont stress the motor, FI will do that for you.

I feel that in an FI motor, you will be adding the power to it, why try to build the motor to make power. I thought thats what the blower was for right?

Rick

 

Using an aftermarket block, bore options are 4.000" or 4.125".
Stroke will be somewhere between 4.250" and 4.500". (9.800" deck height)
If I wanted to rely on boost alone to make the power on a small displacement motor, I would have kept the old 266 cubic inch bullet in there The more displacement you can build into the motor, the less you have to rely on boost to make up the difference.

 

I remember researching optimal rod length several years ago, and veteran engine builders basically said that it was not worth splitting hairs with combos like ours, that rev to 6000-7000 rpms. I was looking into the pros and cons of going to a 6.250 rod.

 

W2W was talkin about this in the Roofer Dave thread, starting on this page https://ls1tech.com/forums/showthrea...&page=18&pp=20 although they are dealing with much higher boost levels than a street car