Selecting engine dimensions for its intended use, effects of R/S ratio

[GrannySShifting]

Or on the other hand, taking the existing engines dimensions and tailoring cam, intake, header setup etc to make best use of such things.

Talking about traditional V8 engines, specifically fuel injected GM stuff that I work with but thats somewhat irrelevant

From the limited amount Ive looked at rod/stroke ratios and its effects on piston speeds, accelerations etc it looks to me that the changing of rod/stroke values will change the max/min accelerations, but the spread from high values to lowest values appears to remain the same. is this true?
Example:

http://e30m3performance.com/tech_articles/engine-tech/rod-ratio/accel-1.gif

If that is the case, which is more detrimental to a motor at high rpm? If the range of accelerations is the same would you rather have more positive or negative acceleration for longevity reasons?
In terms of trying to fill a cylinder (N/A) since you cant generally get an intake valve at full lift where peak velocity occurs (@75 deg vs 110 deg ATDC) would you want to delay the peak piston velocity as much as possible? peak demand is at peak velocity, not necessarily peak accel right? (long rod looks to me like peak velocity would occur latest)

As a side note, you probably will have to run slightly more quench height in a shorter rod (everything else equal) as higher acceleration at TDC=more rod stretch.

http://e30m3performance.com/tech_articles/engine-tech/piston-dynamics/force-graph.gif

Looking at this, I would think that a shorter rod would be more ideal for a high compression NA motor whose power stroke is relatively short. Basically because cylinder pressure is only available for ashort time, using the most mechanical leverage possible in that time period. For a low compression, supercharged motor, the longer power stroke looks like it would be more suited to the longer rod correct? What are your guys thoguhts and how it shoudl be applied?

 

[incandescent]

It is a lot easier to pull a rod apart than it is to crush it so acceleration at TDC is a larger concern. It doesn't help that acceleration around TDC is always larger than around BDC.

The differences are minute between most any combination you would run though (rpm, rod lengths, stroke) so don't get too caught up in rod ratio. IMO anything from 1.5 to 2.0 is fine.

There are some very good people, including at least 1 major F1 engine supplier, that do not design around rod ratio. They choose a block height, perfect the piston and ring package, pick a crank (stroke), and then fit a rod into whatever space is left.

 

[richdubbya]

I agree with incandesant. I do think that the rod ratio will affect optimum port size, camshaft timing, ignition timing etc. but when all parameters are optimized I believe the results will be the same within a reasonable range of rod ratios say 1.6 to 2.2. I think it is more important to get the ring package and piston pin placement right and to use the length rod that fits, rather than compromising the piston to get a certain rod ratio. Changing rod length will add or lose HP if the original r/s, port size cam timing etc were not optimized and the new ratio brings them closer or farther away from optimum. One thing to consider is, on common engines, like the small block chev, the common heads, cams, pistons etc. may be designed around a certain rod length and if you stray to far from the norm the available parts may need more changes to reach optimum performance.

 

[GrannySShifting]

Okay then developing everything to suit the rod stroke ratios used, how would on go about that?

For example on a LS1 motor, stock rod is 6.098, and most aftermarket are 6.125.. with a stock stroke of 3.622 those things are relatively unchangeable. What would that tell me, or possibly compared to a trad SBC what would that mean it likes?

For a pump gas motor, I was considering the idea of using a 5.8 to 5.9 inch rod, since the intake doesnt support much past 6800-7100 rpm anyway and tring to make max power from 4500-6500.

 

[tbuelna]

grannysshifting,

For a relatively low rpm production engine, a long rod is best. It produces less side loading, has a more uniform piston velocity during its stroke and thus lower peak accelerations and inertia loads.

However, for a no-holds-barred racing engine like F1, reciprocating mass becomes the dominant factor. The shorter the rod, the better. Inertia loads increase at an exponential rate relative to speed. Current 3.0L V10 F1 engines likely have a stroke of about 48mm, which would produce a mean piston speed of close to 29 m/sec at 18,000 rpm. A 350small block chevy has a mean piston speed of about 18 m/sec at 6000 rpm.

 

[GregLocock]

Bear in mind that long rods will have an immediate weight and packaging penalty. Long rod=tall block=heavy block.

You might get away with a tall block on a SOHC engine, but DOHCs tend to get unwieldy very quickly - I've seen DOHC V8s that won't fit in full size RWD sedans.

I6 engines tend to be taller than you'd like anyway

Boxers also need careful consideration, otherwise they force the engine bay to get wider.



Cheers

Greg Locock

 

[incandescent]

tbuelna, do you know which particular engine runs a 48mm stroke? I thought they were closer to a low 40s (41, 42mm.. and closer to mean piston speeds of 25, 26 m/s

THanks

 

[GregLocock]

http://forums.atlasf1.com/showthread.php?s=&threadid=76264&highlight=stroke

Note 3 in that thread has some pertinent details, I agree, I remember 41-42mm stroke

Cheers

Greg Locock

 

[incandescent]

The other thing to remember is that engine speed is the single largest "destructive force" causing factor in an engine. The differences in piston acceleration mentioned in mcguire's post would be much smaller if engine speed was closer to (or under) 10,000 RPM as it is with most anything that us non-F1, non-forumlaAtlantic, non-CART, non-prostock, folks might build.

 

[GrannySShifting]

Looking for the purposes of a sub 7100 rpm, pump gas motor. What Ive seen from John Kaase, and few others, short rod is way to go to make fast action up down bore, less detonation prone?-for example the engine masters builds.

What do you guys think about that? longer rod higher r/s ratios seem to be more forgiving of cam timing, but if you get it right may a shorter rod be beneficial?

Just numbers thrown out, but 12:1 short rod on pump gas or 11.3:1 and lon rod on pump gas, whihc would you trade off? I think the higher compression short rod woudl be the best combo... compression helping with driveability, responsiveness of motor, exhaust reversion should be less and early ex opening shouldnt be as detrimental at low rpm to a high compression motor