Compression height and rod ratio issue

What would be the bare minimum compression height possible with todays modern piston technology. I was thinking that with thin rings that are available today, and with a narrower pin than stock, (such as one for a honda motor) I would be able to get a height of .895". Using a 3.25 inch stroke, it would allow me to use a 6.5" connecting rod in a SBC. Is such a short compression height possible?  

anything is possible,but how reliable would it be?  

why not put the second ring and the oil control ring in the (what are those things called...plugs that retain the piston pin  and fill out the side of the piston, pin buttons???) ... i have seen others that had the oil control ring groove machined into it..  so why not put the second ring groove there also...


and how about doing what they do to some 327 chevy power ring pistons..

machine out the ring groove wider weld it back up and remachine it.. this might even make it stronger.   that must be the reason they do it... i think mahle still done that on pistons they make.

the biggest problem i see with such a short skirt is the piston rocking and the rings coming off the walls....

people who want to build super long rod/stroke motors have custom billet blocks machined..

(this site is running slow again)

so if you want to build the optimal rod ratio motor a billet block might be the way to ***

in hot rodding almost anything is possable... it only take money....or the ability to think out of the box...  this idea is probably going to classified above the deck...  

DON'T build this combination. It isn't worth it. Connecting rod length means nothing. Don't buy into the myths, you will only add cost and probably lose power.

I know of some pretty exotic engines but I've never heard of using a shorter length pin. You are going to have to re-engineer these pieces and pay the price of custom pistons and maybe losing engines when deviating so far from proven components.  

The problem of the short skirt causing piston rocking and having the rings come off the walls, therefore losing theit seal, has been corrected through research, and design. They have had those pistons on the market now for years without the problems mentioned. Also, with the added rod length, the rod angle is lessened, making the associated side thrust (the cause of the piston rocking you mentioned) is greatly reduced. Fortunately, I will not be needing a custom machined billet block, where as I have chosen a short stroke, and a suitable rod to fit.

As to the shorter pin mentioned by another writer, I did not mention a "shorter" pin, but a "narrower" pin. (Honda runs a pin diameter of .875 I believe) They have been making pistons using shorter pins for years BTW, with excellent results. As to the combo losing power, and rod ratio meaning nothing, The longer rod ratio will allow the piston to dwell up at TDC for a longer period of time, giving the combustion process more time to build preasure and push more effectively. This process also seems to make an engine more octane tolerant, allowing for a bit more compression.  

The Engine Masters Challenge has been a great teacher when it comes to this kind of stuff. While I'm not necessarily the top expert, I have seen several things. The first is that rod ratio (one way or another) hasn't had an appreciable effect on output like some would have you believe. Secondly, we've seen ring packs where the compression and second ring have been combined, so that there are only a primary ring, and an oil-control ring.

For a street engine, my recommendation is that you use the best rod ratio that is practical. I'd also stay away from oil rings that intersect the wrist pin. You'll find that race motors don't particularly matter, but when it comes to long-term durability, you'll want the better sealing and stability of a full circumference oil-control ring.

As far as Engine Masters is concerned, I can't say for sure if some of the things tried there would translate to a street engine, but these guys really beat these engines up during pre-event testing, and they seem to hold up.  

I agree with the with the points mentioned in favor of using a longer connecting rod but not beyond risking durability of the piston with too close a ring stack. Smokey Yunick did exhaustive practical testing on Small Block Chevy racing engines and made many of the same points regarding piston dwell and dynamic compression as well as better mechanical leverage and less piston side loading on the cylinder walls.
This definitely carries over to street engines because I've been running a "long rod" 351 Windsor in my 65 Mustang since 1994. It was based on an engine assembly that was documented, dyno tested and written about in Mustang Monthly by Peter Saueracker using a stock 3.5 in. crankshaft with 400M connecting rods (6.58 inch) and a custom JE piston set. This of course benefitted from the tall (9.5") deck height of the 351 Windsor block. This engine produces close to 400 hp and maintains over 400 ft. lbs. of torque from 3,000 to 5,000 rpm. What's more - it does this with only 9.5 to 1 compression and runs well on 87 octane gas. I take this street driven car to track meets often and its a real kick.
I like this motor so much more than the 383" stroker in my Corvette that I'm planning on changing it also into a long rod engine with a 3.25" stroke and 6.25" (more common) rods. This will fit easily with a readily available .030 over pistons with 1.125 inch compression height.  

Also regarding Engine Masters and their remarkable, high end power plants and competitors. Although every one of these mills seem to be made from the very best available racing blocks and of course also lean toward as big as possible displacement for the desired high horsepower numbers required to win that competition, even taking this into consideration, you'll still notice these guys choosing the longest rods they can fit with  almost all their choices of crankshaft stroke. For instance, the winning BES Ford Windsor
employed 6.2 inch rods - longer than stock, with about a 1 inch compression height piston. So these engines favor BIG displacement but still try to fit as long a rod as possible
to offset the really low rod ratio you'd get from a stock length connecting rod. Just look around and see if anyone still favors using a 5.565" rod(from a SB Chevy 400 inch) for the
common 383 Chevy stroker motor. Even the Ford 302 to 347 inch motors try to favor longer than stock length rods for the same (proven) reasons.  

Let's see some past examples of Engine Masters combinations -- 4.125" stroke; 6.125" rod length, (which won, by the way) -- 4.250" stroke; 6.385" rod length -- 4" stroke; 6" rod length, (which also won), -- 4.150" stroke; 6.700" length rod -- there are examples either way; long rod as well as SHORT rod.
Even with this proof, misinformation abounds. Just another example of people seeing what they wish to see and overlook the rest.  

I know this thread is old, but i am very intrested in this.  My idea was to run a connecting rod from a 5.4 modular motor that is 6.657 long with a piston from a 347.  They are off the shelf parts nothing custom.  I would have to turn the crank down to 2.086, which is close to sbc size.  What i dont know is, well a lot. Can i turn a 351 crank down that much, if so i would think it would make the crank lighter and after balancing it with the rods and piston, it should be a quicker reving motor.  I would think lighter weight would free up horsepower. longer rod hoefully would improve the compustion efficency.  And the article i read had used 11 to 1 compression with 87 octane gas ( they used custom pistons with the 400 rods). Can i get away with 11.5 or 12 to 1 compression with 92 octane. They made 400 horsepower and 450 lb-ft. Also is the compression height of the 347 piston (1.09) to short for reliability.  

HotRod magazine did two long rod small blocks, a Chevy (ironically matching a 400 block and a 327 crank, which the Olds 350 offers with a 6" long c'rod and an ability to accept aluminum BB heads) and a Ford.  Their claim was the long rod helped high compression avoid detonation.  I suspect aluminum heads and proper chambers would do similar for less work.

David Vizard, Reher Morrison, and others have done dynowork that shows Yunick's theory on long rods to hold little water.  On a NASCAR engine, where a small diameter at the big end c'rod like a Honda's, is used more for oil control to get who knows what small amount of HP, yeah I can see using the million dollar budget to chase c'rod length.  But, they are also using gas porting (holes around the perimeter of the piston crown) to glue the piston rings from the top, and crankcase scavenging to create a vacuum inside the dry sumped engine block, upon thin little piston rings from below.

On a street engine, the gas ports plug up and create glow points, and an evac system made from some AIR pump out of the junkyard creates a lot of oil vapor that has to be stored somewhere and wiped out weekly.  As for using smaller pins to lighten up the reciprocating assembly, and thus reving to your powerband quicker, it used to be said that knifeding, or pendulum cutting, the counterweights of the crank could make the recip ass'y feel as if you were running aluminum connecting rods.  I suspect a lighter flywheel would also be icing on the cake.

I say, make the rods strong but lightweight.  Make certain the cylinder walls aren't overbored, so they won't flutter at high RPM, and do whatever else to keep the rings sealed up, so you aren't blowing your compression into the oilpan.  Focus on making the combustion chamber right, so that every swing of the piston gets its maximum.  Try to minimize as much power being lost to the valvetrain, etc.  Look at minimizing the combustion chamber inside the head, so combustion occurs inside the dish of the piston (why use combustion to push the head off the block?  Let it do most of its expansion, ie pushing, on the piston).  That'll probably do everything and more, that screwing with the c'rod length could give...and then if you decide to give it a shot of nitrous, you don't have to worry if the piston pins are strong enough, if the piston crown lacks enough meat to take the hit, etc.