I am building a Fi 355 this winter and i need to fig out this rod to pistion thing...

my heads a LT1 ported by Ai cc are 53 1/2 cc, for me to get under the 9:0.1 CR i need a 5.85" rod and a je/sro -31cc pistion.

I don,t under stand this why not a 5.7 rod or a 6"

Is a 6" ok in a Fi eng....

 

The height of the crank (1/2 the stroke) + rod length (center to center) + piston compression height (measured from the center of the wrist pin bore to the top of the flat part of the piston crown) has to approximately equal the height of the block (crank centerline to deck surface). The difference between these two measurements is the "piston deck". The deck is often set at ~0 (aka "zero deck") with the thickness of the gasket establishing the "quench height" (distance between the adjacent quench surfaces of the head and piston crown).

If the deck is held constant, the rod length does not influence CR. IOW, the compression height of the piston is varied to preserve the height of the assembly. That's why pistons are alwyas specified in terms of rod length. Most builders prefer shorter rods for blower combos. The shorter rod allows for a taller compression height and thus more room for a thick, dished piston crown above the wrist pin. More room is also left for a thick top ring land, which is a potential weak area in a motor that will see very high peak cylinder pressure and high piston crown temperatures.

Rich

 

So why the 5.85 over the 5.7 rod..the 5.85 rod is longer...., do you allways zero deck a block...

 

He's probably specifying a 5.85 inch rod because the particular piston chosen for your application is only offered with a compression height compatable with the 5.85 inch rod. If you want, you can run pretty much ANY rod, if you have a set of custom pisons made.

What Rich was saying is true: In other words, your shortblock is only so tall from the crankshaft centerline to the deck surface, where your cylinder heads mount. When the crankshaft pushes the piston up to the top of the cylinder (top dead center), the distance doesn't change: The distance in most stock SBC motors is 9.025 inches. So, imagine your crank has swung around, and pushed the connecting rod all the way up into the cylinder. Let's do the math: You have a 3.48 inch crankshaft stroke, of which one-half of the stroke takes you UP the bore, and one-half takes you DOWN the bore. So, one-half is 1.74 inches. Now, add the connecting rod: 5.85 inches. From the centerline, your distance is 7.59 inches. This puts the wristpin 7.59 inches up from the crank centerline. This means the wristpin is (9.025 - 7.59) = 1.435 inches down from the deck surface. If you wanted to put the piston face exactly at the deck height, you would need to build a piston with the distance of the wristpin hole to crown of 1.435 inches.

If you wanted to use a six inch rod, your wristpin hole would need to be moved UP towards the piston crown by exactly .150 inches, to 1.285 inches. Otherwise, the piston will get embedded into the cylinder head surface by,,,oh,,, about .100" and you would not even get the engine to turn over, much less start.

Oh, and you dont always zero deck a block. My new engine is decked to 9.005" so that the pistons are -.005 inches in the bore. I'm using a .041" thick gasket, so there is 0.046 inches between the piston crown and the cylinder head surface. Stock GM engines usually keep the piston between 0.025 and 0.035 inches down the bore. High performance builders like John Lingenfelter suggest that unless you intimately know your piston configuration, operating temperature, and engine application, you should keep your distance from cylinder head to piston crown to no less than 0.050 inches.

Hope this helps,
--Dan

 

if you run the 5.700" rod instead of the 5.850" then your compression ratio will be lower still

 

Yes Dan thx, this help ...one thing i can't get my head around is guench....why do i need this 0.035-0.040 quench if i go to this 0.050 will this real hurt the combo????? and lead to spark knock....

 

First off, AdioSS, you are correct but not totally correct. If you run a 5.7 inch rod with the improper piston, yes indeed the piston will not rise to the top of the bore and compression will suffer. However, it is possible that by using the incorrect piston you'll slam the piston skirts into the crankshaft counterbalancer when they reach bottom-dead-center. That would be a bad, bad thing for engine durability.

If you use the CORRECT piston with the CORRECT length rod, compression ratio will NOT change REGARDLESS the length of the connecting rod. It's all about matching the rod length to the piston's dome to wristpin distance.

As for the issue of quench, here's what quench does: As the piston reaches top dead center on the compression stroke, all that air/fuel mixture that's churning around in there gets squished into the combustion chamber. Since the chamber sits on only a portion of the piston, there is this teeny .050"wide slot where the gases must exit and get to the combustion chamber. The piston slams the mixture up, and it blows out from under the piston / cylinder head into the combustion chamber with incredible force. This creates a swiftly churning, turbulent mixture of air and fuel vapor that spin violently in that tiny 60 cc volume. Now, when the spark plug fires, the mixture ignites, and the violent swirl rips apart the flame front into multiple flame fronts which blossom like cauliflower into and through the mass, speeding up the burning process. In essence, the tighter the quench, the greater potential for fast combustion, and the less need for advanced timing.

As for the .035-.040" of downhole location of the piston, it's done by the factory to accomodate engineering stackup of dimensions for safety reasons. As engines are machined, there may be some core shift that causes the machine to bore the crankshaft mains a little off-center. This might cause the crank center to deck height to vary somewhat, such that one bank might have a .025" piston crown to deck distance at top-dead-center and the other might have .030"distance at top-dead-center. It isnt common, I am sure, but it would be possible. The engine manufacturer puts the piston "downhole" for a safety margin that enables them to make lots and lots of reliable engines for an economical price. However, add yourself a composite head gasket with a .041 to .050"compressed thickness, and you soon have a .075" distance from the crown of the piston to the cylinder head. This renders "quench" that is quite tall, full of air/fuel mixture not prone to efficient burning because of all that cold metal in the piston top and cylinder head surface. Instead, a less than complete burn takes place in this area, leaving a bit of carbon soot which sticks to the piston or cylinder head. Eventually this soot starts to glow with heat, and you soon have the conditions for preignition, known as "spark knock".

By milling your deck such that the piston is flush with the deck surface at top dead center, you eliminate the "in the hole" part of the quench area. Instead, you can completely control your quench area by varying the thickness of the head gasket. Unfortunately, pistons rock side to side, and crank bearings occasionally experience wear. Thus, if you tried to hold the piston to head distance to ZERO inches, you would be pretty much assured of the piston dome kissing the cylinder head at speed. For us folks that want our engines to live longer than one race, we need to build some robustness into the engine by opening up the distance from the piston to the cylinder head. This will enable the engine to eat the occasional bearing withough completely blowing up and destroying everything from the piston to the rod to the cylinder head itself. We can do it by selecting a gasket with our desired thickness. Gaskets vary tremendously in thickness: You can buy teflon coated steel gaskets that are .015" in thickness, and you can buy composite gaskets that are .060" in thickness. Your choices are endless, and the gasket choice will be made on your intended application, as well as the final desired piston crown to cylinder head distance you want.

John Lingenfelter, in his book on building small block chevys, recommends that street engines stick to .050" of piston to cylinder head clearance.

 

You can run any rod you want as long as you can get pistons for the assy you want to run.
Basicly is a long rod is for high RPM's and short is for lower RPM's.This all has to do with your stroke and head flow. There isn't a nickles worth of difference in a 5.7 to a 6.0 you cann't tell.The HP figures are within the margin of error on a dyno so you can't tell.If you are running a "0" deck then the quinch will be determined by the head gasket,otherwise it is the gasket +/- the piston location at TDC.
I have run .030 quinch on the street but it is REAL close..035-.038 would be the safe minimun.
The tighter the quench the more compression you can run without detination.
To confuse you a little more The dynamic compression should stay below 9.0 on pump gas.

 

I know I'm correct I was partially joking, but in reality, some people say that in a forced induction application, quench doesn't make much difference. They intentionally run the piston extra far in the hole.

 

Some people might run the piston a ton down,but that ain't the right way.
Quinch makes a difference on any engine.

 

Well this is a Fi eng combo that i am doing..One thing everyone with there head/cam on stock block there quench has to be piston 0.025-0.030 in the hole then a head gasket ontop of that 0.039....that is a very large quench..

What i am trying to do is leave a little on the block 0.010 what i am looking at here is when i do a rebuild i have so meat there just trying ti save the cost of doing the block ALL over again...

 

ws6transam is saying you can have a guench up to 0.050 is this OK....
what about .045 then....

Why and how will a larger quench lead to spark knock..

What is squench(spelling?)

 

There has been a fair amount of discussion of quench here. In brief, quench, or squish area, refers to the flat area on the top of the piston that's typically almost level with the top of the block deck. It must have a corresponding flat area on the deck surface of the head to provide a squish area. When the piston nears TDC the flat areas on the head and piston approach each other and force the mixture from those areas to "squish" into the rest of the combustion chamber. The result is more through mixing of the mixture in the chamber. The more homogeneous mixture is less prone to detonation and burns more completely. And the more complete combusion adds hp and allows a leaner mixture. Quench also allows you to run a higher compression ratio.

Rich

 

.045 inches is supposedly tight. I'm running .046 inches on my new, yet untested engine. As long as crank and rod bearings stay together, and the piston doesn't rock too much from a worn cylinder wall, it should, in theory, be fine. If, however, you destroy a bearing and the rod gets to knocking, the clearance rapidly goes away as the piston/rod wont accurately track the crankshaft. The result is the potential that the piston could make contact with the cylinder head deck surface, causing further destruction. As least, that is my own understanding and is NOT based on any direct experience. I'm currently running a 0.044 inch clearance on my old Feeble305 and it hasn't spit anything onto the pavement yet, even after eating the lobes off a cam two summers ago.

 

5.7, 5.85, and 6.0" rods will all require different matched pistons. The compression ratio will all be the same, but with a shorter rod you have a bigger piston which is generally better for FI because you have more between the top of the piston and the rings, although it does decrese dwell time, but also reduces your risk for detonation from what I undertand. If I am wrong someone please correct me but this is what I have gathered.
-Mark