rod length 360 LA
Posted November 16, 2009 at 08:45 pm
Posted November 16, 2009 at 10:11 pm
has anyone used longer rods w/let's say a pin to dome clearance of 1.3" or so and have noticeable improvements w/ the same cam or not?
The rod length is 6.123inches, deck height is 9.6inches, so if you used a 1.3inch pin to piston height, and 3.58 half length of 1.790, that comes up to 9.213 from 9.600 deck, or .387 extra rod length, or a 6.510 length rod. equating to a 1.82 rod/stroke ratio, which increases the dwell time on the piston at top dead center vice 1.71 ratio, which will lower the torque on the engine for more bottom end oomph, which isn't a bad thing. Back the timing off a couple more degrees from total advance and you will have a strong motor, but the question is, is $800-1000 cost for the rods worth the effort, or did you just get a good deal on a set of stroker pistons and trying to figure out how to use them with a stock crank?
Personally, domed pistons suck unless they are properly ported, usually restrictive to the back side of the piston, which defeats the purpose of having higher compression overall. They can be fixed, but it does take some work.
Posted November 17, 2009 at 12:07 am
Posted November 17, 2009 at 07:59 am
Posted November 18, 2009 at 08:58 pm
Posted November 18, 2009 at 10:40 pm
So with the deck height, the piston height, and flat tops, I could go near 6.8" for the rods? Also, what I cannot get a straight answer on is- is there a better cam to use w/ high rod ratios? I want torque- not worried about 6000rpms. With higher rod ratios do "towing" cams get even more benefit or can a lopey cam stabilize using these higher ratios?
Rich, what are we really trying to do with the needed words? My post describes how to figure out what length of rods one needs, but here it is again.
Deck height minus half the stroke (it goes up and down, half the amount is the amount it really travels).
9.6inches deck height, minus half the 3.58 is 1.79, which comes up to 7.710 to the top of the deck.
7.710 minus the 1.300 wrist pin to the top of the piston is 6.410 inches will give you a zero deck top of the piston.
With a piston with the noted wrist pin location of 1.3 inches, you can't use a 6.8 inch rod, the oil ring will pop out the top of the block.
What are these pistons out of, given they are a 4.00 inch bore, are they something else other than a Mopar? I know there are about 50 engines with a four inch bore, but you aren't really accomplishing what you are intending to do, it will cost more than sticking with the real plan to fix the real question.
So, since you are looking for more torque and less top end rpm, what you want is a cam that has a very short duration. You can get cams cut to what you want, so they can (the cam companies) can take a big cam and make a smaller cam out of it, so what you want is a 185degree cam at .050inch lift (pretty much stock duration around 245degrees) with a nice medium .450-.460 lift (vice stock .410). This will give you really good bottom end torque because the cam opens and closes quickly, builds lots of low power/torque where you want it. You can go 2bbl, 4bbl, ported heads will make the most for the money, high volume oil pump for longevity, roller timing chain for consistency, electronic ignition and be good to go.
Remember, even though there are 50 different four inch bore pistons out there, different piston pin to piston top heights differ, along with the size and type of wrist pin diameters, and when you start getting into that area, your cost to make them fit each other will outweigh doing things right the first time every time, unless you are trying to do something exotic, and it doesn't sound like you are. You also have the rod change, and Mopars have larger rod journals than the others, thus the crank would still have to be cut down, and from there, you would have an additional stroker engine, but we are really getting exotic and you don't sound like that is where you are going at all.
So, what are the pistons out of, are the wrist pin sizes the same, and where are you going to get these different rods from that aren't going to cost over $1200, unless that price isn't an issue? The cam specs I list above will do more than trying to make a high rod/stroke ratio engine from a stock stroked crank. The minor minor minor gain does not outweigh the cost, but a better cam could, especially if you are looking for bottom end torque.
Edited by dana44, November 18, 2009 at 10:42 pm.
Posted November 21, 2009 at 01:38 pm
Posted November 21, 2009 at 09:45 pm
Sorry for the incorrect math- tried to reply too quickly. So 6.4 is the longest rod-good. What about the cams? Will every cam just react better w/ longer rod ratios or how should a cam be ground for longer rod ratios. P.S. I'm working on another engine also.
Longer stroke will give a little more compression of the cylinder volume when the valves close, raising static compression, giving more power, which can be adjusted as noted by reducing overall timing curve. Stroker engines are liked because of this reason, more than just a bigger cubed engine, but the longer rod itself isn't going to alter this other than increase the dwell time at TDC, which should not be affected by the camshaft itself, or adjustment of a cam profile to alter this. If going with an other than stock cam, check valve clearance with clay to ensure the amount of duration and lift doesn't interfere with clearance, which is actually pretty unlikely until you hit something in the .550+ range.
Double check my math, I think my first post said 6.510 vice 6.400, and mock it all up on one cylinder with bearings and no rings prior to full assembly, you want a deck to piston clearance of .000-.005 or more for good compression, and factory 318s can be as much as .080, .040 deck to piston top is about average (no wonder they are so underrated and so good when built properly).
Posted November 27, 2009 at 08:00 pm
Posted November 27, 2009 at 08:45 pm
Found a calculator for dynamic compression. Rod length really affects this. D.C.'s around 8.0 are about correct-right?
It is affected not in the fact that you have a longer rod, but the fact the piston is closer to the top of the deck of the block. So, if you have a rod that pushes the piston so the deck to piston top is .005, it is no different than having a shorter rod and shaving the deck of the block to get the existing deck to piston top to that same limit.
Other than the rod/stroke ratio, the only other advantage I have seen with longer rods is that the longer the piston is at the apex that makes it seem like the piston isn't moving, more power is actually made because the compression is at that given compression longer. For example, if the compression is 8.0:1 and it is at this compression through a 20degree arc, the explosion in the cylinder explodes and burns for 1/18th rotation (20/360degrees), whereas a shorter rod may have the same compression but a 10 degree arc and burn at 8.0:1 for 1/36th (10/360 degrees) and the compression ratio drops quicker, losing power quicker. So this is one great reason for the stroker engines and longer rods. Additionally, the longer the rod is at that top arc where it appears to be at the top of the bore, the explosion has the ability to cool longer afterwards, thus cleaner, quicker and shorter rod burns lower compression after the arc and burns residual longer and less clean.
Posted November 28, 2009 at 10:05 am
Posted November 28, 2009 at 08:16 pm
OK, I'm getting confused w/ the phrase "stroker engine". To me this means you're using a crank w/ a longer throw. Doing this reduces or at best keeps the rod the same length w/ a better piston/ring package. The rod ratio would usually stay the same or actually decrease. Am I thinking this wrong?
You are right, the stroker engine does have a longer stroke, whereas a longer rod would be a long stroke engine. I hope the rest of what I posted makes sense either way. A stroker engine does make for a shorter rod/stroke ratio, it gets more power through larger displacement by increasing the stroke of the engine, usually keeping the same rods, just adjusting the piston pin higher on the piston.
Posted January 29, 2010 at 02:20 pm
A friend built a 440 into a 496 and he actually increased the rod length along with the stroke. In his case he went with a very expensive piston that has the pin very, very high. He wanted to get the rod geometry back to favorable territory.