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Maximize the stroke and bore, then make the compression height large enough to keep the wrist pin out of the oil groove

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Whats so bad about the pin in the oil ring?

It's just that you need an oil support ring. It's done all the time, but I like to keep the pin out of the oil ring just because I'm weird.

So how does the rod ratio affect your 60' it HAS to somehow!

That article written on the first post was'nt directed @ race engines , it was 'rod ratio & it's effects' in general , so i guess this is an open thread to discuss the subject in general.

A few years ago i made a mock up of a crank & two different sized rods using a dart board & a couple of rulers , i kept hearing over & over again from differing articles that a long rod motor would dwell for a longer period of time & TDC & a short rod @ BDC , this was'nt so , dwell time is EXACTLY the same for a short & long rod @ TDC/BDC , the only difference in the 360 degree cycle between the two different rod sizes was between 20 to 70 degrees before & after TDC & before & after BDC.

If i was building a motor for longevity the long rod would always win.

Some things just grate in my head if i don't fully understand it , hence the reason for the mock up , some may think i'm a bit funny in the head but i just like to see for myself how things work , this rod ratio issue had me real stumped thinking about it , just had to see it for myself.

when you have the pin in the oil ring, it increases the ability of the piston to flex about the center parallel to the pin... you loose integrity in respect to flexing within the ring grooves... for motors that make alot of power or torque this can be an issue but minimal

the effect of rod ratio on a motor is minimal in a .1 change,,, if you go from 1.5 to 1.8 it can start to add up,,

the rod ratio will dictate the size of the intake port given the stroke and operating range

one thing that gets me is when people say bb mopars like tight lobe seperation cams,,, no they dont,,, they dont know the lobe sep really,, all they know is the amount of overlap,, they like alot of overlap due to the high rod ratio and piston dwell about tdc so the added overlap helps draw in new charge where in a typical chevy motor the piston starts to go down in the bore creating a depression to pull in the charge.. so when you stroke a mopar,, you just made it a chevy..

what is "longevity"! Id bet that alone will have a million different answers! Personally I think rod ratio should be in the "theory" category

It is not theory any more beacuse dynos and tracks have proven over and over that a longer rod makes more power at high RPM and shorter rods make more tq at lower RPM. I think the only debates about this are people willing to trade in the rod ratio for more cubes via stroke? Personnaly I would rather have the extra stroke and a slightly worse rod ratio. The other debate is the oil ring in the pin hole, and in my opinion on a N/A motor I would not be afraid to do this to get a better rod ratio, within reasonable costs for the project at hand of course. Now if the rod ratio was really terrible and the rings were already in the pin I may trade a little stroke for a little rod length but not much because the extra cubes out weight the extra rod length unless you are getting a really bad side loading of the piston.

When I build an engine I like to stuff in the biggest bore first, then stroke as much as possible and use either budget, crankcase space, or max allowed cubic inches, as the limit for the stroke. Then I want to determine how long of a rod I can stuff in there by useing the shortest reasonable piston

this is all relative,,, from what rod length to what???

long rods do not make the most hp,, given a long rod,, say ratios of 1.9-2.0,, the cylinder head has to be too small to accomodate the dwell characteristics and the order in which the depression is created as the int valve comes off the seat,, now there all limitations..


ill set a example of a nhra pro stock engine,,,

500 cubic inches,, biggest bore, shortest stroke to get to 500,,

shorter stroke, less ring drag friction

bigger bore, more airflow capacity, larger valves

im not doing the math right now but we will say 3.625 stroke,,, 6.125 rod which is what most run

if we made the rod longer,, we would have to...

-block deck taller,, more weight
-block deck taller,, heads/valve face further from carb,, harder to get throttle bore pressure to act on runner openings due to being further away
-rod ratio higher,, cylinder filling suffers with current head due to low velocity at initial depression because ports are too big, make head smaller,, loose air flow,, loose power

-better piston loading into bore due to rod to crank angle


there are more pluses and minuses,, but im working so im not thinking too well..

our net result from a longer rod

+5hp from friction loss,,, yeah......

-40hp from cylinder head airflow loss, carb signal loss

net -35hp :-(


if you put a shorter rod in a ss hemi ill bet my life it will make more hp

Listen and learn, this man knows of what he speaks!

But In practical street real world terms, I would almost never trade the opp to get more cubes in the package to get a better rod ratio.(up to about 1.45:1 or so). Bigger isn't always better, but Bigger is always MORE!

There does come a practical limit where the rod angle is allowing the crank to try to shove the piston THROUGH the bore instead of UP it, but most mopars don't get anywhere near that.

Als oremember Rod ratio is really a misnomer (in terms of ratio): In reality the rod angle (the true ratio) is calculated from Rod C to C to only 1/2 the Stroke, not the full stroke

Great thread!

so who likes a 4.500 piston with a 4.250 stroke and a 6.760 rod

I'm sure Andy is checking in on this thread, Please clarify the following, Andy:

So we seem to be establishing that the longer rod motor, by virtue of its longer TDC dwell, will make slightly more torque.

So why,then, does Andy say in his book, when outlining the 451 combo, that he prefers the short rod version, because it will make more torque? Seems to me that the long Rod 451 would not only dwell at TDC longer, but would also offer a lighter rotating assembly (???)

Andy........

where are you trying to make this torque at?

if you are drag racing you want your torque curve kinda high,,, if its a street car,, in the middle,, if its a passenger car or tow vehicle,, kinda low..

now when you say it will make more torque dow low,, maybe thats 2500rpm on andy's motor,, but his tq peak is at 4000,, where andy thinks the shorter rod makes more tq,, saying it will make more torque down low doesnt usually mean it will make a higher tq peak on the motor


ill use a ss hemi reference again,,, cause thats what i run,, and i run the [Edited by Moparts - Family Friendly Site - Keep it clean] out of them on the dyno,, if i advance the cam youd think it would gain tq always once you are in your ideal position due to the intake valve closing earlier,, but i can advance the cam and loose tq,, maybe it made more at 5000, but when i peak at 7000,, it lost

get it??

I am sure I will put some noses out of joint in saying this, but I have not seen a shred of data that supports this "long rods always make more power" mentality shared by the often mis-informed. A couple of sayings I have heard that I believe hold true in the Racing Industry.
"Without Data you're just some guy with an opinion"
"If your data doesn't support your theory, you better get a new theory"

The posts made by Jeff, well worth reading.

I bought a 360 many years ago, built back in 1981. The owner/builder went to the trouble of cramming in a 6.625" BME rod with a csutom Venolia piston. All this to acheive some magic theoretical rod ratio.

I think in this day and age, any Engineer designing a new production engine is going to look at physical size and fuel efficiency over anything else.

Lastly, I'll third what Al Alguire said!

kinda like oil ever being the blame for engine failure.... never any "real" proof!

Notice I said thank you for the info to the original poster and then moved on to the important issue - piston speed - and as mentioned, lighter pistons are always better. Still a good info thread from all.

as long as the internet has wrong info posted all over it non informed people will go towards whatever is repeated more. I am tired of this fight as the non educated will believe what they want. As for me i have posted this many times give me the shortest package any day of the week it even works well in my boosted apps.
As for torque in a drag car? i'll just use more rear gear and more transmission gears.