Engine Masters rod ratio test results

[Walter R. Malik]

Below is post from retired Chrysler engineering Dave Zelkowski in regards to rod length.

He was in charge of the 4 cyl Dodge Stratus Engine in the Super Touring series.

Interesting is the lengths OEM's can go to in evaluating changes and testing procedures.

Thanks
Randy



I know in the old Stratus Super Touring Engine we placed zero importance on rod ratio. We talked about it but we couldn't find anyone with good credentials to give any credence to the theory, like Geoff Goddard from Walkinshaw Racing. We ran different rod lengths over the course of the three years we worked on the engine from 136mm to 146mm but these changes were only to accommodate different piston designs. In fact JanSpeed Engineering of the UK, who ran the Nissan super tourers, suggested SHORTER rods. There maybe something to the idea of longer piston dwell time at TDC but it was felt that it would only be worth something if we spun the engine to 10,000 rpm and we were dictated by rules to run no higher than 8500rpm.

Why does no one ever seem to bring-up the breathing effects of Longer piston dwell time near "Bottom Dead Center" ...?

[blackford]

If you ask me, I think the main advantage of longer rod is that you can use a shorter/lighter piston to help reduce rod stress and the longer rod lowers G forces a small amount also reducing rod stress.

[Walter R. Malik]

If you ask me, I think the main advantage of longer rod is that you can use a shorter/lighter piston to help reduce rod stress and the longer rod lowers G forces a small amount also reducing rod stress.

That is exactly true but, it has nothing to do with making horsepower ... the engines ability to breath is the horsepower maker.

[PackardV8]

... the engines ability to breath is the horsepower maker.

Agree, Walter, but the long rod guys will also tell us, it's the long stroke which is the torque maker. ;>)

[CGT]

Shorter rod has more dwell at BDC, I can see that as more of an advantage then more dwell at TDC of the longer rod. Regardless, I haven't seen the test...but it would have to be pretty damn intricate, complex, and precise for me to get on board with results in either direction. And I'm doubtful it was any of those things.

[digger]

anytime i've done sims in ENGMOD4T with a bunch of different combos the short rod always had more VE but it was a small difference not enough to worry about. there are some things that i can foresee that could make either one come out on top though.

its kind of hard to test the rod length alone in practical terms as there are other knock on effects that are probably just as significant.

[pdq67]

Shorter rod has more dwell at BDC, I can see that as more of an advantage then more dwell at TDC of the longer rod. Regardless, I haven't seen the test...but it would have to be pretty damn intricate, complex, and precise for me to get on board with results in either direction. And I'm doubtful it was any of those things.

Again, I guess I always thought a longer dwell at TDC due to a longer rod allowed more push time on the piston.

pdq67

[frnkeore]

Shorter rod has more dwell at BDC, I can see that as more of an advantage then more dwell at TDC of the longer rod. Regardless, I haven't seen the test...but it would have to be pretty damn intricate, complex, and precise for me to get on board with results in either direction. And I'm doubtful it was any of those things.

Again, I guess I always thought a longer dwell at TDC due to a longer rod allowed more push time on the piston.

pdq67

That was Smokey's theory, too but, I've always thought of it as a cylinder wall, friction reducer, as well as a stress reducer on the bottom end.

For me, a win/win for SBF's.

[Old School]

There has been sbc superstock 327 engines built for the auto trans classes with less than 5.7 rod length. With an auto trans the shorter rod was always faster. We changed the lobe separation, duration on both intake and exhaust, and lift trying to get the 6 inch rod to better the 5.7. Tried 4 cams and nothing would run with the 5.7 in a 350.

Never ran a stick shift so I have experience there.

[Stan Weiss]

Shorter rod has more dwell at BDC, I can see that as more of an advantage then more dwell at TDC of the longer rod. Regardless, I haven't seen the test...but it would have to be pretty damn intricate, complex, and precise for me to get on board with results in either direction. And I'm doubtful it was any of those things.

Again, I guess I always thought a longer dwell at TDC due to a longer rod allowed more push time on the piston.

pdq67

Just think about that. ATDC the crank journal is straight up the rod is straight up. All pushing on the piston does is to try and push the crankshaft out of the block.

Stan

[Tom Walker]

Long rod or short, most of the time the difference I believe to be insignificant as a power producer.
Randy, your thoughts about dwell at B.D.C. for the longer rod are not talked about as often and I have wondered why.

[digger]

Long rod or short, most of the time the difference I believe to be insignificant as a power producer.
Randy, your thoughts about dwell at B.D.C. for the longer rod are not talked about as often and I have wondered why.

most people dont know it, thats probably why

[Walter R. Malik]

Long rod or short, most of the time the difference I believe to be insignificant as a power producer.
Randy, your thoughts about dwell at B.D.C. for the longer rod are not talked about as often and I have wondered why.

The longer the rod means the less time its dwell will be near bottom dead center. That is the part of the filling cycle where most of the inertia cylinder filling gets done as the intake mixture is moving at high speed and the piston is moving slowly; comparatively.

Some extra added connecting rod durability and very little less wall friction vs. some extra added cylinder filling and slightly less main web stress is the compromise that needs to be made for what is most important to THAT engine build and the parts in it.

It is a very small "seesaw" but, it is there.

[Tom Walker]

Totally agree with your points and observations Randy, very interesting though why B.D.C. time is not talked about with as much fervor as T.D.C. time as it relates to effects of rod ratio.
By the way, congratulations on your performance at the E.M.C. I saw you in Hot Rod magazine in 3 different pictures. Very cool.
Always appreciated our conversations at the R.E.C. Your experience and passion for this kind of thing is impressive.

[Walter R. Malik]

Totally agree with your points and observations Randy, very interesting though why B.D.C. time is not talked about with as much fervor as T.D.C. time as it relates to effects of rod ratio.
By the way, congratulations on your performance at the E.M.C. I saw you in Hot Rod magazine in 3 different pictures. Very cool.
Always appreciated our conversations at the R.E.C. Your experience and passion for this kind of thing is impressive.

Back when breaking rods was a big issue is probably when that long rod talk started becoming gospel to some.

I gave those 3 guys some help with their entries so, I was on THEIR team.