Engine Masters rod ratio test results

[NORSK]

for the ones that watched it what was your take on it? Were you surprised the shorter rods made more power?

I have to ask if the intake and exhaust tracks as well as the cams matched to the individual rod ratios

Exactly. If the camshaft wasn't optimized for each rod length, The test is meaningless.

Yepp,the cam did suit the short rod better than the long rod,simple as that

[gmrocket]

What I found of interest was the difference in ignition timing requirements. The longer rod ran best in the 34-38* advance range and the short rod ran best in the 40-42* range. What got my attention and had me scratching my head is they said the long rod was insensitive to timing and ran the same from 34-38* btdc, so they went with 38*. Wouldn’t it have made more sense to go with 34-35*?

To me that's the biggest learning point of that particular test. That's huge and is telling something.

Needing 40+ Usually isn't a good thing

Didn't see the tests...what are the bsfc's for each and is there an average given or just peaks?

[Mikej26]

What I found of interest was the difference in ignition timing requirements. The longer rod ran best in the 34-38* advance range and the short rod ran best in the 40-42* range. What got my attention and had me scratching my head is they said the long rod was insensitive to timing and ran the same from 34-38* btdc, so they went with 38*. Wouldn’t it have made more sense to go with 34-35*?

To me that's the biggest learning point of that particular test. That's huge and is telling something.

Needing 40+ Usually isn't a good thing

Didn't see the tests...what are the bsfc's for each and is there an average given or just peaks?

They tested for that, but didn’t disclose the information. So either it was insignificantly different or they felt that presenting that info would be going too far into the weeds for the general audience.

[Tom Walker]

gmrocket, thought the same thing. Needing 40+ degrees is saying something!

[CamKing]

Didn't see the tests...what are the bsfc's for each and is there an average given or just peaks?

They tested for that, but didn’t disclose the information. So either it was insignificantly different or they felt that presenting that info would be going too far into the weeds for the general audience.

Or it would have shown the audience what was really going on, and screw up their narrative.

[Joe-71]

The only way to put this argument to rest is to build two identical engines and balance them perfectly. The ONLY difference is the rod length and piston pin height. Dyno test both engines for the same number of runs to the point of repeating three times, R&R the crank, rods, pistons and swap them to the other block, assemble, and run both engines again same number of dyno pulls to repeat three times. Compare the dyno sheets. The statistics will then be able to be compared. Needs to be engines that are mainstream, and making lots of horsepower. Would take two teams to tear down and reassemble, but could be possible. Lots of money for what? Start another argument. Joe-71

[CamKing]

The only way to put this argument to rest is to build two identical engines and balance them perfectly. The ONLY difference is the rod length and piston pin height. Dyno test both engines for the same number of runs to the point of repeating three times, R&R the crank, rods, pistons and swap them to the other block, assemble, and run both engines again same number of dyno pulls to repeat three times. Compare the dyno sheets. The statistics will then be able to be compared. Needs to be engines that are mainstream, and making lots of horsepower. Would take two teams to tear down and reassemble, but could be possible. Lots of money for what? Start another argument. Joe-71

Or how about using an inline 4 engine, change only the rod length, and use the same piston, by changing the deck height with the rod length(.200" longer rod gets a .200" taller deck height. Maybe use a double overhead cam setup, so you could move the intake and exhaust cams individually to optimize performance for both rod lengths. If only someone would have thought about doing that in the 1960's.

[digger]

I have to ask if the intake and exhaust tracks as well as the cams matched to the individual rod ratios

Exactly. If the camshaft wasn't optimized for each rod length, The test is meaningless.

Yepp,the cam did suit the short rod better than the long rod,simple as that

so what cam specs roughly speaking would have suited the longer rod?

[gmrocket]

The only way to put this argument to rest is to build two identical engines and balance them perfectly. The ONLY difference is the rod length and piston pin height. Dyno test both engines for the same number of runs to the point of repeating three times, R&R the crank, rods, pistons and swap them to the other block, assemble, and run both engines again same number of dyno pulls to repeat three times. Compare the dyno sheets. The statistics will then be able to be compared. Needs to be engines that are mainstream, and making lots of horsepower. Would take two teams to tear down and reassemble, but could be possible. Lots of money for what? Start another argument. Joe-71

Sometimes the dyno will lead you down the wrong path

If the car is quicker down the track and recovers better on each shift..is that good, even though on the dyno it wasn’t?

[kimosabi]

I couldnt care less if one makes slightly more power than the other. A longer rod makes for a happier engine over time. Angles. Pressure. Piston acceleration. Rod bearings. Lighter piston. Stuff like that. Minute stuff that can make a difference.

[dannobee]

I, like a lot of others, have been down that road chasing rod ratios. Fortunately, my "go to" engine guy ran Comp Eliminator and pretty much tried it all on the dyno. His response was very similar to all of the PS guys; it ain't worth much if anything.

With the restricted asphalt circle track cars, you'd be better off working the cylinder heads than the lower end. Especially when a set of undercover heads can net you 30hp and still pass tech.

[ClassAct]

Didn't see the tests...what are the bsfc's for each and is there an average given or just peaks?

They tested for that, but didn’t disclose the information. So either it was insignificantly different or they felt that presenting that info would be going too far into the weeds for the general audience.

Or it would have shown the audience what was really going on, and screw up their narrative.

Mike, do you have any thoughts on rod ratio affecting total timing, or maybe even requiring a change in the curve?

Some short rod circle track stuff I was doing in the late 1990's always wanted 42-44 total. If I built the same engine with a longer rod it might be 38-40.

Any thoughts??

[Mikej26]

Didn't see the tests...what are the bsfc's for each and is there an average given or just peaks?

They tested for that, but didn’t disclose the information. So either it was insignificantly different or they felt that presenting that info would be going too far into the weeds for the general audience.

Or it would have shown the audience what was really going on, and screw up their narrative.

True, but do you think they could properly interpret it?

[n2omike]

Mike, do you have any thoughts on rod ratio affecting total timing, or maybe even requiring a change in the curve?

Some short rod circle track stuff I was doing in the late 1990's always wanted 42-44 total. If I built the same engine with a longer rod it might be 38-40.

Any thoughts??

I agree that in all practical senses, rod ratio in a conventional engine is pretty much meaningless. It's not like you are going to be able to use a rod that is 2X or 3X as long as another one. We're talking 5.7" vs 6.0"..... or in the test 6.1" vs. 6.8". It's not that huge.

As for shorter rods wanting more ignition timing. I think that's pretty easy...
Engines need timing advance because it takes TIME for the flame kernel to propagate across the chamber and build pressure. This is a TIME thing... not necessarily a 'degree' thing. Short rods do not dwell along TDC as much as longer rods. Therefore, to get the same amount of TIME for the pressure to build, it needs to be started more degrees before TDC. A person could do the math to see what the difference would be.
That's my theory.

[Stan Weiss]

Maybe Mike can way in. Based on what I see the long rod might like a later exhaust opening.

All the testing we've done, shows that to be correct. The higher rod/stroke ratio like a later exhaust opening, but seems to like about the same exhaust closing.

so what cam specs roughly speaking would have suited the longer rod?

The exhaust was covered earlier.

Stan