Engine Masters rod ratio test results

[digger]

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

that describes more what is the difference between the requirements of long vs short rod motor, not how do you know that the cam was better suited one vs the other.

i ran some sims in ENGMOD4T with what limited info i had, and for two vastly different cams it showed the same trend as the test that the long rod rod has better VE at lower rpm and at higher rpm the shorter rod made more VE but only a few numbers. the trend was the same with both cams. the average joe is not going to try several cams a degree or two here and there with no other changes to optimise things

[modok]

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.

Indeed true. better to test on a four cylinder or less, for so many reasons.

[RCJ]

We've tested the heck out of different rod lengths in limited class circle track engines. There are a ton of circle track classes that limit you to 358ci, stock stroke, a compression limit, and unported production heads.
In these classes, Rod length is one of the few things that can be changed, so it's a great place to learn about the effects of rod length. One thing we've proven, as the rod/stroke ratio goes up, the engine wants an earlier intake valve closing point, for the same rpm power band. If you run a cam designed for the shorter rod/stroke ratio, with the longer rod, the intake closing is too late, and it'll hurt the power.
When we cam the engines correctly for the different rod lengths, we normally see the power between peak torque and peak hp to be very close, with the shorter rod making more power below peak torque, and the longer rod, carrying the power farther past peak hp.

What made you try or tipped you off that it needed a earlier closing point?

[motormonkey]

Longer rod probably likes later exhaust opening , erlier opening due to dwell time in his testing.
Just want to say thanks to guys Camking Maxrace and Stan for sharing the math.

[Stan Weiss]

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

that describes more what is the difference between the requirements of long vs short rod motor, not how do you know that the cam was better suited one vs the other.

i ran some sims in ENGMOD4T with what limited info i had, and for two vastly different cams it showed the same trend as the test that the long rod rod has better VE at lower rpm and at higher rpm the shorter rod made more VE but only a few numbers. the trend was the same with both cams. the average joe is not going to try several cams a degree or two here and there with no other changes to optimise things

How many different discussions have there been here about how do you select the right cam for a pile of parts? Has there ever been an answer that everyone agreed on?

I do not have ENGMOD4T or I would run the simulations my self. Optimize the cam for a 4.00" bore, 3.25" stroke, 5" rod. Then change to a 6.5" rod and just change the EVO to 4 to 5 degrees later and what happens to the results?

Maybe Mike will way in here again. For the above long rod simulation I thing that the IVC needs to be a couple of degrees earlier.

So overlap is staying the same, while both duration are being shorten exhaust more than intake. Center lines are changed.

Stan

[CamKing]

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?

Normally the higher the rod/stroke ratio, the shorter the seat duration needs to be for both the intake and exhaust profiles, and the intake needs to close earlier, and the exhaust needs to open later. Intake opening and exhaust closing don't change much, if at all.
For example, if you had a had a 12:1 358" SBC, that came off the corner at 4,500rpm, and turned 7,200rpm at the end of the straights, with a 5.7 rod, you may run a 296/300 @ Seat with a 102 ICL and a 110 ECL. If you were switching over to a 6.125" rod in that same engine and gearing, you would go to something around a 292/296 @ Seat, with a 100 ICL and 108 ECL.

[frnkeore]

What would be the estimate of HP lost, using the wrong cam, for the either rod/stroke ratio?

[gmrocket]

Every time I see/hear/read one of these shows/articles that go about testing stuff I get the critical feeling that ratings and controversy are more important and cleverly hidden inside illegitimate ABA type testing. It’s really hard to take any of them serious when something kinda important is glossed over because it doesn’t fit a narrative.

After watching all of their shows and listening to their reasoning I get the idea that they are still doing mostly 1990's magazine articles, for example in this one Frieburger has wanted to do this test since 1997 and he seems oblivious to the fact that this has been hammered to death, both in discussion and testing and that just changing rod length is an invalid test. So I sent him a link to googlesearch/speedtalk/rod length/rod ratio.

They do have some cool stuff on dyno testing basic stuff that the pubic would encounter but they don't seem to take into account what has bee learned over the past decade. I also understand that they probably have a lot of money tied up in one 20 minute dyno video publication.

At the beginning of the video did you notice he mixed up the long rod and short rod every time he pointed too each? Didn’t get it right once

If a total beginner was watching they would be saying,, what the heck??

[gmrocket]

I kinda thought the slight power difference may have something to do with when each was at the point of faster or slower piston movement.

The faster movement doesn’t just happen, another cylinder on its power stroke is being hurt by that...however slight it may be.

When a rod and piston is moving faster, it’s like the rest of the engine suddenly sees a heavier part to move..that should take more power to do that work.

We all know it’s doenst take much muscle to rotate a crank when we have one piston&rod in it around TDC and BDC..it’s the before and after that takes some grunt because of the rod angularity and if your Turing it the same speed , the piston picks up speed and friction

So the faster movement during mid stroke should hurt power being produced by the other cylinders..they are working harder to move whatever piston is mid stroke, and to a lesser degree moving faster ,less dwell time at TDC or BDC

Stan or one of the smart guys can figure out by time, at a particular RPM, which set up spends more time at faster piston movement .

Or maybe not

[Stan Weiss]

For any given combination no matter what the rod length is it will have the same mean piston speed.

Using the same 5" and 6.5" rod as an earlier post. The short rod will have a higher peak piston velocity of about 116 FPM than the longer rod and this will be about 3.2 degrees sooner ATDC than the longer rod.

Bore = 4.0 Stroke = 3.25 Rod Length = 5.0 RPM = 6500
Piston Speed is 3520.833 Feet per Minute
Maximum Piston Velocity 5817.885 FPM @ 73.50556 Degrees


Bore = 4.0 Stroke = 3.25 Rod Length = 6.5 RPM = 6500
Piston Speed is 3520.833 Feet per Minute
Maximum Piston Velocity 5701.309 FPM @ 76.72098 Degrees

Stan

ab-pv-5-65.gif

[digger]

How many different discussions have there been here about how do you select the right cam for a pile of parts? Has there ever been an answer that everyone agreed on?

Stan exactly,

It seems like people have preconceived notions about all this and assume the short rod setup was at an advantage because they like longer rods and didnt like the outcome lol.

i don't disagree that to optimize both setups a different cam is required, the question is which will come out on top and why after optimisation. is it always going to be one or on some applications will the other. why in this case did the short rod make a little more at top end and a little less at the bottom end, what are the physical reasons for this irrespective of the combination being right or wrong.

[digger]

why would a piston dwelling at and around BDC make more power? simply because prior to that the piston was moving faster and the "ramming" is improved?

[GerryP]

...
At the beginning of the video did you notice he mixed up the long rod and short rod every time he pointed too each? Didn’t get it right once

If a total beginner was watching they would be saying,, what the heck??

That's not all that unusual in production situations since you're concentrating on the script and the presentation. It does drive you crazy when you notice it. This shows how much brain power communication takes and why you don't notice an angry boyfriend coming at you with a knife when you're talking to his main squeeze.

One of the car shows -maybe it was Horsepower TV- used a supercharger animated graphic. The rotors were turning the wrong way. The supercharger would be pulling air out of the engine. Hard to build up a head of steam with you don't have any air. It was like that for probably six months or so.

[GARY C]

Every time I see/hear/read one of these shows/articles that go about testing stuff I get the critical feeling that ratings and controversy are more important and cleverly hidden inside illegitimate ABA type testing. It’s really hard to take any of them serious when something kinda important is glossed over because it doesn’t fit a narrative.

After watching all of their shows and listening to their reasoning I get the idea that they are still doing mostly 1990's magazine articles, for example in this one Frieburger has wanted to do this test since 1997 and he seems oblivious to the fact that this has been hammered to death, both in discussion and testing and that just changing rod length is an invalid test. So I sent him a link to googlesearch/speedtalk/rod length/rod ratio.

They do have some cool stuff on dyno testing basic stuff that the pubic would encounter but they don't seem to take into account what has bee learned over the past decade. I also understand that they probably have a lot of money tied up in one 20 minute dyno video publication.

At the beginning of the video did you notice he mixed up the long rod and short rod every time he pointed too each? Didn’t get it right once

If a total beginner was watching they would be saying,, what the heck??

Yeah I kept wondering if it was the camera angle because it looked like he was pointing to the wrong one.

[GARY C]

You guys should love tomorrows useless EMC test.

"Picking a fight between Edelbrock, Summit, and Demon carburetors! Stream an all-new episode of Engine Masters TOMORROW on the MotorTrend OnDemand App!"