Rod ratio and dwell time... doesn't make sense

[Belgian1979]

David,

If there were no limits with regards to engine rpm (not indicated in the above) and airflow as it would perfectly follow the piston, any engine according to you would rev to the moon, which isn't true and you know it.

The volumetric efficiency decreases with increase of rpm and is dependant on Ma (mass of air inducted), p inlet (inlet air density) and V (volume displaced) in the formula VE = 2xMa/(p*V*N) so this uses mass as a basis. If your piston moves you have to move the mass of that cfm number you arrived at which is the theoretical inhaled air and if the air is forced in with the same pressure the Ma will have to drop as I explained above. This explains why you can not rev an engine to the moon.

The difference between acceleration created in which I assume 90° being the point with the highest speed would be (and based on the above number)
A. 17545 m/s²
B. 16.227 m/s²

So the airmass has roughly to accelerate 8,1% faster.

Assuming that the force by which the mass is drawn/pushed into the cylinder remains the same for both, the mass of air would have to loose 8.1% in density.

[groberts101]

Unless something's changed more recently?.. that seems to be the trend used by nascar on various tracks. Superspeedways with big sweepers get shorter stroke/longer rod type deals compared to shorter tracks sharper turns getting longer strokes/shorter rods.

It's also been said that the cylinder head and induction requirements can change slightly due to the way the piston is drawing on it. Makes me wonder how the cylinder heads might change between the short and long tracks too.

correct and the reasons are imo that you have better breathing capabilities of that same engine in terms of how much mass of air it can cram into the cylinder.. Which in its basic form boils down to how many air molecules you end up with in your cilinder during the intake stroke.

No difference in bore or stroke between tracks.

Thanks for the reply. Must be oldschool tech or lower class racing I still have stuck in my head.

[groberts101]

David,

If there were no limits with regards to engine rpm (not indicated in the above) and airflow as it would perfectly follow the piston, any engine according to you would rev to the moon, which isn't true and you know it.

The volumetric efficiency decreases with increase of rpm and is dependant on Ma (mass of air inducted), p inlet (inlet air density) and V (volume displaced) in the formula VE = 2xMa/(p*V*N) so this uses mass as a basis. If your piston moves you have to move the mass of that cfm number you arrived at which is the theoretical inhaled air and if the air is forced in with the same pressure the Ma will have to drop as I explained above. This explains why you can not rev an engine to the moon.

The difference between acceleration created in which I assume 90° being the point with the highest speed would be (and based on the above number)
A. 17545 m/s²
B. 16.227 m/s²

So the airmass has roughly to accelerate 8,1% faster.

Assuming that the force by which the mass is drawn/pushed into the cylinder remains the same for both, the mass of air would have to loose 8.1% in density.

Right, IMO. It has been said many times, and by many a pro racer/builder far smarter than I, that a short rod ratio motor with higher and more quickly achieved rod angularity(whether it be the result of a longer rod or simply via increased stroke length for same rod length) has the ability to pull harder in the earlier phase of the induction stroke which can allow the head port to liven up more quickly earlier on in the induction stroke. Which in turn allows for slightly bigger induction sizing without killing off or trading away lower speed torque production.

And on the flipside, a more restricted induction setup can benefit from slightly reduced induction and head port sizing, primarily to speed up the velocity for better overcoming the slightly reduced lag incurred from the longer rod ratios lazier early induction phase reaction. Or used as crutch for more limited sizing architectures. Think restrictor plated motors and/or 2bbl class racing with OEM style manifolds and cylinder head ports.

So, static cylinder volume(calculated demand) is only one portion of the equation and we must also factor in the starting and stopping speeds of the very sticky mass as well. I'm not implying that one should build around a magic rod ratio number, only that it can and has been used as another tuning tool for specific applications, is all.

[Belgian1979]

By no means there is a fixed good and fixed bad stroke/rod ratio. In effect due to the pistons being at different locations you cannot even keep valve events at the same moment. So any comparison made is at best an estimate.

[Stan Weiss]

David,

If there were no limits with regards to engine rpm (not indicated in the above) and airflow as it would perfectly follow the piston, any engine according to you would rev to the moon, which isn't true and you know it.

The volumetric efficiency decreases with increase of rpm and is dependant on Ma (mass of air inducted), p inlet (inlet air density) and V (volume displaced) in the formula VE = 2xMa/(p*V*N) so this uses mass as a basis. If your piston moves you have to move the mass of that cfm number you arrived at which is the theoretical inhaled air and if the air is forced in with the same pressure the Ma will have to drop as I explained above. This explains why you can not rev an engine to the moon.

The difference between acceleration created in which I assume 90° being the point with the highest speed would be (and based on the above number)
A. 17545 m/s²
B. 16.227 m/s²

So the airmass has roughly to accelerate 8,1% faster.

Assuming that the force by which the mass is drawn/pushed into the cylinder remains the same for both, the mass of air would have to loose 8.1% in density.

So if they both had the same parts and one made 600 HP then based on the difference in air density the other would make 550 HP.

Stan

[groberts101]

An older quote from Darin Morgan... http://www.superchevy.com/how-to/projec ... ding-tips/

Rod Length

"Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero difference in average power and a zero difference in the shape of the horse-power curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length."

I also think we need to keep this in perspective given that these results may not be completely and directly related towards an "average designed" OEM based motor with much thicker rings and considerably larger skirt/bore contact area ratios. Even changing cylinder head and induction designs as well as cam spec's can move results around one way or the other as well. People tend to fixate on and overgeneralize soleley one aspect when it should be based on the system of parts involved for any specific application. Which is why I sincerely dislike and distrust full blanket statements.

[naukkis79]

So if they both had the same parts and one made 600 HP then based on the difference in air density the other would make 550 HP.

No, it means that shorter rod motor in that example has 8.1% more air demand from tdc to 90 degree crank angle. It's about head airflow capability whether that is converted to higher volumetric efficiency or just more pumping losses. And that piston speed isn't only about air demand, at expansion phase it's about combustion - if combustion is slow higher piston speed at tdc will lower average effective pressure - to make low rod engines to work you need both high flow and fast burn heads.

[Warp Speed]

An older quote from Darin Morgan... http://www.superchevy.com/how-to/projec ... ding-tips/

Rod Length

"Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero difference in average power and a zero difference in the shape of the horse-power curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length."

I also think we need to keep this in perspective given that these results may not be completely and directly related towards an "average designed" OEM based motor with much thicker rings and considerably larger skirt/bore contact area ratios. Even changing cylinder head and induction designs as well as cam spec's can move results around one way or the other as well. People tend to fixate on and overgeneralize soleley one aspect when it should be based on the system of parts involved for any specific application. Which is why I sincerely dislike and distrust full blanket statements.

Power difference with a .100 change in rod length is measurable in a Cup engine.

[user-30257]

An older quote from Darin Morgan... http://www.superchevy.com/how-to/projec ... ding-tips/

Rod Length

"Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero difference in average power and a zero difference in the shape of the horse-power curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length."

I also think we need to keep this in perspective given that these results may not be completely and directly related towards an "average designed" OEM based motor with much thicker rings and considerably larger skirt/bore contact area ratios. Even changing cylinder head and induction designs as well as cam spec's can move results around one way or the other as well. People tend to fixate on and overgeneralize soleley one aspect when it should be based on the system of parts involved for any specific application. Which is why I sincerely dislike and distrust full blanket statements.

You do know some oem have .9mm. 9mm 2mm. Right? Most oem now have thinner ring packs and better designed pistons than most sportsmans have in their engines. So much for blanket statements though.

[Stan Weiss]

There have been a number of OEM auto and motorcycle engines produced with 2:1 or greater rod to stroke ratios.

Then we could talk about race engines
1998 v10 3.0l Ferrari F1 engine
91.5 mm bore, 45.6 mm stroke, 110.0 mm rod, 2.41 rod to stroke ratio, 2.01 bore to stroke ratio.

Stan

[groberts101]

Power difference with a .100 change in rod length is measurable in a Cup engine.

Thanks for that contribution. Seems to be the consensus for other lower classed roundy round racers as well.

Can you elaborate further? Maybe what impacts on the induction and cam requirements as well?

Thanks,

Greg

[groberts101]

You do know some oem have .9mm. 9mm 2mm. Right? Most oem now have thinner ring packs and better designed pistons than most sportsmans have in their engines. So much for blanket statements though.

yep I do. And some of those motors are also supercharged too!

maybe I'm not as smart as you dumb I am.. but my overview of most things engine related ain't that narrow minded either. Not your average bolt together "internet guru" engine builder and never was or will be. Always trying to improve on things built around production lines and cost cutting measures no matter what its intended usage model may be. Also not afraid to cut things up and break out the welder either.

[Stan Weiss]

So if they both had the same parts and one made 600 HP then based on the difference in air density the other would make 550 HP.

No, it means that shorter rod motor in that example has 8.1% more air demand from tdc to 90 degree crank angle. It's about head airflow capability whether that is converted to higher volumetric efficiency or just more pumping losses. And that piston speed isn't only about air demand, at expansion phase it's about combustion - if combustion is slow higher piston speed at tdc will lower average effective pressure - to make low rod engines to work you need both high flow and fast burn heads.

Why should I care about part of the cycles air flow? Should not what matters be the trapped air / fuel mass at IVC?

Since the two engines in this example have close to the same volume above the piston at each degree of crank rotation Can you please explain this. One will have greater piston surface area while the other has longer stroke / lever arm. Generated force should be almost identical.

Stan

[ptuomov]

Power difference with a .100 change in rod length is measurable in a Cup engine.

Just because you're very good at measuring those difference precisely in statistical sense doesn't in itself mean the differences are significant in terms of magnitudes!

[Scotthatch]

Stan Weiss your comments in this thread are great

In the two same displacement engine example wouldn't the change in stroke change the dwell time more then the shorter rod ?

Also if you increase the demand on the head airflow at peak piston velocity what you will see is a lower peak rpm .... more torque about the same HP

With just changing the rod length and taking the extra height out of the piston I have seen no difference HP ...though at times I like that the piston is lighter ....

My last thought is that in today's world we tend to add so much more stroke to our builds that it's less about rod ratio and more about what fits .... so the question becomes what else should I change to deal with the change rod ratio ? Camshaft? Clearance?