Yes, I was the one that wanted to discuss flow separation as it relates to
the topic (Very intense reply BTW).

I notice when reading certain posts that a select few tend to believe the
changes made will enhance performance across the spectrum at any given time.

Maybe a 2:1 R/S ratio is better for Engine A between 4000-6000 RPM, where
Engine B will see little, or no change in the same area.

Runner lengths, pulse mixing, RPM , etc. will send waves in and out of phase
constantly which makes it impossible to benefit in every case.

I wish I had the time and money to experiment with motors like some of you
lucky guys. It would make all of this research much easier to understand.

 

Bret, this is a rod length discussion. Are you saying fuel separation from solution occurs because of rod length or rod/stroke ratio?

CamKing, are you saying that a change from a 1.64 to 1.76 rod/stroke ratio with a 3.48 stroke can cause a use 4° smaller cam with a 2° smaller LSA? Is this moving the IVC 4° earlier? If so, is because of the piston speeds at IVC, where the longer rod can be faster?

And going packed to the choked flow, were you saying a choked flow can be induced by high piston speeds?

 

Just to clarify David, I snuck that question in because of the material I have
been reading. It seemed fitting because of the topics coming up during discussion.

Around the point of sonic choke, i have read that you must think outside of
the box to make things happen. Whether it be injector placement closer to the valve to keep the port "dry', or use a slower air speed to promote higher
cylinder filling.

The R/S ratio doesn't have any direct effect on fuel separation as far as I'm
aware. Port shape, surface finish, air speed, fuel type, atomization are the most common
factors from what I've read.

 

Its a valid topic...although CamKing is an authoriative source saying there is a direct effect on valve timing and power curves from rod length (for a fixed stroke). Then choked flow--sonic flow that is no longer dependent on the pressure differential--was mentioned. Followed by a discussion about whether an intake port has sonic flow (I never heard of any part of the intake being sonic, the exhaust is aways sonic when the valve opens). After 3 pages of discussing that rod length didn't matter, I was hoping this topic would be shed light on what affects it does have.

 

Bret, I agree and thats why I am never very concerned with rod ratio anymore. Plus I am around and talk to professional engine builders all the time that can find no real differences when developing an engine around R/S ratio. I've seen pro stock truck pistons too short and they made less power and then they put a better piston back in with the shorter rod and picked up power all the way through 10K rpm. It wasn't the rod ratio going down but that the piston was better, stiffer and sealed up better.

I agree though with Camking that with some heads that are borderline insufficient or that have bad short turns that the longer rods sometimes have made more power in the past or at least even good engine builders say they have. Nowadays though with the crazy heads we have I don't think the rod length matters much anymore. I know engine builders that are worried about having too long a rod more than too short so I know they aren't worried either.

 

The intake flow does not have to become sonic to choke up. The power starts to drop on most engines after a certain average intake velocity usually 300 ft per sec or so in that ballpark. The intake velocity is almost directly tied to piston speed especially when the cylinder is really being filled. Air is not very dense and can react extremely easily to pressure drop. It's not exactly linked like a string of course but there's not the huge lag that some think! What Camking was saying is that instantaneous port speed gets higher with a lower rod to stroke ratio and it does so with a smallish head or a bad port the shorter rod can put the port in stall sooner.

 

He said choked flow...a very specific condition that is related to sonic flow. That's what causes it. His statement was very specific: 355 cid, 5.7 vs 6.125 rod, 4˜ smaller, 2° smaller LSA, and 4500 ft/min average piston speed which would be around 8000-8100ft/min peak. Those aren't the specs for some 1200hp Pro Stock motor, or 19k F1 engine, that is just a strong motor.

If that small a change could have that noticable a change in the cam...I'd like to understand the details.

 

On paper it seems it would have some effect. If the dwell at TDC and BDC
change, I would expect to see IVC change to reflect...possibly close slightly
sooner.

I would also expect to see EVC happen a little later, and IVO happen sooner.
At higher RPM, this might be something a tuner might grind to help prolong
the scavenging period. Along with that, tune the exhaust to keep the exhaust
port at a lower pressure/ Intake higher pressure during overlap within that RPM band.

It would be nice to take one motor and swap just the rods while keeping a
similar profile piston to maintain consistency in weight as much as possible.

From there, dyno test at steady (not accelerated) RPM intervals and measure pressure profiles accordingly.

 

As was pointed out previously, constructing a number of engines with only rod length changes and not piston, deck height, pushrod length, runner length. etc. changes would be nearly impossible. However, with a good engine simulator, you can do just as you suggest.

I ran a strong 355 SBC (600+ hp @ 6500 and 500+ lb-ft @5200) thru EA Pro with rod lengths from 5.0 to 7.5 inches. This gave R/S from 1.44 to 2.16. The results were unsurprising (to me, anyway):

Max torque: 2 lb-ft difference with 5.0 rod showing best and 7.0 worst.

Torque @ 3600 (the typical "hole" in the torque curve of this type of engine) varied 16 lb-ft (~$4.3%) with the 7.5 inch rod best and 5.0 rod worst. This differnce virtually disappeared by 4300 rpm.

Max hp: 2 hp difference with the max around 6.2 inch rod length.

At 7000, past the power peak, there was 7 hp difference with the 7.5 inch rod best, and the 5.0 worst.

In the 4500-7000 rpm designed operating range of this engine, there was almost no difference (0.3% torque and 0.6% hp) with rod lengths from 5.7 to 6.3, the practical range for this engine configuration.

Note that this engine was initially optimized with valve events, intake and exhaust tuning for a 6.0 rod. None of the other parameters was changed during the "rod run".

There's only one engine shop I am familiar with that can measure internal cylinder pressures in real time on a running race engine, and I suspect they have done just about what your suggested. The admit to running about a R/S ratio of just about 1.9 in their 358 cube 9700 rpm Cup engines.

 

The instantaneous gas speed through the port is further complicated by pressure waves present in the port. When the intake valve first opens, a negative pressure wave is created that travels from the valve, up the runner toward the bellmouth opening in the manifold. To make it easier, I'll call a wave travelling away from the valve as going to the left and a wave traveling toward the valve and combustion chamber as going to the right. A negative pressure wave(negative relative to the gas medium it's travelling in-an expansion wave) travelling to the left, will accelerate gas particles to the right. A positive pressure wave(compression wave) travelling to the right will propel gas particles toward the right. So you could have a theoretical gas speed calculated in the port that's sub sonic, but after several waves have passed through the gas, that calculated speed is no longer correct. In fact, at some spots, the theoretical gas speed might go sonic. This is why a port will choke at a lower gas speed than the calculations say it will.

The worst case is when an expansion wave travelling to the left, meets a compression wave travelling to the right. Both waves accelerate the gas to the right. When they meet, superposition takes place and the local pressure will go to near zero. However the combined energy of the two waves might propel the gas particles to sonic or near sonic.

Al

 

????
It's been done over and over again for over 40 years.
I've done it, and my dad did it long before I was born.
You just change the rod, and the pin height in the piston.
My dad first did this on Offy engines. Since they were overhead cams, the deck height could be changed and the same piston could be used.

 

I can tell they aren't working in Indycar of F1.
Rod lenght is still very important in all forms of racing, but the rules may change the lenght of rod needed.
In Cup, you have a max bore size rule that also limits haw small the stroke can be, and a deck height limit. They now also have a gear rule limit that keeps the engine RPM's down.
These rules keep you from building the optimum engine, and you have to start changing specs as a bandaid to make it work better withing the rules.
This is also why they can't run their better heads. The gear rule won't allow the engine to get to the RPM's where the better heads would help.
Before the Max bore, and Gear rule, the engines needed the longest rods they could get. Back when penske was running Fords, they ran a 700 mile test at Charlotte at over 10,000rpm.

 

Forget about the choke flow for now.
Look at flow caused by the pressure differential from the piston moving down.
If you graph out the valve opening curves(use a 240-270@.050" profile), and the piston motion and velocity of both a 1.638:1, and a 1.76:1 rod/stroke ratio, you should see what's happening.
Look at where the longer rod hits max velocity compared to the cam lift.

 

Is an expansion wave as you discussed in the second paragraph the same thing as a negative pressure wave as you discussed in the first paragraph?

I've tried to understand this phenomenon a few times and think i've got a general idea of what's going on here. But that's why i'm getting ready to ask the following questions and am looking for input.

Essentially when the intake valve opens the negative pressure wave journeys to the left (or expansion wave if we're on the same page, let me know if not). This negative pressure wave traveling to the left induces the charge to move to the right toward the combustion chamber. Now, when the negative pressure wave travels all the way to the end of the intake runner and meets the plenum it's my understanding that a posative pressure wave now is enduced and moves to the right from the plenum back down to the right further aiding in moving the charge to the right? Once the posative wave moves all the way to the open valve it once again reflects a negative wave back to the left after passing into the combustion chamber and the cycle continues until the valve closes. Once the valve closes a posative pressure builds in front of the valve and will aid in moving the mixture to the right toward the valve and into the combustion chamber when the valve opens and the negative wave will reflect to the left and once again the cycle continues.

Is this in fact correct or am I playing left out in baseball, ha

Also, as the cycles continue to keep on going within the same IVO and IVC period doesn't the intensity of the negative pressure wave and posative pressure wave start to fall off?

 

I'm just going to throw out some comments, questions and maybe some challenges.
No hitting below the belt, let the flood gates open!


Yes, agreed and I read Bret's post concerning the difficulties when swapping rods.

I'm not aware of how an overhead cam engine changes deck height without
sacrificing quench height as I have never built an OHC motor from top to bottom.

Is there a link to show a picture, or diagram of how this is accomplished?

Can this be done without ICL, or without a lobe profile? Even if we had the
valve motion plotted against piston movement, this doesn't show how the
air/fuel mixture reacts through the port and enters the cylinder. It also doesn't
show pressure differentials within the cylinder and port at a specific RPM.

I think you're getting it for the most part. I say that with much respect as
I'm learning just as much.

If we can break down one cycle of a single cylinder engine (single runner),
this is what I see happening in point form.

- Engine off. No waves. Pressure = atmospheric

(start engine, piston on intake stroke, intake valve is opening)

- Piston moves downward, produces negative pressure in cylinder

- Air within intake runner moves into cylinder

- Piston reaches BDC, intake air still has momentum and may continue
to fill the cylinder if pressure at BDC allows

- Intake valve closes, intake air moving toward valve reflects back toward plenum

- Time passes...compression, power and exhaust stroke (forget overlap period)

- Intake valve opens again and piston is going downward.

* If the negative wave crest has reached the end of the intake runner at the
correct time, the depression created will begin to move air down the runner
as the piston is moving down the bore. This would be a best case scenario
and would typically improve VE.

* If the negative wave has not been timed properly, it may begin to move the
intake air too soon, or too late according to the pressure located at the valve
opening during the start of the intake stroke.

This may cause air in the runner to continue moving toward the plenum as
the piston is moving downward lowering VE.

 

F1 was exactly where I WAS talking about. Dropping the V10 from 72 degrees to wider allowed the cylinder heads to also come in as they were basically hitting each other at 72 degrees and the deck and rods could be shortened quite a bit. This made more power as well on all engine dynos and on the track. Of course the reciprocating weight was also reduced as well. I also talked to several engineers about the Aurora engine in IRL and they really complained that the deck and rod length was way too long and that hurt the power overall.

Also if there was any real horsepower gain from rod length at all you wouldn't see 8.200 deck height pro stock truck engines either. I don't think that anyone runs Ford engine's now at 9.200 deck either do they in cup? I think they all use shorter rods now. Two NASCAR guys told me the same thing that the long rods are just heavier and weaker. I know there is a weird acceleration curve when going really low on R/S but it doesn't seem to even affect 24 hour engines. I used to believe it was a real engine building parameter but no one in high end engine building has ever even mentioned it.

I interviewed several large engine builders and asked them to make a list of the top ten things they would look at when evaluating an engine design on paper to see what kind of power it could make. This included one Formula One engine designer but I got lucky and know some people there and he did it for me. The interesting thing is that not one of the 8 guys I interviewed even listed Rod / Stroke ratio at all. I am trying to write this book and am getting permission to quote all of them and so far it looks very good.

I should say though that many people do believe it affects the engine slightly as Camking is saying especially in the final cam decisions after testing etc as extremely long VS. short rod engines will differ a little. If you shorten the rods a lot and do nothing else you will see a cranking compression increase alone so it does matter but when optimizing an engine and accounting for everything I couldn't find anyone that seemed to think it mattered so far and I've talked to some people that almost have unlimited budgets.

 

CamKing,

I don't know if Rod Length is VERY IMPORTANT like you state but it can have small effects on the system. I've beaten this topic to death for the last 5 years with lots of guys, I still don't think we are going to see anything significant where most of us would give a rats ***. FWIW I'm with about everyone else on this, you can't change it in most motors without changing something else that will have an effect on the output of the motor. You can believe that there are these dead set rules in motors, but i've seen more times that something you think will do one thing actually does another. The best that can be done with rod length is to crutch a small intake port with a longer rod. If the port is too big a shorter rod will make more power, all depends on what you are looking for here. I'm building a 6 1/8" rod 355 now, but it's more about the dewell time and the lighter assembly to stress the crank less at the RPM it will be running at for extened times. Good luck with trying to convince everyone of your thoughts though.

DAPSUPRSLO,

Al's comments on the waves are very good. What you are tring to do with the waves is get them to work the way you want them too in the RPM range that you are running in. If you ever look at the Engine Masters motors and the power curves most have a large dip in the 3000rpm area. This is mostly due to the wave pulses going on in the intake and exhuast doing the wrong thing, that's what he is getting at with a statement like this:

"The worst case is when an expansion wave travelling to the left, meets a compression wave travelling to the right. Both waves accelerate the gas to the right. When they meet, superposition takes place and the local pressure will go to near zero."

Now the other side of it is "However the combined energy of the two waves might propel the gas particles to sonic or near sonic." This is one time when a larger cross section thru the port is more helpfull in making power.

FWIW the most intense pressure pulse period is towards IVC, if you think about it that's when it has to be because you need to build more pressure in the port than there is in the cylinder to have the charge travel from the port to the cylinder.

Eric,

"Air is not very dense and can react extremely easily to pressure drop. It's not exactly linked like a string of course but there's not the huge lag that some think! "

Well the faster you try and spin the motor the wider the seperation in crank degs of peak piston veloicty is and peak port velocity. Makes sense because the charge only has so much time and the faster the crank spins the less time it takes per deg of rotation, the inertia of the air/fuel doesn't change so it reacts at the same speed. FWIW I think this is the dog you want to beat on in terms of which one **** on the carpet.

Bret

 

Forgot to ask, but Adrenaline reminded me here....

So where are you changing the duration? Seated?, .020", .050", .200"? Are you keeping the same nose of the cam and changing the ramp and flank? Are you changing the accelerations over the nose? Is the lobe area increasing or decreasing? Obviously you are keeping the overlap the same, but changing the IVC and EVO points, the problem is you can't just change those and keep the same things happening everywhere else. The valve motion and control will change if you keep the lobe area the same, or the valve motion will stay the same if you keep the same shape but the lobe area will decrease....

Nothing is just one change here, many other things happen.

Bret

 

Yes. A compression wave is pressure wave; an expansion wave is a negative pressure wave. Waves will always refelct when there is a large change in the density of the medium they are travelling in. So if a compression wave, say caused by your voice, were travelling along and hit the side of a cliff, the rock cliff has a greater density than the air the wave is travelling in, causing the wave to be reflected back as an echo. Any wave hitting a more dense medium will reflect back as the same wave. Compression reflects back as compression, expansion as expansion. However, waves can also reflect if they hit a less dense medium. Say you're standing in a fog and cause a compression wave. The wave will travel to the edge of the fog; the air at the edge of the fog, being a less dense medium than the fog itself, will cause the wave to be reflected back. This is why you sometimes hear an echo in fog. Any wave hitting a less dense medium will reflect back as the opposite of itself. A compression wave relfects an expansion wave and vice versa.

In an intake runner, an expansion wave travelling to the left, when it hits the bellmouth(or any increase in runner cross section), it will cause the wave to reflect back as a compression wave because the wave sees the increase in volume as a decrease in the density of the medium it's travelling in.

As long as an expansion wave doesn't hit a closed intake valve, or a compression wave hit the bellmouth, the waves will help propel air down the runner. A compression wave propels air in the direction it's travelling. An expansion wave propels air opposite the direction it's travelling. Note, that this works favorably in a exhaust header, too. The amount of energy that the waves have to move air compared to the piston is small though. If air is ever moving up the runner toward the plenum, it is most likely because the piston is pushing it(low rpm, late intake closing), not because of wave action. The point I was trying to make was that even though you can calculate what the speed of the air in the intake ought to be based from piston speed, the actual speed will most likely be higher because of the effects of wave action. This may cause a port to "choke" even though the calculated speed is less than sonic.

Al

 

That is the best and most concise explanation of wave tuning i've seen thus far. thanks a bunch man