Hey all you Engineers and head porters I'm trying to get some good info on the effect that rod/stroke ratio has on intake runner volume.
Is it true that the smaller the r/s ratio the larger the intake runner volume needs to be because of the piston speed moving away from tdc?
I have a sbc 421 AFR 220's carbed, procharged and intercooled which will be getting a refresh and I need to know so that I can get my heads done correctly btw my r/s ratio is 1.51.

 

C'mon someone has to know the answer to this one, where's Jordon Musser or Chuck Riddick when you need them...:

 

Two threads and 3 posts and no answer to this...... You might want to check how often the guys you mention post.

Now on to the question.

Where would you get the correlation between R/S ratio and intake runner volume?

So many other things are envolved in just those two things. R/S ratio is in your case not much of a option. With a 9.000" deck height and 1" compression height pistons you only have so much room to stuff a rod and half the stroke in there. So there is a extremely small range to work in as far as the ratio goes, which by the way means about zip.

Picking the Intake runner volume has variables in it also. Runner length, min cross sectional area, head flow etc.... all play a part.

I find this correlation between arbitrary numbers in engine design interesting, to me there is no connection. If there is it has escaped me for years.

I would stop barking up a tree with no cat in it, that might be why nobody has hissed back at you, because there is no cat in this tree.

Things like HP level, RPM range, cubes, runner length can give you a target runner volume or min cross sectional diameter. R/S ratio IMHO doesn't effect anything.

We've disscussed rod length on this forum a couple of times, there are alot of things that it slightly effects but runner volume has never been one of them.

Not jumping on your back here, but where did this whole idea stem from?

Bret

 

This came from an article I read in muscle mustag and fast ford where Crawford Racing built a 350 sbf base on 8.2 deck 302 block3.5 stroke and 5.4 rods and they stated that during their r&d they were playing around with different combos of stoke rod length etc and found with the shorter r/s ratios the engines made the most power when they increased the intake runner volume from what would normally be considered correct for the ci of the engine. They felt this was due to the piston speed of the shorter r/s ratio moving the piston away from tdc faster and pulling harder on the intake port and as rpm increases there is less time for cylinder filling.

 

Yes, they are correct.

A shorter r/s ratio will accelerate off of tdc faster and in turn the air outside will sense the pressure differential at a greater rate and in turn will get moving to fill the void faster.

In turn, if there is a restriction in the intake system (be it cylinder heads, intake manifold, restrictor plate, throttle body, etc) a shorter r/s ratio will multiply that restriction. Whereas a longer r/s ratio in a restricted application will accelerate off of tdc slower giving the air a little more time to get moving in the direction it needs to go.

Also, if you have large runners for your cubic inches, a shorter r/s ratio will help.

This is a big part of why we use up to a 6.250 long rod in a restrictor plate Winston Cup motor and as short as a 5.200 rod in a Pro Stock truck motor with the same stroke crankshaft and cubic inches.

Hope this helps...

 

Thanks Compairflow the theory seemed to make sense to me but the other respondant tried to make me seem like some sort of dumb***. Ok so on my 421 with 1.51 r/s ratio I can go much larger than 220 intake runner volume ( up to a point) and not cause myself any problems.

THANKS so much for your informed answer I guess we all can learn something we didn't know can't we Bret.

 

With that many cubic inches especially procharged, you can get away with a larger than 220 cc runner 23 degree SBC. In a blown application, You can worry less about port volume and its effects on port velocity. Port Volume affects airspeed throughout the port somewhat indirectly. There are many othe contributing factors that IMO are as important.

Being it is a blown application, you are not relying on strictly atmosphere to push the charge through the port. You are going to build a positive boost pressure higher than atmospheric and that will build velocity.

Now, dont get me wrong, I believe that you want to have as an effecient a port as possible, Or as small a volume / cross-sectional area for the CFM the head flows especially on a normally aspirated street car with small cubic inches.

For your application, A port volume of around 240-245 and around 330 Cfm would be about right.

Email me if you want to get a little more in depth as to your specific application.....

 

Thanks again CompAirflow I will do just that once I get the engine out of the car and disassembled, those figures sound just about what I was thinking, but it's good to know my thinking wasn't flawed.

 

O.k. so the tests were more of a question of changing the rod length than the R/S ratio.

I would think that the testing is flawed here when it can only effectively take into account one of the two parts of the equation. This isin't a R/S ratio thing, but a rod length question.

In that test they didn't change the stroke/bore combo for the same cubes to get the R/S ratio different, you just said they changed the rod length. (i.e. 3.50" stroke) Which is my point, we are talking about rod length on the effect of intake runner size. You are really confusing what is going on here with taking the rod length and interchanging it with the R/S ratio. The reason being that conclusive evidence when you are changing the stroke in the same test and maintaning the same cubes means that your friction numbers will be less with the larger bore. Therefore you can't say that with X cubes this R/S ratio is going to produce this effect, because the R/S ratio is different. The only thing you can change in that test (as long as it's a slow RPM/sec dyno pull to negate the effects of the heavier and lighter pistons with different rods) is the rod length. You could change the B/S ratio for the same cubes and not change the rod and see the same or different results. So we should look at this from a rod length standpoint not a R/S ratio. That doesn't make sense to change only one variable and say this is why this is.

Now if they did the testing and changed the bore size/stroke length and kept cubes the same. Say a 4.00 x 3.50 = 351 cube engine and a 4.150 x 3.25 = 351 cube engine. Both with a 5.400" rod, we get a 1.54 R/S ratio for the long stroke engine and 1.66 R/S ratio for the short stroke engine. When you find that the shorter R/S ratio setup likes more cylinder head volume/min cross sectional diameter you have to look at that closer. Remember this, we have to go into something else here for a second.

Now I have 2 cylinder heads, one is 215cc one is 230cc, both based on the same casting (or not doesn't matter). To make conslusive evidence that the additonal runner volume/min cross sectional area is needed for the shorter R/S ratio engine you need to look at the flow charts too. My guess is that adding 5-15cc is going to get a different shaped curve. Possibly less flow at the lower lifts and more in the uppers for example. Now we have two different cylinder heads here.

Now take the different bore/stroke ratios from above, couple that with the dyno results and the different cylinder heads. They say that the shorter R/S ratio engine (which if you change the rod lengths is most likely going to be the longer stroke engine) likes more port volume. Did it really like that or did it like the different shaped flow curve. The valve in the large bore engine would be less shrouded so there would be more flow from that head, where the longer stroke (shorter R/S ratio) would be more shrouded because of the smaller bore and probably could use more flow. As Mr Horsepower says a proper cylinder head is devloped for a particular engine combo. My guess is that they got lucky with that finding, the new larger port liked that rod length and bore size, along with that flow and rpm range.

Basically, in the real world you probably can never do this test and come up with these results in any accurate manner. Well at least draw these conclusions from it. They did find something but it's not the R/S ratio vs Port Volume relationship.

On top of that this "They felt this was due to the piston speed of the shorter r/s ratio moving the piston away from tdc faster and pulling harder on the intake port and as rpm increases there is less time for cylinder filling." If you ever take a degree wheel and have a long rod (therefore a longer r/s ratio) you'll see that the longer rod dwells longer at TDC and pulls away faster from there to mid stroke. The longer dwell time causes that. It only has 90 deg to get from TDC to half stroke, which from there the piston is decelerating, also at a faster rate than a short rod engine.

Now if you change the stroke, the piston speed will be much faster in the longer stroke engine, which in turn is the shorter R/S engine. The stroke is going to raise the average piston speed and the max piston speed. Which in turn means that the piston has to accelerate from 0 to that higher velocity at a faster rate since it still has to do it in the same 90 deg of rotation.

I'd have to do the math to see exaclty if there is a r/s ratio and piston acceration rate correlation. It seems to me that a 1.5" stroke with a 3" rod and a 3.0" stroke and a 6.0" rod both with 2.0" r/s ratios will have different piston speeds and acceration rates.

R/S ratio has a ton more to do with dwell time than either component individually. Why, well in the same deck height (9.000 for a std SBC) you can't get a 4.000" stroke and a 6.500" rod. You can get a 6" rod with a 4.0" stroke, but with a 3.0" stroke you can get a 6.5" rod. So that's the difference between a 2.0 r/s ratio and a 1.5 r/s ratio. It's all what you can fit into the parts you are given.

I'm not saying I can't learn anything on here, but that relationship seems extremely odd. Basically it says that I can run a larger port volume if I shorten the rods on a given engine. I'll also have to deck the block unless I want to gain a ton of piston weight. Taking a common JE piston example going from a 5.700" rod to a 6.250" rod means I gain 21-26% in my piston weight. You can't just deck any block even a Bowtie .550" You need a special block like the Pro Stock Truck guys used.

Now going from a 6.250" Rod in a restrictor plate 6000rpm Winston Cup motor which moves about 300rpm in 45 sec worth of lap to a 5.200" Rod in a 10,000rpm Pro Stock Truck motor which has more carb than it needs and accelerates at unbeliveable rpm/sec rates. Hmmmmmmm I would think that the weight you save with the smaller rods would help engine acceration rates. Here again was the example of changing the rod length and assuming that the r/s ratio is the reasoning why it's changed. Yes I'm from the Smokey Yunick School that you "put the longest dam rod in a engine that you can."

While we are on restrictor plate engines, makes you wonder if you are only given x amount of air and if less cubes pulling on that amount would be an advantage. I wouldn't be supprised to see a DEI restrictor plate engine come out to 350 cubes rather than 358.

With 421 cubes, and a blower 220cc is extremely small. In that circumstance you could find power by just increaseing the port volume with the same flow, and not small amounts, but significant amounts. 240-245cc could even be small, but since it's a street car it's not a bad place to start, I agree there. 400cubes and 195cc is way to small on boost. The 240-245 is the right ball park. Even if it was a NA application the atmosphere is not doing most of the pushing on the air, it's the intake tuning pulses that are doing most of it. 1 PSI vs up to 7 PSI is extremely different. I'm not from the school anymore that the smallest cross sectional area I can get for the same flow is always the best. It's the right size that counts. On a big power NA application I would start with around 235cc of port volume with that many cubes, and that would probably be small. Going with boost on top of that would need even a larger port size. Probably something that a 23deg head couldn't get too, even though I have heard of some Pro Actions getting up to 275-285cc.

Mr. Z28 73/97, you are asking a good question. It's just the wrong one was my orginal point. You really need to get closer to the right size based on all the other factors than the R/S ratio one. As I said I can learn something on here, never implied that I couldn't. I just needed more info to answer the question correctly. We still don't know your target operating range for the engine, boost level, HP level etc......... CompAirflow and I agree on some of these things, but on some we don't. That's what you find on here alot. We should talk about it on here, not in private. Then nobody will learn anything.

Bret

 

No Bret go back and read my second post on this thread, Crawford was changing both the stroke length and the rod length trying different r/s ratios to get the info I was referring to.

 

Read my full post, I got to the differing bores and strokes too.

Bret

 

You think you could find that article on the net for us? Maybe scan it or copy it, something. I'd like to read it to get all the details.

 

Brett,

Yes, I am saying, that for any certain combination, FIXED BORE AND STROKE, any time you shorten the rod, you will shorten the R/S ratio. I am not saying that is the only and best way to increase piston speed and make more power with the scenario we are speaking about, but it is one way.

Now, If you increase the piston speed from TDC it will create the pressure differential at a greater rate and therefore a less restrictive intake system will benefit that attribute. (larger cc's)

I was not saying that instead of switching rod length, you change stroke/bore/rod length.

The only reason that I brought up the Winston cup restrictor plate vs Pro stock truck is because of the fact that they are very similar in the bore/stroke/cyl head cc/cylinder head flow. And yes, the decks are a lot shorter on the pro stock truck block than a winston cup block, but Why is this.....Rod length. Yes, the lighter the rotating assembly is, the faster the acceleration rate will be. BUT, LETS LOOK AT WHATS REALLY GOING ON HERE. The answer is airflow. Put restrictor plates on a pro stock truck motor and lets see how that affects the acceleration rate vs the weight issue. Most of the pro stock truck motors and Restrictor plate motors that I have built have very similar compression heights. Normally around 1.050 on a truck motor and 1.150 on a restrictor plate. Even more....The bobweights not considering the rods are also comparable.

I can assure you that DEI's restrictor plate motors do not come out at 350 ci. Last I heard (earlier this season) they were running a 4.190 bore, 3.240 storke motor.

I was only stating that IMHO, The shorter the rod/stroke relationship in a FIXED BORE/STROKE application I have always seen a larger runner work better even with the cylinder heads having a very similar flow curve and CFM Rate.

On the 2 different cylinder heads issue, they may or may not have different flow rates. Obviously, if there is a better flow curve for the application in 1 set it is going to be a better match. So with Mr. Horsepower I agree that you have to have a cylinder head that works with the application, In fact, this is how I make a living.

You are correct that you do not have to have the smallest cc/ Cross sectional measurement per cfm in all out race motors. I do believe that though for the street as there are a lot of other requirements that have to be met on a street car that do not exist on a race only application.

On the Smokey Yunick thing, Back when Smokey was writing those books and was at the leading front of race engine technology, He was without a doubt absolutely correct. This is not the case today. The biggest reason for this is again, Cylinder head technology. Look at what he had to work with back then, compared to what we have today. Back then, everything was restricted as far as airflow is concerned. So I would absolutely agree in most applications, Yes even our Camaros and such is something that I would consider restricted. But, when you go to a venue such as pro stock, pro stock truck, comp eliminator, Winston cup with the cylinder heads and intake systems that you see today, I absolutely disagree with that statement.

Pro-Action, Yes, they are getting HUGE Port volumes in these heads, I have done about 5 sets of the BIG BIG 14* heads and normally end up with a 390-395 cc port volume with about 435cfm@.900@28". In fact, I have NHRA Comp Eliminator record with one set in a Jr. Fuel car. But, another thing to look at Brett is the fuel they are running...Alcohol. You have double the volume of fuel going down the runner and have to compensate for that. Most of the classes that are running the pro action type cylinder heads are burning alcohol. Typical SB2 Winston Cup ports are right about 274-276 cc runners and go just above 400CFM@.800@28". Pro stock Truck around 375-380 cc and flow415-430@.900-1.000. Pro stock car around 470-480cc runners and 550CFM. Now, I am not saying that runner volume is everything, I think that is obvious from my earlier post. I am also not saying that in these Ultra high levels of racing does the CFM always correlate to horsepower. I have many times seen heads that flow less make more power and run better at the track, and that is all due to port design. Now, not to say that on a street car, is that going to be the case because 99.999% of the time if a head flows more air it will make more power. The Pro stock we are talking about is a totally different animal.

As you can probably tell, I am a Big Bore, Short stroke, short rod, lightweight components, Short deck, Great cylinder heads and intake, and turn as much RPM as you possibly can person.

I know that it simply states in physics, "the output of a motor is only limited by the rpm that it can turn" and I totally agree.

I didnt mean to come off like I think you took my first posts. I have always just read, and never posted. I just saw Mr. Z28's question and no replys for days and then your post and I thought that I would add and try to help this guy out. I also agree that we should talk about this on here. That is why I posted to begin with. I just was saying to Mr. Z28 that if he needed any help on the specifics, I could help.

Yes Brett, We do agree on some things, but not on everything and dont take any offense to anything I have said, I am not here to attack anybody, but rather to teach if I can, and also learn what I can. I am not a block guy, I deal with application-cylinder head port-camshaft-components. It has been quite a while since I have looked at some of this stuff so....
Sorry it took so long to post back, been really busy,

-Nick

 

Okay, could you simplify this for me?
The basic idea Mr. Z28 73/97 is looking to verify is that all other things being equal, engines with shorter rods benefit from larger port volumes. Correct?
Since a shorter rod accelerates the piston away from top or bottom center more quickly it would have lower piston speed in the middle of the stroke than a longer rod. Is this correct?
It seems to me that lower peak piston speed will require less port volume to fill the cylinder. Why is my thinking wrong here?
Why would increasing piston speed in itself make more power? Or does this have to with airflow being most effective in mid stroke (where the valve is at max lift)?

 

Nick,

Now this has become a good thread.

I'm going to get into more things, but I wanna think about them a little more. It's interesting how head guys think about this, and how I think about this. Interesting differences.

What's in my head now.

1. The way I see it a longer rod will have a faster acceleration from TDC to 1/2 Stroke, even though piston speed is going to max out at 1/2 stroke and that is 100% determined by the stroke length. If we just wanted to increase piston speed, then we need to take bore out of it, therefore add stroke. So I don't understand the statement of increasing piston speed with a shorter rod. The piston acceleration will be lower and the speed will max out at the same velocity with a shorter rod and the same stroke. On the other hand a shorter rod will have a more gradual pull on the air, which as a fluid is not going to work like anything mechanical because it doesn't just start and stop. The short rod Pro Stock Truck motors most likely had alot of duration so a less aggressive pull by the piston would help with that. At least that's what I'm thinking.


2. Bobweights, we can consider it without the rods but that's not really what's going on. All the differences in the packaging have more to do with weights and stiffnesses. I would think that in the NASCAR rules there is a minimum deck height for the blocks by now too. If not then the short track engines are cut down like PST are, just to get the mass of the components down.

It's a good talk and I don't take offense to any of this. The golden rule on here is that there is alot of knowledge and with that some ego so we all get a little uptight sometimes, or it just seems like that. This is a good topic.

BudM,

"The basic idea Mr. Z28 73/97 is looking to verify is that all other things being equal, engines with shorter rods benefit from larger port volumes. Correct?"

Nick is suggesting this, I'm on the other side of this. I don't see the correlation between the 2, yet. I don't think that there is a rule of if I shorten my con rod I can run a larger port.

"Since a shorter rod accelerates the piston away from top or bottom center more quickly it would have lower piston speed in the middle of the stroke than a longer rod. Is this correct?"

The piston speed in the middle of the stroke will be = for a given stroke. The only way it will go faster is to increase the stroke, which to both of us is a bad thing.

"It seems to me that lower peak piston speed will require less port volume to fill the cylinder. Why is my thinking wrong here?"

With a equal cube engine and having a longer stroke you will get more piston speed and therefore could use a smaller intake runner. I'm going to dig into some stuff first to see if that's right but it seems to me if there is a relationship, it might be there. My rule though on this is that no matter how I get the displacement the optimal size/min cross section for the displacement is going to be the same. (But this can change) I have a long drive today so I'll have something to think about.

"Why would increasing piston speed in itself make more power? Or does this have to with airflow being most effective in mid stroke (where the valve is at max lift)?"

I don't think increasing piston speed would in itself make more power, another thing to think about. RPM will make more power if you can get the parts to work up there. That would mean all else being equal that a longer stroke would make more power/TQ on just this thought alone. Not accounting for the longer arm/mechanical advantage theory, and having the head shrouded more. Air is a fluid and it doesn't react with imediate start/stop reactions like mechanical things do.

Get back to this later,

Bret