Some good info here, but I'm curious about something else. If we assume a given displacement, won't the big bore engine be trying to bring in more air per degree crank revolution than the long stroke engine? Common sense tells me that it would be, which brings me to ponder this; would the ability of a street car's (marginal) heads would be better matched to the larger stroke, in spite of the valve shrouding... which isn't that big of a deal on a bench to begin with.

Of course, on a race motor we push the valves outboard on a big bore engine, but the kind of budget it takes to build serious engines is out of the question for the average street guys. The place I work has a dyno and a bench, all I need is somebody to pay for back-to-back buildups. Actually, I'm going to build a big bore engine over the winter with 14* heads. I'd love to try this theory out, but our old motors are all 23* small bore stuff.

Then again, this is all IMHO, as I haven't ever popped an engine in a dyno competition and therefore have less experience than one other guy here . <-- Just a friendly ribbing.

 

Probably more than one guy

 

There are some things to be said about airflow per crank degree, but in the same block you aren't going to find any big difference because there is just not that much room to change the displacement combos that much.

Valve shrouding is a huge deal. A wider combustion area will greatly deshroud the valves even if they are left at the stock spacing apart. If you take a ported LT1 head and flow it on a 4.00" bore vs a 4.155" bore you will see flow improvements. Now take that same head and change the chamber shape to match the larger bore and help the flow, there is even more flow from the cylinder head.

The big difference is not due to the rate of the pistons movement, or the larger mechanical advantage of the stroke in the RPM ranges we are talking about, it's in AIRFLOW INCREASES because of the larger bore. Period.

Look at those AFR flow results in that last thread. The 4.250" bore vs. the 4.500" bore numbers at the bottom, that's the gain from THE SAME CYLINDER HEAD! That's pretty dam big!

Bret

 

Oldsstroker,
Lets look at piston speed
RPM seems to be more the limiting factor as far as stress goes from the view I have come to see it from.
Lets say you have two engines at the same piston speed one has a stroke of 3.25" and one has a stroke of 4" and piston speed = 5000 ft/min

5000 ft/min x 6 / 3.25 = 9230 rpm

5000 ft/min x 6 / 4 = 7500 rpm

Each piston will probably weigh close to the same because I am considering the bore and compression height constant and varying the rod length

So now we have two pistons that weigh about the same running at the same piston speed and each one will change directions twice each revolution.

Now the short stroke engine is turning 23% more rpm and changing directions 23% more times per minute. So even if the acceleration and deceleration forces were the same the fatigue of the parts would be about 23% more

You must also consider the acceleration and deceleration forces.
The short stroke engine is going from a dead stop to 5000 fpm to a dead stop in 23% less space.

Imagine the G-forces in your car if you reached 0-100-0 in the 1/4 mile versus 0-100-0 in the 1/8 mile. The distance change here is 50% but you get the point.

As far as the friction thing goes Im not saying the big bore short stroke engine will make less power. Big bore short stroke engines will always make more power per cubic inch.

Lets say that the 3.25" stroke at 7500 rpm would equal friction of 1. And the 4" stroke engine at 7500 rpm would equal friction 1.23
if we increase the rpm of the short stroke to 9230 rpm the friction will go up by the square of the 23% increase in rpm. So the overall friction will go from 1 at 7500 to 1.51 at 9230.

Friction increases by the square of rpm increase.
Double the rpm and quadruple the friction.

Now as for the gear ratio changes lowering the rear end gear ratio will multiply the torque to a higher level but decrease wheel speed.
With any gear box there are inefficiencies to deal with. You can not transfer power through a gearbox without losing power. No amount of gear multiplication can get that back. Power out can not exceed power in.
So if you input a certain amount of torque from the engine at a given rpm you can multiply that torque to the wheels at the same engine rpm while reducing the wheel rpm. You now should have more torque but the same overall power because you are applying that torque at a slower rate.

Also when you change to a lower gear you are increasing the step rate of the engine along with the required rpm at the lights to have the same trap speed.

Two good examples are engine and chassis dynos.
When you increase the step rate of an engine on the engine dyno you will see power losses as the step rate is increased. These inertial losses can to some degree be lessened with lighter internals but there will still be some loss.
Also on a chassis dyno when you dyno a car in a lower gear than direct drive or when you put a lower gear in the same car you will always show less power to the wheels.
You also need to consider that with a lower geared rear end running the same trap speed you will be spinning your gears in the transmission and rear end faster and slightly increasing the frictional losses through them

This is why if you have two engines making the same power in exactly the same car but one engine is making peak power at 6000 rpm and the other is making peak power at 9000 rpm the lower rpm engine will be faster due to the fact that it can run a more efficient drivetrain and suffer less inertial losses.

 

Let's look at a 383 for a minute and do a real world example of Bore vs Stroke.

We're not talking about power anymore here, but bottom end durabilty with these bore stroke combos. Power comes from the top end, and the higher head flow from a non-shrouded valve will make more power IMHO.

4.030 bore, 3.75" stroke

RPM/Piston Speed/Piston G's
7500/4688/3930
8000/5000/4470

4.155" bore, 3.525" stroke

RPM/Piston Speed/Piston G's
7500/4406/3620
8000/4700/4120
8500/4994/4650

Now the piston speed at the same RPM is less. So are the piston G's. For a given Piston Speed the smaller stroke has a higher piston G's.

Now one thing you can't do is say that the pistons are all the same weight and size. If we are trying to compare things, equal cubes are important. If we had a 1.125" Compression Height on these pistons then we just have a 6" rod on the 3.75" stroke and a 6.125" rod on the 3.525" stroke.

Looking at the JE catalog we have equal flat top pistons there for us. a 4.030" bore, 1.125" Comp Height piston weights 386g. A 4.155" bore 1.125" Comp Height piston weights 421g.

If we use a Carrillo Rod, then we have the rotating and recipricating portions of the rods here http://www.carrilloind.com/03bro3.pdf

a 6.000" H beam has 195g of recipricating weight
a 6.125" H beam has 194g of recipricating weight

Lets say we have 118g pins, 45g of Rings & oil rail supports, and 8g's of pin locks.

So if we say 7500rpm limit on motor, we can find the Force the piston puts on the rod and the crank.

F=MA,
F=M(Piston+Pin+Locks+Recipricating Weight of Rod IN Kg)A(G's x 9.81 m/s^2)

4.030x3.750

.752Kg x 3930 x 9.81m/s^2 =28992.08 N or 6517 lbs of F

4.155x3.525

.786Kg x 3620 x 9.81m/s^2 =27912.59 N or 6274 lbs of F

That's 3.8% less Force on the crank at the same RPM. That to me says that for a given displacement the larger bore wins.

Now if we look at friction HP at the same RPM, if we have the same rings, bearings and skirt designs then we have esentially the same motor. The longer stroke motor will have more Friction HP. @ 7500 RPM we are only looking at about 6HP more in the long stroke engine.

Another win for the large bore combo.

This relationship doesn't increase at the square of the speed of the motor because there are parts that produce friction at a exponential rate and some that produce it closer to a linear rate. Common thinking would be that the friction HP would increase at the square, as Old SStroker said it is exponetial but not at the square of the RPM.

You'll find that the friction HP increases with RPM exponentially, but not at the square(2). It's closer to 1.7 on a street motor and 1.6 on a race motor.

For example on a 600hp 383, you are going to find the friction HP look more like this.

2000 RPM = 14 Friction HP
4000 RPM = 40 Friction HP
8000 RPM = 130 Friction HP

if it was squared then it would be more like

2000 RPM = 14 Friction HP
4000 RPM = 56 Friction HP
8000 RPM = 224 Friction HP

One more thing. Since we can turn our motor faster with the short stroke, Why do we want more RPM? Well the obvious answer is HP. Or you can look at it like this.

Motors are basically limited in TQ productin per cube thru physics and technology. So we end up with a max lbs ft per cube of around 1.6 for a endurance NA engine. It's not the rule but it's a good place to start.

So say I have a 500 ft lbs 310 cube Trans Am motor. I can make that TQ at 5000rpm or I can make it at 7500rpm. If my motor that makes that 500 lbs ft @ 5000 needs a 3.42 gear I have 1710 lbs ft at the rear wheels, if I drop in my 500 lbs ft motor @ 7500rpm then I have a 5.13 rear gear to achive the same speeds. The higher gear gives me 2565 lbs ft at the rear wheels. That's a pretty big improvement.

Now in reality the extra friciton HP from the higher RPM will hurt my TQ output, the faster acceleration rate of the motor will hurt my TQ output and the higher driveline losses spinning everything 50% faster (ahead of the ring and pinion) will make that 2565 lbs ft much lower, but it will still be more TQ at the rear wheels.

That to me is why high end race motors in F1, Pro Stock and Winston Cup, like larger bores, shorter strokes and more RPM. Even limited RPM classes and rules like the larger bore for a few more reasons.

The only problem with a larger bore is $. Stroking is the budget way to go. A large bore small stroke motor can turn much higher, so that in itself is going to add cost to your motor.

Bret

 

wow.

 

You all probably don't remember a long-ago Advanced Tech thread where I asked "between two motors producing the same torque, which will be faster, one producing the torque at higher or at lower rpm?" Bret just answered that, and much much more! Great post, thanks.

Rich Krause

 

Well that's going to take me a couple more cups of coffee to digest.

Great thread.....it's amazing the things you learn (or attempt to) on this board.

 

Two things to add to that.

1. You have to add the other portions of recipricating mass to the F=MA forumula because using the piston mass by itself will throw the numbers off, and in that example the heavier piston will have more force. I used recipricating mass for that example, but since the highest pistons G's occur at TDC and BDC you could count the whole connecting rod, but that didn't seem as pure of an example.

2. If you look at the example above, when the piston speeds are the same they are obviously at different RPM. At the same piston speed you see that the piston G's are higher in the big bore/short stroke motor. That is because the g's are a function of RPM and Piston Speed.

One thing that is interesting is the piston G's of a Winston Cup Motor. If they used off the shelf parts those things would never live. They use very light parts in those large bore short stroke motors.

Piston 4.180" around 300g
Pin, shorter and smaller around than street stuff, around 75g
Locks, 3g
Rods, They are limited down to 525g, so the recipricating mass there is light let's say 150g
Rings, narrow rings don't just cause less drag but have less weight too. So around 35g for rings.

563g vs the 780g on the normal stuff.

The piston speed of a Winston Cup motor at 9500 is about 5162ft/min, the piston g's are 5260.

.563g x 5260 x 9.81 = 29051 N = 6530 lbs of F

That's a little bit more than what average 383 will be doing a 7500rpm.

You'd be hard pressed to get 850hp NA out of a 383 @ 7500 RPM. So you gotta lighten up the parts, which costs alot. Just the pins for a WC motor are around $100 a peice. Your not even paying that for a piston/pin combo per hole in a street motor.

Bret

 

bret-
are you saying the pistons are accelerating at 5260 g's? i'm getting lost just following this, but g's as in grams and g's as in acceleration are confusing me

:::back to writing a paper::::

 

Yeah the piston g's are that high.

All those g's are pretty confusing.

Bret

 

Just wanted to respond to a few points

Now one thing you can't do is say that the pistons are all the same weight and size. If we are trying to compare things, equal cubes are important. If we had a 1.125" Compression Height on these pistons then we just have a 6" rod on the 3.75" stroke and a 6.125" rod on the 3.525" stroke.

In my example I was attempting to show the difference between long and short stroke engines at the same piston speed with the same bore to keep the recip weight as close to the same as possible.

That's 3.8% less Force on the crank at the same RPM. That to me says that for a given displacement the larger bore wins.

Thats true if you are looking at it with the same rpm's but the larger bore engine will breathe better so you need to compare piston speeds. With a larger bore, shorter stroke and better head flow you will probably be able to run close to the same piston speed that the small bore engine's heads will support.
That is why I was trying to use the same piston speed in my example because the same head on the same bore will only support so much piston speed before it overspeeds the air in the port. Changing the bore size and head flow complicates things more. And besides if the two engines were the same cubic inches at the same rpm with the same BSFC they would be making the same power. And since no one would make the same cubic inches with a short stroke big bore and not spin it harder to take advantage of the better airflow your force and friction comparisons arent really applicable.

You'll find that the friction HP increases with RPM exponentially, but not at the square(2). It's closer to 1.7 on a street motor and 1.6 on a race motor.

Friction and Inertial forces increase at the square of speed increase. Thats a fact.

One more thing. Since we can turn our motor faster with the short stroke, Why do we want more RPM? Well the obvious answer is HP. Or you can look at it like this. Motors are basically limited in TQ productin per cube thru physics and technology. So we end up with a max lbs ft per cube of around 1.6 for a endurance NA engine. It's not the rule but it's a good place to start.

I agree completely..

So say I have a 500 ft lbs 310 cube Trans Am motor. I can make that TQ at 5000rpm or I can make it at 7500rpm. If my motor that makes that 500 lbs ft @ 5000 needs a 3.42 gear I have 1710 lbs ft at the rear wheels, if I drop in my 500 lbs ft motor @ 7500rpm then I have a 5.13 rear gear to achive the same speeds. The higher gear gives me 2565 lbs ft at the rear wheels. That's a pretty big improvement.

If your motor makes 500 ft/lbs at 5000rpm thats 476 hp.
If it makes 500 ft/lbs at 7500 rpm thats 714 hp.
Sorry but thats a pretty poor example. Of course the higher hp car will go faster. But thats because of power not torque. As far as racing is concerned Torque may as well not exist.
Despite what some people say Torque has never won a race.
What is torque without rpm? Potential for work.
If it is not applied there is no work being done.
All that matters in a race is rear wheel power. I can have a little lawnmower engine and if I gear it right I will have 5000 ft/lbs of torque. But is it really getting much work done? No. Because it will be turning such a low rpm that it will hardly be moving.
So I dont see how rear wheel torque can be used as an example without the rpm of the wheels. And then you will just be referring to power again so you might as well start out quoting rear wheel power numbers for comparisons.

Now in reality the extra friciton HP from the higher RPM will hurt my TQ output, the faster acceleration rate of the motor will hurt my TQ output and the higher driveline losses spinning everything 50% faster (ahead of the ring and pinion) will make that 2565 lbs ft much lower, but it will still be more TQ at the rear wheels

Lets compare apples to apples. Two engines with the same HP at the crank one with 700 hp @ 6000 rpm and one with 700 hp @ 8500 rpm. The different friction HP losses dont really count because we have already accounted for those on the dyno. They both end up with the same power at the flywheel.

Now the acceleration rate and higher driveline losses come into play and the higher rpm engine starts to come out with less rear wheel hp due to these losses. If you doubt it test it on a chassis dyno. Now since we cant gear up power there is no way one car can come out with more power at the wheels than it has at the crank. The only thing we can do is minimize losses through the drive train and inertial losses from the acceleration rate of the engine.

That to me is why high end race motors in F1, Pro Stock and Winston Cup, like larger bores, shorter strokes and more RPM. Even limited RPM classes and rules like the larger bore for a few more reasons.

The classes you mentioned above all have cubic inch limits. And we have already established that when cubic inch limited the big bore short stroke engines can make better power per cubic inch due to the fact that they can breathe better and turn more RPM. Not because they can run a deeper gear. None of those classes turn any higher rpm than they have to to make good power. Pro Stock could turn higher rpm's with the same peak power but they dont because they would have to run deeper gears and actually be slower.

 

thats insane. I figured the units came to that but i still coulnd't believe it

 

I know but then we are not talking about the same cubes. The only context where bore vs stroke makes any sense is in a cude limited application. A 406 vs 331 or a 383 are not the same motors.

Now we are talking about power again.

Again air is not on a string attached to the top of the piston. We are looking at a complex fluid model that has varying pressures acting on it. The port only cares about the volume that is pulling on it. The piston is folowing a standard mechanical path, the air/fuel is not following any path it is just flowing from a high pressure to a low pressure. Sometimes it gets helped by tuning frequencies.

True too high of a port velocity will limit HP and in turn HP. More CFM from the unshrouding of the valves will give you a potential for more HP. The 2cfm per HP rule would apply here right? The addtional CFM will bump TQ at all RPM, and that will obviously help the higher RPM more.

The problem I have is since we are talking about pressures and not air on a string, then the rate at which the volume of cylinder is exposed to the port is going to give you a pressure differential. The stroke does play a part in the piston speed, but the difference of the pressures created by the low pressure cylinder are only one part of the equation.

If they had the same flow and the same friction HP numbers at a certain RPM then yes they would be the same. Since the larger bore allows more flow, and less friction HP then we will gain power and TQ at the same RPM.

Not really, take a look at any RPM limited class. If you are given a gear rule or a RPM limit of a RPM band then, you are limited to a certain max RPM. Usually those classes have a cube limit too, so the little advantages add up there too. If there is not a bore rule then it wouldn't bother me to run a larger bore at any point in time.

For example, we will go back to the 4.50" bore vs the 4.25" AFR flow results.

On a 4.50" test bore
LIFT .100 .200 .300 .400 .500 .600 .700 .800
CFM (Intake) 79 170 253 315 350 375 386 390
CFM (Exhaust 67 135 198 255 290 302 314 323

On a 4.25" test bore
LIFT .100 .200 .300 .400 .500 .600 .700 .800
CFM (Intake) 76 162 232 282 321 348 368 370
CFM (Exhaust 62 133 199 256 284 290 293 300

That's the same cylinder head. You can see thru the entire flow curve that you gain CFM. Based on the simple 2HP per CFM, that's 40HP. It's not just going to be all at the engines max RPM either, you are going to gain thru out the RPM range.


Inertial Forces yes, Frictional Forces no. If you ever have a motor on a Electrical Brake dyno you can test this. Basically it's a really big alternator, but it can also drive the engine and tell you how much HP it takes to drive the motor. The friction exponent can be variable, if all of your motors have a exponent of 2 then you need to solve some friction issuses in those motors. The lower the friction in the enigine is the lower the exponent. That's why low drag rings, small bearing surfaces and coatings help so much, they lower the exponent of the friction HP.

If you want to belive yours go ahead. I've seen enough evidence to prove otherwise.

Yep, we do need the wheel speed too. Going at the same wheel speed we see why RPM is so important. It really isisn't a bad idea to quote RWHP vs RWHP, that gives us all the advantages and disadvantages of a particular system.

Funny thing about this. There is a famous head porter who patents his ports and then sells them to high end race teams. His stuff doesn't show much if any of a gain on the engine dyno but on the chassis dyno a magical 20hp appears. That same setup also does faster lap times. I have no idea why that is, and neither do alot of guys I know. It does go to show that a chassis dyno does give you a better estimation of performance output.

Your right my example was a simplification of alot of things.

Since dynos measure TQ and then apply them to a given RPM after they are accounted for by taking out the rear end ratio, we will see lower RWHP numbers, but they will still be a faster car. That's all due to the gear ratio multiplication.

TQ is what moves the car, the RPM is a bonus since I can multiply the TQ more for a given wheel speed. Those guys don't run faster than they can make "good power" because the TQ is falling off so fast that their is a lower average TQ in the RPM range they are running in. If they could make the same HP at a lower RPM they would have more TQ to pull that off. Everyone of those classes strives to turn those motors higher and higher RPM. They are a limited by valvetrain, if they can get those motors to turn faster then they will develop the heads and other parts to work at that RPM to make more HP (or just keep the TQ from falling off at higher and higher RPM) Once they are limited on bore sizes and such it's the technology of the bottom end parts that also becomes a barrier since they need to make parts lighter and stronger.

Not to bust your ***** here dano, but you called me out.

Bret

 

I dont consider it busting my ***** at all. I do enjoy good debates.

I know but then we are not talking about the same cubes. The only context where bore vs stroke makes any sense is in a cude limited application. A 406 vs 331 or a 383 are not the same motors

I wasnt going for cubic inch comparisons just differences if friction and inertial loads at the same piston speeds with different strokes and as close to the same recip weight that I could think of for my example. I wasnt trying to make a point for power with this example.

Again air is not on a string attached to the top of the piston. We are looking at a complex fluid model that has varying pressures acting on it. The port only cares about the volume that is pulling on it. The piston is folowing a standard mechanical path, the air/fuel is not following any path it is just flowing from a high pressure to a low pressure. Sometimes it gets helped by tuning frequencies.

The air is flowing through the port. That is a path to the cylinder. The low pressure area is created by the piston traveling that mechanical path. I consider the physics of induction pulse tuning to be beyond the realm of the average engine builder's control besides just picking the best parts combinations available to complement the desired use and operating range of the engine they are building. Most of the true tuning is better left to professionals with signifigant resources and/or fabrication skills.

True too high of a port velocity will limit HP and in turn HP. More CFM from the unshrouding of the valves will give you a potential for more HP. The 2cfm per HP rule would apply here right? The addtional CFM will bump TQ at all RPM, and that will obviously help the higher RPM more

It is true that airflow improvements will help torque until the limits of Naturally Aspirated induction tuning have been reached. Torque is directly related to displacement and cylinder pressure. NA engines will only make so much Torque per cube. Beyond that the only thing you can do is improve breathing and maintain as high a cylinder pressure as possible at the rpm's your engine is capable of turning. This of course equalling more hp while peak torque stays limited.

then the rate at which the volume of cylinder is exposed to the port is going to give you a pressure differential.

The rate at which the volume of the cylinder is exposed to the port IS piston speed. The faster the piston travels the bore the higher the pressure drop seen at the valve. This is the main factor in moving air from outside the engine through the induction path. Any consideration of scavenging, or pulse tuning is secondary and used for further optimization.

Originally posted by dano73327 And since no one would make the same cubic inches with a short stroke big bore and not spin it harder to take advantage of the better airflow your force and friction comparisons arent really applicable.

I should have been more clear with the intention of this statement. I was in fact refering to classes without RPM limitations (other than valvetrain of course) I totally agree with building big bore short stroke engines when power output is the prime consideration and cubes are limited this is the way to processing more fuel and air.

Since dynos measure TQ and then apply them to a given RPM after they are accounted for by taking out the rear end ratio, we will see lower RWHP numbers, but they will still be a faster car. That's all due to the gear ratio multiplication.

Chassis dynos measure power and calculate torque. They work backwards of an engine dyno. They measure the acceleration of the drum and can calculate how much power it takes to accelerate it at that rate then take the rpm and figure the torque.
Now let me get this straight. Your saying that if I have a car that runs a certain speed in the quarter mile and I reduce the rear wheel power that I can go faster. If every other factor remained the same then that is impossible.

TQ is what moves the car, the RPM is a bonus since I can multiply the TQ more for a given wheel speed. Those guys don't run faster than they can make "good power" because the TQ is falling off so fast that their is a lower average TQ in the RPM range they are running in. If they could make the same HP at a lower RPM they would have more TQ to pull that off. Everyone of those classes strives to turn those motors higher and higher RPM. They are a limited by valvetrain, if they can get those motors to turn faster then they will develop the heads and other parts to work at that RPM to make more HP (or just keep the TQ from falling off at higher and higher RPM) Once they are limited on bore sizes and such it's the technology of the bottom end parts that also becomes a barrier since they need to make parts lighter and stronger.

All I can say here is that your idea about torque moving the car is not correct. Power is the only consideration. It would be quite easy for me to have the same torque at the wheels as another car yet have more power. Are you saying that power doesnt matter and that all you have to do is have the right gear to run fast? Im not trying to be rude but to me this is what you are saying. If I have a comp eliminator 302 chevy and put it in a pro stock car then gear it low enough that it has more torque at the wheels than a Pro Stock 500 incher that I will be able to run faster and have all that RPM as a bonus?
Because I could turn that comp eliminator engine higher than that pro stock engine.

Yes those classes do try to turn more rpm. When you are displacement limited that is about the only way to increase power besides reducing inefficiencies. But from what you are saying if they could turn more RPM even at the same power they could run a lower gear and multiply torque more right?