Does Stroke Matter?

[olddog]

I considered calling this mental masturbation for engine geeks.

I read a book a while back. I believe it was a college text book for a course on internal combustion engines. It was written in the late 1950’s. A very difficult to read book, it was. It mainly looked at commercially successful engines, and pointed out research done at several universities.

The professor pointed out that the overwhelming factor for making power is the bore diameter – not the displacement. To illustrate his point he compared the Hp of a model air plane engine with a ¼” bore to marine diesel with a 3 foot bore and 9 foot stroke. The Hp per piston area was very close on both engines; however the Hp per displacement were not even in the same ball park. He did take into account the BTU/lb of the fuels being used.

His explanation for why this is the case, boils down to a few simple principles. The bore limits the size of the valves. The valves limit the air flow into the cylinder. The amount of air dictates the amount of fuel. The amount of fuel per unit of time is the amount of power per unit of time available. The efficiency of converting the available power to Hp changes very little, in piston engines.

He went on to explain that on a given engine, the amount of air being pulled into the cylinder increases with piston speed until the flow through the port and valves become the bottle neck. As piston speed increases beyond the maximum flow point less air is pulled into the cylinder. Since air flow is proportional to power, this makes perfect since. We all know there is a peak in Hp and spinning the engine faster results in less Hp.

Now here is where it gets a bit interesting. He always talks in Mean Piston Speed. He said in controlled laboratory experiments, changing the stroke does not significantly change the Hp. In fact the peak Hp will be reached at the same piston speed regardless of the stroke, when all other factors are kept the same. A long stroke reaches a given piston speed at a lower rpm than a shorter stroke. So the long stroke will have more torque at a lower rpm than the short stroke, but both will make the same Hp.

So if we had a 302 cid (4x3 bore/stroke) and its peak was 300 Hp & 197 lb-ft at 8000 rpm and we change only the stroke to 400 cid (4x4 bore/stroke), we could expect 300 Hp & 263 lb-ft at 6000 rpm..

Today we have canted valve heads that allows a bigger valve for a given bore. We also have another 50 years of advancement. However I have to wonder if the old boys understood some things pretty well. His work did not include radical cam shafts or engines designed for racing. His work suggests that if I stick a set of heads that would work best on a 400 cid stroker on a 302, I could make the same power at a higher rpm with less torque. I wonder if this is true today.

[blykins]

Originally Posted by olddog

His work suggests that if I stick a set of heads that would work best on a 400 cid stroker on a 302, I could make the same power at a higher rpm with less torque. I wonder if this is true today.

Well sure...

Take a 351C with 4V heads. It makes some great horsepower and great torque. Now put those 4V heads on a 302 (Boss 302). You get great horsepower at a much higher rpm, but less torque and bottom end response.

That rule has always applied. Extra displacement eats up camshaft duration and port volume.

I can make 500 easy horsepower with a 408W using 205cc heads. I can also make 500 easy horsepower with a 347 and those same heads. However, the horsepower peak will be much, much higher and the peak torque value will be much, much lower. Port velocity will be worse for the smaller engine as it doesn't have enough butt to take advantage of the larger head. Therefore throttle response and low end performance will suffer.

[fordracing65]

Blykins, what engine are you going to use, when you get back in the game.

[blykins]

Well, the game has changed a little. I'm still doing the 445FE that Patty supplied the cam for, but I switched my thinking from a Cobra to an older Ford pickup....say '64-66. Always wanted one and I've been patiently watching the locals for a good deal.

The 445 is a smaller bore, longer stroke, around 4.080" bore, 4.250" stroke.

[Barnsnake]

I have a bit of a problem with the author's example. The model airplane engine is probably a cross flow two-stroke and the marine engine is a four-stroke deisel.
The airplane engine might be good for 100 hours, while the marine deisel is designed to run 50 years. Comparing apples to oranges (or is that peas to watermelons?).

Torque is force times lever arm. A longer stroke results in more leverage and therefore more torque at equal bore and cylinder pressure. I don't want to rekindle the torque-vs-horsepower argument, but horsepower is a function of torque.

Whether you want a long stroke or short stroke motor is a matter of application.
Personally, I think a big, broad stroker powerband is nice in a road car. For the drag racer that only runs below 6000 rpm in the staging lanes, the broad powerband is useless.

[Dirty Harry]

I've always been a little concerned about much of an increase in stroke because of the increased angularity of the rod. Also, doesn't the piston pause, ever so briefly longer, at the bottom and top of its travel when stroke is increased? It would seem to me that, in a stroker, the forces applied to the bearings, rod, and wrist pin would increase as the piston "parks" at TDC and BDC and then gets yanked back into the next stroke. In otherwise identical engines, does the stroker wear piston and cylinder wall surfaces faster due to side loading from increased rod angularity? This does make for interesting thought processes. Thanks for bringing it up.

[olddog]

Originally Posted by Barnsnake

I have a bit of a problem with the author's example. The model airplane engine is probably a cross flow two-stroke and the marine engine is a four-stroke deisel.
The airplane engine might be good for 100 hours, while the marine deisel is designed to run 50 years. Comparing apples to oranges (or is that peas to watermelons?).

Both engines were two strokes. He discussed that bore limits port size in 2 strokes, as well as valve size in four strokes. He gave examples of large 4 stroke comparisons, too. About a half dozen examples in all. I just happened to remember the one mentioned, because the huge size difference was so striking.

[fordracing65]

For a 427 stroker is it better for the street to have more bore or more stroke to achieve 427 cubes.

[Wbulk]

Good discussion. If possible, it would be interesting to see what the numbers would be for a 302 vs. a 331 using the same cam and compression ratio. Then you could tell what the impact of increasing the stroke only would be.

[PDUB]

Originally Posted by Wbulk

Good discussion. If possible, it would be interesting to see what the numbers would be for a 302 vs. a 331 using the same cam and compression ratio. Then you could tell what the impact of increasing the stroke only would be.

Brent ought to be able to nail this down pretty close with simulation software.

[blykins]

Originally Posted by Barnsnake

Personally, I think a big, broad stroker powerband is nice in a road car. For the drag racer that only runs below 6000 rpm in the staging lanes, the broad powerband is useless.

You wouldn't believe what the stroke is on an 800ci Mountain Motor Pro Stock engine....

The cam (duration and LSA) determines the powerband.

For a street engine, my motto is go with as many cubes as you can get.

To get a 427, it's easier to go about it with an aftermarket block and crank, otherwise, you have to do a lot of machining to get exactly 427 with a production block. You either have to use an offset ground 400M crankshaft, or you have to bore the block out past .060" and use a 4.100" stroke. Even then you're only at 425ci. It's not "better" to have a bigger bore/shorter stroke, that's just the way it comes out with the aftermarket block combos. You guys shooting for the "magic number" have no clue on what you're missing out on with the larger displacements.

Think about it in terms of power per cubic inch. Builders often use that as a "benchmark" for determining how strong an engine is. A lot of my engines are in the 1.4hp/ci range.

If you have a 331 that makes 1.4hp/ci and a 347 that makes 1.4hp/ci, which is going to have the most power?

Keeping all other items the same, increasing the stroke will generally increase torque and broaden the curves.

[olddog]

Originally Posted by PDUB

Brent ought to be able to nail this down pretty close with simulation software.

You inspired me to finally track down where my Dyno2000 launch icon disappeared to.

I had a file on my 347 stroker, so I just changed the stroke on it.

T=Torque

stroke - T@2000rpm --- peak T --- Hp@2000 - Peak Hp
3" ----- 314 -------- 426T@5000 --- 120 --- 460@6500
3.4" --- 367 -------- 464T@4500 --- 140 --- 454@6000
4" ----- 446 -------- 510T@4000 --- 170 --- 448@5000

I put in the Hp and torque at 2000 rpm to show what Brent is saying about the torque curve is fattening up. Notice that at 2000 rpm the 3" stroke has 73.7% of its peak torque, while the 3.4 and 4.0 strokes have 79.1% and 87.5% respectively. So the lower rpm torque comes up, making a flatter torque curve.

In this case it is showing that the peak Hp is slightly less as the stroke increases. I believe this is because it is a single plane intake. As the rpm comes down do to the longer stroke, I suspect the single plane is hurting the power. Although the air flow should be the same, so maybe not. Brent may have a different opinion.

I did do one extra comparison. I compared the 3" and 4" stroke Hp curves. I then looked at the 3" strokes Hp at 4/3 the rpm of the 4" stroke to simulate changing the rear end gears. Specifically the 4" has 170 Hp @2000 rpm and the 3" stroke Hp is 170 Hp @ 2666 rpm. I had to use linear interpolation to get that number, as the chart is in 500 rpm increments, however I believe it is close enough. The point is that the Hp is the same if you shift the rpm by the same ratio of the stroke change. So if you use a 3:1 gear ratio on a 4" stroke and a 4:1 ratio on the 3" stroke the performance of the car should be about the same, theoretically. No doubt there are some other real world factors that I have not accounted for that will skew these numbers.

It is looking to me like the stroke just lets you run in a different rpm range.

[blykins]

Larger engines will take advantage of a single plane intake because they can increase the velocity through the intake, so that's not necessarily the issue. A 302 with a Victor Jr can be downright fussy, whereas a 445ci Windsor LOVES that intake and more.

The issue is that when you increase displacement, you need to increase the camshaft duration as well. Cubic inches eat up duration as the cam has more volume to fill.

If you increase camshaft duration to keep the peaks the same, you'll notice that you still have more torque, but the peaks move toward the right and the horsepower will go up as well.

Stroke lets you run easier in a specified range, but also adds more horsepower across the curve. Run your sim again and look at average hp/tq between the 3 combinations.

[olddog]

Brent my software does not show average Hp/Tq, or I don't know how to find it.

[blykins]

You can do it the old fashioned way....

Add up all the numbers and divide by how many entries you have.

[blykins]

Here's some data from EA Pro:

347 SBF: 512hp @ 6500, 464 lb-ft @ 5000, 367 average horsepower, 392 average torque

408 SBF: 539hp @ 6000, 543 lb-ft @ 4500, 414 average horsepower, 457 average torque

427 SBF: 556hp @ 5500, 571 lb-ft @ 4500, 427 average horsepower, 475 average torque

460 SBF: 570hp @ 5500, 612 lb-ft @ 4500, 435 average horsepower, 495 average torque


Now, some caveats: sims are sims....not real data. But the trends work the same as real life.

I used the same camshaft, head, intake, and carb profile in every one of these engines. I started out big so you could see how high you have to spin a small engine to make similar power and even then, you don't get the same peak power, nor do you get anywhere near the same amount of torque or average figures.

Now if I were to bump up the camshaft and valve sizes in those larger engines, the differences would be astounding. For instance:

460 SBF: 638hp @ 6000 rpm, 621 lb-ft @ 5000 rpm, 474 average hp, 513 average torque

[blykins]

The higher average number is what allows you to cruise comfortably and have power available at any rpm. It also makes an engine "feel" like it's got more power.

[olddog]

Originally Posted by blykins

You can do it the old fashioned way....

Add up all the numbers and divide by how many entries you have.

I was going to copy paste the chart to Excel and do that, but the software will not let you copy the chart. I cursed the program writer for that.

Anyway you already did it for me. Thanks

[Keithc8]

When possible always run more cubic inches to a given point. You get into application, how long you want to use the engine, power range, parts used and a few other things. But as Brent's sample shows in a street engine with almost everything the same the larger longer stroke enigne will make more torque for sure a little more power to a given point, but if we were going to do the longer stroke engine we would change some things to make it work better.

Thanks, Keith