Saw this thread and decided to propose a thought experiment.

http://www.chevelles.com/forums/showthread.php?t=101641

I’ve been thinking about this for some time, but was too lazy to type it all out until now. My respect and thanks to Greybeard for motivating me.

In particular, I’m curious about ring friction of long stroke vs. short stroke engines. Let’s assume two engines with the same bore size, but the stroke (and therefore displacement) is different. So, let’s imagine a stock 3.48 stroke 350 +.030 vs. the typical 383 using a 3.75 stroke. I’d prefer to not get caught up in rod length and other geometry issues, and for the purpose of this comparison I’m assuming that RPM/mile is the same, too (same gearing and tire diameter) but if the thread goes that way and there’s useful information, so be it.

I’m proposing this hypothesis: Stroke length has little to do with the amount of parasitic ring friction during actual engine operation because most of the ring friction happens in the first inch of downward piston travel after the plug fires. That first inch exists whether the stroke is 3.48 or 3.75 inches long.

The longer stroke engine does have more foot-per-mile piston movement. I project that this makes little difference in practice because:
Moving the piston away from TDC and BDC involves static friction. Static friction tends to be greater than dynamic friction, but both engines have a TDC and a BDC, so there is no advantage to either stroke length here.
Once the piston changes direction, we’re dealing with dynamic friction on an oiled surface. The rings and pistons slide on a film of oil. The oil film (oil wedge?) exists until the piston stops at the other extreme of the stroke, where we change back to static friction.
The top of the cylinder is where the friction (evidenced by bore wear) occurs because of 1) gas loading of the top ring causes higher loading of the ring/cylinder wall interface, which means more piston speed is needed to get the rings to “hydroplane” on that oil film. In addition, the cylinder is hotter at the top, which means the cylinder wall material is softer, but also the oil viscosity would be thinner. More pressure and thinner oil, mean more friction at the top relative to the bottom. The bottom of the cylinder is still an area of static friction, but with little gas loading of the ring pack, cooler (harder) metal and more oil with greater viscosity and film strength. This accounts for higher friction at the top of the bore, as evidenced by the tendency for the cylinders to wear in a taper. But as we said, long or short stroke, both engines have a TDC and a BDC, therefore, (I think), we can assume no advantage to long or short stroke.
In between, the higher average piston speed of the long-stroke engine would actually help oil film strength. That may not completely make up for the additional length of piston travel, but it would offset it some.

I would test the ring-friction vs. stroke length hypothesis by building two short-blocks as identically as possible in terms of cylinder wall finish, ring tension, bearing, piston, and oil pump clearances etc. (EDIT: One short block having a short stroke, the other having a long stroke) Both short-blocks would be tested to verify similar low-speed friction by hand-cranking them with a torque wrench and comparing both the break-away and constant-speed torque required to rotate the crank.

Once two “specially calibrated” test short-blocks were built, I would spin them at selected RPMs using an electric motor. To make the tests as “real world” as possible, the blocks and the oil would be heated in a controlled fashion so that temperature were the same between the two engines. This is to simulate operating conditions.

So, both short blocks would be motored (tested) at 1000 RPM, 2500 RPM, 5000 RPM etc. I would measure and compare the current draw of the electric motor as it drives each short-block at those speeds. Lower current draw for one short block should mean lower friction.

Anyone care to offer an opinion?

Very interesting. Let's see if I am following your line of thinking. If we build two engines withthe same bore size the frictional losses will be very similar regardless of stroke. If we build two engines of the same size, one with 3.48 bore and 3.715 bore (302) the other with a 3 inch stroke and a 4 inch bore (302) the 4 inch bore may (should) have greater friction losses due to the increased surface area at the critical first inch of acceleration from TDC. I hadn't ever thought of it that way. Makes sense, might sorta explain how the pro stockers are making almost as much power/cu. in. as many of the smaller engine combos. Thinking along the same line- Would reverse cooling lead to less wear at the top of the cylinder? Thats another thread. Someone else-just hope I have not misunderstood the hypothesis.

Not an opinion, but a question.

What is your reason for doing this experiment? Purely as a curiousity feeder or for some engine performance advantage?

The reason I ask is because it has been shown on tracks and on dyno's that the stroker motors just plain make more power. So if the longer stroke is at a disadvantage friction-loss wise, it is more than making up for it with its displacement and increased levering ability.

Now where this would get interesting is if it was found that a shorter stroke produces less friction loss as opposed to a longer stroke, then building motors with big bores would be the new wave. That is, instead of building the conventional 383ci by using a 350 block and a stroker crank, we would start using 400s and aftermarket blocks with short cranks to make the same displacement.

On the other hand, the big bore motors would have big bores (makes sense, huh, lol) and thus rings with more surface area contacting the bore, causing more friction.

So no matter which way you look at it, more displacement equals more friction because you need either a bigger bore or a longer stroke. Not even using more cylinders for the same displacement would change this.

The best outcome would be if the longer stroke actually produced less friction. That would be a win-win situation. I'm guessing this might be what you want the test to show us? That would be awesome.

Something you might look into is the formula for piston speed.

(Stroke x RPM's) / 6 = Piston speed in feet/minute (fpm)

He who makes more piston speed has a greater chance of making more power. Makes sense when you think that a bigger stroke generally means more cubes and if you turn it higher....well, you get big power.

Whoops, just caught that you mentioned piston speed. Well, if you didn't know the formula, it's there for you anyway.

That's a good way to check for friction, using the torque wrench. I don't think I can give away a specific number for fear of reprisal, but an engine built here has such low rotational torque (amount of torque it would take to turn over the short-block), it would knock your socks off. This engine runs like a sonovagun, too.

The ring pack, clearances and cylinder wall finish would be key to much of this. I would also agree that after about an 1" of piston travel following TDC, the piston is just along for the ride, however, the stroke length could still provide a small difference.

Friction is not linear and over 7000 rpms or so, that amount really takes off. I can only think that the same would be in regards to piston speed. Since piston speed is calculated with stroke and rpms, there should be something there.

Anyway, what was the question?

Good topic, btw. :thumbsup:

About Reverse Cooling. GM tried it in the mid-late 1990s and then dropped the technology when switching to the LS1-style engines. One argument might be that the LS1 also introduced an aluminium block but the truck blocks are iron.

I think if it would've been substantially better, GM would've stayed the course.

The trend IS to use the biggest bore possible. In the professional ranks, they use the biggest bore allowed and let the stroke fill in the blanks to achieve the maximum c.i.d. Big bore size allows for more breathing.

So.....big bore, relatively short stroke to achieve a certain engine size. Then use the lightest piston possible with the shortest compression height that will last as long as required. This also makes it easier on the rod, not having to carry so much weight.

Now, the good ones will also go with a short rod. Rod/stroke ratio means nothing. They cut the decks down to as low as is allowed or as low as they can physically take it. The piston/rod/stroke package fills in the blanks from there.

Pretty much:

Big bore
short stroke
short piston
short rod
short deck

Tight, efficient engine package that makes gobs of power.

Something you might look into is the formula for piston speed

Hmmm, not heard a discussion about piston speed since my days with 2 stroke out-boards. I remeber reading a book about and this and the mystical barrier of 4000 whatevers and you could not operate there etc. There was one of the Jap bikes, a 400 that did and kicked big A.

So much for that. Big bore small stroke and long rods, very nice combo.

Justin,

You're completely correct. When going for max allowed displacement, you want to go with a big bore and if possible a big stroke. That is when going for MAX displacement. But for a set displacement, like 377ci or 383ci, everyone seems to be choosing the long stroke version over the short stroke version. Of course, this is probably because 327/350 blocks are much more plentiful than 400 blocks. Yet, the long-arm producing more torque has been a selling point of the stroker.

Of course, going with 400ci+ is a better way to go all around.

And.......I've gotten off topic......

So Schurkey, are you actually planning on doing this or will this stay as a though experiment?

You don't have to go to all the trouble of building two engines to figure this out.

Get a wheelbarrel, drop the tire pressure to 5 lbs, and fill it with gravel. Now go out in the street and mark off 100', and then make another mark 10' further. When you start out you'll notice that it's hard to get it moving just like you say about the rings. Now, when you reach the 100' mark, see if that last 10' are free.

I would agree that people are directed to the 383cid combo, due to the availability of the 350 blocks and low end torque it has. This is good for the street/non-serious strip crowd. Those people, and there's nothing wrong with them as I'm one myself, won't put the gear into it to achieve max performance, since some streetability is a concern.

My Chevelle has a 700R-4 with 3.91's. It's the equivalent of a 350-TH with a 4.75 gearset in the rear. Monster launch and it's nice on the street/highway too. However, I don't have the engine to take advantage of it. But, I digress.....

NOW, if it's an extremely competitive class, where you have a car that does NOT drive on the street and you have some money to win, I'd wager that the person will find the block that will allow for the maximum bore and then go from there.

With no regards to street manners, you get that big bore/short stroke and then add more gear. Now, you do want torque, but you want to move the torque peak as HIGH as you can get it. So, you gear for this. Then you get alot of multiplication off the line and you turn RPM's.

After that, the thing that a person would be concerned about is how good of a valvetrain do they have? Can they afford to check/change springs often, etc.?

The race car we have now is a naturally aspirated '95 Cobra. It WAS a 395cid Windsor-based engine. It went 9600 rpms with a valvetrain that doesn't even use springs. So, we know we aren't pushing the envelope yet. Plans are to increase the cubic inches with a bigger bore, using a different block. We will keep the stroke the same, to not go over the magical 400cid mark, since we would take on a weight penalty. Our current block will become the back-up block/engine. Might sound like a lot of work for a 5 cid increase, but when you factor in that we are making 2.2hp/cid, it could be an 11 hp increase just for that. Not to mention the limits will be tested in other areas that should hopefully make more power.

I look for this engine to turn 10,000 rpms someday. Crazy for sure. Anyway, it goes 8's NA, which ain't bad for 3150lbs.

To be honest, I'd like to see us go up to an even bigger bore and stroke to attain the class limit of 440cid. I have faith that we have competent people here that can turn RPM's even with that size of an engine. However, that get's back into piston speed.

Theoretically, (as in, according to some scientists :rolleyes: ) around 4000/fpm is the max, but we all know that's incorrect. I THINK Pro Stock is turning about 5800/fpm and I would think that F1 is over 6000+/fpm.

So, in a max effort deal, you'd have to look to keep it under those kinds of numbers. We have a different project going together now, that will have about 5800/fpm.

Nice example involving the wheelbarrow, Greybeard. :D

Oh, Crap!!!!

He, He, He, He!!

Now, bttt...

I figure it doesn't matter all that much IF the cam is matched to your rpm running range you want and that the heads breath free enough to allow the wanted rpm's IF the rod/stroke ratio is such to allow a good ratio!

AND I am with Smokey here b/c he always said, "Run the longest rod you can fit in her!!"...

I say this after looking the spec's on Honda's little S2000, 9,000 rpm corn-popper motor over REAL CLOSE is all.

AND remember, on the street, GRUNT is where it is at so build the biggest, highest winding motor you want it to rpm to AND then GEAR and TIRE accordingly, imho...

pdq67

Now, while you've got the wheelbarrel out, you can do another test to determine the effect of the longer stroke and the increased piston speed.

Go back out on the street and have someone "time" the 100 ft.
Now, do the 110 ft in the same amount of time. Decide wheither it took more effort.

Justin

Some day when you've got a little time on your hands, e-mail me and tell me about school and how things are going for you.

Greybeard, check your TC private messages. :)

Schurkey, I'd still like to see you build 2 engines and then give one away, to a lucky TC'er, in the name of science. :D