It isn't really and a lot of todays motors are undersqaure and make way more power than yesterdays stuff. The actual stroke length will tell you more about an engines revability than the bore/stroke ratio. An undersquare 1" bore X 3" stroke engine will turn more rpm than an oversquare 4" bore X 3.5" stroke engine if they both have heads that go the same piston speed. If you hold displacement and number of cylinders equal then you will see as bore increase the stroke MUST decrease which will raise rpm, hp and hp/per inch. If you kept the same or more stroke with that better and bigger bore you would have even more power but not necesarily more power per inch. More power beats more power per inch anyday at the same weight though. This is a concept that the ricers don't always understand.

 

What in holy hell are you talkin' aboot?

Heads that go the same piston speed?

 

Where in the world did you get this from?? They were talking about huge 700+ BBs. The m6 revs high because they are dohc and have to in order to make decent power. That doesnt make it a race engine though.

 

Read from the beginning. And I mentioned the M3 engine, not the M6 engine.

 

Read the post again and you will see. He is saying that rod length is more important for revs then bore/stroke ratios. If both pistons are forced to travel at the same speeds, then the shorter rod will always rev higher.

Using an example of a huge 700+ cid engine runing 8k doesnt prove that a longer stroke engine can outrev the shorter stroke engine. I mean, comon, 69 z28s were hitting 8k rpm on flat tappet solid lifter cam, stock internals, and stock rockers. Some of the chevy 302 and 327 race only engines are hitting 10-12k on the track... Never heard of a long stroke engine doing those kind of revs.

 

Sorry, i had a typo. And no one ever mentioned that the m3 can rev 8k because it is a racing engine! You read from the beginning

No where in there do i see that the m3 can rev over 8k and do it well becuase it is a racing motor

 

When I re-read that post it makes less sense then the last time. Not to mention that I never said anything about a 700cid engine.

How does rod length affect piston speed? And heads have absolutely nothing to do with piston speed.

Those 302 and 327 race engines you mention are built for a purpose and that purpose doesn't require the same amount of torque that a 700cid engine is designed for. If you need to spin that high you design the engine with less stroke so that it doesn't come apart at those rpm's. Stroke doesn't limit rpm, it just creates problems at those higher rpm's.

 

There's a point here, whether the readers want to see it or not. You bring up the 69 Z-28 and one of the most famous rev-ers of all time, the '69 Chevy 302DZ. What is special about that 302DZ you ask?

The 302DZ motor was created by taking a 327 with a 4" bore and 3.25" stroke, and de-stroking it with a 3" stroke crank from a 283 chevy. The result was an engine that you could stand in up to 8000 RPM right off the showroom floor. These principles are as old as hot-rodding itself, and muddying the waters with concepts that only exist on the most extreme end of ultra-high-performance seems counter-productive, and only stands to confuse the people here that are only concerned with grasping the basic concepts. I mean Smokey Yunick isn't asking these questions here, it's just guys trying to get a foundation in the principles of engine dynamics. For some reason though, someone always has to take that "baffle them with bullshit" approach and mysify these topics to the point of confusion.

 

Having an impact is one thing, being a "primary limit to rpm" is another.
Baloney... I'm very interested in where you obtained the specs you're referencing for the LS1's maximum recommended piston speeds. Share?
I never said you didn't... The argument wasn't whether or not the way to correct the issue had been stated previously. The purpose of my statement was simply to show that ring flutter can be remedied, and has less impact on shaping the engine's rpm range than virtually any other component choice you'll make for that engine. As far as I'm concerned, it's the last hurdle in the race. Bottom line, at 9000 RPM, I will have given a lot more thought to keeping my lifters against the lobes than keeping pressure from building between my first and second compression rings.

The simple fact of the matter is, this is just verbal masturbation at this point, and has been magnified WELL beyond the scope of the original question. The point of this thread was to explore what effect varying bore/stroke combinations have on an engine's performance. I think that's been accomplished here, and then some.

 

That 2nd paragraph talking about 700+ cid engines revving wasnt pertaining to you, someone used that as an example though.

Piston speed - Ok, lets assume you have 2 different engines that are running at the same rpm. One has 3.25 inch stroke, while the other one has 4 inch stroke. The piston on the 3.25 stroke is moving much less distance then the 4 inch stroke piston. Since they are both rotating at the same rpm, the longer stroke piston must move at higher speeds to keep up with the same rpm as a shorter stroke piston. The piston acceleration definately starts having a huge impact the higher revs go. Now remember, the piston needs to stop in order to change directions. Higer piston speeds hinder this, and transfer much more load onto the rods.

Yes, the 302 and 327 engines are purpose built, so are the 700+ cid engines that someone used as an example of running 8k rpm. The example was used to prove that long strokes can rev high. People have been saying that longer stroke engines can rev as high and higher then the shorter stroke, I am just trying to say, no they cant. You said that "Stroke doesn't limit rpm, it just creates problems at those higher rpm's." Yes, stroke does limit rpm, the limiting factors are those problems that you refer to. Any engine is going to run into problems with high revving, the point is, the longer stroke engine will encounter them sooner. Obviously any trick parts that a long stroke engine uses to try to ward off these problems for as long as it can are still available to shorter stroke engine. Because of this, the shorter stroke engine will always have the higher rpm potential, no matter what. This doesnt mean that long stroke engines cant rev high, just means that shorter stroke will always have the potential to rev higher.

 

I asked what does rod length have to do with piston speed...nothing. We're pretty much on the same page, but my response to racer7088's post was legit. Fact is if an engine with a long stroke can rev to 10k, then stroke doesn't limit rpm. The fact that it may come apart after 30 minutes of abuse doesn't refute the claim that stroke doesn't limit rpm. You just won't be satified with rebuilding the engine every 30 minutes.

 

"Fact is if an engine with a long stroke can rev to 10k, then stroke doesn't limit rpm." This is not a fact! How can you claim that as a fact? Im talking about maximum rpm. Take the long stroke engine and floor it in a low gear. Dont stop until the engine blows. Do the same thing with the short stroke engine. If both are using the same parts, the short stroke engine WILL rev higher before letting loose!
And how does rod length not have anythign to do with piston speed??
https://ls1tech.com/forums/showthrea...4&page=8&pp=20

 

LOL.. come on now. That's like saying that irreversible brain damage doesn't limit you from smashing your head repeatedly with a ball-pein hammer.. you just won't be satisfied shitting into a bag, and eating through a tube for the remainder of your vegetated existence.

Like I said before, just because you "can" or you "have seen it done" doesn't make it a good idea, or an efficient way to go about it. Stroke DOES limit RPM, like it or not. You always have someone looking to invalidate sound Physics, and Mathmatics, but some people like to learn the hard way.

The inertial forces involved in stopping, and reversing the piston/rod in that long stroke engine are ALWAYS going to be greater than a similar, shorter stroke engine. Take two IDENTICAL engines, one with a 3.5" stroke, one with a 4" stroke. All things being equal, as you increase the RPM's of these engines to infinity, THE LONGER STROKE ENGINE WILL FAIL FIRST. Even if you're moving objects of EQUAL MASS within the rotating assembly, we've already agreed that the longer stroke engine's piston strokes the bore FASTER than the shorter stroke engine. Force=Mass x Velocity. Greater force is being exerted in the longer stroke engine, and thusly, it will reach the weakest component's yeild point first... or in layman's terms, it's going to **** parts.

The argument seems to have become what you CAN do, as opposed to what experience teaches us works best... that's an argument in semantics, and I have no interest taking part in it. <see ball-pein hammer analogy> You can do what you like, but I have no interest in blowing up motors just to prove that I can destroy a very expensive engine by mis-using it well beyond it's intended limits, in complete defiance of clearly demonstrated fact.

 

Actually, rod length has a lot to do with piston speed. It determines where max piston speed occurs. A shorter rod engine has max piston speed closer to TDC than mid stroke where as a long rod engine has max piston speed closer to mid stroke. Max piston speed occurs when the crank shaft centerline and rod journal make a 90* angle with the rod.

Some engine builders believe short rod length allows the intake stroke to estabish momentum of the air moving into the cylinder early in the stroke allowing a raming effect when the cylinder is near BDC just before the intake valve closes.

Others believe the added engine stress from the short rod negates these factors.

And some believe the raming effect is not significant.

Just my .02

 

You can only put so big an intake valve in so big a bore so your pistons speed limits are independent of bore size. Basically the stroke tells you how high the engine will make power to. Piston speed limits are from heads not shortblocks usually.

 

I don't disagree, I just didn't know what you were trying to say the first time out.

 

I stand corrected. I didn't put too much thought into the "where" aspect.

 

Sorry, I tend to lose myself in the semantics every now and again, it's a side effect of my job and needing a vacation...which I just got back from.

 

Hey, no harm no foul.... I thought this was a pretty good thread, aside from some incorrect information being touted as fact... since that was never followed up on though, I guess we can consider it a moot point.