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Old 03-28-2012, 11:56 AM
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Basic Engine Questions

Hi All,


I've been doing some reading on engines and engine math. Being new to this stuff, not everything makes sense (immediately), or I just can't find answers to some questions.


So I decided to create a general thread on engines where others and myself can ask our questions and easily refer back to later.


Today, my question is: How are an engine's maximum rpm and redline determined? Is there a general rule of thumb, a formula, or is it determined via testing?


Thanks in advance!

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Old 03-28-2012, 12:08 PM
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Lets see, when the rod's and pistons blow out the bottom of the pan.......you've reached or better yet, you've past REDLINE. Here's your sign!
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Old 03-28-2012, 12:31 PM
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I don't know the answer to the question being a dumb bodyman myself and not knowing a whole lot about motors. But I do know that there are many motors that you could never blow the pistons out of the pan because the maximum RPM would be limited by things like floating valves and such that would keep the motor from revving so high.

I am dying to see the answers these questions ask though.

Brian
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Old 03-28-2012, 12:50 PM
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Max RPM is a function of several variables. The combination of head/induction flow, cam duration and the physical strength of the parts used are among the most important. Exhaust flow has an effect as well, but not as "pronounced".

It's a pretty broad question...

Jim
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Old 03-28-2012, 01:07 PM
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I suppose one could separate the question into (at least!) two parts:

Powerband redline, where the engine has reached a point of diminished returns for added RPM. Track or dyno testing can determine this fairly easily.

Mechanical redline, where the physical limits of the parts are reached before they yield or fail. Here's where you want use caution. Parts manufacturers will use things like "good to 500 HP", and similar terms. But too much RPM will kill an engine as quickly as anything. In a lot of cases the valve train will be the limiting factor.

I believe most engines used for competition are over-built to some extent to give some leeway, but the exact point of failure can be very tricky to estimate. Fortunately, most cam profiles used in streetable engines will put a ceiling on the RPM needed to make the most power (as a broad statement it's uncommon to need more than 7000 RPM, say, and bottom ends and valve trains can manage that w/o too much trouble) but with racing engines, this goes out the window.

EDIT- But yeah, broad question. And an interesting one.

Last edited by cobalt327; 03-28-2012 at 01:20 PM.
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Old 03-28-2012, 02:52 PM
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Thank you everyone for the responses.


Mr. P-Body and Cobalt327, since both of you have pointed out that my question is quite broad, it would explain why I did not find much information on it. However, the information both of you posted in terms of what is involved in computing max RPM and redline does help put things in some perspective and gives me some food for thought. And my thought process has definitely been more on the mechanical side of things (when does it all go kaboom!).


I imagine the calculations are quite complex for an average person working on a street engine, and must rely on manufacturer specs and recommendations. For race engines, would programs like DynoSim be of some help? (I've never used one.)


On a related note, what really sparked my interest was finding out that a Chevy 302 will rev higher than a 350, and I would like to understand why. Or is it just that the 302s I've read about just happen to be designed to rev higher the average 350?


Thanks again!
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Old 03-28-2012, 04:35 PM
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I think the 302 will rev easier because it uses a 3" stroke crank (350 is 3.48", 400 is 3.75"). The longer the stroke the faster the piston and rods move (angular momentum). A shorter stroke reduces angular velocity making higher revs easier and safer.
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Old 03-28-2012, 04:42 PM
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Yes, the bore to stroke ratio is exactly what we are looking at. As a general rule, the stroke shorter than the bore is wide makes for higher RPM, higher horsepower less torque. While the stroke that is longer than the bore is wide makes for lower RPM torque with less horse power.

An old inline six for instance will usually have a very long stroke to bore ratio and of course we all know the torque they produce compared to a very low max RPM.

Brian
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Old 03-28-2012, 05:20 PM
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Hey Silver Surfer and Brian,


Thanks for the responses. I am trying to visualize what you guys have said in my head. Boy I could really use a cut-away engine lol!


The analogy I am coming up when thinking of short stroke vs longer stroke, and how they relate to RPM is boxing.


Lets say that:
- Torso = crankshaft
- Arms = piston rods
- Fists = pistons
- Speed of punches = horsepower (might be way off on this one)
- Power/force delivered by each punch = torque
- Punches per minute = RPM


If the opponent is at a close distance, I can throw alternating short-distance punches (short stroke) that are very quick (high rpm), but don't deliver too much power (torque). If the opponent is a little further away, I have to throw long punches - longer swings? - which carry heck of a lot more power, but I can't throw them as fast as the short-throws (lower RPM}.


Sound about right? My terminology might be off but I hope this makes some sense.


Thanks!

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Old 03-28-2012, 05:41 PM
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A good point. Now lets look at say a Indy race car motor. I cant tell you what the bores and stroke are but I do know they use a very short stroke and develop amzing rpm like 12,000. Thats more than any small block chevy could.
A good question though. I bought a cam, it said MAX power 6500 rpm. So now I have to build the rest of the motor to sustain that. Quality heads, steel crank, forged pistons and forged rods.
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Old 03-28-2012, 07:28 PM
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One more thing to add to the stroke equation is piston speed. A shorter stroke has a slower piston speed all else being equal.

The lesser centrifugal forces and acceleration rates favor a shorter stroke for ability to rev before the crank begins to deflect too much to sustain an oil wedge, or some other design shortcoming enters the picture.

I bookmarked an article some time ago, it had to do w/the above but I only skimmed over it before saving it for a rainy day. If I find it I'll post it. It had some great crank manufacturing photos: raw forgings, machining and the like.
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Old 03-28-2012, 10:21 PM
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Contemporary Crankshaft Design. Articles under "Technical Articles and Product Descriptions" are worth taking a look at, too.
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Old 03-29-2012, 08:22 AM
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Over the years, we've learned the bore/stroke ratio is not nearly as significant to the ability of an engine to "rev" as once thought. Modern Top Alcohol engines are good example. The "blown/alcohol" engines usually have a 4.350" bore and a 4.375" stroke. That "makes" 525 or so CID (526 "limit" by rule). They routinely rev those to near 10K.

One of the more important internal parameters for longevity is the rod/stroke ratio. The "longer" the rod, the "shorter" the rod angle (relationship to the crankshaft at "full swing"). A steeper angle increases stress from "side-loading" the piston against the cylinder wall. Smokey Yunik advocated as high a r/s ratio as physically possible. Even 2:1 in some "short stroke" engines. It is "accepted" that a r/s ratio of 1.7:1 is "ideal" (no coincidence that 327 Chevy with a 5.7" rod has a r/s ratio of 1.7:1!). Anything under 1.5:1 is considered a liability. There IS the argument that a "short rod" engine can "catch revs" more quickly, and there's evidence to back that up. In "short track" applicatoins, shorter rods are often employed. Current "thinking" at the highest levels are to use a piston that "fits" the requirements and use a rod that will "connect" the crank, regardless of length.

The reason 302 "out revs" 350 is beacuse they have the same (maybe not identical, but overall) heads and bore size, but a much "smaller" engine to "feed". That alone, increases the RPM range. A 302 won't make more power than a 350. And with internal changes and "enough" cylinder head, 350 will "rev to the moon" as well.

"Mean piston speed" is the term. It means the "average" speed from a "dead stop" at the top and bottom. Most engines with a stroke under 4" won't have much issue with MPS, at least in street situatioins. MPS is a significant factor in how "long" (in time on a clock) a given combination can resist damage from sustained higher revs. Drag engines and street engines seldom reach those limits because they don't spend much "time" at the peak. Road racers and circle track engines must pay close attention to this.

Next... (:-

Jim
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Old 03-29-2012, 09:00 AM
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Another Link

Here is a link from Wallace Racing that I believe will be helpful to the discussion.

http://www.wallaceracing.com/enginetheory.htm

bt
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Old 03-29-2012, 03:08 PM
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A big thank you to all the members who took the time to provide the detailed answers, real -life examples as well as the links. You guys have given me so much to read up on!


For anyone reading this, after you've read the responses here, start with the link posted by beertracker as it serves as a good primer that is easy to follow, and then move onto the on by cobalt327 which has tons of info and actually deals with the physical attributes of the materials used in engine building.



Quote:
Originally Posted by Mr. P-Body

Next... (:-

Jim
LOL! I have many, but none that I am ready to ask at the moment. First, I need to find some dedicated quiet time to read all this information at least two or three times, and then see where I end up. But it is good to know you and the rest are ready to help and share your knowledge! Thank you.


Again, thanks everyone! With you guys around, who needs auto class?

Last edited by lt1silverhawk; 03-29-2012 at 03:15 PM.
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