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Old 07-09-2007, 10:35 AM
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smooth engine vs. fast revving?

I understand that straight sixs and v12s are the smoothest engines around and naturally balanced but they are not so popular for high performance. The 4 liter in my jeep does not rev out as quick as my 351 W ford but is much smoother at 4-5000 rpm. I would think that a long crank of a straight six or V12 would be heavy and would take longer to get to the same rpms. I hear they do not make straight 8s anymore so I guess it is not as smooth as a straight 6. I do realize that straight 6s and v12 take up a lot of space. Am I missing anything?

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Old 07-09-2007, 11:51 AM
richard stewart 3rd's Avatar
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Hi,
I have had in the far past,quite
a few straight 8s, you can't even feel them running,
& the torque is smooth, right up to where you run out
of rpm's, on both the OHV & the
Flatheads, standard & automatics.
Take care,
rich
I've also been told that the 16cylinder was a smooth
running engine, but I can't attest to it, as I have never
been around when one was running, & never rode in one.

Last edited by richard stewart 3rd; 07-09-2007 at 11:59 AM. Reason: forgot the 16cylinder
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Old 07-09-2007, 02:49 PM
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With that longer crankshaft of the straight 8, v10,V12, 16 cylinder would it take longer to get to say a hypothetical 7000 rpms compared to a v6/v8 of the same displacement with the shorter crankshaft?


thanks...
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Old 07-09-2007, 04:00 PM
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Yes, but the length of the crank isn't the reason. It all has to do with rotational mass. If both cranks were of the same weight they would spin up at the same rate,all other things being equal.
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Old 07-09-2007, 06:17 PM
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Thankyou; that is what I wanted to know. So it is the mass of the crank and not the length. I would think that a longer crank would be heavier but that is beside the point that the length does not inheriently make the engine spike up in rpms slower. So the crankshaft on my straight six is probably heavier than the crankshaft on my 351w. I imagine a stroker crank is heavier than its original counterpart such as a 289 to a 347 ford.
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Old 07-10-2007, 05:41 AM
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It is probably but not necessarily true that you six crank is heavier than the 8 cylinder crank because it has 6 journals rather than 4. A stroker is not necessarily going to be heavier . When going to a stroker, other components will likely be lightened so not as much mass is required in the counterweights for balance. A small bore short stroke motor will spool up quickly but makes its power at the top end conversely a big bore long stroke motor will make its power at lower rpm and will not rev as quickly. That is just a kinda guideline. There are so many variables involved that some guys have made a career out of trying to build the perfect engine.
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Old 07-10-2007, 07:52 PM
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The problem with using a straight six for performance is torsional vibration in the crankshaft. A long crankshaft tends to twist back and forth more than a short one, and all that vibration tends to shake flywheels & dampers loose, and can destroy the crank in extreme cases.
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Old 07-11-2007, 11:13 AM
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The AMC six cranks are pretty good, but do get a torsional vibration around 6K rpm. I've never run one much over 5K, but some of the racers have told me as long as you don't try to run at the vibration point it's fine. Say the point is 6K on your particular engine (it will vary slightly between engines, but you know it when you hit it!). You can run it at 5800 rpm or 6200 rpm (leaving +/- 200 rpm for safety) with no problems. Running through the vibration point isn't a problem. All straight sixes are the same way, though the point varies with bore/stroke/length/mass. I've never heard of anyone breaking an AMC crank due to torsional loads, but that's because most are built for torque and not high rpm power. They are seldom run over 5K rpm.

Stroke has a lot to do with how fast an engine gains rpm also. Short stroke engines will generally spin up a lot quicker than long stroke models.
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Old 07-11-2007, 07:53 PM
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I think maybe some of the older 6's and straight 8's took a while to spin up because they had heavier flywheels on them to assist in getting the vehicle moving from a stop.

Perhaps straight 8's fell out of favor because of the difficulty in packaging them. A car has to be fairly long from the firewall to the radiator to accomodate them.
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Old 07-11-2007, 10:28 PM
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If you are comparing the weight of a 240/200 CI Ford truck crankshft to that of a 351W, I can tell you the 300 crank is quite a bit heavier. I have one of each in the shop, that 300 crank stays put. It's no fun to move around.
As far as "smoothness", a high quality rebuild of a 351W should include dynamic balancing. The most noticable benefit is a very smooth running engine. A factory 351W was built with "weight matched" parts but wasn't actually balanced. You could even try different 351W powered vehicles on the lot when new that you could tell some had smoother running engines than others (this was actually true for many years on all kinds of cars). That's been a while ago though, newer vehicles and other brands may have balanced engines from the factory.
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Old 07-11-2007, 11:50 PM
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Many years ago I read that a six cylinder crank was naturally smoother -- something to do with natural cancellation of some of the vibration modes.

I've had a few sixes, and they always seemed pretty smooth -- a little more so than the V8's, but nothing to write home about. They can do a lot of things to make engines smoother.

Most sixes are designed to run slower and be torquey. Mainly because the longer crankshaft can be 'whippy'. But, the Jag XKE was initially a 6
(later a V12). It revved pretty good. The Mercedes 300SL was also a 6, although I don't know that it revved as high as the Jag.

The straight six seems to mainly lose out for space reasons -- and the crank being whippy doesn't help. The straight 8 is the same -- only more so.
Plus, the long 8 is probably not as naturally smooth. (Not sure about that, Buick quit making straight 8s when I was 9)

ford2go
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Old 07-12-2007, 06:31 PM
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With 7 main bearings, I doubt the crank in my Ford 300 six is very whippy.
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Old 07-12-2007, 09:59 PM
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I had trouble keeping the flywheel on my old 292, which got revved past 7K on a regular basis. I went to a foreign car shop where a friend worked, and he showed me the backside of an XKE motor. I don't know exactly how many bolts there were holding the flywheel to the crank, but it was more than 6, for sure. Looked like there was a bolt everywhere there was room for one, all the way around. I figure the Jag motor also might have been a little better balanced than my old backyard-hopped-up truck motor.
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