[font size="1" color="#FF0000"]LAST EDITED ON 21-Oct-01 AT 09:51 PM (EST)[/font][p]A guy told me he can tell a 360 from a 390 by the way it revs in nuetral?
I have a FE 4v engine in the dually and it revs super quick.He said it was a 390 garunteed.I have had 360s and they didn't run like this but is he pulling my leg??
Thanks~




'66 F250 Custom Cab blue/white
'70 F250 4x4 Sport Custom blue/white
'73 F350 dually red
'79 F150 daily driver

 

oleJasonblue, yes and no. No, he is not pulling your leg when he says he can tell. But he only THINKS he can. In his mind he is telling the truth, but that is not by any stretch of the imagination a reliable way to tell. DF

 

usually, at least in the motorcycle world, the shorter the stroke the faster it will rev, so technically a 360 will rev faster, but engine condition and state of tune make a big difference too, I agree, not a reliable way.
Justin

 

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I hope you told him you can tell the difference between a Horse and Jacka...


Dennis

 

The way I always understood it in the motorcycle (dirt bike) world, as well as the automotive world, was that an under square (smaller bore than stroke) engine would make better low rpm power and an over square engine (larger bore than stroke) would make better high rpm power.

I would say that how fast an engine revs up is based on the relationship between the torque curve and the rotating mass of the engine.

If it were assumed that a 360 and a 390 had the same rotating mass (which they don't), a 390 would rev faster off idle than a 360 would because a 390 makes more low end torque The 390 makes more low end torque for two reasons: It has a larger displacement. And it is a more under square engine.

If a 390 really would rev faster at low RPM would just depend on the difference in mass between the crankshafts, rods, pistons and the difference in torque at a given RPM. For simplicity lets just focus on the crankshaft mass. The added mass of the 390 crank would make it slower reving, but the added torque of the 390 would make it faster reving. The question is which has more of an influence, added mass or added torque? I don't know, but my subjective opinion is that yes a 390 would rev faster off idle. Would there be enough difference to notice? Again I don’t know.

So this guy might not be a fruit after all for thinking he can tell the difference between a 360 and a 390 by reving them out of gear.

It would be interesting to get a good running 360 and a good running 390 next to each other and rev them up to see if the 390 reved faster at low RPM than the 360 did.

Does anyone have access to a 360 and a 390 both in excellent mechanical shape and properly tuned?

 

Saying you can tell the difference is just stupid. I used to have a 360 in my truck. It is getting replaced by a pretty hot 390. I'm sure in this case the 390 would rev a lot faster. Stock engine vs. stock engine since the later 390 & 360 truck motors share everything except crank, rods and pistons the 360 should rev faster in neutral because of its smaller stroke but if you told me that you could tell the difference between the two by the way the motor revs I would probably spend the next ten minutes of my life verbally abusing you. Stick a dowel down in the spark plug hole and measure the stroke. This is a sure way to tell and doesn't require some crazy mechanic's theory.

 

Blue it would be interesting to line up 10 side by side and let him do the blind (uh taste) test. That one just don't sound right, huh? :-)
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John

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In the cool still quiet of night you can hear chevies rusting away.

 

Thanks for the replys.I think it was kinda weird too.The idea of listening to them and knowing sounds cool but i suppose he justs thinks he knows.
Hey,I did rev it with the 352 i have.THe engine in the dually revved almost twice as fast and actualyy sounds like it isn't stock.I have had alot of trucks and this one likes to idle about 700 rpm and when you give it some fuel it really gets on it.You know how some engines have to build there rpm's?Not this one,I can crack that 4v Motorcraft wide open and the rpms are right there quick.
Dowel rod check this week.

 

John,
At the least i would like for him to read this thread.I think any engine that runs great will rev better than the next??Some are just weak to build rpm's.

 

More fuel for the fire. My friend threw a rod in his GT500 so while it was down we got a 390 and put all the CJ goodies in it like the heads, cam, intake, carb. and exhaust. The first thing we all noticed is that it reved quicker than the 428 with the same parts. Not to say it had more power, it just would “blip” quicker. I always thought the CJ in my Stang was a slow rever but I was used to Windsors and Clevelands before that.

Barry

 

It's not how fast it revs in neutral its how fast it revs in gear. I think it is usually referred to as horsepower.

 

HaHa.No foolin?

I have built many engines including some high horsepower strip engines.
I can take my point one step farther:
The reason it matters in nuetral is that it dont care what size vechicle it's sitting in!You could put a Nascar engine in a tank and you wont get any revs.This is in a 1-ton,it has lotsa power but i was just refering to a "no load" situation.Understand?
All engines will rev up differently regarless of whether there is a load or not.All i can say is this engine is a stock 4v 390 and it revs real quick.Unlike a 351M which is a slug.I am not saying i belive the guy who said he could tell by ear just making sure you all know where i am coming from personally.