piston to valve clearance prob...HELP!

[Jeff Chambers]

Exactly correct Unit. More revs = higher volumetric efficiency. That's how I get 600+ crank horsepower from my little 316ci SBF with a .500" lift camshaft......spinning it to 9400 on every shift!

[Unit 5302]

Hahaha, not really (Einstein). Basically, it comes down to shorter strokes are better for higher rpm applications. The 5.0L (302) has an oversquare engine design, meaning the bore is wider than the stroke is. If we look at the 302, the 3.0" stroke means that the piston must travel 3.00" up, and 3.00" down for each revolution of the engine. That's 6.00" per rpm. Now, at 6,000rpm, a 302's pistons have to move a total of 6.00" x 6,000 = 36,000". Converting that to a speed you can take 36,000 / 12 / 5,280 = .56818 miles per second. Now take .56818 x 60 x 60 = 2,045mph. That's the average speed of the piston in a 302 at 6,000rpm including the stopping and redirection of the piston at the top and bottom of it's stroke. Starts to sound like a lot of velocity, doesn't it? So the connecting rod has to stop a 302's connecting rod and piston assembly travelling at speeds far in excess of 2,000mph. Eventually, as the speed of the rotating assembly increases, the force will cause a component to fail.

Look at the 351 in comparison. 4.00" x 3.50". Now the piston is moving at an average speed of 2,386mph for 6,000rpm. Adding that much velocity creates signficantly more force on the components like connecting rods.

Given the same strength rotating assembly, and assuming the engine isn't limited in rpm by valvetrain or power production at higher rpms, a 351 will always fail at an earlier rpm than a 302 because there will be greater forces acting against the internal connections. That being said, there are so many differences between the engines, that it's hard to duplicate an equal environment.

Don't make the mistake of assuming you can simply put beefier, stronger components into the engine because beefier components weigh more too. It's not just the piston that's moving. All the rest of the rotating assembly is also moving. Somewhere in the middle of heavier, stronger metals and lighter, weaker metals is the perfect balance of strength and weight reduction for an ultra high rpm engine. It's pretty hard to hit that level of rpms with a pushrod engine because of the valvetrain.

In summary, smaller stroke engines can theoretically rev higher than long stroke engines because they don't put as much force on the rotating assembly.

What Jeff is commenting on is my note about rpm = potential hp on a naturally aspirated engine. Since his engine is a finely tuned and balanced setup, he's able to spin his rotating assembly to extremely high rpms without whipping a rod through the side of his block (that's the plan anyway ) With his engine spinning at 9,400rpm, his engine is essentially pumping as much air/fuel through it as a 460ci engine at 6,500rpm. Of course, we're going to ignore the fact his engine will probably make more horsepower than that 460 at 6,500rpm because of all the power that's necessary to spin the heavy 460's engine components.

Engine dynamics are extremely complex, and that's why the folks that understand them so deeply make so much money.

[goodyear1984]

hey you need a min of .100 with steel rods and .130 with alum rods...and im guessing u have a stock bottom end so your looking at .100 .....get some good springs so theres no valve float...good luck

[Rev]

I spin my 306 to 6000 rpm quite frequently with no problems over the last 10 years. It is balanced and uses forged TRW flat top pistons. It does use stock rods but with stronger after market Pioneer rod bolts. I think the piston speed is around 2700 f/sec. It's the g forces on that rotating assembly that makes it come apart. That's directly related to piston speed and the mass and strength of the components, just as Unit said.

Rev

[goodyear1984]

hey rev what kind of valve train do you have