RPM Questions

duo_z

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Jan 24, 2009

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Feb 23, 2011

#1

• Feb 23, 2011

• #1

What determines safe top RPM for an engine. How do you know when is the best time to stop revving other then being past the power band?

 

dodgestang

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Dec 15, 2003

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Feb 23, 2011

#2

• Feb 23, 2011

• #2

duo_z said:

What determines safe top RPM for an engine. How do you know when is the best time to stop revving other then being past the power band?

Click to expand...

two things will tell you have reached max RPM for an engine

1. When the valves 'float' meaning they no longer have time to close on the intake side prior to the spark igniting the mixture thereby robbing additional power.
2. Because the valves are floating the piston makes contact with them and terminates all engine function

It is usually a good idea to stop revving before either of these really happens.
The type of cam, intake, heads, and springs in use all go into the calculation of what your power band and max RPM should be.

 

C

CMEFLY

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Dec 11, 2003

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bonney lake WA.

Feb 23, 2011

#3

• Feb 23, 2011

• #3

This method has always worked for me, I shift just before it blows up.

 

blown65

Founding Member

Jul 7, 1999

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Queen Creek Arizona

Feb 23, 2011

#4

• Feb 23, 2011

• #4

I know most shift around 600-800 past peak HP when drag racing, but like what dodgestang said most will be limited to your valvetrain keeping up.

 

J

j69302

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Jan 31, 2006

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Feb 23, 2011

#5

• Feb 23, 2011

• #5

Cam manufacturers will usually recommend valve spring ratings and will usually list a valve float RPM for those springs. These are specific to each cam.

Also pushrods com into play too as cheap ones can bend.

connecting rod bolts too.

 

PoppyMod

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Jun 27, 2010

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Severna Park, MD

Feb 24, 2011

#6

• Feb 24, 2011

• #6

j69302 said:

Cam manufacturers will usually recommend valve spring ratings and will usually list a valve float RPM for those springs. These are specific to each cam.

Also push rods com into play too as cheap ones can bend.

connecting rod bolts too.

Click to expand...

+1 on your explanation!
Revving until it blows up is just not acceptable, is it?
Good cam grinders, will offer the "red line" with the usable shift point.
Always install hardened P/Rs and check the geo.

 

duo_z

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Jan 24, 2009

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Feb 24, 2011

#7

• Feb 24, 2011

• #7

What is the "geo"?

 

J

j69302

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Jan 31, 2006

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Feb 24, 2011

#8

• Feb 24, 2011

• #8

geometry.. Check the valve train geometry for proper pushrod length and rocker arm to valve tip contact.

 

PoppyMod

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Jun 27, 2010

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Severna Park, MD

Feb 24, 2011

#9

• Feb 24, 2011

• #9

j69302 said:

geometry.. Check the valve train geometry for proper pushrod length and rocker arm to valve tip contact.

Click to expand...

Spot on! Bud.
Thanks!

 

duo_z

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Jan 24, 2009

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Feb 24, 2011

#10

• Feb 24, 2011

• #10

j69302 said:

geometry.. Check the valve train geometry for proper pushrod length and rocker arm to valve tip contact.

Click to expand...

Oh, ok, sorry I missed that.

 

rbohm

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Apr 12, 2002

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tucson,az

Feb 25, 2011

#11

• Feb 25, 2011

• #11

as you get to know your car, you will feel when you are past your engines horsepower peak, as the engine will "nose over", meaning it will feel like it is starting to lose power, because it is. at that point you want to pull the next gear. contrary to popular opinion, valve float does not always mean the valves are hitting the pistons. that depends on the rpm the engine is turning at the time. if you float the valves at say 5000 rpm, chances are the valves wont hit the pistons, but much beyond that they will.

 

robbz28

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Sep 23, 2009

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Epps, LA

Feb 25, 2011

#12

• Feb 25, 2011

• #12

To add to all the good info you guys gave about the valve train and cams....your rotating assembly is also a factor, not so much as the valve train, but if you are running stock cast internals they arent as strong as the good forged stuff, but if they are balanced properly and you arent racing all the time and keeping things below 6500 they should be fine.

 

blown65

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Queen Creek Arizona

Feb 25, 2011

#13

• Feb 25, 2011

• #13

Kinda depends on piston to valve clearance.