high revving engine ejamakashun

[Guest nevarmore]

I was flipping through one of the Mopar mags at work today, they were testing out a new 340 build that had 48 degree heads instead of 53 degree. They said this would allow the engine to rev higher. Presumably, this had something to do with it running on a lower octane fuel, the article didn't specify.

 

How does the valve angle allow an engine to rev higher?

 

What sets something like a crotch rocket engine that can run way up above 10,000 rpms compared to a big block that redlines at less than 6,000? What about the little screamers that are used in IRL and CART?

[Guest stevil]

One of the biggest factors for RPM and redline, is the bore versus stroke. Piston speed. Two engines with the same bore, the one with the shorter stroke would be able to rev higher. Torque is dependant on stroke as well, HP is affected more by bore. Like when you hear someone that strokes a 350 to a 383. They gain HP, but gain alot more TQ.

 

My car has a 4" bore, 3.48" stroke for example, 6000 rpm redline. A CART car probably has a 2" stroke or something, maybe a smaller bore, and it can rev up to like 15,000 rpms. Smaller displacement, different cams, heads and such... but the key factor is the shorter stroke. I'm sure street bikes have a much shorter stroke than a Harley.

 

Airflow affects how much power you can make, so the new Mopar design probably makes power higher up in RPMs, where the old design the power fell off and revving higher wasn't needed? Not really sure how valvetrain would limit RPMs much, but I can see it limiting power.

[SupraGlue]

This is to answer some of what makes a high-revving motor different from a "normal" one.

 

As engine rotation speeds increase, stresses on internal engine components will tend to increase exponentially. The lower the rotating mass, the lower the stresses are, and the higher the RPM potential can be. As Steve mentioned, bore x stroke is a big factor, but not the only one.

 

Some of the limiting factors that need to be addressed in an engine revving higher are:

 

- Valve mass and springs. Heavy valves need stiffer springs to keep them on their seats (otherwise they will float). Hence the use of multivalve heads on high revving motors, and the use of sodium-filled valves, etc.

 

- Pushrods. Higher spring pressures will put more stress on pushrods, tending to bend them. If you ever saw the old Penske-Mercedes pushrod Indy motor, it had something like 3/4 diameter pushrods that were very short. Shim and bucket overhead cam eliminates this factor. On traditional domestic V8s it's addressed with hardened pushrods.

 

- Rocker arms and studs. Higher spring pressures will put more stress on the rocker arms and rocker studs. Again, this factor is eliminated with shim and bucket OHC. On a high-revving traditional domestic V8 you'll use a rocker shaft and needle bearing rockers, which can be machined from billet or forged aluminum.

 

- Cam (and lifters, if any). Will need to be able to last under higher spring pressures, and also deliver higher duration and lift needed to allow the engine to breathe at the higher revs. There is also a lot of friction at the lifter/cam interface, so for all of those reasons, roller-tip lifters will be used on high-revving cam-in-block engines. Again, shim and bucket OHC eliminates this as well.

 

- Rod material. Higher RPM will cause the rod to stretch or fail. The rods need to be made of a material that will do neither. This also applies to rod bolts. Usually this means a forged aluminum or forged steel alloy.

 

- Crank. Same as rods, except that the material will be a forged steel alloy, or sometimes machined from a forged billet. Shorter stroke means less rotating mass for the crank. Fewer cylinders *sometimes* means fewer crank throws and less rotating mass for the crank. Shorter cranks will be stiffer and less likely to stretch in the plane of rotation, which is one advantage of the V cylinder configuration.

 

- Main bearing caps. These needs to be more securely fastened (typically more bolts, cross bolted or girdled) to withstand the stresses of high RPMs. Some blocks need to be machined for that. This is much easier to engineer on an inline motor.

 

- Head ports. These need to be large enough to flow the air the engine now requires, both on the intake and exhaust side. Valve area will need to be larger also, which means bigger valves or more valves.

 

Of course, on a purpose-built race motor, all tolerances and clearances will be much, much tighter than a production street motor.

 

Sorry. That's kind of a long answer. smile.gif

 

As far as the valve angle of those heads goes, that could affect a couple of things:

 

- It might allow for larger ports and a less shrouded intake valve, so that the engine can breathe at higher RPM.

 

- It might allow for higher valve lift before piston-valve contact occurs, also helping high RPM breathing.

 

- It might allow for a better spark plug location, giving a better burn at the same compression ratio, and lowering octane demand.

 

- It might alter the valvetrain geometry so that it is less prone to float the valves, or reduces stress on valvetrain components.