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Feel of HP & Torque

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Since Gollum Necro-Posed to this one, it gives me a chance to comment on something said earlier:

 

"Exactly, its not just engine torque, its the torque at the wheels. 4.62 gears, 275 ft. lbs. in my old 2,100 lb. NA race car pushed me and my passengers back in the seat very well from 4,000 to 7,500 rpms."

 

Cheap narrow-seat buying, short belted SOB! Not this prospective passenger it didn't! :(

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Since Gollum Necro-Posed to this one, it gives me a chance to comment on something said earlier:

 

"Exactly, its not just engine torque, its the torque at the wheels. 4.62 gears, 275 ft. lbs. in my old 2,100 lb. NA race car pushed me and my passengers back in the seat very well from 4,000 to 7,500 rpms."

 

Cheap narrow-seat buying, short belted SOB! Not this prospective passenger it didn't! :(

 

Cheap! Cobra Imola Pro S seats were $750 back in the day and I even bought the GT (wide butt) model for the passenger side. I offered to grease your ass up so you could slide into it but you demurred. :-)

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Hey I FIT in the seat, those damn non-adjustable belts set up for Spadanoesque midriffs is what did me in.

 

Ask Steve McQueen: The Blob chased him places he NEVER thought it would fit, but DID!

 

Same with me, I fit in stuff, but generally restraint devices defy me...

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Comparing a new Nissan 4 valve per cylinder engine to an older L series engine isn't fair....

 

Low end torque requires displacement and the ability to fill those cylinders at lower valve heights.(high VE%) The Nissan Titans 5.4 has heads that flow well

at a reasonably wide powerband, the 4 valve per cyl heads have this characteristic.

 

The valves are smaller but there are more of them...and the ports behind the valves are also more in number....this is good for high port velocities at low engine speeds and because there are two intake ports per cylinder....the engine can also breathe well at higher rpms.

 

My comment is oversimplified but I hope it makes sense.

 

If we could get our hands on 4 vale per cyl heads on our L series engines.....forget it...we'd be making some serious power.

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Comparing a new Nissan 4 valve per cylinder engine to an older L series engine isn't fair....

 

Low end torque requires displacement and the ability to fill those cylinders at lower valve heights.(high VE%) The Nissan Titans 5.4 has heads that flow well

at a reasonably wide powerband, the 4 valve per cyl heads have this characteristic.

 

The valves are smaller but there are more of them...and the ports behind the valves are also more in number....this is good for high port velocities at low engine speeds and because there are two intake ports per cylinder....the engine can also breathe well at higher rpms.

 

My comment is oversimplified but I hope it makes sense.

 

If we could get our hands on 4 vale per cyl heads on our L series engines.....forget it...we'd be making some serious power.

I don't think 4 valves or 2 valves has as much to do with torque as the cam. You can have higher port velocities with a 4 valve head, but if you have a cam that breathes well at higher rpm it's going to open the valve farther, reducing the port velocity and it's not going to have much torque. Likewise you can have lower port velocities with a smaller cam and a 2 valve head and it won't breathe as well up high but it will have a lot of torque. What would work best would be adjustable valve lift so that you could have low lift and duration at low rpms and high lift and long duration at high rpms. I know they have experimented with electrostatic valves, but I haven't heard anything lately on that front, but that's the holy grail of valve operation IMO. If it were possible to program whatever lift and duration at whatever throttle position and load is present at the time, then I think you'd see a real revolution in the internal combustion engine.

 

It may be true that 4 valves is better than two, but I don't think the 4 valve head is so far superior that "we'd be making some serious power" with one but we can't with a 2V head. Just look at some of the threads on max hp. You'll see things like Electromotive getting 1100 hp out of the L engine with a 2 valve head back in the early 80s. Maybe you could get more with a 4 valve head, but at some point it's going to be more of a hindrance than a help to overall speed unless you're talking about drag racing on wrinkle wall slicks, and then you have much better engine choices than the L, regardless of what head you put on it.

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I don't think 4 valves or 2 valves has as much to do with torque as the cam. You can have higher port velocities with a 4 valve head, but if you have a cam that breathes well at higher rpm it's going to open the valve farther, reducing the port velocity and it's not going to have much torque. Likewise you can have lower port velocities with a smaller cam and a 2 valve head and it won't breathe as well up high but it will have a lot of torque. What would work best would be adjustable valve lift so that you could have low lift and duration at low rpms and high lift and long duration at high rpms. I know they have experimented with electrostatic valves, but I haven't heard anything lately on that front, but that's the holy grail of valve operation IMO. If it were possible to program whatever lift and duration at whatever throttle position and load is present at the time, then I think you'd see a real revolution in the internal combustion engine.

 

It may be true that 4 valves is better than two, but I don't think the 4 valve head is so far superior that "we'd be making some serious power" with one but we can't with a 2V head. Just look at some of the threads on max hp. You'll see things like Electromotive getting 1100 hp out of the L engine with a 2 valve head back in the early 80s. Maybe you could get more with a 4 valve head, but at some point it's going to be more of a hindrance than a help to overall speed unless you're talking about drag racing on wrinkle wall slicks, and then you have much better engine choices than the L, regardless of what head you put on it.

 

While valve lift is somewhat dependent (metallurgical/physical issues) on duration, I don't see why you'd want lower lift at lower rpm unless maybe you factor in some mixing processes (tumble/swirl). Ideally, you'd have maximum valve lift over the duration of the valve's open period, or essentially very fast ramp rates in order to increase cylinder filling (VE).

 

A lot of people put too much focus into port velocity. The main function of port velocity is to keep the mixture distributed and the fuel atomized in a carburated or throttle-body fuel injected engine. There are kinetic effects that occur at higher speeds that are slightly dependent on port velocity, but really you want those ports to be big.

 

Cam duration (along with port and manifold design) is what really controls VE over the operating range of the engine, given a high enough valve lift. At lower speeds, you want lower duration because longer durations encourage gas reversion, where exhaust pressure is higher than cylinder pressure or cylinder pressure rises above intake pressure. At high speeds, you want longer duration since gas speed is higher which increases both intake kinetic energy as well as exhaust scavenging, making longer durations more efficient.

 

A cam is really designed for one operating point where it is most efficient. Being able to alter this point as the engine accelerates is a huge bonus. Changing it continuously would be even better. Many modern engines already do this.

 

If I had a choice of constantly varying a part of the valvetrain, variable valve duration would be on top! Although variable lift is cool as well, especially when the intake valve doubles as the throttle, like BMW!

 

I think I've digressed a bit, but it's an interesting topic! I've had some free time to dig into Heywood's IC Engine Fundamentals...

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