8 Valves vs 16 Valves at Low RPM

[NICAT]

Hi.
16 valves let engine breathe easily at high RPMs means more power.
But how about at low RPMs ? is the power same?
I have found some contradicting answers to this question. Some says power is same, while some even claims that 8Valves are performing better at low RPMs than 16Valves.

Copy-paste below:
Quote
There's no real differene in FE just from the number of valves. A lot of 4v/cyl engines close off half their intake valves at lower rpms to improve FE and torque. Restricting the intake increases turbulence and helps fuel mix better. At higher rpm they open to allow more air in. With more air coming in a 4v/cyl engine can burn fuel faster than a 2v/cyl engine. If you drive the car hard you can get lower FE out of a 4v/cyl engine than a 2v/cyl one, at least in theory. The key word here though is can. It just gives you the option of going faster at the expense of less FE.

 

[OldSparks]

Yamaha employed 5 valves per cylinder in their litre bikes for many years, but now the R1 has 4 valves per cylinder. I believe BMW leads in HP for production NA litre bike engines at the moment...

 

[CR94]

16 works much better than 8 in an 8-cylinder engine.

 

[dogememe]

Originally Posted by CR94
16 works much better than 8 in an 8-cylinder engine.


Hahahahaha yeah I don't know how one valve per cylinder would work... Maybe some sort of blend door above the valve to determine whether the port is connected to the intake manifold or exhaust manifold? Except probably people would install Dorman replacements that would go bad after a week. Though I'd rather change a valve cover gasket than rip out a dashboard to change a blend door.

 

[CT8]

Originally Posted by dogememe
Originally Posted by CR94
16 works much better than 8 in an 8-cylinder engine.


Hahahahaha yeah I don't know how one valve per cylinder would work... Maybe some sort of blend door above the valve to determine whether the port is connected to the intake manifold or exhaust manifold? Except probably people would install Dorman replacements that would go bad after a week. Though I'd rather change a valve cover gasket than rip out a dashboard to change a blend door.

2 stroke diesel.

 

[dogememe]

Originally Posted by CT8
Originally Posted by dogememe
Originally Posted by CR94
16 works much better than 8 in an 8-cylinder engine.


Hahahahaha yeah I don't know how one valve per cylinder would work... Maybe some sort of blend door above the valve to determine whether the port is connected to the intake manifold or exhaust manifold? Except probably people would install Dorman replacements that would go bad after a week. Though I'd rather change a valve cover gasket than rip out a dashboard to change a blend door.

2 stroke diesel.


Just pretend my Christmas joke was funny ok? LOL I didn't think of a 2-stroke. Oops.

 

[cven]

Originally Posted by OldSparks
Yamaha employed 5 valves per cylinder in their litre bikes for many years, but now the R1 has 4 valves per cylinder. I believe BMW leads in HP for production NA litre bike engines at the moment...



The new V4 Ducati might be the new big dog.


Smaller intake runners tend to make more low end ..... IIRC.....I think the same thing applies here???

 

[UncleDave]

Originally Posted by cven
Originally Posted by OldSparks
Yamaha employed 5 valves per cylinder in their litre bikes for many years, but now the R1 has 4 valves per cylinder. I believe BMW leads in HP for production NA litre bike engines at the moment...



The new V4 Ducati might be the new big dog.


Smaller intake runners tend to make more low end ..... IIRC.....I think the same thing applies here???



port velocity gives you good low end -through whatever combination of runners, valves, lift /durations you get it from

UD

 

[CT8]

No mercy !!

 

[Oildudeny]

Yesterday's big blocks made plenty of power with 2 valves.

 

[Oildudeny]

If I'm not mistaken the corvette has 2 valves per cylinder

 

[LotI]

2002 Honda VFR800 Interceptor introduced a VTEC design that would use 2 valves under 6800rpm and open the other 2 above it. I have a 2003 and there is a distinct power and sound difference when the other valves kick in.

It means that this engine is usable from 2000 rpm to it's 11,750 rpm redline. It also means that adjusting valves is a multi-hour job, and requires special shims for the VTEC valves. I've done it twice.

VTEC Yo!
The VFR800 was the first non-JDM motorcycle to use VTEC valve-gear. Honda used VTEC to meet tightening noise and emissions standards and to increase the peak engine horsepower. Based on the VTEC-E system, the simplified motorcycle version of VTEC employs only two of the four valves per cylinder when operating at lower engine speeds. All four valves per cylinder are engaged above approximately 6,800 rpm. This is initiated by an electronically actuated oil spool valve, which send oil pressure to the lifter actuators, which then move the engagement pins into place above the valve stem, allowing the remaining two valves to open. This design allows for variable valve timing as well, since the cam lobe profiles can be made different. After much criticism of the abruptness of power transition, Honda lowered the VTEC activation rpm threshold to 6,400 rpm in 2006. The VTEC disengages two cylinder valves when the engine speed drops again below 6,100 rpm.

 

[ragtoplvr]

Both corvette, camero, and hemi chryslers in charger and challanger have pushrods and 2 valves per cylinder. They all have world class perfromance, The newest camero had Nuremberg record for a while the corvette and camero are both in the top 10. The science of port flow and camshaft design, has reduced the difference to a minimum, only the ability to time exhaust and intake seperately is the big advantage and that can be fixed. I have to say a nice short cam chain is a huge advantage over the EXPENSIVE and shorter lived OHC cam drives if you are kieping a car or buying used.

Rod

 

[Silk]

The Yamaha XT600 had twin carbs, one a slide type, the other CV. At low throttle openings only the slide carb worked, and as the throttle was opened more the CV carb came in. So at low engine speeds (off road) only one intake port was working, with higher air speed.

 

[Wagonmaster261]

When accounting for same cubic inch displacement, 4v heads make more power across the range than 2v heads.
https://www.enginelabs.com/news/jon-kaase-racing-wins-amsoil-engine-masters-with-409ci-ford/
They let the Ford 4v engines in the one time. 1-2-3 finish over the LS types, and then the rules changed to not allow them anymore...

An in-depth look at the winning build. Mr. Kaase goes over the advantages he found in the development. https://www.hotrod.com/articles/jon-kaases-ford-four-valve-mod-motor/

There has to be good reason when even Toyota went to 4v heads on 4 cylinders for greater efficiency a long time ago.

 

[CR94]

Excuse my first silly reply. The only simple serious answer to the original question (which evidently assumed a 4-cylinder, 4-stroke engine) is "It depends on the details." That's why you found "conflicting answers."

 

[alcyon]

I think I get what the OP is trying to ask. Since supposedly a 4 valve per cyl breathes better at higher rpm, would it actually use less fuel if driven a bit harder versus babying it ?
I know toyota had TVIS back in the 80s that blocked off intake runners at lower rpm.

 

[HangFire]

What CR94 said...it depends.

In theory, you can get faster intake charge velocity with 1 intake valve (and proper induction tuning) then with 2 always-on valves tuned for higher RPM power. That describes the SOHC VTEC engine in my Honda Pilot.

But with advances in induction tuning and now with cam phasing/variable valve timing, there are other ways to improve low-end response.

 

[alcyon]

Originally Posted by HangFire
What CR94 said...it depends.

In theory, you can get faster intake charge velocity with 1 intake valve (and proper induction tuning) then with 2 always-on valves tuned for higher RPM power. That describes the SOHC VTEC engine in my Honda Pilot.

But with advances in induction tuning and now with cam phasing/variable valve timing, there are other ways to improve low-end response.


Lets keep it to 1980s and 1990s vehicles since thats what I am driving and I would like to know.

 

[supton]

With port injection (let alone direct) does intake velocity matter that much on a normally aspirated setup? I know if you have sufficient velocity you can get a dynamic boost in pressure (more air, think supercharging ala tunnel ram), but is it that much of an effect? Does high velocity get that much swirl that port injection gets a real big boost in mixing? Seems like it must, otherwise why go to lengths to disable an intake valve (or lower lift) at lower rpm. Although not everyone does that (not aware of it on my two Toyota's, outside of playing with valve timing).