Push Rod vs. DOHC engines

[Rayme]

Originally Posted by ZDan

4.6 liter DOHC Ford mod motor vs. 5.0 liter OHV engine it replaced:


4.0 liter DOHC Toyota 1UZ V8 vs. 5.7 liter LS1 (same external dimensions as current 6.2):


Benefits of OHV cam-in-block design are pretty much limited to V-engine layouts, where there's a big valley with plenty of room for a camshaft and pushrods. Inline engines definitely want to be SOHC or DOHC.

Presumably talking about the old air-cooled H-D twin and not the later Rotax mill (which was DOHC/4vpc). Suffice it to say, the very long-stroke air-cooled Harley engine was never by any stretch designed to be a performance engine. Not really a valid comparison.

LS3 vs. S65 is a MUCH more reasonable comparison.


Yup. I'm not averse to any engine architecture that gives me the power I want for as little weight as possible.
The LS V8 does lend itself to being swapped into smaller/lighter-weight cars much more readily than physically bigger DOHC V-engines.

And thanks to Camaro SS sales (nothing but donor cars to me!), there will be a good supply of LS3s and T6060 transmissions for some time to come

Damn those are SMALL... or is it the other ones that are BIG ?

EDIT: So, is GM the only company doing pushrods engines for cars now?

[LSxJunkie]

Originally Posted by dorifuto

Gm dosent liscence magnetic suspention to anybody. Delphi the gm division responsible for the technology was sold to the damn red Chinese.

The Ferrari 599, Audi R8, and current Audi TT were all released in 2006. They all feature MagneRide. The Ferrari California was released in 2008. Delphi was sold to BeijingWest Industries in 2009. GM owned the patent when it was licensed out.

Originally Posted by ottopilot

Damn those are SMALL... or is it the other ones that are BIG ?

EDIT: So, is GM the only company doing pushrods engines for cars now?

And Chrysler in the new HEMI (5.7-6.1-6.4) and 8.4L Viper V10.

[rice_classic]

I think the only thing GM has done recently that was "bold" was fly their private jets down to their meeting with congress to ask them for their bailout. That was pretty bold if you ask me.

Edit: that was a lowblow, I'll admit.


These pictures Zcan posted of the comparisons of V8's was terrific. That's excellent argument for pushrods if I've ever seen one.

[Calum]

I always figured GM kept the push rod in their flagship for the same reason Porsche keeps rear engines their flagship. It gives their customers that warm fuzzy feeling.

[Dimman]

Originally Posted by LSxJunkie

This is the 1UZ vs. the LS1. The 7.0L, 505hp LS7 has the same external dimensions. The 638hp LS9 has nearly the same external dimensions, save for the blower and intercooler mounted where the intake manifold is on the LS1. Extra couple of inches at the top of the motor.

Someone just posted about their old ZR1.

A side by side if the LT1 and whatever the DOHC version is called would be interesting. Back in the day that made 4 valves look like they dominated OHVs.

[blu_]

wow this thread. GM makes some amazing pushrod engines and if you look at how much these engines have improved over the years it makes the "selling outdated tech" argument invalid.

There is a good argument to made that dohc engines will get better fuel economy in theory, but as far as NA hp per the weight and total size of the engine GM's pushrod v8's are pretty damn good.

[serialk11r]

Originally Posted by blu_

wow this thread. GM makes some amazing pushrod engines and if you look at how much these engines have improved over the years it makes the "selling outdated tech" argument invalid.

There is a good argument to made that dohc engines will get better fuel economy in theory, but as far as NA hp per the weight and total size of the engine GM's pushrod v8's are pretty damn good.

Well in theory the DOHC design should get you more power per unit mass since the valves no longer become as much of a limiting factor and as we all know short stroke gives more rpms and more power. It's just that the few engines revving past the point where pushrods become a problem don't rev that much higher so they don't fully utilize the DOHC in some sense.

[ZDan]

Originally Posted by serialk11r

Well in theory the DOHC design should get you more power per unit mass

DOHC will get you more power per unit *displacement*, not necessarily more per unit mass. For the same engine *mass*, for V-configured engines my money is on OHV cam-in-block over DOHC.

For the same engine *mass*, you can have a much much larger-displacement OHV engine, as DOHC adds a lot of mass (and physical size).

[ZDan]

Originally Posted by blu_

There is a good argument to made that dohc engines will get better fuel economy in theory, but as far as NA hp per the weight and total size of the engine GM's pushrod v8's are pretty damn good.

Actually, there are very good arguments to be made that OHV get better fuel economy. Reduced frictional losses with OHV vs. DOHC, and reduced viscous flow losses drawing air past/through fewer, bigger valves.

The bigger, heavier, less aerodynamic Camaro SS gets better mileage than the smaller, lighter, more aerodynamic M3 with the same power.

Buicks used to get amazing mileage for big/heavy luxury cars with OHV V6s. Mileage went down when they went DOHC.

[GTB/ZR-1]

Originally Posted by Dimman

Someone just posted about their old ZR1.

Me...

I'm very partial to my old DOHC LT5. My motor is an ex-World Challenge built w/ slightly higher duration cams & 368ci (vs stock 350 sitting in the garage).

It pulls like a freight train to the 7550rpm cutoff. Most cars I've had started laying down while approaching redline--not this one... I have to watch the tach like a hawk, or I'll smack the limiter because it's still @ peak HP.
I've had several C5 & C6 Z06s & they've all been sold & my old ZR is still standing after 18yrs...

It is much larger/heavier than the LSX & new LT variants coming out in the C7, no doubt; but nothing sounds like that WOT pull to redline like an LT5

[Dimman]

Originally Posted by ZDan

Actually, there are very good arguments to be made that OHV get better fuel economy. Reduced frictional losses with OHV vs. DOHC, and reduced viscous flow losses drawing air past/through fewer, bigger valves.

The bigger, heavier, less aerodynamic Camaro SS gets better mileage than the smaller, lighter, more aerodynamic M3 with the same power.

Buicks used to get amazing mileage for big/heavy luxury cars with OHV V6s. Mileage went down when they went DOHC.

The valve thing is completely wrong. There is no question that 4 valves per cylinder dominate 2 vpc in pretty much every practical category.

The only area where single intake valves out perform dual is at flow at 0.25D lift rate (when the valve head is theoretically no longer an obstruction) with the same port area. Basically for the same un obstructed flow area, a single large port will outflow 2 smaller ports with the same combined area as the big single. This comes from turbulence between the two air streams, and ends up with around 90-95% of the single.

However, there are a few catches. First is that the 2v will need more cam lift to reach the same .25D since the valve is much larger. This means it takes slightly longer to reach max flow, is harder on the valvetrain, and can compromise compression. Second is the 4V heads dominate at low lift since in that situation, when the valve is still the primary obstruction, it is lift x valve circumference that is the flow area, rather than port (minus stem). This gives 4V much better midrange for the same max power. It sounds strange, but there are few modern situations where we can directly compare them on motors of the same displacement. Third is combustion chamber shape. The layout of a pent-roof 4V allows better flame propagation/efficient burn from the same compression ratio, which means either more power or less fuel.

So in short the OHV's advantages are not from 2V heads, but from its cost and packaging.

[ZDan]

Originally Posted by Dimman

The valve thing is completely wrong. There is no question that 4 valves per cylinder dominate 2 vpc in pretty much every practical category.
This gives 4V much better midrange for the same max power. It sounds strange, but there are few modern situations where we can directly compare them on motors of the same displacement.

Of course they won't be comparable for engines of the same displacement.

For V-engines of the same cylinder count, 2v OHV will generally be comparable to or superior to 4v DOHC in terms of power/engine size (external dimensional engine size, not displacement), power/engine weight, and fuel economy for the same power levels.

Third is combustion chamber shape. The layout of a pent-roof 4V allows better flame propagation/efficient burn from the same compression ratio, which means either more power or less fuel.

Comparing performance engines of the same power level and engine size/weight, the larger-displacement OHV will generally more than make up for any burn efficiency advantage by having lower frictional and pumping losses operating at the lower rpm at which it makes the same cruise power.

So in short the OHV's advantages are not from 2V heads, but from its cost and packaging.

Cost and packaging are of course tremendous advantages as well...

For fixed displacement, no doubt DOHC 4v pentroof is the way to go. But if you're not displacement limited...

[Dimman]

Originally Posted by ZDan

Of course they won't be comparable for engines of the same displacement.

For V-engines of the same cylinder count, 2v OHV will generally be comparable to or superior to 4v DOHC in terms of power/engine size (external dimensional engine size, not displacement), power/engine weight, and fuel economy for the same power levels.


Comparing performance engines of the same power level and engine size/weight, the larger-displacement OHV will generally more than make up for any burn efficiency advantage by having lower frictional and pumping losses operating at the lower rpm at which it makes the same cruise power.



Cost and packaging are of course tremendous advantages as well...

For fixed displacement, no doubt DOHC 4v pentroof is the way to go. But if you're not displacement limited...

I don't think the valvetrain friction is accurate because of rocker arms and pushrod flex. But I'm not 100% certain, so more research will be done.

Here's a good example of why GM clinging to OHV has a bad rep. Vortec 4.3L V6, 190hp @ 4500 rpm, 260 lb-ft @ 3400 rpm. Compare with Toyota's 5VZ-FE of the same era. 3.4L V6, 190 hp @ 4800 rpm, 220 lb-ft @ 3600 rpm.

There was no need for Toyota to go bigger to meet their requirements. If they matched the displacement it would be an embarrassment for GM.

Now GM can talk about the packaging advantages all they want, but in this example it is complete bullshit. Because GM just showed how cheap and lazy they were, and made that motor by chopping two cylinders off of their small block V8. Which ended up with a 90° V6. Not exactly compact...

So there is a lot more than just OHV = smaller motor to consider. Kind of like the solid axle = suck argument.

[serialk11r]

Lower frictional losses? If you make a 2 valve OHC engine, you'll have less frictional losses. The extra valves are useful though :P

I'm not convinced there's less friction. Pushrods add a lot of mass to the valvetrain, and the valvetrain is one of the largest sources of friction power in the engine.

What is the practical rev limit of an OHV motor? I imagine even with titanium pushrods, titanium lifters, titanium valves, the limit is a lot lower because of the mass. DOHC motors with normal steel valves can hit 15000rpm on motorbikes, they just have very stiff springs in there. Over 200hp/L might possibly make up for less displacement per unit mass. This is obviously an extreme case, but I bet at the same reliability levels, OHV can tolerate much less revs.