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dwightlooi

Pushrods = better fuel economy

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If you need a real world example, here's one... Despite 1.2 liters of extra displacement, 60 more horsepower and 80 more lb-ft of twist, the pushrod engine matches the DOHC V8 in city fuel economy and exceeds it's highway rating by 1 mpg.

Ford F150 4WD -- 5.0L DOHC-32v V8 -- 360 bhp @ 5500 rpm / 380 lb-ft @ 4250 rpm -- 14 mpg (City) / 19 mpg (Hwy)

GMC Sierra Denali 4WD -- 6.2L Pushrod-16v V8 -- 420 bhp @ 5600 rpm / 460 lb-ft @ 4100 rpm -- 14 mpg (City) / 20 mpg (Hwy)

Now, let's also compare a 5.3L Pushrod V8 with a 3.5 liter DOHC Bi-turbo V6. To its credit, the Twin Turbo V6 has 10 more hp and 37 more lb-ft of twist. But, the Pushrod V8 is actually the more economical engine with 1mpg better fuel economy both in city streets or on the freeway despite carrying a whopping 51% greater displacement (1.8L).

Ford F150 4WD -- 3.5L DOHC-24v Ecoboost V6 -- 365 bhp @ 5000 rpm / 420 lb-ft @ 2500 rpm -- 15 mpg (City) / 21 mpg (Hwy)

GMC Sierra Denali 4WD -- 5.3L Pushrod-16v V8 -- 355 bhp @ 5600 rpm / 383 lb-ft @ 4100 rpm -- 16 mpg (City) / 22 mpg (Hwy)

Still think that down sizing engine displacement is a good way to produce higher MPG numbers?

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Sources:

Edited by dwightlooi
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nice theory for a heavy large blocky vehicle.

does the same theory apply for a small, light, aerodynamic vehicle?

prove that a 2.5 litre pushrod 4 will have greater net benefit than a DOHC 2.0 or say a 1.5 turbo three.

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for 'smaller' vehicles it'd be like comparing the 4.3L v6 to a 3.5L dohc (or sohc) v6. or his theoretical ohv 3.6L v6 with ~280hp to a 2.2-2.5L I4 Turbo.

oh how i do kinda wish for a resurgence of wagons that could use engines like these for the hard working grocery getter/smaller toy pulling.

Edited by loki

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nice theory for a heavy large blocky vehicle.

does the same theory apply for a small, light, aerodynamic vehicle?

prove that a 2.5 litre pushrod 4 will have greater net benefit than a DOHC 2.0 or say a 1.5 turbo three.

It's already been proven.

The 2005 Malibu with the 3.5 V6 4-speed auto, 200hp, 220 lb-ft @3200 rpm

The 2005 Camry with the 2.4 I4 4-speed auto, 157hp, 162 lb-ft @ 4000 rpm

Both rated at 23//32, and while I don't know about the Camry, I know the Malibu could beat the highway number easily.

Edit: But there is no advantage to a pushrod inline engine... except, I suppose, that you could increase the stroke slightly. Pushrods have a substantial packaging advantage in V engines.

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Yea, larger displacements often work under relatively lighter loads, burning surprisingly less fuel.

That 5.3 posts a nice power/MPG combo.

If there was a way to do 32v pushrod V8, how would that change things?

It's not that large displacement engines run under light loads and are hence more efficient. In fact, with gasoline engines running under light loads increases specific fuel consumption -- gallons/hour/horsepower. So while running a V8 at 10% throttle at 1600 rpm saves fuel over running that same exact engine at a higher speed and bigger throttle openings, running a smaller displacement engine at even bigger throttle openings and load theoretically saves even more fuel. This is because gasoline engines are at their most efficient with the throttle wide open and are least efficient with the throttle closed. If you take the same V8 and remove 4 cylinders it is running under higher load and becomes more efficient -- that is the reason AFM is implemented.

If two engines of different displacement are otherwise identical, the smaller displacment one will always be more efficient. However, if two engines are otherwise identical, the smaller one will make less power. If the displacement is halved, so does the power output.

What happens over the last several decades is that manufacturers try to reduce displacement while increasing specific output, so that smaller displacement engines can still provide the same or similar outputs as the larger displacement engines they replace. They do so by increasing adopting DOHC heads, multiple intake and exhaust valves, turbocharging, etc. The problem is that while these design choices improve specific output and makes the smaller displacement engine more powerful, they also reduce fuel economy. And, oftentimes you end up with an engine where the fuel economy losses to the implements that increases specific output exceeds the gains from running at a higher load with a smaller displacement.

It's really not that complicated. If you take the Ford 5.0 Ti-VCT DOHC V8. It's reduction in displacement vs the GM 6.2 increases it's load and throttle opening at cruise. This improves fuel economy. But having four camshafts instead of one creates more friction, having 4 cam sprockets vs 1 creates more friction, having 32-valves instead of 16 creates more friction. All these things that are put in place to allow it to be powerful enough at 5.0 liters instead of 6.2 liters ends up causing it to lose more efficiency than is gained from the displacement reduction.

The more efficiency robbing things can be found on say the ecoboost V6 engine. Turbochargers = high exhaust back pressure and poor exhaust efficiency = lowered compression ratio and reduced thermal efficiency. Sure the turbos allow it to make more power than a naturally aspirated V6, but they do nothing at cruise while the low comprssion and high exhaust pressures reduce efficiency at cruise.

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nice theory for a heavy large blocky vehicle.

does the same theory apply for a small, light, aerodynamic vehicle?

prove that a 2.5 litre pushrod 4 will have greater net benefit than a DOHC 2.0 or say a 1.5 turbo three.

Well... OK... how about this... and here GM is on the losing side so maybe it'll help you dispel any notion of a bias.

Ford Focus 2.0 -- 2.0L DOHC-16v DI I-4 -- 160 bhp @ 6500 rpm / 146 lb-ft @ 4450 rpm -- 6-speed Automatic -- 27 mpg (city) / 38 mog (hwy)

Chevy Cruze 1.4T -- 1.4L DOHC-16V Turbo I-4 -- 138 bhp @ 4900 rpm / 148 lb-ft @ 1850 rpm -- 6-speed Automatic -- 26 mpg (city) / 38 mpg (hwy)

We are comparing two DOHC engines because there is no contemporary Pushrod I-4 to compare with. But as you can see, going to 1.4L while adding specific output with a turbo did not result in better fuel economy. Highway numbers are the same, city numbers are actually worse. And, I am using the numbers for the traditional 6-spd Auto Focus as opposed to the dual clutch Focus just to keep it fair.

BTW, you don't really need a 2.5L pushrod to be in the mix here. There is currently not that big a gap between the specific output of a pushrod engine and the typical DOHC mills you find in mainstream cars. At the same specific output as the LT1 V8 (74.65 bhp/L) a 2.0L pushrod V4 or I4 will be 149 hp and smack in the middle of this contest. Honestly, though, for an Inline-4 you are better off with an SOHC 8v design rather than a pushrod. The main reason for a pushrod design is so that both banks in a Vee configuration can share one camshaft. If you are an inline engine an SOHC configuration will allow you to eliminate the actuated mass from the pushrods, rev a little higher and/or use slightly lower tension valve springs.

Edited by dwightlooi
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What would the 6.3 liter produce in power and fuel economy if it's displacement were reduced to 5.0?

Uh... just look at the 5.3 Ecotec V8. It's not 5.0, but it;s close.

You pick up 2 mpg on both city and highway cycles.

  • GMC Sierra Denali 4WD -- 6.2L Pushrod-16v V8 -- 420 bhp @ 5600 rpm / 460 lb-ft @ 4100 rpm -- 14 mpg (City) / 20 mpg (Hwy)
  • GMC Sierra Denali 4WD -- 5.3L Pushrod-16v V8 -- 355 bhp @ 5600 rpm / 383 lb-ft @ 4100 rpm -- 16 mpg (City) / 22 mpg (Hwy)

There is also a negligible difference between the truck tuned Ford 5.0 (360 bhp / 380 lb-ft) and the GM 5.3. Howeverm the GM engine wins the fuel economy battle with 2 mpg (city) / 3 mpg (hwy).

  • GMC Sierra Denali 4WD -- 5.3L Pushrod-16v V8 -- 355 bhp @ 5600 rpm / 383 lb-ft @ 4100 rpm -- 16 mpg (City) / 22 mpg (Hwy)
  • Ford F150 4WD -- 5.0L DOHC-32v V8 -- 360 bhp @ 5500 rpm / 380 lb-ft @ 4250 rpm -- 14 mpg (City) / 19 mpg (Hwy)

BTW, here's the Toyota Tundra with it's 5.7L DOHC V8 for comparison; it's worse than all of the above. Heck, even the 4.6L DOHC is worse than all of the above...

  • Toyota Tundra 4WD -- 5.7L DOHC-32v V8 -- 381 bhp @ 5600 rpm / 401 lb-ft @ 3600 rpm -- 13 mpg (City) / 17 mpg (Hwy)
  • Toyota Tundra 4WD -- 4.6L DOHC-32v V8 -- 310 bhp @ 5600 rpm / 327 lb-ft @ 3400 rpm -- 14 mpg (City) / 19 mpg (Hwy)
Edited by dwightlooi
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side note Dwight et al, the ford 5.0L doesn't have DI., the eco boost 3.5L does. for a better comparo of valve actuation....

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side note Dwight et al, the ford 5.0L doesn't have DI., the eco boost 3.5L does. for a better comparo of valve actuation....

Yes, but if you look at the 2012 Sierra 4WD with the port injected 5.3L it's 15 (city) / 22 (hwy) mpg...

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How much of that highway number is coming from AFM though?

Probably about 1 no more than 2, remember that on the LT1 in non-AFM mode posts 28 mpg Hwy whereas with it on it posts 30 mpg hwy. On the Camaro it the L99 was worth 1 mpg on the freeway. City numbers were unchanged. With a heavier vehicle with lower fuel economy the effect will be less not more. This is so because in a heavier vehicle, the engine will be operating at higher loads without AFM anyway and the opportunities to use AFM will be less frequent hence the difference will be less pronounced. Also, the same percentage gain in fuel economy works out to less mpg(s).

Edited by dwightlooi
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To be fair, I would hope that the brand new engine would be more economical than the 3-year-old ones.

And Toyota's engines... have they done anything with them since they debuted?

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Sheldon

When are you going to ever learn to use all the facts and not just cherry pick only what works for your wild a$$ theories.

You also need to add in less weight, and other parameters like Direct Injection cylinder drop etc. that help the GM truck.

Sorry but it is the whole package not just the push rods that have delivered the MPG. If it were only the push rods then everyone would have them.

Bazinga!!!!!

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Sheldon

When are you going to ever learn to use all the facts and not just cherry pick only what works for your wild a$$ theories.

You also need to add in less weight, and other parameters like Direct Injection cylinder drop etc. that help the GM truck.

Sorry but it is the whole package not just the push rods that have delivered the MPG. If it were only the push rods then everyone would have them.

Bazinga!!!!!

Actually, the weights are about the same, The F150 Crew Cab Medium Bed 4x4 5.0V8 has a curb weight of 5461 lbs. The GMC Sierra 1500 Denali 6.2 V8 is 5370 lbs. That's a 1.7% difference. It's like the difference between a 3500 lbs car and a 3559 lbs car. Essentially negligible -- in the sense that it isn't enough to register as 1 mpg on the any test cycle. The Ford Ecoboost V6 is DI as well and it was in the comparison.

Edited by dwightlooi
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This really brings me back to the question I asked on the other thread, of why GM made the decision to put the 3.6L into the new Colorado instead of the 4.3L. Rhetorical question at this point since it does not seem as if there was (is) any engineering rationale behind said decision.

I have another question. Does the better ability for DOHC engines to breathe make them more amendable to forced induction, or is that irrelevant since the air has already been compressed before it enters the combustion chambers? Likewise with diesels, do they lend themselves better to a DOHC setup? I understand that the Euro V6 diesels that have made it here in some SUVs (and soon the Ram 1500) are DOHC because they are displacement tax limited to 3.0L, my question is if smaller V shaped diesels became popular in the United States, with no corresponding (and stupid) displacement taxes, would the same general rules hold true - pushrods would offer a smaller / lighter package that would likely offer efficiency advantages for similar power over its DOHC counterparts. I know that diesels are made heavier from the getgo so perhaps the added weight of the cams is fairly miniscule in relation to the overall engine weight?

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This really brings me back to the question I asked on the other thread, of why GM made the decision to put the 3.6L into the new Colorado instead of the 4.3L. Rhetorical question at this point since it does not seem as if there was (is) any engineering rationale behind said decision.

I have another question. Does the better ability for DOHC engines to breathe make them more amendable to forced induction, or is that irrelevant since the air has already been compressed before it enters the combustion chambers? Likewise with diesels, do they lend themselves better to a DOHC setup? I understand that the Euro V6 diesels that have made it here in some SUVs (and soon the Ram 1500) are DOHC because they are displacement tax limited to 3.0L, my question is if smaller V shaped diesels became popular in the United States, with no corresponding (and stupid) displacement taxes, would the same general rules hold true - pushrods would offer a smaller / lighter package that would likely offer efficiency advantages for similar power over its DOHC counterparts. I know that diesels are made heavier from the getgo so perhaps the added weight of the cams is fairly miniscule in relation to the overall engine weight?

If you can breathe naturally aspirated air better, you can breathe compressed air better. DOHC 4-valve engines will make more power with a surpercharger or turbocharger than a pushrod 2-valve engine. That is a given.

The question becomes more complicated if it pertains to whether the increase in output from DOHC 4-valve heads in forced induction application is worth the extra parasitic drag from the valve train, or the extra bulk & weight from the fat heads. Another thing that one needs to ponder is fuel economy. At cruise, the extra valvetrain drag, engine mass and reduced compression (if force induced) are all there, but the increase in output is not. At cruise you are operating largely off boost and you are choked by the throttle body not how well or how poorly the heads flow air. At cruise, the more cams, the more valves you have the worse your fuel economy becomes (for any given displacement). But then, again, in markets where you pay a displacement tax, legislators may have forced you to choose between paying the government or paying for fuel!

It may surprise many, but DOHC designs are actually better for diesels than they are for gasoline engines. The reason is simple. Diesel don't have a throttle body! At cruise, it runs with a wide open throttle (actually there's no throttle plate period) just like it does with the pedal to the floor. So while gasoline engine only benefit from four valve heads and it's freer flowing aspiration when you floor the pedal, diesels benefit from it all the time! Still, the extra drag, bulk and mass stays.

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