Interesting DOHC vs. pushrod 500+ motors

[todesengel]

Originally Posted by Paul Workman

Well, assuming they both had the same "output" (a BIG IF), that would depend on which one was the higher maintenance, I recon!

well, it may be a big if, but it is more possible than the engine posted scenario.

I will bow out now so the big kids can talk and keep it civil

[Aurora40]

Originally Posted by todesengel

To your question about hp/l, if an engine is one liter larger, physically the same size, and made the same power it means it is less efficient.

Efficiency requires something to be used, and low efficiency means something is being squandered. What is that thing in the case of low hp/liter?

hp/liter doesn't relate to fuel efficiency, nor does it relate to power/mass or power/physical size.

Is "displacement" a thing that needs to be conserved or carefully consumed? Can we run out of it?

[Aurora40]

Originally Posted by Paul Workman

The underlying supposition is fuel economy AND hp being the elements in the equation. But, those are some pretty big "IFs" thar! IF the same power, IF they drove the same, IF they get the same economy, IF they are the same physical size...Then hp/l would not be a factor. But, is that possible?

There are tons of examples of smaller displacement not translating to smaller size, or better economy.

One of GM's, and supposedly the auto industry's, most efficient engines in terms of brake specific fuel consumption, was the "lowly" Buick V6. It's an ancient design that was often criticized for being of large displacement and old pushrod technology compared to its peers. In the late 90's it made about 200hp from 3.8L when Honda and Toyota were making that from 3.0-3.5L engines.

Was it less efficient? No. Was it less powerful? No. Was it heavier or larger? No. It just had more displacement. It also was a hell of a lot cheaper. People viewed it as inferior, and that matters for sales. The engine is gone now.

Look at the LS4 vs the FWD Northstar. The 303hp 5.3L LS4 gets substantially better fuel economy than the 300hp 4.6L L37, and it is small enough to fit in W-body/midsize cars. The Northstar could only fit in full size cars. Unfortunately they were never in the same car for a direct EPA economy comparison, but a look at a Bonnie GXP (probably the lightest car to get the N*) to something like a Buick LaCrosse Super (the largest car to get the LS4) and it is no contest.

It is not a big "If" to have a larger displacement engine that makes the same power as a smaller displacement one, while still having similar economy, size, and weight. DOHC setups are physically large and heavy. You may be able to do more with less displacement, but why do you need to? Take the space you saved in head size and turn it into displacement.

In spite of the low power/weight of something like the LS1/2/3/7, you would be hard-pressed to name a "smaller" production engine that makes the same amount of power as it, is lighter and physically smaller, and uses fuel as efficiently.

[Paul Workman]

Originally Posted by Aurora40

There are tons of examples of smaller displacement not translating to smaller size, or better economy.

One of GM's, and supposedly the auto industry's, most efficient engines in terms of brake specific fuel consumption, was the "lowly" Buick V6. It's an ancient design that was often criticized for being of large displacement and old pushrod technology compared to its peers. In the late 90's it made about 200hp from 3.8L when Honda and Toyota were making that from 3.0-3.5L engines.

Was it less efficient? No. Was it less powerful? No. Was it heavier or larger? No. It just had more displacement. It also was a hell of a lot cheaper. People viewed it as inferior, and that matters for sales. The engine is gone now.

Look at the LS4 vs the FWD Northstar. The 303hp 5.3L LS4 gets substantially better fuel economy than the 300hp 4.6L L37, and it is small enough to fit in W-body/midsize cars. The Northstar could only fit in full size cars. Unfortunately they were never in the same car for a direct EPA economy comparison, but a look at a Bonnie GXP (probably the lightest car to get the N*) to something like a Buick LaCrosse Super (the largest car to get the LS4) and it is no contest.

It is not a big "If" to have a larger displacement engine that makes the same power as a smaller displacement one, while still having similar economy, size, and weight. DOHC setups are physically large and heavy. You may be able to do more with less displacement, but why do you need to? Take the space you saved in head size and turn it into displacement.

In spite of the low power/weight of something like the LS1/2/3/7, you would be hard-pressed to name a "smaller" production engine that makes the same amount of power as it, is lighter and physically smaller, and uses fuel as efficiently.

Well, without casting dispersions, this thread has evolved to become less of an engineering epiphany than it has a morass of anecdotal meanderings. Interesting points have been made by you and other, to be sure. But some hard core engineering stuff is needed here - must go find some.

In the mean time, whenever a car "sucks the paint off" when it passes another, or some NA 500 hp engine delivers 35-40 mpg and has stop and go manners and meets emissions requirements - make a note.

gotta run

P.

[Aurora40]

Originally Posted by Paul Workman

less of an engineering epiphany

Considering production car engines are designed around numerous compromises, I don't think you'll ever have an epiphany of engineering that states one engine design is unequivocally better than the rest.

[tpepmeie]

Didn't plan to jump into this thread, but....

one way race engines of various sizes and types are compared in the industry is BMEP (Brake Mean Effective Pressure). It is an often used measure of how well developed / efficient the engine is.

The equation is
BMEP = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)

This is typically compared at peak power RPM, although can be used at peak torque as well. The torque value is as measured at the crankshaft. It is essentially "hp per liter, per rpm", and reflects volumetric, thermal, and mechanical efficiencies in a single yardstick.

I wrote an article years ago which was never published in the newsletter comparing the LT5 to various other production engines.

More info here... http://www.epi-eng.com/piston_engine..._yardstick.htm

But as some have pointed out, racing engines are constrained by rules, so efficiency can be an important differentiator. On the street, shear power usually results in more fun (to hell with efficiency, right?)

[Jagdpanzer]

Todd,
Please send me the article you wrote.

[Jagdpanzer]

Generally speaking for modern naturally aspirate four cycle gasoline engine designs BMEP numbers in the range of 200-215 psi (14-15 bar) is as good as it will ever get. However, let's not confuse high BMEP numbers with total efficiency. Multivalve overhead cam engine designs coming out today are more fuel efficient per unit of power produced, especially when considering emission regulations that production engines must also meet. Together with the latest developments in direct fuel injection and engine management hardware/software systems, the overhead cam engine design affords better conditions for the combustion process within the cylinder and also achieve lower "pumping" and internal mechanical losses compared to pushrod engine designs of comparable power output.

[tpepmeie]

Originally Posted by Jagdpanzer

Generally speaking for modern naturally aspirate four cycle gasoline engine designs BMEP numbers in the range of 200-215 psi (14-15 bar) is as good as it will ever get.

Highest I'm aware of was the Super Touring cars in the 90's, early 2000's. Rev limited, so high BMEP was the only way to make competitive power. 17 bar (246 psi) not uncommon there. Have to do the math, but 330 hp, 2 liters, 8500 rpm. The old Cosworth AC F3000 was 500hp/3L/9000 rpm=16.5 bar. This kind of development is usually only present in rev-limited classes... otherwise, it's easier to chase higher revs for more power.

Originally Posted by Jagdpanzer

However, let's not confuse high BMEP numbers with total efficiency.

Not sure I follow you here.

Originally Posted by Jagdpanzer

Multivalve overhead cam engine designs coming out today are more fuel efficient per unit of power produced, especially when considering emission regulations that production engines must also meet. Together with the latest developments in direct fuel injection and engine management hardware/software systems, the overhead cam engine design affords better conditions for the combustion process within the cylinder and also achieve lower "pumping" and internal mechanical losses compared to pushrod engine designs of comparable power output.

All true. Breathing (VE), Burning (TE), Friction (ME) are the key ingredients. I was lumping pumping losses in with Friction out of convenience, but you are right it is an important variable.

[Paul Workman]

Thanks for the link, Todd. That link, and others contained therein, supply a lot of useful information for frustrated mechanical engineers like me. After getting more familiar with the information contained there, I can perhaps graduate from "smart azz" to "intelligent posterior" maybe?? - (Then I could be really be dangerous! )

Obviously, you have pondered engine design waaaaay beyond that of all but the most devout enthusiasts; save maybe for an automotive engineer (such as yourself). You've made a significant investment in time, testing and materials for the LT5 project. I'd be curious as to what engineering characteristics and or potentials made you decide to explore the LT5 further, rather than some other platform?

I hope to get to meet you at BG this spring. If you wanted to hold court on your 427 project, I'm sure you could pack the auditorium. I know I'd be there in the front row, as would most of the FBI gang.

P.