If OHC is so great then why.....

[Win]

quote:

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Originally posted by CBDFrontier06:
Your LS1 is certainly not smaller or lighter than my 4.0 all aluminum V6.

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The numbers I found for the LS1 are 383 lbs, with oil, intake, coil packs, Water pump, pan, filter, TB, you name it, except for the exhaust manifolds and clutch. The cast iron exhaust manifolds are 13 lbs each, I believe.

I couldn't find any numbers on that Nissan engine. Are they embarrassed to publish them?

The wikipedia article on the VQ indicates that the earlier Nissan engines (a VE?) were better. Of course Nissan, like many car companies, was going down the tubes financially and would be out of business now if the French (Renault) had not bought them out.

 

[CBDFrontier06]

Wikipedia is nothing but a publicly updated info site, so anybody was welcome to give their input on which engine is better while they're providing specs. I could go in and counterpoint it if I though it was worth my time.

Don't hate on Nissan because they, along with Toyota and Honda are taking market share away from GM hand over fist. I guess GM can only hope for a similar rescue, but Renault isn't likely to touch them. Shaping them up would be more akin to an act of ***.

 

[mechtech]

Matt - A pushrod design has more torque than an OHC?? Why would more parts and weight = more torque?
It all depends on the cam grind.
Other than that, an OHC is more efficient.

 

[KW]

OHC more efficient, how so? Maybe with a shim under bucket set up, but they are a pain to adjust the valves due to having to pull the cams.

 

[Lazy JW]

quote:

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Originally posted by Spitty:
[QB] Could somebody please explain why a Pushrod engine would produce more torque than an OHC???...

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An OHC could easily be designed to produce low-end torque in abundance...but why go to the trouble of building a more expensive engine when tractors have used pushrods quite effectively for decades. I have some old Popular Science and Popular Mechanics magazines that have articles debating the relative merits of flatheads vs. OHV. Not many flatheads survived though.

Efficiency is usually measured in horsepower hours per pound of fuel.
Joe

 

[tamu_man]

quote:

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Originally posted by CBDFrontier06:
I'll use this as an example because I own both.

2005 Ford Sport Trac 4.0 SOHC V6, 2 valves per cylinder. 210 HP, 242 FT Lbs 16 / 21 MPG

2006 Nissan Frontier 4.0 DOHC VVT V6 24 valve. 265 HP, 284 Ft Lbs. 16 / 20 MPG

Why not extract as much HP as possible from a given displacement? What's preventing the Ford from producing more HP? Breathing is one factor. Ford had to put a V8 in the next generation of Sport Trac to approach the power of the Nissan V6. That is antiquated American thinking - fix our lack of technology with cubic inches.

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1. The Frontier needs premium gas to attain those figures
2. This makes it even more expensive to drive.
3. When the Sport Trac starts to come into a good supply, they will be cheaper due to massive domestic rebates.
4. The Sport Trac is a ULEV-II while the Nissan is a LEV-II. All that technology and then crappy emissions.
5. The Nissan has higher compression.
6. The list could continue for a while but I'll get to the point.

Yet another apples to oranges comparision

The only apples to apples comparision I can think of is the Chrysler V6's. The 2.7 and 3.5 DOHC motors are derived from the 3.3 and 3.8 OHV.

 

[G-MAN]

Anyone remember how Mercedes-Benz dominated the 1994 Indianapolis 500? I watched that race and kept thinking it was like when Chrysler unleashed the 426 Hemi on NASCAR tracks in the early 60s: Nothing could touch it.

From Wikipedia:

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The Indy 500 race in Indianapolis each year bears some vestige of its original purpose as a proving ground for automobile manufacturers, in that it once gave an advantage in engine displacement to engines based on stock production engines, as distinct from out-and-out racing engines designed from scratch. One factor in identifying production from racing engines was the use of pushrods, rather than the overhead cams used on most modern racing engines; Mercedes-Benz realized before the 1994 race that they could very carefully tailor a purpose-built racing engine using pushrods to meet the requirements of the Indy rules and take advantage of the 'production based' loophole but still design it to be state of the racing art in all other ways, without any of the drawbacks of a real production-based engine. They entered this engine in 1994, and, as expected, dominated the race. After the race, the rules were changed to prevent a recurrence, and the engine became obsolete after just the one race, as Mercedes-Benz knew it would when deciding a victory at Indy was worth it.

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[G-MAN]

quote:

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Originally posted by tamu_man:
The only apples to apples comparision I can think of is the Chrysler V6's. The 2.7 and 3.5 DOHC motors are derived from the 3.3 and 3.8 OHV.

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That is so completely wrong it's not even funny. The 2.7 DOHC and 3.2/3.5 SOHC engines that were introduced with the second generation LH cars were completely new designs from the ground up.

 

[Matt89]

quote:

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Originally posted by mechtech:
Matt - A pushrod design has more torque than an OHC?? Why would more parts and weight = more torque?
It all depends on the cam grind.
Other than that, an OHC is more efficient.

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I'm sorry I wasn't clearer on this - they don't put out more torque in and of themselves. I just meant that when a pushrod and DOHC have the same hp rating, the pushrod engine MUST put out more torque.

Let's look at it this way - two engines rated at the same hp...I'm making up these numbers to illustrate the point:

Engine A, pushrod V6, 200hp @4500 rpm. 3.5L displacement.

Engine B, DOHC V6, 200hp @6600 rpm. 2.5L displacement.

The pushrod engine's larger displacement means that the crankshaft is being pushed around more forcefully during each revolution = more torque. On the other hand, the DOHC engine's pistons will be pushing the crankshaft around less forcefully (due to smaller displacement) = less torque, but can do it at a much higher rpm due to more valves per cylinder, overhead cam etc.

Since horsepower is a function of torque and rpm, given two engines of the same hp rating, the pushrod engine will have more torque. The DOHC makes up the difference by being able to spin a lot faster. Both engines skin the same cat, but the driving experience is different.

Hope that makes sense and/or wasn't a painful statement of the obvious!

 

[G-MAN]

Doesn't your theory that the pushrod engine will always produce more torque fall apart with the following scenario?

quote:

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Engine A, pushrod V6, 200hp @5500 rpm. 2.5L displacement.

Engine B, DOHC V6, 200hp @6600 rpm. 2.5L displacement.

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[Fatboymoe]

On the Indy Mercedes engine of 1994, by using the "Stock Block" loophole, Penske was allowed much more displacement and more turbocharger boost. It's almost like Toyota coming into NASCAR. Toyota was given a clean sheet of paper to design a 355ci pushrod V-8. I don't recall Toyota ever having a pushrod V-8.

I don't think that Mercedes could of won if they had the same displacement and boost rules with a 2 valve , pushrod motor. It would have never had the RPM potential that the DOHC, 4 valve motors had.

 

[tamu_man]

quote:

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Originally posted by G-Man II:

quote:

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Originally posted by tamu_man:
The only apples to apples comparision I can think of is the Chrysler V6's. The 2.7 and 3.5 DOHC motors are derived from the 3.3 and 3.8 OHV.

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That is so completely wrong it's not even funny. The 2.7 DOHC and 3.2/3.5 SOHC engines that were introduced with the second generation LH cars were completely new designs from the ground up.

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If so, why does this site call it a series of engines.

LINK

This site even says that the 3.5 OHC was derived from the 3.3 OHV.

LINK

And again, halfway down on this page.

LINK

Dude, seriously, I work for Dodge.

 

[brianl703]

quote:

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Originally posted by Quest:
Also note that Sentras in the 80s and early 90s have hemispherical combustion chamber designs in all their 1.6L engines also.

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The Ford 1.9L/2.0L SOHC engines used in the now-discontinued Escort are known as "compound valve hemispherical combustion chamber" (CVH) engines.

 

[brianl703]

quote:

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Originally posted by Dominic:
Furthermore, DOHC can benefit from using variable valve timing and lift to give it better low-end flow without killing the high-end flow capabilities.

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In the 2.5L Duratec engine, which is a DOHC V6 engine, the intake is composed of two parts. There is an upper intake with a long and short "runner" for each intake valve in each cylinder (the short runner goes to one intake valve, the long runner goes to the other).

There is also a lower intake with a shutter assembly controlled by an actuator. At lower engine speeds, the shutter assembly closes off the short runners from the manifold. This provides better torque at lower engine speeds since air is only being admitted through one of the intake valves for each cylinder (and, I would guess, the camshaft profile for that intake valve is optimized for torque at low engine speeds).

At higher engine speeds, the shutter assembly opens up the passages (secondary intake runners) for the additional valve for each cylinder, to which is attached the short intake runner. This provides better torque at high engine speeds.

The usage of short and long runners also improves performance, since the optimal length of the runners depends on the engine speed.

EDIT: The actuator is controlled by the PCM and apparently the PCM takes throttle position as well as engine speed into account when deciding whether to open the secondary intake runners.

[ May 06, 2006, 11:54 PM: Message edited by: brianl703 ]

 

[Last_Z]

quote:

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Originally posted by windnsea00:

quote:

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Originally posted by greenjp:

quote:

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Originally posted by windnsea00:
... Just saying, "My LS2 6-spd gets 28mpg on the freeway" doesn't mean it is a more efficent motor in making power which I think a lot of people tend to confuse...

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I really need some help here. What efficiency, other than fuel efficiency, is relevant?

jeff

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Well for example an LS2 getting 28mpg is capable of doing so because of its low torque curve and high gear ratios. It's chugging along at 2k or less rpms. Wind that up to 3k-4k rpms and it will in no way be able to keep up in fuel mileage to a DOHC engine. Also 6.0 liters for 400hp while commonly a European brand can do that with a 4 to 5 liter engine that is DOHC. I know there is more technical info that I'm missing that hopefully someone can chime in. Generally speaking, a DOHC engine is more efficent at making energy per gallon of fuel to a pushrod design. That's why most of the world uses that setup.

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Although you are not wrong, the way you are explaining things are a bit misleading. Yes the LS1 will consume more fuel at 3-4k RPMs, but the fact is it doesn't need to be there for highway use. DOHC engines do need such RPMs because their peak torque is quite higher than OHV engines. It's an inherent OHC flaw

Yes, the European engines do produce more power from less, but I think this is due to different engine design doctrines. Americans prefer not to stress the engines, therefore going bigger. Europeans and Japanese designs do produce more HP/lt, but at the price of reliability and/or low end torque. A good example is the BMW 3.0 making 333HP (M-3)....how many owners have blown their engines?! That's 110HP/lt!
Lastly, my LS1 made 300HP and 315lb/ft of torque to the rear wheels. That's about 345/360 respectively to the flywheel....on a basically stock engine. Full exhaust and a cam swap puts these engines at over 400HP without breaking a sweat. Not bad for a 60 year old design. The fact is both engine types have their use and applications.

 

[G-MAN]

quote:

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Originally posted by tamu_man:
Dude, seriously, I work for Dodge.

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Allpar is a great site but it is notorious for having inaccurate information. Nevertheless, the very first paragraph on the page of your last link says:

quote:

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Chrysler's 2.7 and 3.2 liter V-6 engines were designed to replace the Iaccoca-era 3.3 and 3.5 liter V6. They are a new design using aluminum blocks and heads. The 3.5 litre, released at the same time as the 2.7 and 3.2, has been modified and given an aluminum block to produce 242 to 253 hp, depending on induction systems and tuning.

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The all aluminum 2.7 DOHC and the 3.2/3.5 SOHC that were introduced with the 2nd generation LH cars were clean sheet designs. Period. The old 3.5 SOHC that was offered in the 1st generation LH cars was derived from the pushrod engine, and like that engine had an iron block.

As for Wikipedia, that article is simply wrong.

 

[Matt89]

quote:

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Originally posted by G-Man II:
Doesn't your theory that the pushrod engine will always produce more torque fall apart with the following scenario?

quote:

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Engine A, pushrod V6, 200hp @5500 rpm. 2.5L displacement.

Engine B, DOHC V6, 200hp @6600 rpm. 2.5L displacement.

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G-Man;

Horsepower = (Torque * RPM) / 5252

Even in your scenario, per the formula above the pushrod engine would HAVE to be putting out more torque because it is putting out the same HP at a slower RPM.

How the pushrod engine could do that at the same displacement as a similar HP DOHC engine would be up to the engine builder

I guess all was trying to explain is that pushrod engines rely on torque because they are limited in max rpm, where a DOHC can spin faster, so can afford to have less torque per stroke (smaller displacement). I'm sure there are exceptions to this...

I agree with LastZ that both types of engines have their place...I know that I've enjoyed driving both.

 

[G-MAN]

Yeah, I should have equaled the rpms.

My point is that all things being equal, a pushrod engine is not going to produce more torque than a OHC design just because it's a pushrod engine.

 

[road_rascal]

quote:

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Originally posted by brianl703:
In the 2.5L Duratec engine, which is a DOHC V6 engine, the intake is composed of two parts. There is an upper intake with a long and short "runner" for each intake valve in each cylinder (the short runner goes to one intake valve, the long runner goes to the other).

There is also a lower intake with a shutter assembly controlled by an actuator. At lower engine speeds, the shutter assembly closes off the short runners from the manifold. This provides better torque at lower engine speeds since air is only being admitted through one of the intake valves for each cylinder (and, I would guess, the camshaft profile for that intake valve is optimized for torque at low engine speeds).

At higher engine speeds, the shutter assembly opens up the passages (secondary intake runners) for the additional valve for each cylinder, to which is attached the short intake runner. This provides better torque at high engine speeds.

EDIT: The actuator is controlled by the PCM and apparently the PCM takes throttle position as well as engine speed into account when deciding whether to open the secondary intake runners.

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I'm not sure what other engines have these but the 3.0 Duratec and 3.8 OHV also has the secondary intakes (think Windstar). Clever design but poorly executed on Ford's behalf. Woe be the owner that has a $.10 o-ring fail or actuator clip break only to find out the entire lower intake has to be replaced to the tune of around $300.

 

[Lazy JW]

quote:

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... DOHC engines do need such RPMs because their peak torque is quite higher than OHV engines. It's an inherent OHC flaw...

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Utter nonsense. Pushrod engines have long, heavy valvetrains, and they compromise the location and shape of the ports. These are definite disadvantages.

OHC can be designed with much shorter and lighter valvetrains, plus they allow greater latitude in port design. Clearly an advantage that ALLOWS, but does NOT REQUIRE higher rpm. That said, most OHC engines are indeed designed to take advantage of the high rpm POTENTIAL (not a requirement).

An overhead cam engine COULD be designed with a long-stroke, smaller bore design and appropriate valve timing to allow wonderful low end torque as good as any pushrod engine. In fact, the Chebbie Vega was indeed designed that way. I owned a Vega wagon with the little Holley progressive two-barrel carb and it gave excellent low end performance, plus it revved up quite nicely. Too bad the car fell apart around it

Joe