Push Rod vs. DOHC engines

[serialk11r]

Originally Posted by Dimman

@Allch Chcar I asked in an older thread about pushrod 4v engines, and someone linked to one. Consensus seemed to be that it wasn't that great (losing the complexity advantage of 2v ohv and not being able to rev like an ohc).

The cam timing, I knew that GM had phasing for the whole cam on their last ohv V6, but didn't know that Chrysler had developed a system to operate intake and exhaust lobes separately. But that cam-in-cam sounds very complex, and well... Chrysler.

Yea cam in cam sounds pretty silly to me. If you're going that far, might as well just use separate exhaust and intake cams yea?

[Allch Chcar]

Originally Posted by Dimman

@Allch Chcar I asked in an older thread about pushrod 4v engines, and someone linked to one. Consensus seemed to be that it wasn't that great (losing the complexity advantage of 2v ohv and not being able to rev like an ohc).

The cam timing, I knew that GM had phasing for the whole cam on their last ohv V6, but didn't know that Chrysler had developed a system to operate intake and exhaust lobes separately. But that cam-in-cam sounds very complex, and well... Chrysler.

You two are of course correct that they can have 4 valves. That was a mistake on my part. My point was merely that they don't due to little to no advantage over 2 valves for OHV.

As for cam timing, I didn't know about the Viper VVT, the Mechadyne cam in cam. It sounds like it's excessively expensive and over complicated.

[RogerR]

Sorry, I'm a bit more than fashionably late to this party but I'm going to pretend I was on time... (oh, I also am answering largely in the order I see them. I'm sure some were answered before I got to this post, if so, sorry for the repeated info)

Something that we should keep in mind, a LONG time ago the French (yes, blame them) decided to regulate the power of these new fangled car things. More power should be taxed more. OK, how do we measure power? The Dynojet was just a number of decades away from being invented so they got the idea that displacement == power. Thus was born the idea that we needed to really note engine displacement. Interestingly if you look at airplane engine specs displacement isn't such a big deal since airplane care about power, weight and efficiency, not displacement.

Of course as soon as more displacement cost you more money you the buyer wanted more power without more displacement.

Originally Posted by ottopilot

4/5 valves per cylinder will always be more efficient than 2.

Not exactly true but really just a complex issue. More valves do allow more breathing and can typically be made to work over a wider range of engine speeds. However, more valves don't always mean more efficient in terms of brake specific fuel consumption, especially at part throttle. If the 2 valve engine is tuned for say lower RPM cruising power it should be just as efficient as the 4 valve motor. It typically will lose out on peak power. Thus you have say a 3L motor that gets the same highway mileage but has inferior peak power.

As far as I know, pushrod engines are limited to 1 intake and 1 exhaust valve.

Yes, no (don't you hate that answer) again. Historically pushrod motors have been 2 valve engines. However, pushrod diesels today and for quite a while have almost all been 4 valve engines. A pushrod 3-4 valve setup is quite possible but for what ever combination of factors these motors haven't made it into production. BTW, as an interesting example consider the GM XV8. This was a 4.3L 3 valve, double cam pushrod V8 that was even smaller than the LSx motors. Lot's of interesting ideas though it never made it into production.


And more info:
http://www.mfba.org/showthread.php?7...tal-engine-XV8

Originally Posted by Draco-REX

The first production Dual Over Head Cam engine was in 1924.

Even older than that. I think 1912.

Originally Posted by serialk11r

Neither has cam phasing ability that way.

Cam phasing with a single cam is quite possible. Typically you will adjust both the intake and exhaust together. However, Chrysler has a unique cam in cam that allows independent intake and exhaust control with a pushrod motor. It's used in the Viper. Check it out:
http://blogs.motortrend.com/top-10-t...nerd-1435.html

Originally Posted by 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.

Read All Corvettes are Red. When GM was looking to come out with the LSx family of motors all options were on the table. They did know that this motor would be used in both the Corvette, other cars, and the trucks so it had to be a platform that worked for all of them. In the end they decided their clean sheet design would not follow in the footsteps of their most recent clean sheet V8 (the 32V Northstar) nor that of Ford's recent OHC/DOHC V8s but instead would be a pushrod motor. It's interesting how the benefits of the pushrod motors worked for both the trucks and the Corvette. The trucks really just needed a cost effective option with decent torque. Well the pushrod motors were cheap and had good low end grunt thanks to lots of displacement. The Corvette needed a compact, low profile motor. Again the pushrods delivered.

Originally Posted by Allch Chcar

That's because the M3 has very aggressive cam and gearing compared to the Corvette. It's also well known that GM has been purposely trying to game the EPA cycle for the Corvette's MPG for years. Remember the 1st-4th gear automatic shift?

First, I have to say the Corvette, when you don't use the power does deliver class leading mileage. I recall a Car and Driver road test which included both an S2000 and Corvette (C5 Z06 IIRC). The two cars got basically the same mileage. The Corvette bested the Honda by 1 on the highway but was 1 short in town. Interestingly, if you calculate the displaced volume of the motors per revolution X the RPM they turn on the highway you will see both motors are pumping about the same volume of air per mile down the highway. The Corvette simply does it with fewer, bigger breaths.

Originally Posted by 7thgear

because it works

it's reliable and i you want more power just add displacement

GM is a cheap company that didn't want to innovate since they made their first car, focusing on re-using existing technologies hoping that consumers still think that anything American is the best in the world.

As for efficiency.... Japanese engine builders have been making reliable engines which produce over 1 hp per liter for the last two decades, what has GM made?

Hmmmm....
GM did seem to be adrift for a while but to say they aren't innovative is really... well... wrong. Over it's long history GM is perhaps the most innovative car company of all time. They also innovated in areas that reached far beyond cars. Just to hit a few:

The mag shocks that are now used by Ferrari, Audi and of course a number of GM cars since GM first put them on the Caddy STS in 2002. Yes, modern Ferrari's are using Cadillac suspension

They have shown great creativity such as when they decided to use a balsa wood sandwich panel for the floor of the C5 and C6. This created, in effect, a composite, honeycomb structural panel. How many cars that cost less than $250k use honeycomb structural panels... or wood for structure?

The hydroformed frame rails were a neat engineering feat as were the first alloy control arms on a volume production car ('84 Corvette).

I think GM was the first to use touch screens in cars as well as ABS and air bags. GM had heated seats in the 1950s.

Remember R-12, you know Freon. That was a GM invention. With that they launched a brand called Frigidaire.

GM was a pioneer in aluminum engine castings. The Rover aluminum V8 was a design that Buick cast off. GM was not only the first to have a turbo charged car, they were the second. Chevy and Olds both produced turbo cars (not the same designs) in the same model year. GM had fuel injection in the 1950s.

GM was a pioneer in the world of multi-link IRS. While the Germans were using swing axles the Corvette got a multi-link IRS (1962).

GM is the only car company with cars broken down on the moon. The rest leave their broken down cars on Earth.

Gm helped invent the first heart bypass machine. Yes, that's not car related nor are GM's various jet, helicopter and gas turbine engines. Well, unlike Honda's jet engine, GM's had afterburners.

Even many of GM's failed cars are actually examples of GM being creative. The Corvair was a classic example. The later Vega was actually a car with MANY innovations but a misguided corporate structure pushed it out the door before two key technologies, technologies that are basically standard today could be tested. One was the dipping process used to rust proof the chassis (oops we didn't noticed that trapped air... that kept the dip from working in the fenders) and another was the aluminum engine block without pressed in iron liners (oops, sulfur in the gas kills the nikasil cylinder linings... good thing we showed BMW so they will avoid that same problem in the late 80s... oh wait).

Yes, GM certainly has dropped the ball more than once but they also have a long history of innovation and other companies such as Honda and Toyota have learned new manufacturing and design idea from GM (as GM certainly has from them).

GM has screwed up and has been backwards thinking many times in their history. However, it's foolish to claim they never looked forward and never innovated. GM didn't become the largest car company in the world and hold that title for 70+ years without some innovations (not to mention the marketing and business organizational innovations pioneered by GM).

[serialk11r]

Well there is one good reason for looking at displacement, which is city driving. You could say that it's not just displacement that matters, and various things can improve efficiency, but for the most part among light vehicles that are built within 10 years of each other, idle consumption is directly correlated to displacement.

Start stop only takes care of that at dead stops, while you're rolling along at 20mph a giant V8 is inevitably guzzling large quantities of fuel for no reason.

I don't understand why people need to bring up ship engines and plane engines. Those engines are operated at near maximum load all the time, so it makes sense to have high displacement. Car engines are operated barely above idle in many cases.

[RogerR]

Originally Posted by serialk11r

Well there is one good reason for looking at displacement, which is city driving. You could say that it's not just displacement that matters, and various things can improve efficiency, but for the most part among light vehicles that are built within 10 years of each other, idle consumption is directly correlated to displacement.

Start stop only takes care of that at dead stops, while you're rolling along at 20mph a giant V8 is inevitably guzzling large quantities of fuel for no reason.

I don't understand why people need to bring up ship engines and plane engines. Those engines are operated at near maximum load all the time, so it makes sense to have high displacement. Car engines are operated barely above idle in many cases.

Actually at low speeds like 20 mph the V8 probably isn't consuming much more gas because it can spin at a lower RPM. Only when you are truly talking idle would I expect this to be more true. Also with displacement on demand (not exclusive to pushrod V8s but now nearly standard on them) the near idle consumption is better than you might suspect.

Things like aero engines are brought up because it helps people to realize the perspective though which we often think of cars and engine technology is colored in part by the rules and regulations that we are subjected to. We think of displacement as one of the most critical features of an engine. "It's a 2.5L H4". The same is not true on other areas. What type of motor does a boat have? 120 hp V6. What about that plane (180hp H4). Displacement is a rather artificial measure of a motor yet because of a choice made a long time ago it has some how become one we think is critical. What would we do if the world had gone to the gas turbine so long ago?

[Dimman]

Originally Posted by RogerR

Actually at low speeds like 20 mph the V8 probably isn't consuming much more gas because it can spin at a lower RPM. Only when you are truly talking idle would I expect this to be more true. Also with displacement on demand (not exclusive to pushrod V8s but now nearly standard on them) the near idle consumption is better than you might suspect.

Things like aero engines are brought up because it helps people to realize the perspective though which we often think of cars and engine technology is colored in part by the rules and regulations that we are subjected to. We think of displacement as one of the most critical features of an engine. "It's a 2.5L H4". The same is not true on other areas. What type of motor does a boat have? 120 hp V6. What about that plane (180hp H4). Displacement is a rather artificial measure of a motor yet because of a choice made a long time ago it has some how become one we think is critical. What would we do if the world had gone to the gas turbine so long ago?

Been living with tragically poor throttle response?

[Draco-REX]

Just for the record, the new LT1 engine in the C7 vette GM just unveiled has cam phasing and displacement on demand.

[n2oinferno]

I saw a lot of M3 vs Vette engine efficiency argument, but no real numbers, so I looked them up.
http://www.fuelly.com/car/bmw/m3/gas%20v8
http://www.fuelly.com/car/chevrolet/corvette
Unfortunately you can't sort the Vettes by year, but looking at the numbers it appears the Vette gets about 3mpg better overall, at ~17 to ~20.

[serialk11r]

Originally Posted by RogerR

Actually at low speeds like 20 mph the V8 probably isn't consuming much more gas because it can spin at a lower RPM. Only when you are truly talking idle would I expect this to be more true. Also with displacement on demand (not exclusive to pushrod V8s but now nearly standard on them) the near idle consumption is better than you might suspect.

That is a load of crap. My 1ZZ runs at 1400rpm at 25mph real smooth, gets about 60mpg like that. If I shift to 4th instead then it drops to 50mpg, but in 4th the engine can hold speeds well under 20mph and that is about 60mpg too. If you want to talk fuel flow rates, it's something like 0.4 gal/hour. A 4L V8 idles at ~0.5 gal/hour, nevermind a 6L V8. I'd expect a 6L V8 spinning at say a bit over 1200rpm to be using 1gal/hour with zero load, and that's a huge amount of fuel.

With 8 cylinders if you're not doing at least 40, you are losing BIGTIME on fuel.

[serialk11r]

Originally Posted by n2oinferno

I saw a lot of M3 vs Vette engine efficiency argument, but no real numbers, so I looked them up.
http://www.fuelly.com/car/bmw/m3/gas%20v8
http://www.fuelly.com/car/chevrolet/corvette
Unfortunately you can't sort the Vettes by year, but looking at the numbers it appears the Vette gets about 3mpg better overall, at ~17 to ~20.

Well the M3 is heavier and has crap aero and short gears, so it's somewhat expected. But hand it to a hypermiler, I bet you they can do better in the M3. Or maybe not, because that engine sounds so good one might be tempted to rev it for fun

[RogerR]

Originally Posted by serialk11r

That is a load of crap. My 1ZZ runs at 1400rpm at 25mph real smooth, gets about 60mpg like that. If I shift to 4th instead then it drops to 50mpg, but in 4th the engine can hold speeds well under 20mph and that is about 60mpg too. If you want to talk fuel flow rates, it's something like 0.4 gal/hour. A 4L V8 idles at ~0.5 gal/hour, nevermind a 6L V8. I'd expect a 6L V8 spinning at say a bit over 1200rpm to be using 1gal/hour with zero load, and that's a huge amount of fuel.

With 8 cylinders if you're not doing at least 40, you are losing BIGTIME on fuel.

Please don't bring this to a personal level by claiming others are full of crap. You are comparing an individual I4 motor to a much larger V8 with no comparative data. Your comparison of V8s is based on speculation and somehow you have decided that a 50% increase in displacement results in a 100% increase in idle fuel consumption. To really make your point you need some additional detailing to try to make sure we are really comparing apples to apples.

[serialk11r]

Originally Posted by RogerR

Please don't bring this to a personal level by claiming others are full of crap. You are comparing an individual I4 motor to a much larger V8 with no comparative data. Your comparison of V8s is based on speculation and somehow you have decided that a 50% increase in displacement results in a 100% increase in idle fuel consumption. To really make your point you need some additional detailing to try to make sure we are really comparing apples to apples.

First of all I didn't bring it to a personal level, I was just saying that your statement was a load of crap, and I still stand by that.

If larger motors really could spin slow enough to make up for the difference, then no one would be talking about downsizing. That's clearly not the case.

1200+rpm is not idle. I'm comparing apples to apples.

[RogerR]

Originally Posted by serialk11r

First of all I didn't bring it to a personal level, I was just saying that your statement was a load of crap, and I still stand by that.

If larger motors really could spin slow enough to make up for the difference, then no one would be talking about downsizing. That's clearly not the case.

1200+rpm is not idle. I'm comparing apples to apples.

Sorry, you could have said disagree. Load of crap, is an insulting phrase. Given the poor support you offered in your previous post and your insulting manor I'm not going to bother with this discussion other than to say you are grossly over simplifying and have not proven your case in the least.

[Dimman]

Going to quote myself from the 'Vette thread:

I've thrown a bit of a challenge @ZDan a couple times, with regards to what may be a practical displacement limit to a V8 engine, but got nothing from him.

We've seen the Ford Coyote put some nice power and weight numbers recently. Now with the low lift flow advantage of multi intake valves, and ease of implementing cam phasing (particularly LSA control) and even lift and duration lobe-switching (not on the Coyote, afaik) of DOHC, giving it a head start on power to displacement and emissions, is it possible that the OHV GM setup will no longer be able to 'grow' itself competitively? Rotating inertia increase, heavier internals, physical block size increase, higher mounting due to longer stroke, could start to have rapidly diminishing returns on a V8 over 7 liters or so. A DOHC rival may decide to look at that and just make an efficiently sized smaller displacement with far higher specific output. (Say a 99x86 5.3L putting out a somewhat conservative 550bhp and 425 lb-ft.)

Is there a realistic practical displacement limit on the OHV V8?

Thoughts?