[metros11]

We've seen the argument before. Push rods are old technology, outdated, not as sophisticated as overhead cam design, built for trucks, not sports cars... and so on.

It's just easier when someone else explains the advantages and the argument for using that specific design.

https://www.roadandtrack.com/new-car...ocialflowFBRAT

[Hasti]

Cause its better, Lower COG, weight.
Engineering explained video this article is about

[crysalis_01]

If ICE survives for the foreseeable future i can't help but imagine that it won't be OHC or CIB but camless.

https://youtu.be/FJXgKY2O4po

[BlaqWhole]

I think we're starting to see that standard pushrod engines are superior to OHC engines.

[whiteboyblues2001]

Neither the article or the video really gets to the meat of the argument here. I'll try to explain:

First, you have to understand a few fundamental facts of internal combustion engines (ICE's).

1. For a normally aspirated engine (non-boosted) the total amount of torque is a function of displacement. And when I say the total amount of torque, I mean the area under the torque curve on a dyno sheet.

2. The simplified equation for power (HP) for and ICE is P = (RPM * Torque(at that RPM)/5252. Why is it divided by 5252? Just so that when you put in the torque in ftlbs, the power number is in HP. If you wanted different units of measure the constant (5252) would be a different number. SO, for our discussion we can simplify even further by just dropping the constant (we are just looking at the relationships not measuring anything. SO, HP = RPM * Torque(at that RPM).

So we can see from fact #2 that if you want to increase power, you can either increase RPM's or increase torque. To get more torque you need more displacement (from fact #1). To get more RPM's, well you have to spin the motor faster.

So, lets suppose that we all work for an auto manufacturer, and we have a 3.0L N/A V6 pushrod engine that puts out 250HP. And our main competitor just put out more HP in their latest offering. So we decide we want to up the power and compete.

Let's now suppose that one engineer wants to achieve more power through more displacement and one wants to achieve more power through more RPMs. Let's explore what that would do to our current engine.

If we want to get more RPM's, we would have to go to OHC. This will add weight, size, and complexity to our motor. And there will be no extra torque, just extra HP at the highest end of the RPM band. BUT, remember that HP= RPMs * Torque(AT THAT RPM). So now, if we just add the cams and change nothing else, we won't get to the HP numbers we want because torque is dropping in the higher end of the RPM range. So if we increase RPMs and lower torque, we don't get anything. SO, we HAVE to re-tune things like camshaft profile (lift, duration, lobe separation) and intake runner length and diameter to increase torque in the upper RPM range. BUT, that would reduce torque in the lower (and sometimes mid) RPM range. But now we have our peak HP number.

Let's look at option two, which is to increase displacement. When you increase displacement, sometimes you have to go to a bigger block to accommodate, so it could (but not always) increase the size and weight of the engine. BUT unlike the first example, we are not decreasing torque anywhere in the torque curve. In fact, we are INCREASING torque throughout the whole RPM range, and thereby increasing HP throughout the entire RPM range.

Which would you rather have? More HP in the highest RPM range at the sacrifice of less torque down low? Or, more HP and more torque at EVERY RPM.

I hope that explains it a bit better.

Another way to look at it is the folks who love DOHC engines say that they have greater HP/Liter numbers. But the HP/Liter measurement is only meant for engines of the same design. If HP/Liter is the best metric to use, all the mustang folks should buy the turbo 4 cylinder. It has a much greater HP/Liter number than the coyote. AND, you really would want to have the smallest displacement you could get with the most boost you could get. Who wants that???? That would be a very peaky engine with tons of turbo lag and non-linearity. Hell an electric vehicle would have the best HP/Liter number because it doesn't even have displacement. So it approaches infinite HP/Liter numbers, again, why by a Coyote if HP/Liter is the best metric?

Another way to look at it is this: when you divide by the displacement in the HP/Liter equation, you are normalizing the numbers with respect to displacement. Another way of phrasing that is the number that is calculated represents how much HP you would get if each engine had only 1 Liter of displacement (the same displacement). BUT, that is NEVER the reality. The pushrod engine of equal size and weight will ALWAYS have more displacement than the OHC engine.

So applied to our examples above, that is like saying, "Okay, OHC design is a better way to get more HP than more displacement because if we don't add more displacement the OHC engine would put out more power" WHAAAT???? How would OHC fans react if someone said, "okay, you can go OHC, but you are not allowed to rev any higher than our pushrod engine of higher displacement". That wouldn't make sense either. How come we don't hear about HP/RPM? A pushrod engine of greater displacement than an OHC engine will put out higher HP at all RPMs that they have in common.

Using the HP/Liter argument is just taking away the advantage of a pushrod engine, namely: you can have more displacement in a pushrod engine than a DOHC engine of the similar size and weight. And that more displacement gives you more torque and more HP across the entire RPM range, but the DOHC engine only gives you a good peak HP number at the cost of low and sometimes midrange torque and HP.

Make sense?

[whiteboyblues2001]

Now, I want to make the counter argument for going with DOHC engines. DOHC engines are nothing new, in fact DOHC and OHV engines go way back to the 1910's or so. So, neither is, by any stretch of the imagination, a new technology.

BUT, getting rid of the pushrods lowers the weight and thereby momentum of the valve train. That allows for higher RPM operation.

Back in the late 80's and early 90's cars started going over to DOHC for fuel efficiency reasons. Because a smaller displacement engine would usually have better fuel efficiency (and sometimes better emissions) than a larger displacement engine because there is a minimum amount of fuel needed to burn to keep everything going in a low load situation. So, a DOHC engine of the same peak power numbers (back in those days) would have a smaller displacement, and thereby have better fuel efficiency as a larger OHV engine. BUT, direct injection allows for the "ultra lean mode" which reduces the minimum amount of fuel needed for low load situations. When you add in variable valve timing, cylinder deactivation, etc. the fuel efficiency advantage for the DOHC pretty much went away.

Also, suppose for some reason, you CAN't go with more displacement. Like, you are building an engine for a racing class that limits you to a certain displacement. Or if your country has a tax on displacement that you pay yearly (like Europe). If you want more power, you HAVE to go with either more RPM's or go to boost (like turbo or supercharging). That way, you can have more power without using extra displacement.

Most car companies went over to 2.0L Turbo engines because of the European and Asian regulations regarding displacement. And since all the major automakers want to sell global vehicles, they have build engines that meet the requirements EVERYWHERE, not just the US. So, the pushrod engine is still not making a comeback anytime soon.

But, I don't care so much about meeting regulations (I am not running a auto manufaturing company), I just care about the how much power, where the power is in the RPM band, and the size/weight of the engine relative to power. I prefer N/A engines for the throttle response, and I prefer pushrods because it gives torque and power everywhere, not just up high. But that's just me.

[SSDan]

Thank you Whiteboyblues for the in-depth details and the time it took to do that.

My abridged version:

Big Cubes = Big Torque = Big Fun = LT1

[whiteboyblues2001]

Originally Posted by SSDan

Thank you Whiteboyblues for the in-depth details and the time it took to do that.

My abridged version:

Big Cubes = Big Torque = Big Fun = LT1

No worries! And yes, as they say, there is no replacement for displacement!!!

[Mr. Wyndham]

Says it all:

http://gmauthority.com/blog/2014/02/...engine-design/


"I rue the day we would ever have to go to overhead cams. Bigger, heavier, giant heads. Cams in head, higher center of gravity, bigger package, in most cases more weight. All the additional parts like chains, pulleys, and gears—you just increase the opportunity for things to go wrong." - Doug Fehan, Corvette Racing Program Manager

[SpeedIsLife]

Well GM is slowly starting the transition to OHC motors. Isn't the new Caddy TTV8 an OHC? Along with the rumored 4.5/5.5L V8's for the C8?

[ST1LE]

Originally Posted by SpeedIsLife

Well GM is slowly starting the transition to OHC motors. Isn't the new Caddy TTV8 an OHC? Along with the rumored 4.5/5.5L V8's for the C8?

Indeed! I think in the end of the day, it is tough to attract Exotic Car buyers with a pushrod motor, due to the characteristics of the engine.

GM is not making the switch because the OHC is better, but rather because it provides the driving characteristics the C8 buyers will demand.

That's MHO of course, and one I would not have had 2 years ago. My view has been changed with my recently purchased daily driver.

[hotlap]

Originally Posted by whiteboyblues2001

Now, I want to make the counter argument for going with DOHC engines. DOHC engines are nothing new, in fact DOHC and OHV engines go way back to the 1910's or so. So, neither is, by any stretch of the imagination, a new technology.

BUT, getting rid of the pushrods lowers the weight and thereby momentum of the valve train. That allows for higher RPM operation.

Back in the late 80's and early 90's cars started going over to DOHC for fuel efficiency reasons. Because a smaller displacement engine would usually have better fuel efficiency (and sometimes better emissions) than a larger displacement engine because there is a minimum amount of fuel needed to burn to keep everything going in a low load situation. So, a DOHC engine of the same peak power numbers (back in those days) would have a smaller displacement, and thereby have better fuel efficiency as a larger OHV engine. BUT, direct injection allows for the "ultra lean mode" which reduces the minimum amount of fuel needed for low load situations. When you add in variable valve timing, cylinder deactivation, etc. the fuel efficiency advantage for the DOHC pretty much went away.

Also, suppose for some reason, you CAN't go with more displacement. Like, you are building an engine for a racing class that limits you to a certain displacement. Or if your country has a tax on displacement that you pay yearly (like Europe). If you want more power, you HAVE to go with either more RPM's or go to boost (like turbo or supercharging). That way, you can have more power without using extra displacement.

Most car companies went over to 2.0L Turbo engines because of the European and Asian regulations regarding displacement. And since all the major automakers want to sell global vehicles, they have build engines that meet the requirements EVERYWHERE, not just the US. So, the pushrod engine is still not making a comeback anytime soon.

But, I don't care so much about meeting regulations (I am not running a auto manufacturing company), I just care about the how much power, where the power is in the RPM band, and the size/weight of the engine relative to power. I prefer N/A engines for the throttle response, and I prefer pushrods because it gives torque and power everywhere, not just up high. But that's just me.

Nailed it. European regulations on displacement have influenced the direction engine development there but modern OHV engines have closed the fuel efficiency gap. Chevy isn't chasing that market so they are using the most power dense technology, in a smaller/lighter physical package.

[SpeedIsLife]

Originally Posted by hotlap

Chevy isn't chasing that market so they are using the most power dense technology, in a smaller/lighter physical package.

Er...

Maybe not at the moment, but there's a good chance the LT1 and LT1 family will be the last of the OHV V8's for GM outside of truck applications.

[hotlap]

Originally Posted by SpeedIsLife

Er...

Maybe not at the moment, but there's a good chance the LT1 and LT1 family will be the last of the OHV V8's for GM outside of truck applications.

Er...

I wrote present tense. You got a crystal ball? In your view of the future, why would Chevy continue to use OHV in trucks (only)?