Why DOHC/Flat Plane in C8

bobzilla said:

In reply to Harvey :

i think you're discounting that almost 200ftlbs of torque between those two. IT's like honda engines. They rev and make a ton of power up top. But you're not there every shift. You're usually half that where the torque makes a massive difference in driving feel. 

HP is great, but its that torque you feel day to day and where I'll put my money. 

When pointing to the torque difference, don't ignore the huge difference in displacement and the fact that torque output is mainly a function of displacement.

Turns out that the BMW beats the Chevy on the torque/liter basis as well (~74 for the BMW vs ~67 for the Chevy).   It's those 3 extra liters on top of the BMW's 4 that you'd be feeling, not the fact that pushrods were involved instead of more cams and massive heads.

FWIW, those DOHC Ford vs OHV Chevy pictures wouldn't look quite as heavily in favor of the LS had Ford kept the same bore spacing that their OHV Windsors used, the way Chevy did for the LS series from their Ed Cole based predecessor.   What Ford basically did do was design a 4.4L V8 and the only thing that kept it from being a mild stroker motor from the get-go was that its original displacement was held to 4.6L.

More bore => less stroke => shorter block deck height from both the shorter stroke - and also from the rod lengths themselves being shorter because of that shorter stroke.

Norm

In reply to NormPeterson :

That is a Ford engine, not an LS.  It has a deck height significantly shorter than an LS - it's probably the smallest commonly available V8 that isn't valve-in-block.

The 4.6 also has a significantly longer stroke than a 302, and even longer than a 351.  The 302 is old school heavily oversquare engine to get max valve area per displacement.  Modern engines are undersquare for optimal combustion efficiency, and get airflow from 4v/cylinder.

pointofdeparture said:

Shadeux said:

In reply to pointofdeparture :

Which one has the flat plane?

Neither, but one is DOHC and the other isn't, and the DOHC engine is certainly not smaller or lighter as your post stated.

Note that the Ford modular motor in the photo has a smaller bore spacing and a taller deck height than the Windsor, it's got the "undersquare" ratio that is popular these days for various reasons.  The front view in that photo emphasizes the taller deck (part of why the mod motor is both taller and wider), while not showing the shorter overall block length.  (The other part is the head, and yes, DOHC ones are bigger because of the cams).

There are various ways I can think of to do dual cam/VVT in a pushrod motor.  There's probably room to stack the cams vertically, or if you're thinking like a German engineer you could make them concentric. :)

Ultimately, though, it's not that running OHC moves the rev band upwards -- the cam profile does that.  OHC is just a design that allows running a higher-rev cam profile without blowing the valvetrain to bits and/or spending a gazillion dollars making it bulletproof.

My whole point about the Ford modulars is that Ford basically painted themselves into a corner with somebody's insistence on 100 mm bore spacing, when they could just as well have followed Chevy's lead and run with Windsor bore spacing (or at least closer to that - perhaps 4.25" or 108 mm, take your pick).  Small bore spacing basically forces a long stroke design from the get-go - at least roughly 'square'.  Chevy's LS engines have tended to be at least mildly oversquare, with the LS7 apparently being Chevy's production shot at maximum displacement and still being ever so slightly oversquare.

Ford could have built a 108 mm bore spaced engine stretch-able to an easy 5.3L with a shorter stroke than the 5.0 Coyote.  5.9L ought to be possible without going past 'square' or involving spray-arc technology..

Honestly, you'd think that square and undersquare designs would be more consistent with OHV engines from the standpoint of rpm suitability, and that oversquare would fit better with DOHCs.

Norm

NormPeterson said:

Honestly, you'd think that square and undersquare designs would be more consistent with OHV engines from the standpoint of rpm suitability, and that oversquare would fit better with DOHCs.

AIUI the modern ratios have to do with emissions and the desire to make shorter engines for transverse installation.  The latter is probably less critical for the  V8s, but they did want the bore spacing to be common with the V10 and there are length concerns there even in longitudinal installation.

I believe the mod engines are basically square. The 4.6L mod engine was very slightly oversquare and the initial 5.0L coyote slightly undersquare, while the newer ones are very slightly oversquare. I looked it up, the 4.6L has a slightly longer stroke than the 351.

In reply to NormPeterson :

2v/cyl kind of requires an oversquare design if you want to be ale to flow air, because valve curtain area is very bore-limited.  Horsepower ends up being limited by how rough an idle you can tolerate.

4v/cyl, it turns out, has more than enough valve area to make good high RPM power with remarkably mild cams, to the point that many modern port designs deliberately have a negative short side radius, to "ski jump" the airflow all to the backside of the port, which has combustion efficiency benefits.  They're literally throwing away a large percentage of the airflow capability so that they can get more efficiency, and the engines are STILL making crazy good power per displacement.

Knurled. said:

In reply to NormPeterson :

That is a Ford engine, not an LS.  It has a deck height significantly shorter than an LS - it's probably the smallest commonly available V8 that isn't valve-in-block.

 

The 4.6 also has a significantly longer stroke than a 302, and even longer than a 351.  The 302 is old school heavily oversquare engine to get max valve area per displacement.  Modern engines are undersquare for optimal combustion efficiency, and get airflow from 4v/cylinder.

There's nothing wrong with chasing the valve area/displacement metric even in a DOHC design (perhaps more accurately, valve curtain area/displacement).  See Ferrari (3.62 x 2.96. among others).

Norm

In reply to NormPeterson :

​​​​​​But, there's no need to, when you can get sufficient airflow without such measures.  You can have your cake and eat it too, and get great combustion efficiency.  Which is kind of mandatory in today's regulatory environment, and racing environment.  The less fuel you use, the more laps you can turn.

In reply to pointofdeparture :

That's true. I admit all I read was "flat plane crank" of the title. I only realized today that "DOHC" was part of the title. Thus my post was about the block, not the heads, which explains why I was confused with about half of the following posts.

My bad! 

Typically crank is lighter in flat plane.  Less rotational weight and more responsive high Reving engine.   Also shorter stroke allows for shorter crank case which allows motor to be mounted lower.   They are just embracing similar approach to Ferrari and Porsche.  Lighter internals and higher revs.  There is a reason those little 4 liter 6 cylinder Porsche's pump out over 500 hp and hang with big pushrod v8 race cars.

bobzilla said:

Shadeux said:

Smaller, lighter & rev higher. 

Gunning after Ferrari and Lambo, etc.

in what world is a similar displacement DOHC engine smaller and lighter? 

Wait for it.... wait for it.... F1.

_ said:

bobzilla said:

Shadeux said:

Smaller, lighter & rev higher. 

Gunning after Ferrari and Lambo, etc.

in what world is a similar displacement DOHC engine smaller and lighter? 

Wait for it.... wait for it.... F1.

There's 6.2L F1 engines that weigh less than an lsx?

If F1 mandated 6.2L naturally aspirated V8s there would be!

Curiosity:  Is the port layout traditional, or is it a hot vee engine?

Y'know, if it was a hot vee, they could get the benefits of flat plane crank without the hassles...

RE- bore-stroke...

FWIW, as efficiency requirements get more and more demand- you will see longer strokes than bores- seems like the ideal B/S ratio is pushing .8-.88 or so.  

Not for racing motors, but for real motors.

Racing motors will stay way over 1.

Fun fact- thanks to the difference in strokes, mean piston speed for an 18k rpm F1 motor is virtually the same as a 9k NASCAR motor.

Anyway, that was just about one of the thread tangents that has been brought up.

It's pretty interesting that GM has a 100% unique motor for their race car.  Expensive as all heck, though.  While the GT's motor has a crank made from unobtanium, the rest of the engine was mostly off the production line.  So this new engine puts HUGE pressure on the car to win.  Given the previous history of the Vette racing, my gut tells me they will win, a lot.

alfadriver said:

It's pretty interesting that GM has a 100% unique motor for their race car.  Expensive as all heck, though.  While the GT's motor has a crank made from unobtanium, the rest of the engine was mostly off the production line.  So this new engine puts HUGE pressure on the car to win.  Given the previous history of the Vette racing, my gut tells me they will win, a lot.

GM has stated that the DOHC engine is "a future, production-based engine". Likely headed for the ZO6 and/or ZR1 models. LeMans rules state that the engine has to be based on at least 300 road going units to compete in the GTE class too, so at some point there will be some version of this engine in a road going C8. There were also some pretty legitimate looking cad images of a DOHC, twin turbo engine in a C8 engine cradle leaked at some point too...

However, GM did also design and build a bespoke, handbuilt 4.2L twin turbo, "hot vee" V8 for the Cadillac CT6-V, then canceled the CT6 a couple of months later and hasn't mentioned putting that engine into anything else yet, so there's precedent for them to spend tons of money on engine development for a very small handful of engines.

Knurled. said:

Curiosity:  Is the port layout traditional, or is it a hot vee engine?

 

Y'know, if it was a hot vee, they could get the benefits of flat plane crank without the hassles...

Traditional port layout, but the the Mercedez engine has a "split-turbo" with the turbine and compressor wheels on opposite sides of the V with the MGU-H in between.

https://www.motorsport.com/f1/news/honda-set-for-mercedes-style-split-turbine-compressor-864024/864024/

In reply to D2W :

You're missing the point.  It's springs.  
When  you shove a pushrod and a rocker arm around at higher RPM your spring pressures have to increase.  
Squeezing stiffer springs takes more horsepower than squeezing softer springs.  Just look at spring pressures of OHC engines with similar size valves and Operating RPM and compare that to OHV engines.  
In fuel mileage/ emissions  testing very high RPM is not used so they can effectively cheat their way to better numbers.  

_ said:

bobzilla said:

Shadeux said:

Smaller, lighter & rev higher. 

Gunning after Ferrari and Lambo, etc.

in what world is a similar displacement DOHC engine smaller and lighter? 

Wait for it.... wait for it.... F1.

So now we are comparing an $11 million race engine that will last a few thousand miles to a production street car engine that is expected to go 100k miles?

In reply to frenchyd :

It's not the rocker and pushrod weight so much as the valve weight and how single valves require a lot more lift, therefore more acceleration.  A DOHC with .5" of lift would be considered a massive race setup...

Keith Tanner said:

If you're worried about a lack of bottom end with the new Corvette engine, I'd be more concerned about the flat plane than the DOHC architecture.

Can you expand on this?  How would the flat plane affect torque production?

It's my understanding that having a power pulse every 90* of crank rotation instead of every 180* means more torque.

In reply to Keith Tanner :

Cross plane and flat plane both have firing intervals every 90 degrees, the difference is that the flat plane crank will have even firing pulses per cylinder head, instead of having two firing 90 degrees out of phase to the other two.

Either way, torque output is displacement and BMEP, makes no odds as to how many boings there are per revolution.  Or even how evenly spaced they are, thinking of oddfire V6s, V-twins, big bang motorcycle engines, other oddities.

Keith Tanner said:

It's my understanding that having a power pulse every 90* of crank rotation instead of every 180* means more torque.

A Knurled pointed out, there are still firing pulses every 90 degrees.  Its just that the firing order becomes even.  Instead of a traditional 18436572 where you get odd, even, even, odd, even, odd, odd, even, the firing order goes odd, even, odd, even, odd, even.... but still every 90 degrees.

This has big benefits to both intake and exhaust harmonics since they aren't sharing volumetric demand.  This is also what causes the secondary and tertiary overtone harmonics that tend to vibrate the snot out of things.

But, a flat crank still pushes 8 pistons that fire at 90 degrees, so torque production shouldn't be affected by the crank plane dimensions... other than a possible slight benefit to breathing in and out which may improve torque.  I'm not sure.