Why DOHC/Flat Plane in C8

codrus said:

Why flat-plane crank?  Probably because Ford had one in the GT350, so the Chevy marketing department decided they have to have one too.

I thought it was because the kind of person who would dump $100k on a car would prefer it not sound almost identical to a Silverado.

Knurled. said:

codrus said:

Why flat-plane crank?  Probably because Ford had one in the GT350, so the Chevy marketing department decided they have to have one too.

I thought it was because the kind of person who would dump $100k on a car would prefer it not sound almost identical to a Silverado.

 

Murica!

Shadeux 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? 

Those are the pro's for a flat plane crank from what I've read. I've never been near one.

There may be more valve train but there's less block is what I thought.

The crankshaft itself is what is lighter, because it has fewer counterweights. So in otherwise identical engines, the version with a flat plane crank would be lighter than a version with a cross plane crank. But the difficulty of reducing NVH with a flat plane crank may actually add some of that weight back in the form of balance shafts or additional bracing. When you switch to a DOHC design, with larger heads, more cams, and more valve train any weight saved from the lighter crank is surely offset, and you took weight out of the bottom of the engine and moved it to the top, which raises the CoG of the whole package too.

I think the switch to DOHC has a lot to do with motorsports regulations as already mentioned, but I think there will be benefits for the road cars too if the engine makes it that far.

DOHC engines are more stable at high rpms, but often at the expense of low end torque. It's pretty evident that the C8 was designed to accommodate hybrid capabilities from the start. I think GM will eventually use the DOHC engine in a high performance C8, and they'll add that hybrid tech for "torque fill" on the bottom end, and then let the engine spin much faster/flow more air  up top for big power numbers.

STM317 said:

I think the switch to DOHC has a lot to do with motorsports regulations as already mentioned, but I think there will be benefits for the road cars too if the engine makes it that far.

DOHC engines are more stable at high rpms, but often at the expense of low end torque. It's pretty evident that the C8 was designed to accommodate hybrid capabilities from the start. I think GM will eventually use the DOHC engine in a high performance C8, and they'll add that hybrid tech for "torque fill" on the bottom end, and then let the engine spin much faster/flow more air  up top for big power numbers.

It's better said that they CAN be than are.  What you point out is true mostly because DOHC engines make it easier to rev higher, therefore make more power.  To do that, the cam profile compromises the low end power, which has the effect as you point out.  DOHC engines CAN have cam timing that does allow low end power- but since a OHV engine is a cheaper solution to do that, it's rarely done.

The major difference between cam in block and DOHC engines is the DOHC engine allows for cheaper high RPM operation, which just adds a different layer to the compromises.

Virtually all DOHC engines end up with the exact compromise that you point out- just because they can.  There are a handful of them out there that have cam timing to do the opposite- the 2.3 Duratec Ranger is one I can think of.

STM317 said:

DOHC engines are more stable at high rpms, but often at the expense of low end torque. 

Explain plz.

Edit:  Alpha beat me by like 5 seconds.  *shakes fist*

In reply to ProDarwin :

Yeah, I guess I worded that poorly. Didn't mean to imply that a DOHC design was incapable of low end torque, simply that designing it to do so would require tradeoffs that limit it's biggest advantages. So for practical reasons, we rarely see a DOHC design with significant torque low in the rpm range when compared to an OHV competitor.

Knurled. said:

D2W said:

What is the advantage of this engine in the C8. It makes exotic noises, but I really can't understand any other reason for developing this engine. Chevy has proved for years it can make its typical pushrod V8 engines work. Why move to a more complicated engine, that will also be used in production versions.

The time is rapidly coming when it no longer can work.  Things have progressed markedly in the past 24 or so years since the LS1 was designed.

As far as I can tell the GM 6.2L is as capable as ever of achieving the same ore better hp, same or better fuel economy, and meeting the same emissions standards as the Ford 5.0L...In exactly what way is the pushrod engine lacking or falling behind?

Shadeux said:

Smaller, lighter & rev higher. 

Here is a Ford 4.6 DOHC Mod motor on a stand next to a 5.0 OHV for visual reference.

Here is a ford flathead v8, putting those valves on top just take up too much space!!  lol

ProDarwin said:

STM317 said:

DOHC engines are more stable at high rpms, but often at the expense of low end torque. 

Explain plz.

 

Edit:  Alpha beat me by like 5 seconds.  *shakes fist*

I'm looking for the dyno chart comparing a 5.0 HO engine to a Coyote, and the DOHC engine makes more torque at literally every point on the chart.

Knurled. said:

ProDarwin said:

STM317 said:

DOHC engines are more stable at high rpms, but often at the expense of low end torque. 

Explain plz.

 

Edit:  Alpha beat me by like 5 seconds.  *shakes fist*

I'm looking for the dyno chart comparing a 5.0 HO engine to a Coyote, and the DOHC engine makes more torque at literally every point on the chart.

Not really fair to a compare architecture designed in the 70s to something released in the 2010s.

The out going C7.R was an LS7 based 5.5 Liter engine, obviously with 2 valves per cylinder.  I don't think it was revved hard at all, maybe even under 6k RPM.  The new engine design may end up leading to a better BOP in favor of Chevrolet.

In reply to pointofdeparture :

Which one has the flat plane?

I'll talk out my ass a bit about things I know nothing about.

DOHC I've always heard allowed you to rev the engine higher, because you don't have to deal with pushrod flex. Yeah, I've got titanium rods in my C6 Z06, but it still only revs to 7000rpm and it makes peak power at 6300rpm. The S65 motor in the E92 BMW M3 revs to 8400rpm with peak power coming at 8300rpm and doesn't have any titanium components.

The other thing is that variable valve timing is not something that is practical with a pushrod motor and there are both performance and emissions advantages to having it.

C6 Z06 LS7 pushrod motor
7.0 liter
2 valves per cylinder
7000 rpm redline
505hp at 6300rpm
470lb/ft torque at 4800rpm 
Dry weight 454lbs

E92 S65B40 DOHC motor
4.0 liter
4 valves per cylinder
8400 rpm redline
414hp at 8300rpm
295lb/ft torque at 3900rpm
Dry weight 445lbs.

Yeah the S65 motor is larger in package size, but it weighs just about the same amount. Yeah it's more complex with individual throttle bodies and variable valve timing, but that complexity pays off. The car breathes better and makes power all over.

You're making 103hp per liter with the S65 motor vs 72hp per liter with the LS7.

You reach peak power right before redline with the S65 and max torque comes lower on the rev range than the LS7. The only negative I can pull is that it only gets 19mpg highway vs 24-25mpg for the Vette, but the BMW is also a heavier luxury car, curb weight of around 3500lbs vs the Z06 at 3150 or so.

I'm not trying to badmouth my own car, but there are definitely advantages to DOHC and I'm looking forward to seeing what they can put together for the C8.

In reply to Shadeux :

Neither one of those in pointofdeparture's picture has a flat plane crank.  You have to go to the 5.2 Voodoo from the GT350 to get that from Ford..  This one

Edit.. Bing gave me a bad photo, here's specs for the Voodoo

Nascar pushrod motors turn over 9K rpms, so DOHC is not really needed for high speed.  The DOHC is needed for 4 valve variable timing engines, which have a much broader torque/power band and when coupled with direct injection give the best BSFC. As already posted, in 24 hr races fuel economy is important.

The stock crankshaft has but 4 counterweights, so likely weighs less than a 2 plane crank of the same stroke. However, I think a competition version 8 counterweight crankshaft would be found in the race car. The flat plane crank also makes a tuned exhaust much simpler, or in some cases possible.

Harvey said:

I'll talk out my ass a bit about things I know nothing about.

DOHC I've always heard allowed you to rev the engine higher, because you don't have to deal with pushrod flex. 

 

Pushrod flex is not even the main issue.  It's getting all of the valves to do what you need them to do with the additional weight.  You can use 3/8" titanium pushrods and they'll maintain integrity through 8000 rpms, but the additional weight might mean that you hit valve float at 7000.  There are physical limitations and you reach a point where the strength/weight you need to hold together eventually requires a valve spring so tight that you begin requiring more tension than the components you have used can handle.

For instance, let's say 180 lbs seat pressure will get you to 5000 and 210 lbs will get you to 6000, getting to 7000 is not just another 30 lbs... more like another 100 lbs.  It's because inertial forces increase by the square of the speed.

OHC simply removes a very large amount of inertial weight from the valvetrain and can allow for higher RPMs without that weight penalty.

ChasH said:

Nascar pushrod motors turn over 9K rpms, so DOHC is not really needed for high speed.  The DOHC is needed for 4 valve variable timing engines, which have a much broader torque/power band and when coupled with direct injection give the best BSFC. As already posted, in 24 hr races fuel economy is important.

The stock crankshaft has but 4 counterweights, so likely weighs less than a 2 plane crank of the same stroke. However, I think a competition version 8 counterweight crankshaft would be found in the race car.

True about the Nascar stuff, but they get there with blocks that have raised cams to limit pushrod length and they cost $50k to build.  They are also completely wasted after 500 miles... if they make it that far.

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.

The power density of the pushrod LS and LT motors is fantastic. Those pushrods are what make them such great swap candidates, and I'm sure the vehicle engineers appreciate the packaging freedom as well. I suspect the bulk of GM V8 applications will stay OHV until emissions requirements force the use of independent variable valve timing, unless the vehicles all grow so large that packaging just isn't a factor anymore. The Z06 will be like the old ZR1, a special case.

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. 

ChasH said:

Nascar pushrod motors turn over 9K rpms, so DOHC is not really needed for high speed.  The DOHC is needed for 4 valve variable timing engines, which have a much broader torque/power band and when coupled with direct injection give the best BSFC. As already posted, in 24 hr races fuel economy is important.

Since you are bringing racing engines into this, F1 motors used to spin at over 18,000rpm- and made a ton of power up there- don't really see a pushrod doing that very long.

But the less snarky reply- the operative word is cost.  DOHC gives the OEM a pretty inexpensive way to high engine speeds- so production engines are tuned to use it.

Doesn't intake velocity play into this? You have a larger ratio of valve area/displacement with four valves which you get with with those pesky overhead cams. You get a certain amount of power for the amount of air ingested, you can only suck so much air through a given tube etc. And all of this plays into charge mixing and emissions and lots of stuff. 

Just to clarify, because some posts seem a bit unclear or ambiguous, the flat plane, DOHC engine is only confirmed for the C8R race car at this time.

The only currently confirmed engine for the road going C8 is the 6.2L LT2 with traditional GM pushrods and 2 valves per cylinder

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.