[whiteboyblues2001]

Originally Posted by ST1LE

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.

There are engineering reasons behind Cadillac (and perhaps Chevy with the rumored mid-engine vette) going DOHC.

The HP numbers that are being put out are no longer going to be met with normally aspirated (N/A) engines. So, they have to go with boost. Chevy (and GM) has had great success supercharging their pushrod engines, but supercharging is quite inefficient in terms of fuel efficiency. Turbos do a better job here. So to get the ultra high power numbers (current goals my be north of 800HP at this point), and if you still want to have reasonable MPG's turbos are the way to go.

But now, if we are talking about a V-8 engine, you will probably need two turbos and a lot of plumbing, which leaves less room for the engine. So, you have to downsize the engine to fit all this stuff in the engine bay, which means lower displacement. As described above, if you have displacement limitations, RPMs can help (along with more boost in a boosted engine).

Now, if you read my long diatribe above, you know that the torque of an N/A engine is determined by displacement. But in a boosted engine, it is determined by both the amount of displacement and the amount of boost. The good thing about turbos is you can tune them to add boost where you want it in the RPM range. So you can make turbos that kick in early to make extra torque down low where it is missing from a DOHC engine, and use waste gates to spill extra boost in the upper RPM range where it would be too much. This (along with many other engineering tricks) helps get remove most of the turbo-lag and non-linearity inherent in a turbo design.

The new "hot-V" designs that the Germans have been using has been able to strike a nice balance between getting the good low end grunt we like, and still having HP in the upper RPM range (a lot of the 2.0L turbo 4 bangers run out of steam a bit too early in the RPM range for my taste). The downsides are very high complexity, reliability, price, and heat management. That's why these only come in very high priced cars.

But in reality if you want to get 700 or even 800+ HP, an N/A engine would either have to have huge displacement or have to rev to the moon (probably both). So the twin turbo hot-V V-8 is a nice solution.

It will be interesting to see what Chevy is up to now that Cadillac has showed it's hand. The Cadillac engines are Cadillac exclusive, so there is room for Chevy to go with a larger displacement, or go with a flat-plane crank like Ferrari and other supercars do.

[whiteboyblues2001]

So, now you may ask, "well, why can't they to a twin-turbo hot-V OHV engine instead of OHC?". The short answer is that the two valve design won't give enough flow though the heads to accommodate a boosted application.

The guys who love the DOHC engines always say that they have better flowing heads because they have four valves instead of two. But that is only true when comparing two engines of SIMILAR DISPLACEMENT. Remember that the advantage of the OHV engine is you can have more displacement. More displacement means bigger cylinders. Bigger cylinders can accommodate bigger valves. So when you go over to DOHC, you not only have to make the valves smaller because there are more of them, you have to make the even smaller still because the cylinders are smaller too. You couldn't physically fit an LT1 intake valve in a Coyote engine, so they HAD to go to four valves or lose flow. But if you compare the cross sectional area of the on LT1 intake valve and compare it to the two Coyote valves, they provide almost the same amount of flow. So, in reality, the Coyote heads don't really have some huge advantage in flow capability compared to the LT1 engine. It would if the LT1 was the same displacement, but it's not. Bigger cylinders accommodate bigger valves.

But in this case, they are downsizing displacement AND adding boost (which requires even better flowing heads). Hence, DOHC is the way to go for this application.

[SpeedIsLife]

FYI...the Corvette getting a mid-engine OHC engine isn't a rumor..the CAD drawings have already leaked.

[SpeedIsLife]

Originally Posted by hotlap

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)?

Well namely that the Chevy engine chart leaked a few months ago doesn't list the LT1 past..I think 2022? I forget where that was..might have been down in general discussions. The V8's that are marked for production past 2020/2022 are all of a smaller displacement which would indicate either a DOHC N/A or boosted applications.

[ST1LE]

Originally Posted by whiteboyblues2001

There are engineering reasons behind Cadillac (and perhaps Chevy with the rumored mid-engine vette) going DOHC.

The HP numbers that are being put out are no longer going to be met with normally aspirated (N/A) engines. So, they have to go with boost. Chevy (and GM) has had great success supercharging their pushrod engines, but supercharging is quite inefficient in terms of fuel efficiency. Turbos do a better job here. So to get the ultra high power numbers (current goals my be north of 800HP at this point), and if you still want to have reasonable MPG's turbos are the way to go.

But now, if we are talking about a V-8 engine, you will probably need two turbos and a lot of plumbing, which leaves less room for the engine. So, you have to downsize the engine to fit all this stuff in the engine bay, which means lower displacement. As described above, if you have displacement limitations, RPMs can help (along with more boost in a boosted engine).

Now, if you read my long diatribe above, you know that the torque of an N/A engine is determined by displacement. But in a boosted engine, it is determined by both the amount of displacement and the amount of boost. The good thing about turbos is you can tune them to add boost where you want it in the RPM range. So you can make turbos that kick in early to make extra torque down low where it is missing from a DOHC engine, and use waste gates to spill extra boost in the upper RPM range where it would be too much. This (along with many other engineering tricks) helps get remove most of the turbo-lag and non-linearity inherent in a turbo design.

The new "hot-V" designs that the Germans have been using has been able to strike a nice balance between getting the good low end grunt we like, and still having HP in the upper RPM range (a lot of the 2.0L turbo 4 bangers run out of steam a bit too early in the RPM range for my taste). The downsides are very high complexity, reliability, price, and heat management. That's why these only come in very high priced cars.

But in reality if you want to get 700 or even 800+ HP, an N/A engine would either have to have huge displacement or have to rev to the moon (probably both). So the twin turbo hot-V V-8 is a nice solution.

It will be interesting to see what Chevy is up to now that Cadillac has showed it's hand. The Cadillac engines are Cadillac exclusive, so there is room for Chevy to go with a larger displacement, or go with a flat-plane crank like Ferrari and other supercars do.

Excellent post, thanks for the info and education.

[shaffe]

Originally Posted by hotlap

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.

Ok so OHV are more efficient and make more power and are lighter/ better packaging.

Why is every other GM motor an OHC design then? Why does the colorado have an OHC engine?

[whiteboyblues2001]

Originally Posted by shaffe

Ok so OHV are more efficient and make more power and are lighter/ better packaging.

Why is every other GM motor an OHC design then? Why does the colorado have an OHC engine?

Because GM shifted over to OHC way back in the day for the same reasons that everyone else did. It gave the MPG's needed for the regulations given the power they wanted to compete. HOWEVER, the Corvette was so low to the ground, they wanted to keep the OHV design (it is generally a shorter engine due to the smaller heads and cam in block design). They also figured that the family of engines that would ensue would be great in their pickups.

Fast forward to today, and the newer technology (especially direct injection and variable valve timing) made the pushrod engine design able to compete in MPG's. But I doubt GM or anyone is going back to OHV engine design. Manufactures are trying to make global cars that can be sold everywhere in the world, and much of the world has a displacement tax. The key advantage of the OHV engine is that it allows for more displacement given the size and weight limitations from the chassis it's going in. GM isn't really selling full size pickups around the world, so OHV is fine for now in that application. But for other vehicles, it is to GM's benefit to reduce displacement for overseas sales.

By the way, Chevy does make a 4.3L OHV V6 engine, and puts it in the Silverado (it's an LT1 with two cylinders missing, kind of). What is fun, is that that would make it a 90 degree V6 rather than the usual 60 Degree V6. Which I think sounds much better than the 60 degree V6.

[shaffe]

Originally Posted by whiteboyblues2001

Because GM shifted over to OHC way back in the day for the same reasons that everyone else did. It gave the MPG's needed for the regulations given the power they wanted to compete. HOWEVER, the Corvette was so low to the ground, they wanted to keep the OHV design (it is generally a shorter engine due to the smaller heads and cam in block design). They also figured that the family of engines that would ensue would be great in their pickups.

Fast forward to today, and the newer technology (especially direct injection and variable valve timing) made the pushrod engine design able to compete in MPG's. But I doubt GM or anyone is going back to OHV engine design. Manufactures are trying to make global cars that can be sold everywhere in the world, and much of the world has a displacement tax. The key advantage of the OHV engine is that it allows for more displacement given the size and weight limitations from the chassis it's going in. GM isn't really selling full size pickups around the world, so OHV is fine for now in that application. But for other vehicles, it is to GM's benefit to reduce displacement for overseas sales.

By the way, Chevy does make a 4.3L OHV V6 engine, and puts it in the Silverado (it's an LT1 with two cylinders missing, kind of). What is fun, is that that would make it a 90 degree V6 rather than the usual 60 Degree V6. Which I think sounds much better than the 60 degree V6.

So that I get, but the 3.6L DOHC in the colorado isn't exactly small, why not make a 3.6 OHV and have more power if OHV are so much better? I totally get it for the smaller engines but the V-6's? Why are those OHC? Also you said back in the day wasn't the old 3.8 an OHV engine and still used till like the mid 2000s? It seems fairly recent to me that GM started using OHC in cars with V-6 engines. Like in the last 10 years

I am not trying to be argumentative just trying to learn.

[SpeedIsLife]

Well the OHC motors, from a position of being easier to configure, offer better control over emissions to a degree. Mind you this can be mitigated if GM went to a "cam in cam" design like the Viper had.

[whiteboyblues2001]

Originally Posted by shaffe

So that I get, but the 3.6L DOHC in the colorado isn't exactly small, why not make a 3.6 OHV and have more power if OHV are so much better? I totally get it for the smaller engines but the V-6's? Why are those OHC? Also you said back in the day wasn't the old 3.8 an OHV engine and still used till like the mid 2000s? It seems fairly recent to me that GM started using OHC in cars with V-6 engines. Like in the last 10 years

I am not trying to be argumentative just trying to learn.

If GM made a 3.6 L OHV engine it would make LESS power than he current one because you would lose some RPMs, BUT it would be smaller then the current engine. So, they could use the extra room to make a larger displacement engine that would make about the same peak power and be he same size. The advantage would be it would have more torque throughout the RPM range, especially down low. The disadvantage would be that the displacement tax would be higher in much of the world for any vehicle that used it. Since GM’s 3.6L engine is in sooooo many of their vehicles, this could impact sales outside of the US. Keep in mind in places like Europe, the displacement tax is paid every year, not just when you buy. It’s a stupid regulation but it is what it is. This is why so many cars are turbo now. Same power but less annual taxes.

[shaffe]

Ok that makes sense then! Thanks!

[whiteboyblues2001]

Originally Posted by shaffe

So that I get, but the 3.6L DOHC in the colorado isn't exactly small, why not make a 3.6 OHV and have more power if OHV are so much better? I totally get it for the smaller engines but the V-6's? Why are those OHC? Also you said back in the day wasn't the old 3.8 an OHV engine and still used till like the mid 2000s? It seems fairly recent to me that GM started using OHC in cars with V-6 engines. Like in the last 10 years

I am not trying to be argumentative just trying to learn.

As far as the timing of things go, that probably has to do more with the business cycle than engineering. The 3.8 is considered one of the best engines in history. Perhaps GM felt it was more profitable to keep it around as long as they could to reduce the costs associated with developing a new family of engines. Also, GM has turbocharged the 3.6 which mates better with DOHC. I don’t know as much a about the history end of things or the business end of things. I’m more interested in the engineering aspect.

[DevilsReject97]

Originally Posted by whiteboyblues2001

As far as the timing of things go, that probably has to do more with the business cycle than engineering. The 3.8 is considered one of the best engines in history. Perhaps GM felt it was more profitable to keep it around as long as they could to reduce the costs associated with developing a new family of engines. Also, GM has turbocharged the 3.6 which mates better with DOHC. I don’t know as much a about the history end of things or the business end of things. I’m more interested in the engineering aspect.

The 3.8 was a beast of a motor, and still is. That thing with a stock bottom end was capable of well into the 500-600hp range...

Unfortunate that GM canned it....saw a lot of fast cars with it...even in FWD setups..

[whiteboyblues2001]

Originally Posted by DevilsReject97

The 3.8 was a beast of a motor, and still is. That thing with a stock bottom end was capable of well into the 500-600hp range...

Unfortunate that GM canned it....saw a lot of fast cars with it...even in FWD setups..

I've never seen a real Typhoon (turbo 3800) believe it or not!