OHV vs. OHC

[Michael_S]

I'm not really knowledgeable about engines, but I have a handle on the differences between an Over Head Valve (aka Pushrod) engine and an Over Head Cam engine (usually referred to as SOHC or DOHC, single or dual over head cam).

I don't mean to pick on GM. I own a GM car. I just have noticed that GM uses a lot of 3.4 liter OHV V6s for 180 horsepower/205 torque, many 3.8 liter OHV V6s for 205 horsepower and 225 torque, and the 5.7 liter OHV V8s in the Corvette and new GTO with 350 horsepower and 365 torque.

Those are all respectable power figures, but you can get that much power from significantly smaller (and I'm guessing, lighter) engines with DOHC. The 3.3 liter DOHC V6 Toyota just released gets over 225 horsepower and torque. Nissan's 4.5 liter DOHC V8 in the M45 gets 335 + horsepower and torque. That's about 20% smaller than the Corvette engine for only 4% less power. The Dodge Viper uses an 8.3 liter V10 that gets 500 horsepower and 525 pound feet or torque. Those are impressive numbers, but not when you consider that most other exotic cars with naturally aspirated engines get nearly as much horsepower and torque with a quarter less displacement.

[454Casull]

Heavier rotating/reciprocating accessories limit RPM. The higher the RPM, the higher the power, given the same torque throughout the RPM range.

BTW, OHV includes OHC. Just like how a square is always a rectangle, but not all rectangles are squares.

[higgimonster]

the high displacement pushrod engines make a lot of torque really low in the rev range. that is primarily why Gm and Dodge use them. plus they are cheaper to make and most backyard mechanics are familiar with them.

[Michael_S]

Originally Posted by 454Casull

Heavier rotating/reciprocating accessories limit RPM. The higher the RPM, the higher the power, given the same torque throughout the RPM range.

BTW, OHV includes OHC. Just like how a square is always a rectangle, but not all rectangles are squares.

Thanks for the response.

I understand that the OHC engines can rev higher because of lighter accessories. That's what confused me. It seems like OHC engines are better than Pushrod engines in every way except price to manufacturer, and I wondered if price was the only reason they were still being produced.

Okay, so now I know OHV includes OHC. I guess I should have compared Pushrod vs. OHC.

[Michael_S]

Originally Posted by higgimonster

the high displacement pushrod engines make a lot of torque really low in the rev range. that is primarily why Gm and Dodge use them. plus they are cheaper to make and most backyard mechanics are familiar with them.

Thanks for the reply.

So an equivalent displacement OHC engine would have less torque in the low RPMs? I can see why performance car drivers would want to use big pushrods, then. Thanks.

[454Casull]

I suppose R&D of new technology always costs too much for the bean counters. Plus, if the engines currently in use deliver enough power/torque at an acceptable mileage, then there really isn't a problem. When I was younger, though, I did have a problem with the Viper's 8L engine making only 1 hp per cube...

[Michael_S]

Originally Posted by 454Casull

I suppose R&D of new technology always costs too much for the bean counters. Plus, if the engines currently in use deliver enough power/torque at an acceptable mileage, then there really isn't a problem. When I was younger, though, I did have a problem with the Viper's 8L engine making only 1 hp per cube...

I understand that businesses sell product to make money, and I don't begrudge GM or any other automaker their profit.

I just wondered whether OHC engines are inherently superior to Pushrod engines across the board. As it turns out, each type has its own benefits and drawbacks, and that's fine. I thought OHC were superior, and GM continued to make pushrods just to save money.

I would think that any money spent developing a truly superior engine would be offset by the profit from increased sales it might bring.

What I really think GM should do is make a production version of the Cadillac 16 concept engine, the 13.6 liter V16 with 1000 horsepower and torque. I'll take two, please

[ivymike1031]

an equivalent displacement OHC engine would have less torque in the low RPMs

I don't agree with that statement- if the valve lift profiles were the same, and the basic engine geometry was the same, then the torque curves should be very similar (if not identical).

GM seems to prefer timing chains over timing belts, and timing chains are hard to do with OHC engines. GM also seems to have a whole lot of customers who think that pushrod engines are somehow superior to OHC engines...

[Michael_S]

Originally Posted by ivymike1031

GM seems to prefer timing chains over timing belts, and timing chains are hard to do with OHC engines. GM also seems to have a whole lot of customers who think that pushrod engines are somehow superior to OHC engines...

Well, this particular car buyer just wants the most mileage efficient and powerful engine possible for any given price, regardless of the way valves are open and closed or whether it uses chains or belts. It's all theoretical anyway, since I intend to run my current car into the ground before replacing it.

I bought an Impala in 2001 right after my college graduation because, at the time, the only thing that mattered to me was a spacious interior, reliability, and good crash test scores. It's a decent car, don't get me wrong, but now I'm getting a lot more interested in performance.

I started wondering about the advantages and drawbacks of different engines when I compared the 2004 Impala SS with the 2004 Nissan Maxima. I realized that the 2004 Impala SS uses a supercharged 3.8 liter Pushrod V6 to get 280 torque and 240 horsepower while the Nissan DOHC VQ engine, a naturally aspirated V6, gets 265 horsepower and 260 torque out of 3.5 liters in the Maxima. The cars weigh about the same and are both FWD, but the Maxima is supposed to be significantly faster. On first glance, all that seems to indicate that DOHC is vastly superior to pushrod for generating power.

[SaabJohan]

BMW and Saab are both using timing chains with overhead cams, I believe Ford are also using it on a few engines so this shouldn't cause any problems if the engine is designed for this.

It have at least been shown that a 4 valve head will give higher torque during the whole rpm range than a 2 valve head. A 4 valve head with pentroof type chambers will be difficult to make with pushrods if we want the intake and exhaust valve timing to be adjustable.

[ivymike1031]

BMW and Saab are both using timing chains with overhead cams, I believe Ford are also using it on a few engines so this shouldn't cause any problems if the engine is designed for this.

It's more difficult & expensive to design it this way than to use a belt drive, at least in my experience. With a dohc v engine, chain dynamics get pretty hairy pretty fast - belts behave much better.

[higgimonster]

Originally Posted by ivymike1031

an equivalent displacement OHC engine would have less torque in the low RPMs

I don't agree with that statement- if the valve lift profiles were the same, and the basic engine geometry was the same, then the torque curves should be very similar (if not identical).

GM seems to prefer timing chains over timing belts, and timing chains are hard to do with OHC engines. GM also seems to have a whole lot of customers who think that pushrod engines are somehow superior to OHC engines...

After writting that I regreted it because i knew it was not complately true. I think it was the Corvette grand sport that chevrolet put DOHC on their 350 and it signifigantly increased power.

And as for the chain drive thing I think that belt drive is generally quiter and probably signifigantly cheaper.

[454Casull]

Originally Posted by higgimonster

After writting that I regreted it because i knew it was not complately true. I think it was the Corvette grand sport that chevrolet put DOHC on their 350 and it signifigantly increased power.

And as for the chain drive thing I think that belt drive is generally quiter and probably signifigantly cheaper.

Although not as reliable, I should think.

[454Casull]

Originally Posted by SaabJohan

BMW and Saab are both using timing chains with overhead cams, I believe Ford are also using it on a few engines so this shouldn't cause any problems if the engine is designed for this.

It have at least been shown that a 4 valve head will give higher torque during the whole rpm range than a 2 valve head. A 4 valve head with pentroof type chambers will be difficult to make with pushrods if we want the intake and exhaust valve timing to be adjustable.

In the higher RPMs, definitely. But in the lower RPMs? With more valves (and hence more valve area) the intake charge velocity lowers, and cylinders don't fill as well (at low speeds).

Of course 4-valve heads have the potential to best 2-valve heads at all speeds, but IMO variable valve control would be needed.

[swimtx]

advantages of a pushrod not mentioned-lower center of gravity due to lack of parts above engine block, torque and HP in everyday usage range (great for trucks etc. which need it to get a load moving). the c5r was one of the best GT cars in its class, it used a pushrod. keep in mind that a lighter rotating assembly will reduce the noticeable differences, but would be less capable of holding up to the heavier GM vehicles, exceptiong the use of space-age materials i.e. Ti alloys