gotta agree with the guy who said you have no idea what you are talking about. hp/L only matters in racing (where engines are restricted by displacement). in the real world we care about overall fuel efficiency, so hp/kg is important as well as total area under the torque curve (not peak HP), not to mention packaging.
Mouse- The Viper has VVT on its V10. Will probably end up on the GM V8, too. Why would you think VVT is restricted to only OHC engines? Mitch- Thanks for the updated crate motor prices. It has been a while since I looked. Think you could buy an AMG 6.2L for $12K? Taz Terry Phillips
oh, i thought one of the GM V8's already had VVT. I didn't mean VVT was restricted to OHC engines, I meant that you have more options, such as separate intake and exhaust timing and lift control. I suppose there are ways you could achieve that with IBC (?) but I haven't heard of it yet.
Brian- Believe you are right if one looks at the price differential on the AMG cars with the 6.2L. Saw a new 360 engine offered the other day. $40K. MC- The Viper V10 has a hollow outer camshaft with another one nested inside. Presto, VVT possible for both intake and exhaust. That kind of innovation will keep the pushrod engine going for a while. Four valves per cylinder can be done with pushrods too, several examples going all the way back to WW-I. Note Ford has had OHC engines in the Mustangs for years, but they never performed as well as the Chevy small block (if up to 7 liters can be considered small) unless blown. Their new one is quite a bit better, so we shall see. Taz Terry Phillips
MC- You could alter both, just like VVT does for the OHC engines. The two cams can be independently varied if desired, because they are separate from each other. You just need an advance/retard mechanism for the outside cam and one for the inside cam. Imagine one VVT gear train/chain behind the other with the inside cam longer than the outside cam on the drive side. Will have to do some more research and see if the Viper V10 is doing both or if they are only varying one cam, like the 360 did. That is how they got a Viper as fast as a C6 Z06. Turns out they are only varying the exhaust timing, like the 360. Taz Terry Phillips
Depending on the design in question, the big problems are sealing and HC emissions. There are very high pressures in the combustion chamber that the valves have to withstand, and it apparently proved to be too difficult to develop rotary designs that were as good as normal valves. Second point is that, again depending on the design, the shape of the combustion chamber. Some designs with rotary openings lead to very complex shapes where the flame doesn't reach every part of the chamber, the unburned fuel will then drive emissions through the roof.
Two variators, one alters the cam overall orentation (advance,retard), the second alters the intake to exhaust orrentation (lobe separation).
Start trying to propel an airplane through the air with a piston engine, and you see real fast where technology goes. HP per pound. The most powerful aircraft engines in WW1 were liquid cooled, overhead cam 4 valves per cylinder engines. WW2 brought us the Allison and RR Merlin. Sure, the 3350 and 4360 radials were huge, and made up to 4000 HP, but their power per pound wasnt as critical as cost, and survivability. You can shoot the jugs off a big radial with a .50cal and the engine will keep running. Pushrod engines were basically obsolete by WW1. The only thing thats kept them around is cost and simplicity.
Paul- That is a ridiculous statement. In WW-I the Mercedes D-IVa SOHC engine with four valves per cylinder was routinely outperformed and replaced by the pushrod Maybach MB-IVa, originally developed for Zeppelins. Comparing automobile engines to constant velocity aircraft engines, nearly all of which were supercharged or turbo-charged in WW-II for altitude performance is a losing proposition. Aircraft engines were maximized for performance at a specific revolution range to go with propellers of the day and cost was a secondary consideration. Not so with automobile engines. Cost and simplicity are what kept pushrod engines alive, like you said, plus a constant influx of technology that has kept the pushrod V8 competitive with the OHC engines in perfromance, size and weight. The example of the LS7 engine proves they are not obsolete and certainly were not when the modern ohv V8 engines were introduced post WW-II. Those engines were so much more efficient, pound for pound, than anything else available that they were widely used by sports car builders through the McLaren Can-Am racers and their aluminum block and head pushrod V8s. They routinely outperformed all OHC engines until the steamroller of the turbo-charged Porsches arrived. Technology like the concentric cams of the Viper V10 and all aluminum construction will keep the pushrod V engines competitive for years to come. Their compactness also makes them attractive for cars with limited space. The V12 in my 575M, for example, is about twice the size of the LS7s in the C6 Z06s I owned, for essentially the same output, 515/434 vs 505/470. I prefer the V12, but not because it of any lack of performance from the LS7, which can kick sand all over my V12 with nearly double the fuel efficiency. While on the subject, remember one other thing tht drove the European manufacturers to smaller engines, tax structure. With engines taxed on displacement, there was a motivation to make as much hp as possible in as small a displacement engine as possible. This again made cost a lesser factor than it was in the States, where no such tax structure existed. European taxes on gasoline caused the same emphasis with cost again a secondary factor. Taz Terry Phillips
hmmmm. Could someone please identify a competitive gp engine since the 1930's that used pushrods or that was not OHC?
Paul- Not the point, since GP engines have never had cost as an input parameter. You said pushrod engines were obsolete, and had been since WW-I, an obviously overarching and unsupportable statement that was incorrect. Can you name an OHC engine currently competitive in top fuel or funny car dragsters? An equally absurd analogy, but you get the point. Remember the Mercedes engine with pushrods that won Indy by a huge margin because Penske and Mercedes took advantage of a loophole in the rules? Think about it. Taz Terry Phillips
I think that your point was the racing has rules that dictate specific solutions. GP is no exception: they limit RPM, displacement, etc. and they doesn't produce the most HP for the various classes of racing. All I can tell you guys is a small displacement OHC just doesn't have the raw grunt that a large displacement pushrod has and you can't use that high RPM HP on the streets because it doesn't come into play until very high speeds. This is a religious argument just like Ford/Chevy/Mopar so you really can't win.
Name a competitive GP engine in the past, ohhh, 20 or so years that has used a valvetrain like on any DOHC engine you can buy? Or name a competitive Nascar engine that uses OHC? It's a false argument... the formula of GP is such that pushrod engines don't fit. Neither do naturally aspirated engines. Neither do valve springs. If that is the basis for the argument, then every passenger car engine is outdated technological junk, because GP engines are so far ahead. But the truth is that the formula part of F1 dictates the technical requirements. The displacement is set. The # of cylinders is set. The # of valves is set. This dictates the optimum bore and stroke and valvetrain arrangement - none of which have any relation to street cars. So all street cars are obsolete junk? Of course not. What matters in a street car is reliability, power, weight, efficiency, cost and longevity. In that regard, show me an engine that is superior to the Chevrolet LS7. I am not sure one exists. The "old, obsolete pushrod engine" is smaller, lighter, more fuel efficient, cheaper, lasts longer and is easier to service and above all, makes more power per pound than any OHC engine. Not bad for obsolete old junk
GP does not regulate the kind of valve train used, what it does regulate is (now) and 18K RPM limit in the ECU. In the resent past it was 19K RPMs, and a few years ago unregulated with the engines spinning up to 20K RPMs (with questionable reliablity at 20K). Motorcycle engies (600cc) operate up to about 16K RPMs with spring operated valves, and this seems to be the mechanical limits of poppet valves and springs. Thus F1 needed a means to obtain higher RPMs. This was provided by Renault wth the pneumatic poppet valve in the late 1980s. Recently Ducati have been having success with desmodromic valve operation up into the 18K range (800 cc GP bikes), so maybe there are solutions available other than pneumatic valves. NASCRAP, however, DOES regulate the valve train to an single in block cam and pushrod system pushing a maximum of 2 valves. The road worthiness of pneumatic valves is forever questionable due to the high air pressure needed to pump up the valves before turning the engine over the first time after sitting for days/weeks without use. And since the pneumatic system pumps air through the poppet follower as the engine runs, the air pump needs to run continuously at some 3K PSI. Air pump failure would destroy the engine.
If cost is no object to GP racing, the most powerful design would have always been chosen, regardless of cost. They all chose OHC. If cost were an object, they would choose the simplest and cheapest design, yet they still chose OHC. There are no OHC engines in Top Fuel because rules dictate that particular type of engine. There are no OHC engines in NASCAR because the rules dictate a pushrod engine. This could be argued endlessly, but when you take a hard look and note the manufactures that have a choice, yet choose OHC, you have to ask why. Seriously. When the playing field is leveled, when displacement is held level, and when performance is pushed, builders choose OHC. Start limiting displacement in GT racing to something resonable, 5 liters lets say, and they will all suddenly be sporting OHC. But as long as companies like Dodge can run 8 liter engines alongside 5 liter Ferrari's, Dodge will keep running stone age pushrod technology. GP racing cars started using OHC's and 4 valves per cylinder around 1900, even in huge displacement engines, and have continued doing so for over 100 years. If pushrods were superior you can be sure thats the direction things would have gone, but its just not the reality. Not in Motorcycles, not in GP cars, and not on the top sports and GT cars of the world.
Yes, of course I realize that F1 does not specify pneumatic valves, but since pneumatic valves are the only way to be competitive, then they are not a choice made because it is the best available in the world, but rather because they best fit the very narrow requirements of a GP engine - which is worlds apart from a street engine. I believe the point stands... one cannot point to technology used in racing (especially one single type of racing) and say "that's what they do, therefore it must be the best". Especially when that formula specifies a very narrow set of design parameters that can only be solved a single way (with our current technology). In the real world, reliability, cost, weight, packaging, fuel efficiency and other factors are important, when some of those factors are completely unimportant in a race application.
Why only choose GP? There are plenty of race series where pushrod engines are more than competitive - to say the least - over OHC engines. Le Mans for one. It's clear that turbocharging is a superior method for power from a given displacement than naturally aspirated. So any engine that is not turbocharged is an inferior design? And since turbocharged engines were also used in WW2, then NA engines are also outdated obsolete junk? Hardly. There is a completely different set of constraints and requirements for a GP car vs a street car. Comparing them is meaningless. Especially when GP engines operate within a specific set of artificial constraints which lead the designers to one solution, where that set of constraints does not exist in a street car. So I am not sure what the point you're making is? That pushrods are an inferior design? Ok, so explain why the LS7 makes more power, weighs less, is cheaper, more fuel efficient, more reliable and smaller than comparable OHC counterparts? The only answer could be that an inherently superior OHC design that performs worse must be due to subpar engineering, right? This is an age old argument, and IMO was as silly back then as it is today - that pushrod engines are an inferior design. Not inferior, just different. They have their advantages, as do OHC engines.
I think OHC is pretty superior across the board once displacement has been held constant in racing activities. I tend to think OHV is a good starting point for street applications though - for all the reasons Mike has stated.
An interesting thought: It has been said that the air intake flow is harmed by the area in the heads where pushrods pass brm the cam in the block to the rocker arms in the head. It has also been said that the exhaust port only needs about 60%-65% of the flow of the intakes, because most of the outward flow is under significant pressure. This being the case, one could bring in the intake from the sides in a V8 engine and route the exhaust out the tops of the head. This arrangement would bring significant flow benefits to the pushrod engine, but is only applicable in a mid-engine or rear-engine configuration. Thus, overall, one cannot make arguments about engine architecture without ALSO taking into account where the engine will be placed in the car.
I suppose you could boost the power on a Ford Flathead with turbo/supercharging until its power surpassed many modern engines, does that make flatheads superior technology? They could certainly be built light, maybe GM could lead the way with new, smarter flathead technology. I would also imagine that splash oiling may take less power than a hydraulic pump, maybe thats the way to go. No "normally aspirated" 4 cycle engine can produce the same power per cube as an OHC engine. The only engines that come close are pushrod Hemi heads. It was true in 1900 with Fiat, true in WW1 fighter aircraft, true in the late 1920's with Alfa, true in the late 30's with Maserati, true in WW2 with the RR Merlin 4 valve V-12 which produced as much as 2700 HP from 1650 CI, true when Ferrari starting building his little V-12's, true when Cosworth built the F1 V8, and its just as true today. But there are other factors. Most shade tree mechanics in the US cannot understand much more than a simple pushrod V8. Manufactures like GM have a far easier time selling antiquated technology to those kinds of people, and stone age racing organizations like NASCAR, backed financially by companies like GM, help promote it by governing that all engines be constructed that way. Seeing that virtually no manufacturer outside the United States builds cast iron pushrod V-8's, there is really no competition, and no need to improve technology. The engines in NASCAR and Top Fuel have not advanced in 50 years. And by the looks of things, they arent about to anytime soon.
The other factors that Mike and others are talking about are weight, overall size, complexity, and cost. your post here touches on none of those. Please address these four concerns. I don't think "shadetree mechanics in the US" are incapable of understanding OHC engines. Are Americans somehow dumber than the great and mighty yuropeans?
