# What advantage is offered by overhead cams or even DOHC? **URL:** \ **Category:** Factual Questions\ **Created:** [July 17, 2005, 8:51pm UTC](https://boards.straightdope.com/t/what-advantage-is-offered-by-overhead-cams-or-even-dohc/312901 "2005-07-17T20:51:23Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [July 18, 2005, 9:49am UTC](https://boards.straightdope.com/t/what-advantage-is-offered-by-overhead-cams-or-even-dohc/312901/21 "2005-07-18T09:49:29Z")
> [@Metacom](#): > > Ehrm, how critical is this? > > I have an older OHV engine in a tractor and I’ve never adjusted anything valve-related (although I’ve done just about every other standard maintenance chore) on it. What are the consequences of not doing the adjustment, and what exactly is adjusted? I wouldn’t worry too much about it, though you might want to get the valves adjusted soon. I’d also check the compression and get the timing set. Tractor engines are low compression and run at low RPMs under relatively light loads. There just isn’t that much fuel getting burned in the cylinders, and the valves may be able to lose excess heat through the valve stems. The exhaust valves in higher compression engines wear when the exhaust heat the valve face almost to the point of melting and then the valve is shut. The valve welds itself slightly to the exhaust seat, and the softened metal gets knocked off as the exhaust goes past. Retarded timing can aggrivate this, as the fuel may still be burning when the exhaust opens. ---
**Author:** ![Foaming\_Cleanser](https://avatars.discourse-cdn.com/v4/letter/f/f05b48/32.png) [@Foaming\_Cleanser](https://boards.straightdope.com/u/Foaming_Cleanser)\ **Post date:** [July 18, 2005, 11:18am UTC](https://boards.straightdope.com/t/what-advantage-is-offered-by-overhead-cams-or-even-dohc/312901/22 "2005-07-18T11:18:44Z")
The valves in my parents’ '72 Dodge Dart slant six were mechanical. But they never clattered so probably never were adjusted. Years later when I bought my mother’s '78 Aspen slant six, I took it in to a dealership to get them adjusted and had to argue with the white lab coat writing the order that the valves weren’t hydraulic, then persuade him that the engine had to be cold. Studebaker Lark sixes had mechanical valves, too. A '62 belonging to the parents of a friend clattered like mad, but they ignored it until any adjustment was useless. It scared the neighbours for a decade. I had the Aspen’s done because of that. Adjusting the valves in my '82 four-cylinder Yamaha 500 was murder; it had 16 of them. But because of that Studey, I plugged away. Hours later I gave up, and because they had to be adjusted cold, I walked the thing to the bike shop a couple of miles away. It was good I didn’t ride it because I had made a couple or three too tight. I would have had the only Yamaha Lark in existence. The inventor of hydraulic valves should have won a Nobel Prize. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [July 18, 2005, 5:26pm UTC](https://boards.straightdope.com/t/what-advantage-is-offered-by-overhead-cams-or-even-dohc/312901/23 "2005-07-18T17:26:01Z")
FTR some engines are going away from hydraulic to solid lifters so that the ECM does not set missfire codes from the lifter pumping up slightly. No question that hydraulics are easier, unless they fail. A lifetime adjustment beats the hell out of replacing lifters. ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [July 18, 2005, 6:04pm UTC](https://boards.straightdope.com/t/what-advantage-is-offered-by-overhead-cams-or-even-dohc/312901/24 "2005-07-18T18:04:35Z")
> [@Boo Boo Foo](#): > > The system was first pioneered by Renault in 1986. Apparently Ayrtton Senna used to say that he could hold a given gear if he wished by an extra 2,000 rpm. The redline back then was 15,500 rpm. That was VERY impressive for the era. In that time, the engines are now nudging 19,000 rpm quite successfully for an entire race. The idea goes back farther than that, though. Tucker had a similar idea for his 589 engine, using oil instead of air, however, he never had the time to get the kinks worked out of it. I suspect that the idea goes back even farther, but I don’t really know. One of the high end Japanese car makes has valves controlled by an electromagnetic solenoid, IIRC. ---
**Author:** ![Boo\_Boo\_Foo](https://avatars.discourse-cdn.com/v4/letter/b/e274bd/32.png) [@Boo\_Boo\_Foo](https://boards.straightdope.com/u/Boo_Boo_Foo)\ **Post date:** [July 19, 2005, 12:41am UTC](https://boards.straightdope.com/t/what-advantage-is-offered-by-overhead-cams-or-even-dohc/312901/25 "2005-07-19T00:41:01Z")
> [@Tuckerfan](#): > > The idea goes back farther than that, though. Tucker had a similar idea for his 589 engine, using oil instead of air, however, he never had the time to get the kinks worked out of it. I suspect that the idea goes back even farther, but I don’t really know. One of the high end Japanese car makes has valves controlled by an electromagnetic solenoid, IIRC. Interesting. I wonder what the great man’s plans were for timing actuation? Obviously, digital processing didn’t exist back then, so some form of analogue mechanism would have been needed. I wonder what his plans were? Theoretically, the idea still has lots of merit. I’d personally trust a hydraulic pneumatic system more than a compressed air system if it was bulletproof. Something nice and ultra reliable like the Bosch K-Jetronic system. That is, not the best in the world, but just wonderfully simple and reliable. ---
**Author:** ![Nunavut\_Boy](https://avatars.discourse-cdn.com/v4/letter/n/e95f7d/32.png) [@Nunavut\_Boy](https://boards.straightdope.com/u/Nunavut_Boy)\ **Post date:** [July 19, 2005, 12:50am UTC](https://boards.straightdope.com/t/what-advantage-is-offered-by-overhead-cams-or-even-dohc/312901/26 "2005-07-19T00:50:55Z")
> [@Boo Boo Foo](#): > > In Formula One engines they’ve done away with camshafts entirely. > > When it’s time for the engine to start, multiple sensors on the crankshat indicate to the engine’s digital management system where the crankshaft is, in relation to the engine’s firing order. A software map then activates pneumatic compressed air via solenoids to open every valve in the engine precisely the right amount per cylinder per compression stroke and voila! She kicks over. > > From that point onwards until the session is finished, the valves are being blown open or blown shut by solenoids and highly compressed air stored in pressure tanks almost identical to a scuba tank. If your scuba tank springs a leak and runs low, you’re in trouble! > > The system was first pioneered by Renault in 1986. Apparently Ayrtton Senna used to say that he could hold a given gear if he wished by an extra 2,000 rpm. The redline back then was 15,500 rpm. That was VERY impressive for the era. In that time, the engines are now nudging 19,000 rpm quite successfully for an entire race. > > There’s no way that springs could handle such rpm without creating valve bounce. Well, in 250cc four stroke motorbikes they can, but when you get into 3 litre plus engines, your moving mass gets much higher and longer and the springs needed to spin 19,000 rpm would be gargantuanly big and heavy. So, a digital pneumatic system is the way to go. > > Getting back to the OP however, Rick summed it up best. A nicely designed SOHC system can be a very, very reliable machine. It’s a nice compromise which produces good revs and good horsepower without a limiting degree of unreliability brought on by complexity. And there’s the rub you see. Engines like the Cosworth DFV were magnificent masterpieces, but they required regular rebuilds. That was what, 1968 and some 165 Grand Prix wins ago? > > The technology isn’t new - with the exception of the pneumatic solenoid system - but the compromise has been easier to justify in terms of greater complexity these past 20 years due to greater levels of quality control. That’s fascinating. Do you have a link to website that explains more F1 tech? I googled a little bit and didn’t find anything too good. ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [July 19, 2005, 6:15am UTC](https://boards.straightdope.com/t/what-advantage-is-offered-by-overhead-cams-or-even-dohc/312901/27 "2005-07-19T06:15:08Z")
> [@Boo Boo Foo](#): > > Interesting. I wonder what the great man’s plans were for timing actuation? Obviously, digital processing didn’t exist back then, so some form of analogue mechanism would have been needed. I wonder what his plans were? Theoretically, the idea still has lots of merit. I’d personally trust a hydraulic pneumatic system more than a compressed air system if it was bulletproof. Something nice and ultra reliable like the Bosch K-Jetronic system. That is, not the best in the world, but just wonderfully simple and reliable. AFAIK the complete blueprints for the engine no longer exist, however, at least two of the engines survive and are in the hands of a Tucker owner. If I can ever scrape up the money, I’m going to try to sweet talk the guy into letting me take a peek at the thing, so I can do some CAD drawings of it. No idea of the mechanism, but I do know that they had problems with air getting in the lines and screwing up the valve timing (which is why they went with the 335 Franklin Engine). ---
**Author:** ![Jeff\_Lichtman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jeff_lichtman/32/1531_2.png) [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)\ **Post date:** [July 20, 2005, 8:32am UTC](https://boards.straightdope.com/t/what-advantage-is-offered-by-overhead-cams-or-even-dohc/312901/28 "2005-07-20T08:32:09Z")
> [@mike1dog](#): > > Really, it’s common knowledge that pushrod engines make more torque. I suspect you have cause and effect reversed: that slow-turning, torquey engines are usually designed with pushrods. That is, the main reason to have an overhead cam is to allow the engine to spin fast without valve float. Stump-puller truck engines and the like don’t need overhead cams because their engines are not tuned to develop power at high RPMs. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [July 20, 2005, 2:21pm UTC](https://boards.straightdope.com/t/what-advantage-is-offered-by-overhead-cams-or-even-dohc/312901/29 "2005-07-20T14:21:36Z")
> [@Jeff Lichtman](#): > > I suspect you have cause and effect reversed: that slow-turning, torquey engines are usually designed with pushrods. That is, the main reason to have an overhead cam is to allow the engine to spin fast without valve float. Stump-puller truck engines and the like don’t need overhead cams because their engines are not tuned to develop power at high RPMs. Now that makes sense. I can see pushrod engines being **associated** with high torque, but I don’t buy the notion that they have more torque **because** they have pushrods. I stand to be corrected if the facts are provided, but “common knowledge” does not a persuasive argument make. There are lots of widely believed misconceptions about automotive technology. [Previous page](https://boards.straightdope.com/t/what-advantage-is-offered-by-overhead-cams-or-even-dohc/312901.md?page=1)