# Why interference engines? **URL:** \ **Category:** Factual Questions\ **Created:** [January 12, 2008, 7:28pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320 "2008-01-12T19:28:56Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 12, 2008, 10:06pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/21 "2008-01-12T22:06:19Z")
[QUOTE=Joey P] I couldn’t find it in the manual. It did say to _inspect_ the “drive belt” at what I think is 15,000 mile intervals, is a drive belt the same as a timing belt? I’ll have to ask the service writer next time I have my oil changed. [/QUOTE] Drive belt(s) refers to the belts that drive things like the water pump, alternator and alike. What year / model Honda are we discussing? ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [January 12, 2008, 10:08pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/22 "2008-01-12T22:08:20Z")
[QUOTE=Rick] Drive belt(s) refers to the belts that drive things like the water pump, alternator and alike. What year / model Honda are we discussing? [/QUOTE] :smack: I figured that out after I wrote it, I was just coming back to edit it. It’s a 2006 (LX I beleive) 1.8L 5 Speed ---
**Author:** ![Throatwarbler\_Mangrove](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)\ **Post date:** [January 12, 2008, 10:26pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/23 "2008-01-12T22:26:38Z")
It’s a timing chain then (see previous link). ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 12, 2008, 10:40pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/24 "2008-01-12T22:40:02Z")
Learn something new every day. ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [January 13, 2008, 2:30am UTC](https://boards.straightdope.com/t/why-interference-engines/433320/25 "2008-01-13T02:30:13Z")
I’ve been told that _most_ car makers have switched to interference engines because it’s easier to bring them into compliance with emission regulations. ---
**Author:** ![Sockmunkey](https://avatars.discourse-cdn.com/v4/letter/s/f17d59/32.png) [@Sockmunkey](https://boards.straightdope.com/u/Sockmunkey)\ **Post date:** [January 13, 2008, 4:44am UTC](https://boards.straightdope.com/t/why-interference-engines/433320/26 "2008-01-13T04:44:18Z")
I \<3 desmodromic valving. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [January 13, 2008, 3:46pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/27 "2008-01-13T15:46:35Z")
Double the mechanical complexity, double the moving parts, double the moving mass. No operational benefit. Where’s **SpinGears** when we need him? ---
**Author:** ![Sophistry\_and\_Illusion](https://avatars.discourse-cdn.com/v4/letter/s/b3f665/32.png) [@Sophistry\_and\_Illusion](https://boards.straightdope.com/u/Sophistry_and_Illusion)\ **Post date:** [January 13, 2008, 5:18pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/28 "2008-01-13T17:18:14Z")
[QUOTE=Sockmunkey] I \<3 desmodromic valving. [/QUOTE] Sorry; I think that’s the asshat emoticon. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 13, 2008, 5:34pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/29 "2008-01-13T17:34:15Z")
[QUOTE=LSLGuy] Double the mechanical complexity, double the moving parts, double the moving mass. No operational benefit. Where’s **SpinGears** when we need him? [/QUOTE] You are correct unless you are going to spin the engine at some insane RPM. Like [Ducati](http://www.ducati.com/en/bikes/my2008/FamilyPage.jhtml?family=DesmosediciRR) does (scroll down click on engine details) ---
**Author:** ![Sockmunkey](https://avatars.discourse-cdn.com/v4/letter/s/f17d59/32.png) [@Sockmunkey](https://boards.straightdope.com/u/Sockmunkey)\ **Post date:** [January 13, 2008, 8:00pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/30 "2008-01-13T20:00:48Z")
[QUOTE=Sophistry and Illusion] Sorry; I think that’s the asshat emoticon. [/QUOTE] Ah crap, I was going for a heart. You want simplicity? How about rotary ball valves? ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [January 13, 2008, 9:01pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/31 "2008-01-13T21:01:33Z")
[QUOTE=Throatwarbler Mangrove] Are there any pushrod engines where the camshaft is directly actuated with gears? Wikipedia says there might be, but I doubt any are used in production cars. [/QUOTE] The old aircooled VW flat four uses gears to drive the camshaft. That _is_ the engine in the Beetle. Always seemed very sensible to me! ---
**Author:** ![Sophistry\_and\_Illusion](https://avatars.discourse-cdn.com/v4/letter/s/b3f665/32.png) [@Sophistry\_and\_Illusion](https://boards.straightdope.com/u/Sophistry_and_Illusion)\ **Post date:** [January 13, 2008, 9:15pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/32 "2008-01-13T21:15:17Z")
[QUOTE=Sockmunkey] Ah crap, I was going for a heart. [/QUOTE] That’s okay; it added a pleasing element of surrealism to your post, something usually lacking in automotive threads. ---
**Author:** ![Throatwarbler\_Mangrove](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)\ **Post date:** [January 13, 2008, 9:22pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/33 "2008-01-13T21:22:14Z")
Well, I think it’s some kind of consipiracy by the timing belt manufacturers and car makers. Looks like my next car will be a nice1985 Toyota Corolla GTS with a non-interference engine, like god intended them to be. ---
**Author:** ![Sockmunkey](https://avatars.discourse-cdn.com/v4/letter/s/f17d59/32.png) [@Sockmunkey](https://boards.straightdope.com/u/Sockmunkey)\ **Post date:** [January 13, 2008, 11:11pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/34 "2008-01-13T23:11:29Z")
[QUOTE=Sophistry and Illusion] That’s okay; it added a pleasing element of surrealism to your post, something usually lacking in automotive threads. [/QUOTE] And here I thought I was going to have to bring out my tie-dyed coveralls. 😃 Here is a link I meant to post before. > **[Home](http://www.coatesengine.com/index.html)** > > Welcome to Coates International, Ltd. developer of the Coates Spherical Valve Engine ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 13, 2008, 11:22pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/35 "2008-01-13T23:22:26Z")
[QUOTE=Sockmunkey] And here I thought I was going to have to bring out my tie-dyed coveralls. 😃 Here is a link I meant to post before. > **[Home](http://www.coatesengine.com/index.html)** > > Welcome to Coates International, Ltd. developer of the Coates Spherical Valve Engine [/QUOTE] :dubious: There is a lot of stuff on that website that requires more than just a grain of salt. A 25 lb. bag might be more in order. ---
**Author:** ![Sockmunkey](https://avatars.discourse-cdn.com/v4/letter/s/f17d59/32.png) [@Sockmunkey](https://boards.straightdope.com/u/Sockmunkey)\ **Post date:** [January 13, 2008, 11:31pm UTC](https://boards.straightdope.com/t/why-interference-engines/433320/36 "2008-01-13T23:31:48Z")
[QUOTE=Rick] :dubious: There is a lot of stuff on that website that requires more than just a grain of salt. A 25 lb. bag might be more in order. [/QUOTE] Naturally, but the concept of rotating valves is sound. ---
**Author:** ![Throatwarbler\_Mangrove](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)\ **Post date:** [January 14, 2008, 12:12am UTC](https://boards.straightdope.com/t/why-interference-engines/433320/37 "2008-01-14T00:12:38Z")
Reason why spherical valves won’t work is that there’s no way to make it seal properly, compared to a poppet valve where the cylinder pressure actually helps it seal. Much like non-interference engines, if it worked, someone would be using it, I suppose. [History of rotary valves](http://www.dself.dsl.pipex.com/MUSEUM/POWER/RotaryValveIC/RotaryValveIC.htm#lotus) ---
**Author:** ![JRDelirious](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jrdelirious/32/9531_2.png) [@JRDelirious](https://boards.straightdope.com/u/JRDelirious)\ **Post date:** [January 14, 2008, 12:15am UTC](https://boards.straightdope.com/t/why-interference-engines/433320/38 "2008-01-14T00:15:25Z")
My current vehicle (99 Camry) has a noninterference engine. The current equivalent seems to have an interference engine. As to the OP question which is why make interference engines, yes, it seems that these lend themselves better to technological tweaking in order to squeeze more HP for the same or better MPG out of the design. A search of automotive forums led me to a thread [here](http://www.omegaowners.com/forum) about the issue, which I have tried to summarize/paraphrase: > [@Omega Owners Forum Message Board](#): > > Simultaneously maximizing power output, fuel economy and combustion efficiency (=pollution control) involves optimizing 1) the air/fuel mix, 2) the spark timing, 3) the compression ratio, 4) the combustion chamber shape, 5) the flow of mix and gases through the intake and exhaust. 1 and 2 can be handled by the control module but the remaining are still dependent on physical geometry of the combustion chamber. This means that in most modern engines, you design the combustion chamber at the top of the head, and the valve openings, according to some combustion/mix flow optimization geometry that means the valves open wide so the mix flows in and the gases flow our fast, and the the cylinder head combustion chamber is specially shaped for higher efficency so the valves are also set at an angle, rather than straight up at the top (plus there’s probably 4 instead of 2 of them). This means the valves intrude quite a bit into the chamber when opening. > > Now, consider the additional performance element of high-rev speed vs. low-rev torque. This is affected by stroke-to-bore ratio, that is, how long is the stroke relative to the bore of the cylinder. Long stroke = lots of low rpm torque, but will not rev so freely; short stroke = less low down torque but will rev much higher. So most road cars will compromise with a “square” ratio where the stroke is close to the bore. This means that for a higher compression ratio and a more effective exchange of mix and gases, the piston must come damn close to the head at the top of the stroke, specially in relatively low-displacement engines. If the valves have failed to properly retract… Seems that would mean an engineer can take the same displacement and get more power out of it by using an interference design. Or something like that, I’m myself dizzy from the searching. ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [January 14, 2008, 12:30am UTC](https://boards.straightdope.com/t/why-interference-engines/433320/39 "2008-01-14T00:30:15Z")
[QUOTE=Throatwarbler Mangrove] Reason why spherical valves won’t work is that there’s no way to make it seal properly, compared to a poppet valve where the cylinder pressure actually helps it seal. Much like non-interference engines, if it worked, someone would be using it, I suppose. [/QUOTE] Not necessarily. For one thing, it could be that they’re mandated to use certain things which proclude the use of other, better methods. This happened to the car industry when pollution controls were first introduced. The ones that they were mandated to use were worse than other technologies that were available. (Of course, one is free to wonder why the car makers didn’t lobby to use _those_ technologies instead of simply trying to block the whole thing.) Then there’s the cost of switching over to a new technology, which can easily run into the billions of dollars. If the savings of the new technology aren’t significant, then car makers are going to be unlikely to invest the kind of money necessary. Mazda’s use of the Wankel engine is an example of this, and if one of the execs at Mazda hadn’t been a protege of Wankel, they probably wouldn’t be building them. [Chrysler was all set to introduce a mass production turbine car](http://www.allpar.com/mopar/turbine.html), but was blocked from doing so because of conditions placed on it in order to get the government bail out. That they haven’t revisited the idea probably has a lot to do with their shakey finances. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 14, 2008, 12:49am UTC](https://boards.straightdope.com/t/why-interference-engines/433320/40 "2008-01-14T00:49:17Z")
[QUOTE=Tuckerfan] Not necessarily. For one thing, it could be that they’re mandated to use certain things which proclude the use of other, better methods. This happened to the car industry when pollution controls were first introduced. The ones that they were mandated to use were worse than other technologies that were available. (Of course, one is free to wonder why the car makers didn’t lobby to use _those_ technologies instead of simply trying to block the whole thing.) [/QUOTE] Umm, cite? To the best of my knowledge the US government has never mandated the technology used to achieve a particular emission goal. Having worked in the industry during the formative years of emission controls, trust me when I tell you there were plenty of different approaches tried. Plenty. [Previous page](https://boards.straightdope.com/t/why-interference-engines/433320.md?page=1) [Next page](https://boards.straightdope.com/t/why-interference-engines/433320.md?page=3)