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Engineering/Technical Ask technical questions about cars. Do you know how a car engine works? |
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04-17-2008, 06:58 PM | #16 | ||
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Re: Whats a "cam" (DOHC, single cam, that stuff...)
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#1. im talking purely theoretical, with efficiency and flexible output being the only goals. in other words, not in the real world. #2. how would you incorporate VVT into a pushrod engine? why arent more engines using it? i mean the 3800 series III is still pushrod and they dont have any sort of VVT system. the viper is the only pushrod VVT system i know, and its only variable timing on the exhaust, with no lift control.
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04-17-2008, 10:51 PM | #17 | |
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Re: Whats a "cam" (DOHC, single cam, that stuff...)
There was something in motortrend about GMC offering VVT on the small block v8's now.
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04-18-2008, 12:08 AM | #18 | |
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Re: Whats a "cam" (DOHC, single cam, that stuff...)
Honda VTEC system would be a little complicated in a pushrod engine, but variable valve timeing simply needs a solenoid fitted between the cam and its drive mechanism, usually as part of the cam gear.
That is very easy to do on a pushrod motor. If the Viper engine is a single cam, push rod engine, then the variable timing will effect both exhaust and inlet events In purely theoretical applications how far the valves open, and how long, is more important than what is used to open them, and I have yet to see the performance of a pushrod engine limited by its pushrods. I believe the biggest reason multi over head cam engines only become better when you start to use more than 2 valves per cylinder. The more valves you have the more cam lobes you need and the more connections you need between the cam lobes and valves. Could you imagine the cam and block design you would need to run a 32 valve V8 on a single, in block camshaft?
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04-18-2008, 03:39 AM | #19 | ||
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Re: Whats a "cam" (DOHC, single cam, that stuff...)
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The biggest reason for OHC and DOHC that I know of is to reduce the weight in the valve train, if you intend to rev a motor up very high. If the engine can do those revs safely with OHV though, there is no reason to just move the cams for kicks. Alternately, if you have two exhaust or two intake valves, and want the pair of them to run on different profiles, generally multiple cams is required. |
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04-18-2008, 05:17 PM | #20 | ||||
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Re: Whats a "cam" (DOHC, single cam, that stuff...)
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http://www.mechadyne-int.com/vva-med...mbly-animation Quote:
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04-18-2008, 05:35 PM | #21 | |||
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Re: Whats a "cam" (DOHC, single cam, that stuff...)
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That is pretty clever, I wonder how easy it would be to retrofit to other engines. Quote:
In theory yes, but there are plenty of real world examples that prove otherwise.
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04-18-2008, 05:36 PM | #22 | ||
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Re: Whats a "cam" (DOHC, single cam, that stuff...)
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The rules tie them down to australian sedan bodies, rwd and pushrod V8's of I think 5 litre capacity. They spin to 7,500rpm and produce a little under 500kw. I know these figures don't compare with motorbike engines, but they show what's possible with comparitively ancient technology. They're racing in New Zealand this weekend. It doesn't really interest me though. http://en.wikipedia.org/wiki/V8_Supercar The major benefit IMO of pushrods is space. You can fit a bigger capacity engine in a smaller hole. Even many slow revving diesels (limited to 2800rpm) have gone to OHC recently. |
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04-18-2008, 05:44 PM | #23 | |
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Re: Whats a "cam" (DOHC, single cam, that stuff...)
yes. i agree with the above statements. like i said from a purely theoretical performance standpoint DOHC is the best one out there right now. but in the real world and for other goals besides pure performance, there is no definitive answer as to whats best.
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