Whats a "cam" (DOHC, single cam, that stuff...)

[slideways...]

Originally Posted by Moppie

Only if you can justify it.




I would disagree with that.
I think it is simply that more and more modern engines are overhead cam, and it is the same modern engines that also have VVT systems fitted.
But they are not built as over head cam engines just so they can be fitted with VVT systems.

#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.

[vgames33]

There was something in motortrend about GMC offering VVT on the small block v8's now.

[Moppie]

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?

[Nereth]

Originally Posted by Moppie

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?

Thats been done often I believe. It's really quite simple, you just make the rockers split into two on the valve side.

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.

[slideways...]

Originally Posted by Moppie

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.

apparently not. this is the only system in use right now. mechadyne makes it for dodge for the viper. this website is very informative on the topic.
http://www.mechadyne-int.com/vva-med...mbly-animation

If the Viper engine is a single cam, push rod engine, then the variable timing will effect both exhaust and inlet events

look at the animation. seems to me only the one set of lobes are variable.

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?

what about high revs? by the simple definition of horsepower, higher revs makes higher power. pushrods cant do this because they start snapping. and such. plus the whole idea that more than 2 valves per cylinder is unrealistic for a pushrod motor. and the more valves you have the better the engine can breath. thus more performance.

[Moppie]

Originally Posted by slideways...

apparently not. this is the only system in use right now. mechadyne makes it for dodge for the viper. this website is very informative on the topic.
http://www.mechadyne-int.com/vva-med...mbly-animation




look at the animation. seems to me only the one set of lobes are variable.

That is pretty clever, I wonder how easy it would be to retrofit to other engines.

Originally Posted by slideways...

what about high revs? by the simple definition of horsepower, higher revs makes higher power. pushrods cant do this because they start snapping. and such. plus the whole idea that more than 2 valves per cylinder is unrealistic for a pushrod motor. and the more valves you have the better the engine can breath. thus more performance.

In theory yes, but there are plenty of real world examples that prove otherwise.

[KiwiBacon]

Originally Posted by slideways...

what about high revs? by the simple definition of horsepower, higher revs makes higher power. pushrods cant do this because they start snapping. and such. plus the whole idea that more than 2 valves per cylinder is unrealistic for a pushrod motor. and the more valves you have the better the engine can breath. thus more performance.

Have you heard of an Australian Institution called the Taxi Race (aka Bathurst )?
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.

[slideways...]

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.