Valve Overlap ? - PeachParts Mercedes-Benz Forum

leathermang · #1 01-13-2011, 01:55 AM

4 x 4 Welder mentioned performance cams..

and said something about both intake and exhaust being open at TDC...

TDC occurs twice in the cycle... once between the compression /power stroke and
once between the exhaust / intake .

Lets confine this to TURBO 617 engines.

On Gas engines increasing valve overlap is one way of increasing power from the engine. This involves some waste of fuel compared to the lesser power same engine... but gets more air /fuel mix into the bore to be compressed.

Are the intake and exhaust valves both open at either TDC position of the crank and will a change in that relationship make more power on a turbo 617 ?

If some amount of air increase is possible by that method on a turbo 617 won't the amount of fuel per power stroke have to be increased to afford any extra power ? Is that feasible or is the relationship controlled by a cam profile in the IP ?

josha37 · #2 01-13-2011, 01:59 AM

Your getting into a very complicated topic, if you need to ask chances are it is the wrong place for your to be looking at custom cams and you might want to find something pre-built. But to answer your question overlap in a turbo motor amounts to nothing more than wasted boost so... wasted power. high rpm na motors use overlap to encourage cylinder scavening which helps clear out exhaust gases. That is kinda the short of it.

leathermang · #3 01-13-2011, 02:42 AM

I think trying to improve on the MB turbo 617 cam is probably a losing proposition.
With the extreme positive displacement of a diesel piston.. I can not see that valve overlap as used by gas engines would give any significant advantage.
Are you calling the 617 in NA version ' high rpm ' ?

R Leo · #4 01-13-2011, 04:13 AM

Quote:

Originally Posted by leathermang

I think trying to improve on the MB turbo 617 cam is probably a losing proposition.

Cam or no cam, the 617's biggest challenge is that both holes are on the same side of the head. Hard to overcome that.

60x are such a totally different breed of cat: holes on both sides of the head, long intake runners, stupid aluminum oil pan, etc...some of the changes actually made a better engine.

Stretch · #5 01-13-2011, 04:42 AM

I've just measured this on a non-turbo OM617 - which I understand to be the same as a turbo cam but without as much lift.

http://www.peachparts.com/shopforum/showpost.php?p=2619802&postcount=18

josha37 · #6 01-13-2011, 11:16 AM

why was there never a factory turbo 616? i want a bigger cam... With flat tappets in order to get more lift you have to have more duration, Duration makes power not lift. And if you was asking me i was not refereing to a 617 as a high rpm motor... that would be silly.

leathermang · #7 01-13-2011, 11:25 AM

Quote:

Originally Posted by josha37

why was there never a factory turbo 616? i want a bigger cam... With flat tappets in order to get more lift you have to have more duration, Duration makes power not lift. And if you was asking me i was not refereing to a 617 as a high rpm motor... that would be silly.

Josha37, There was a factory turbo 616 and one of our members in India has one.
I do not believe that on our turbo diesels that more duration will produce any worthy power or torque effect.

leathermang · #8 01-13-2011, 11:37 AM

Quote:

Originally Posted by R Leo

Cam or no cam, the 617's biggest challenge is that both holes are on the same side of the head. Hard to overcome that.

60x are such a totally different breed of cat: holes on both sides of the head, long intake runners, stupid aluminum oil pan, etc...some of the changes actually made a better engine.

Now Randy, did you throw me this softball just to allow me to mention Gravely Two wheeled walking tractors which have the same configuration that INDY 500 cars of the 1950 WON so often ? ( Offenhauser )

http://en.wikipedia.org/wiki/Offenhauser

Old Gravely engines had huge valves exactly on opposite sides of the head and long stokes ( relative to the bore ) for high torque at low rpm production .One of our members is a descendant of Ben Gravely.

http://en.wikipedia.org/wiki/Two-wheel_tractor

http://www.youtube.com/watch?v=Y-TqhL41clk

More than 50 attachments were available for them.. here is a mean saw mounted on the front... only four bolts to change most implements..

http://www.youtube.com/watch?v=yIaS6t7pVFI

josha37 · #9 01-13-2011, 11:37 AM

Oh i was just trying to point another member in the right direction im not in the market for a bigger cam at the moment. Power does not drop off drastically in the upper rpm range so i would assume head flow and cam profile is appropriate for my little 2.4, thats not to say that more power could be had from a larger cam i dont think it is where I will be looking

. Just took 5 seconds and did a quick search and... http://www.tildentechnologies.com/Technical/CamPerformance.html read if you have cam questions, how cams work is not a industry secret there are plenty of reliable sources of information on the internet and air flow does not care if you are burning gasoline or diesel.

leathermang · #10 01-13-2011, 11:46 AM

I am not looking for a better cam... I was doubting that any significant valve overlap occurs on these engines since evacuation at 21 to one compression ration produces almost complete ' positive displacement ' exhaust..... as compared to 8 or 9 to one on gas engines.. which can benefit by using intake to help get the spent air out.
The upper limits of rpm are controlled by the physics of needing to keep the valves closed to burn all the fuel efficiently..... the same rpm limiting effect can be had by ' detuning' a gas engine... this will typically lower the power curve also... but when trying to gain something like mpg... all the package has to fit together.. particularly the GEARING... if you change the power curve but do not maximize the fit of the power curve to take advantage of that...then you may even harm your engine by lugging or overspeeding it...

josha37 · #11 01-13-2011, 12:08 PM

R Leo · #12 01-14-2011, 02:16 PM

Quote:

Originally Posted by leathermang

Now Randy, did you throw me this softball just to allow me to mention Gravely Two wheeled walking tractors ...

Not on your life would I have done that intentionally. Greg, you're convinced that the next thing after the Big Bang was that Gravelys appeared in the universe and that since then, all things mechanical evolved from them. LOL.

leathermang · #13 01-14-2011, 02:51 PM

Quote:

Originally Posted by R Leo

Not on your life would I have done that intentionally. Greg, you're convinced that the next thing after the Big Bang was that Gravelys appeared in the universe and that since then, all things mechanical evolved from them. LOL.

I was just SURE that you knew how satisfying that would be for me and pitched it high and easy ! Thanks anyway.

pawoSD · #14 01-14-2011, 04:48 PM

Quote:

Originally Posted by R Leo

Cam or no cam, the 617's biggest challenge is that both holes are on the same side of the head. Hard to overcome that.

60x are such a totally different breed of cat: holes on both sides of the head, long intake runners, stupid aluminum oil pan, etc...some of the changes actually made a better engine.

Do I detect a hint of bitterness in that description?

603's are indeed like a cat, they are more needy, and don't care

617's are like a dog, always loyal and dependable, and will work hard without complaint

leathermang · #15 01-14-2011, 04:53 PM

Martha Stewart's dog just raised his head and put 12 stitches in Martha's LIP...
I bet she wishes she had a CAT...
Well, HE did not put the stitches in... he just made the cut which required the stitches....