Valve bounce

Re: unloaded

I'm sorry! I was basking in the glow of your brilliant and incontrovertible elucidation of the finer points of engine technology. I should have realised. You've explained it. No need for any more discussion. We'll all shut up in deference to your pearls of wisdom.

jfm and I simply got into an interesting discussion. You are quite entitled to ignore us and jfm did add "not recommended reading" to the title. I did also point out that it was not relevant to the thread topic.

Clearly you have explained the problem fully by drawing on your wealth of experience with engines. I bow to your obviously vast knowledge and timorously beseech you to explain to this "fule" the meaning of "extended TDC".
 
Hello again Jfm
I never said anything about under load. What I said was reving the nuts of a petrol engine whilst not under load is the best way to wreck it. Others have said this engine is only designed to run at 4800max. Maybe I should have added this.
No I cant give you any engineering facts. only practical experiance. Were not talking engineering data provided in a pristine enviorment with brand new componants ect, were in the real world with an engine that may be a little tired. may possably have had the wrong spec of oil put in it. timing may be slightly out ect ect. & some dope comes along who knows nothing of its past history & revs 10 bells out of it & holds it there. Im sorry, but that for me is a recipe for disaster.

Just to wander off the subject part of the MOT test on diesels requires the customer to read & agree to a disclaimer as regards to causing damage to the engine by reving it whilst not under load for the emmission test. Now as a diesel is a far more robust design than its petrol cousin & most will quite happily run at wot settings without damage. What does that tell you? It tells me that what engineering theory says & what happens in practice are two different things....Nat
 
Re: fatlady only - not recommended reading :-)

Thanks for the trig explanation. Yes that does indeed explain the W curve

Offset cranks are common. I don't mean a swashplate or anything, just a crank that's half an inch off the centreline of the cyinders. Quite a lot of engines are designed this way - the 2.4 litre Toyote Avensis engine frexample. The offset changes the shape of the piston accel/velocity graphs but actually it's not done to manage that, it's done to reduce friction losses. The offset puts the pistons more vertically above the crank arms on the power stroke, which reduces sideways forces that make the pistons rub on the cylinder walls, so reducing frictional losses. Of course you lose a bit becuase there's MORE friction on the compression stroke, but net net you win slightly. At least, that's the thinking behind the idea...
 
Re: fatlady only - not recommended reading :-)

Offset gudgeon pins are also used to produce the same effect, known as DeSaxe.
Hate to tell you all this but you won't over rev one of these V8's because they're all hydraulic tappets which float at high revs & stop the valves opening. Still not a clever thing to do though & can occassionally pop a pushrod out from a rocker if you're unlucky.
Most people who rev engines hard in neutral do so when nice & cold, thereby massively increasing engine wear.
 
Re: fatlady only - not recommended reading :-)

Well, well! There is a logic to offset cylinders. You learn something every day. My education is more than twice your 18 years ago!
Thanks for that.
 
[ QUOTE ]
But if you think running a motor at 5000rpm loaded is better than 5000rpm unloaded, mebbe explain why using engineering principles?

[/ QUOTE ]

Always used to puzzle me why drag racers would blow their engine at lift off at the end of the run - never heard an explanation of why though - coincidence???
 
Could that be more to do with the Nitro system, remove the load and the revs go sky high.
It's all a matter of controlling the revs. A motor revving to it's safe limit, is safe. Remove the load when you have introduced a supercharged volatile mixture and you no longer have any means of controlling the potential engine revs.
 
Like I explained, I'm new to boating, and thought I'd be safe in the hands of the surveyor, I didnt go to the boat and try and run it at 5k, its a test the surveyor was doing, thinking he's checking to see if the engine's ok.
 
No offence taken Dave, yes I realise something is "wrong" with the surveyor now. I didnt question it too much at the time because I was in the presence of three people who "know" about boats - the broker, a marine engineer and the surveyor, and its my first purchase of a mobo so I thought I'd let them get on with whatever it was they were doing! Hopefully this will all end up ok, Im going to seatrial for the second time this week and talk to the engineer a bit more, then the deal should be done by the end of the week /forums/images/graemlins/blush.gif
 
Misfires are most likely, as others have said, Ignition related. If engine is revving to 4500 without misfire or probs, I would be happy.
Don't take it to 5000.
I have seen the rocker studs pulled out due to over revving, The Mercruisers I have seen mainly seem to use screw in studs.
Also comments on risers/ Manifolds are valid.
 
Misfires are most likely, as others have said, Ignition related. If engine is revving to 4500 without misfire or probs, I would be happy.
Don't take it to 5000.
Some rocker studs are press fit and I have seen them pulled out due to over revving, The Mercruisers I have seen mainly seem to use screw in studs.
Also comments on risers/ Manifolds are valid.
 
[ QUOTE ]
So good he said it twice /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

[/ QUOTE ]

.......no, just old age kicking in. Had to happen one day just suprised it took this long!! /forums/images/graemlins/wink.gif
 
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