If you don't have some overlap it ain't gonna make it's proper power.
12-15* of overlap would be on the low side of my scale.If I was to use this small overlap it would be on a not so well designed engine and low boost 6-8PSI and the customer wanted some more HP down low without doing it right and he had the wrong set up to start with.

 

Here is a link the harmonics chart I made. It is at the bottem of the page.

 

Si if I'm interpretating that chart correctly, at 8K RPM the *perfect* runner length would be just shy of 18"...is this on the V6 only, or just a general chart??? That would also be tuning for the 2nd pulse which is obviously stronger than the 3rd or 4th. And I'm also assuming that would be from the valve? or just the intake runner length? If it's just the intake, you then have to account for the variances in the cyl. head runners as well.

URGH, I'm begining to think this whole thing was over my head...

Chris

 

IT is.

 

Thanks for the confidance boost

 

Yeah that's my thought too.

Sometimes overlap is not needed and in situations when you are looking at average power especially in lower RPM ranges it might actually hurt.

Bret

 

That chart I made is for all piston driven 4 stroke engines.
I'll post the big cam theory again, too.

 

awesome, thanks...I think I may be starting to understand this.....maybe..

Chris

 

As for the position of the piston and valve.
On a big cam it start with over lap.
The intake valve opens as much as 15 and some times as much as 20 degrees (20 is more for 14,000rpm motor cycle engines) BTDC.
BUT, I have also herd of some weard custom grinds mostly for turbo that this would not apply to, not even close. mainly because they are weard.
The exhaust valve closes lets say 8-10 degrees ATDC when the piston is moving down there is still exhaust getting blown out.
At low rpms this is bad. exhaust gets sucked into the intake runner at low rpms while there is vacuum in the intake. To take full advantage of this there must be positive pressure on the intake valve at high rpms when the intake valve starts opening at 15 degrees BTDC There is a slight vacuum in the chamber. because the exhaust valve opened before BDC, letting the exploding gas blowing its self out. but I'm not going to get to far into that yet.
As the intake valve starts really opening with the piston at 5-8 degrees (I mean open enough to let some air move) BTDC. There is intake air being sucked in and getting forced into the chamber chasing out the exhaust while the piston is moving up in the cylinder.
As both valves are open there most (about 1/16'') at the same time and the piston is at TDC the exhaust is sucking all of its self out and the intake air is starting to force its self and it is taking over.
Once the exhaust valve closes all the way the intake air is trapped and the piston is moving down.
The intake valve is open its most well after 90 ATDC. Its open it's most around 30-50 degrees before BDC depending on if its a big turbo cam or big N/A cam.
You get your best fuel mixing when the valve is only open below .2''
And closes as late as 40 degrees ABDC (There are some that close later then that like the 306 grinds), that is when most of the air is sucked into the cylinder (ABDC). Yes there is a lot of air still getting sucked into the chamber because the runner air was accelerated during the time when the piston was between around 90 ATDC and full valve open. So now that the piston is moving up and air is getting into the chamber by means of some unseen force.
At this point the engine is super charging its self. The amount of self super chagrined is determined by a lot of factors, rpm, runners, intake harmonics, valve timing (mainly intake open ABDC), spark timing comes into play a lot when you do FI and more.
The self super charging really starts with the exhaust opening well before BDC on the power stroke and the exhaust blows its self out creating a slight vacuumed sucking the piston up the bore and some times contaminating the chamber with oil if you don't have good oil control. I think that is why the 2nd ring is some times shaped as a scarper, to scrape oil off the sides when going down and keep chamber vacc form pulling oil off the oil ring. That is why you want a vacc pump for realy low tention rings (2lb.-4lb.), keep the crank case vacc near what the max chamber vacc will be.

 

Some of this is pretty close, but some of it might not be. Some of your conclusions aren't necessarily correct.

Consider what happens when you lift (close the throttle) at high rpm. Think about the pressure (+/-) in the intake tract and cylinder then.

I'm not going to pick it apart bit by bit, Seth, but I wouldn't suggest taking it as gospel.

It's always good to get discussions going.

 

This was purely based on at or near WOT from what I under stand is hapening to a big cam in its operating range.
If it is wrong I would like to change it, I like to post it for all to see so they can know what is hapening.
I added that scraper ring part because we have some new diesel engines that have a scraper second ring. They burn no trace of oil, unlike all the other deisel engines.

 

Crash,

The head port length must also be considered when figuring intake runner
length.