[screamingchief]

Originally Posted by Half-Inch Stud

WE all agree that:

Intake Closing is the most important parameter ( Hp & TQ-RPM gradient sensitivity is highest than other events ).

I dunno about that,,,ask me this is one of those things where you'll find there is no 100% agreement across the board,,,ask ten different guys,,,and your likely to get a few different answers...

You'll get all sorts of opinions on what valve events are most important.

With most folks typically just regurgitating something they read somewhere,or heard from someone who's opinion they respect.

Bruce mentioned Harold Brookshire,well guess what,he does'nt feel intake closing is the most important event,hell he puts it third in terms of it's importance.

Originally Posted by UDHarold

I consider the intake opening and the exhaust opening points to be so close in equal importance, I do not know which is MORE important. Here are some thoughts:
The most important single degree in your camshaft may be the degree before the intake valve opens. At that point, the exhaust valve is open and closing, there is a volume of exhaust gases left in the cylinder, being pushed out the exhaust port by the piston. These exhaust gases have a positive pressure, known as back pressure. They cause reversion when the intake valve opens. The volume and pressure of the exhaust gases will determine how the cylinder fills on the intake stroke.
Proof? Most engine builders have tried 2 cams with the same intake and exhaust grinds, but different LSAs. Put both cams in the same engine, on the same intake center line, and dyno. Result? Two different power curves.
You say, but they were different LSAs! I said put them in on the SAME intake center line--Same intake cam grind, same opening and closing points, 2 different cylinder filling rates, 2 different power curves. The exhaust opening and closing points were different, though. The exhaust reversion/back pressure governed how the intake would fill the cylinder.

No matter what the duration or LSA, I rank the importance of valve timing in the following order:
1ST(& 2ND!)---Exhaust opening(which occurs first) and Intake opening.
3rd---Intake closing.
4th---Exhaust closing.

I personally dont see valve events as a popularity contest,so I dont play favorites,I say watch all of the valve events equally,and consider them all based on their own merits,as different applications are typically going to have differing needs.

FWIW.

Bret P.

[imlgnd]

Haven't we all read cam theory up to our eyeballs?!? I've been reading theoretical posts for years in this forum. It's not that I don't wanna learn... I love to learn that's why I've been reading this forum for years. But... i did ask for specific changes in your combos (twice) that helped or hurt. I understand that your combo may require something different than mine and I will most likely not see the same result. That doesn't mean there is nothing to to learn from it or that at the very least I won't be entertained by a great blunder=) I gave my combo Because I want to read about changes you've made with similar combo's. I know most of you guys haven't run d port in 20+ years cuz there are so many good head designs to chose from now, but I gotta run what I got for at least a little while longer =(

So I hope not to kill this tread but... changes you've made is search of power did what to your power band or et's?

[Ron H]

Seems to me you could learn by retarding your cam 4* and running it at the track.

[imlgnd]

I have that in mind. I have a new converter with a totally different car so I'm gonna set a base line next season (chase out the bugs) then if I can make that change I don't expect it to change more that .05 et change but dreams are free=)

[BruceWilkie]

Originally Posted by Ron H

Seems to me you could learn by retarding your cam 4* and running it at the track.

X2 thank you Ron.

The most direct comparison was a few months ago with a friends dohc turbo'd 4 cylinder. Changing cam positions made for interesting results. Advancing the intake cam was a loser across the board. Retarding it a full tooth(about 13.5) showed improvement but nearly 1000 rpm higher power band. Then tried advancing exhaust cam. More power than with both cams in stock position but dead at 6500 it wouldnt make power beyond that. Slight increase in tq better driveability. Fuel curve went rich and had to be leaned too. Tried putting intake back at stock position with ex advanced and it went lean readjust fuel curve power worse than stock. Final test both cams retarded 1 full tooth about 30 degrees underlap. readjust fuel curve again gained 40 ft lbs and 70 hp at the wheels over baseline, power band shifted over 1000 rpm higher and was showing no signs of dropping off at the 7500 redline he didnt want to exceed. Low end driveability suffered. New cam sprockets are being made to better fine tune the cam positioning. Two different cams will be tested too and they can be swapped side to side. We will find out more when everything is in hand and have a few days of dyno time. (Jeff trades work for that dyno time. Sold it to present owner because he wasnt getting his customer work done. Like a kid in a candy shop!) Goal keep the same tq and power but at a lower rpm and same amount of boost. Improve driveability and boost response also. Peak boost in all tests was 18 btw.

We arent so lucky to be able to alter just intake or just exhaust this much with out a cam change but if solid cam, we can adjust lash on just one side or both and cam positions for clues.

Seems my personal real world cam changes have all also been different profiles so LSA and ICL comparisons would be pointless. However simulations done after the fact pretty much showed the results obtained. And I've obtained expected results with cams I simmed prior to using. Most important with sims are the trends and not the actual HP numbers they come up with. Though the hp numbers often are very close its the power curves/ rpm ranges that really show up in the sims that show you where youve been or are going.

For sure wider LSA doesnt always mean wider power band and Narrow LSA narrow powerband.(Last years engine masters Kaase threw that myth under the bus running something like a 92 deg LSA. The engine masters motors are built for very wide powerbands)

Actual valve motion relative to flow curve quanities relative to piston motion/location/speed and support systems like intake and exhaust determine the engines "behavior".

Thats why I felt cam theory was far more important than throwing LSA and ICL numbers out there, its more useful.

[Half-Inch Stud]

Originally Posted by Half-Inch Stud
WE all agree that: Intake Closing is the most important parameter ( Hp & TQ-RPM gradient sensitivity is highest than other events ).

I should clarify: Intake Closing being the most Sensitive Parameter. HP & TQ result versus degree movement

[BruceWilkie]

Originally Posted by Half-Inch Stud

Originally Posted by Half-Inch Stud
WE all agree that: Intake Closing is the most important parameter ( Hp & TQ-RPM gradient sensitivity is highest than other events ).

I should clarify: Intake Closing being the most Sensitive Parameter. HP & TQ result versus degree movement

True but get the exhaust opening wrong and it affects what happens at the 3 other cam timing events thus impacting greatly how much HP and TQ result you get. Open the exhaust too soon too fast, you dump power right out the exhaust. Open the exhaust too late or too slow and you delay incoming charge movement and increase amount of exhaust dilution of the incoming charge. Once an engine is running and at WOT, exhaust leaving sets up the conditions to allow the atmosphere to push the charge in. The piston movement does not suck the charge in.

Ideally you want combustion pressure until the point of max piston speed and then residual combustion pressure to be relieved bdc dwell. That leaves only swept volume for the upstroke portion of the exhaust. Less pressure on the up sweep allows intake pressure to begin moving the charge sooner btdc dwell. Less residual allows the exhaust to be closed sooner atdc dwell.
During overlap the pressure wave from back at ex opening is beyond the ex valve and the pressure behind that "front" is lower, creating a condition where intake pressure(atmosphere) is higher than the pressure in the chamber and exhaust port and pushes charge out the intake valve across the chamber and purges and fills chamber with fresh charge. The piston is in dwell at tdc and fresh charge is already filling the space. Exhaust closing the piston begins descent, and atmospheric pressure moves more charge in, and it gains velocity with piston velocity. Because the charge has mass it wants to keep moving. The cylinder can fill beyond bdc untill either the intake valve gets closed or the pressure rise from the cylinder fill velocity becomes less than the pressure from piston rise. Once the intake closes full compression begins.

Power is directly related to density of the charge from compression, AND DENSITY OF THE CHARGE BEING COMPRESSED!

You CAN NOT get your densest charge if the exhaust timing was off!

Early intake closings at bdc and after increase compression pressure. Too early bdc or after decreases the amount of charge available to be compressed. Way too early (BBDC) loses both pressure and volume .

BTW the purge I speak of at overlap is NOT just a high speed scavenge effect. Overlap purge is present even on a regular econobox type engine.

High speed scavenge effect exhaust tuning can/does create conditions where the exhaust creates a reflected pressure wave that allows greater charge flow than would normally occur and boosts VE (often times greater than 100%).