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Originally Posted by Puddles the Pontiac
as far as the cam goes, i had always thought the wider lsa cams had more cylinder pressure because of the lack of overlap bleeding off cylinder pressure.
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Please folks,stop repeating this gem of information,it is not remotely accurate,overlap has nothing to do with cylinder pressure,not in the practical sense of this stuff.
Overlap happens at the end of the exhaust stroke,and the begining of the intake stroke,nowhere near the compression stroke.
The only events that significantly matter to the engine in terms of cylinder pressure is the intake closing,and possibly the exhaust opening events.
The only way overlap affects cylinder pressure is by influencing the VE and helping the engine get a dense intake charge to trap when the intake valve closes,that is the effect called "exhaust scavenging".
But that happens mostly at higher RPM's,not so much at lower RPM's.
And besides,speaking of overlap independantly of duration,LSA,and the actual valve events would be a complete waste of time for the most part.
A fella can have a ton of overlap and still crank out significant cylinder pressure numbers.
And he can do that with
either a narrow or a wide LSA if he knows what he's doing.
Especially when taken in context with the given static compression ratio of the engine in question...
Now some honest food for thought.
New cam:Custom;
Spec = in/ex
Adv. dur. ~ 289°/297°
Dur. @ .050" ~ 260°/268°
Lift ~ who really cares...
LSA ~ 110° ICL ~ 106° ECL ~ 114°
Events:Seat (.020");
IO = 38.5° BTDC EO = 82.5° BBDC
IC = 70.5° ABDC EC = 34.5° ATDC
O/L = 73°
Events:@ .050"
IO = 24° BTDC EO = 68° BBDC
IC = 56° ABDC EC = 20° ATDC
O/L @ .050" = 44°
Now the 290b6:
Spec = intake/exhaust
Adv.Dur. = 290°/304°
Dur. @ .050" = 255°/266°
Lobe lift = .360"/.356" Rec.lash = .026"/.028"
LSA = 106° ICL = 106° ECL = 106°
Seat events:
IO = 39° BTDC ~ EO = 78° BBDC
IC = 71° ABDC ~ EC = 46° ATDC
Total O/L = 85°
.050" events:
IO = 21.5° BTDC ~ EO = 59° BBDC
IC = 53.5° ABDC ~ EC = 27° ATDC
O/L @ .050" = 48.5°
The new cam is not what I consider "bigger" than the 290b6.
Now,here's the clues,the intake events are'nt a whole lot different if indeed it did get installed @ the 106° ICL,that is in spite of the added .050" duration,that's all because of the similarity in the seat durations between the two cams,but you definitely lost some overlap everywhere,and honestly that's just what a fella who's gonna use spray normally wants to do,but that's also gonna make it seem a bit "sluggish" on the motor alone,especially if it is/was slightly overcammed to begin with or such.
By widening the LSA you effectively gave the exhaust lobe more time to do it's job.
You do so at the expense of the intake side,and at the expense of the exhaust scavenging when running NA,open the exhaust earlier it's gotta close earlier too for a given duration..
By "adding exhaust" to the cam you shoved the power curve upwards slightly,this also happens when you increase the intake/exhaust split as SOP.
So what happened,while you made the intakes lobes seem "smaller" for that given cam,so understand what does'nt make it into the engine to begin with,really cant be compressed to make that power later on,ie: if the intake side cant feed the HP,the exhaust side wont make that situation any better.
That's why nitrous and boost are generally less dependant on overlap,they help feed the intake side copious amounts of intake charge!
But in a NA combo,barring a restrictive exhaust system,that extra exhaust scavenging really helps get the intake charge moving in the right direction.
More charge,more power,it's as simple as that.
With big flow heads,and lots of cubes you dont need the scavenging nearly as much as you do with restrictive heads and such,it's almost like boost or juice in the most basic sense of all this.
Like I said in my first post,I would try advancing the cam to 102° to 104° ICL,and understand that absolutely all cams of this sort need to be degreed when installed,I simply cannot stress this strongly enough!
I would do that and as I said earlier maybe try adding higher ratio rockers (at least to the intake side) and try to tighten up the lash,these changes all should add back some of what was "lost" by widening the LSA and not going adding more seat duration,and that should hopefully help make similar NA power as the 290b6 made.
If it slows it down,then I dunno what to tell you,likely something else is amiss,as that should add power to that combo while trying to bring the power curve back down a few hundred RPMs or so at the same time.
BTW,I did'nt say I dont like this new cam,I just said it gave up some in the NA department,,,myself,,,I would have just sprayed the 290b6 and not lost a moments sleep over that deal...
I also think the car should have run a little better with the 290b6 combo,but you did'nt really give a bunch of info on that deal,please tell us more about the whole combo,what gears,convertor,tires,exhaust system,fuel system,that sorta stuff...
Was the intake port matched?
Tried any spacers or such?
The more you tell us the better!