[imlgnd]

I would love to hear from experience. I have a 455 260int/210ex cfm d-port heads 6.8 rods 8.5:1 compression and 278int 280ex @.050 .700 lift lsa 104 icl 102 Best time 10.81 at 128.8 over a mile high on the worst track in America=)

Who's done something like this? Results/thoughts

[Half-Inch Stud]

I used to try 257/263 Flat HYD cams with 104 LSA and advance 4DEg to result in 100/108 Lobe Centers. High Compression. Never got a clear 1/4time to uphold the Cam merits (Fuel starve, wrong converter, no money).

I did not try that on Low Compression. YET your results seem pretty good. What weight, gear&tire, & Octane?

[GOAT490]

276/286 .680" 106LSA 100IC

PHILLYGOAT

[johnta1]

I used a roller that was 316/316 advertised, 104 LSA .720" lift.

It was for my SD 455, ran 6.40 in 1/8th with 1.38 short times.
That cam was awesome.

[cgeise]

I ran a similar deal years ago in a light car - the tork was brutal - but that was befor the big flowing heads that need a diff deal now.

[imlgnd]

I only got 1.5* 60 ft times (1.52 best) 3000 lbs 4.11's with 31x12.5W I ran out of gear just after 1/8 n rapped it out til I got to the big end. That's why my mph was high the combo made all it's power under 5200 rpm. I think I was rolling through the traps at 6500 to 6700 rpm. pump gas 87 oct.

I still have the same motor. My GTO is almost done I'll be running it this spring. It's a much heavier car!

So thats what I wanna talk about. With these heads I'm very limited. the combo seems to be good. but when I get those kre's I want should I widen lsa? has anyone done the narrow lsa on a set of heads that can flow?

[David Holmberg]

With compression that low, you need to throw some nitrous at it, and it will really wake up!

[BruceWilkie]

Some things to keep in mind. Cams are NOT designed based on LSA. LSA is ONLY the final coincidence of the timings chosen. Intake centerline same deal coincidence of timings chosen. Duration at .050" cam lift a very small part of the picture. ALL 3 BEST USED for checking purposes!

Whats important??

Intake closed point determines not only when you stop filling the cylinder, it determines the NUMBER of DEGREES in COMPRESSION

Exhaust open point. Too soon you lose combustion efficiency, too late and you create pumping losses on the upstroke portion of the exhaust event. When the exhaust valve 1st cracks open all the charge "wants" to leave the cylinder. Ideally you would want to relieve all the pressure at bdc dwell leaving only swept volume to be cleared on the upstroke. It also affects overlap requirements and overlap bias.
Less pressure on the up sweep allows the intake charge to begin movement sooner and the exhaust valve can close sooner because the lower pressure created a better purge. Way back when the ex valve first opened a high pressure "slug" was created. By the time you get to overlap period that "slug" is somewhere beyond the exhaust valve. Behind that high pressure "slug" is a low pressure "front". This lower pressure area allows higher pressure atmospheric pressure to push the intake charge in and the residual exhaust in the chamber out. Your chamber plus very small piston movement tdc dwell and you are already starting cylinder fill. The exhaust closes and atmospheric pressure continues to fill your cylinder. The piston is still near tdc.
If incoming charge has enough velocity the cylinder can continue to fill AFTER bdc dwell. Closing the intake too soon can/will limit rpm/power. The cylinder pressure power increase from increasing time in compression would be negated by insufficient charge for the power/rpm desired. Without increasing displacement and/or rpm you will hit a point where no further power can be made.
Rpm range and flow character of the head, intake and exhaust(more so than intake)AND STROKE, primarily influence the valve movement requirements.

Generally speaking higher flowing heads require less duration for the same power band. If a similar powerband is wanted this usually translates to greater valve open/close rates with reduced seat timings. Higher flowing heads allow for higher rpm power ranges.

The timings required for your specific application determine where the LSA or ICL best ends up. The .050" cam lift duration is only a function of the lobe requirements based on flow requirements and the intended rocker ratio plays a bigger part than one would think.

BTW early intake closings do help low compression motors pick up cylinder pressure but can limit cylinder filling at higher rpm.

A dual purpose street/strip combo will require more compromises than for all out drag use. A short track oval combo will have different timings as well due to part/full throttle operating conditions at various speeds and loads.

Lastly I suggest you bolt a degree wheel on your current combo and map some of your net valve movement on #1 cylinder. Have the head off the even bank and measure/look where #6 piston is (same as looking at #1 as far as piston movement relative to the degree wheel).
That little exercise will give you a much better perspective of why timing events and valve movement rates are more important than an ICL, LSA, or duration at .050" cam lift.

Keep in mind piston speed is max somewhere around 72-75 degrees bbdc depending on stroke/rod length.

Keep in mind big gains in VE at certain speeds more than offset small combustion efficiency losses.

With your current combo it is entirely possible that different exhaust timings with the very same intake positions could have extended your power band with little to no loss in the lower race rpm band width(possibly gain there too). It would certainly result in a different LSA with same ICL. It could also be possible that the same intake lobe would want different position timing with revised ex timing and the new resulting LSA.

Again its best to NOT choose your cam by LSA ICL or .050" cam lift duration.

[performerrpm]

i running an ud 700 lift cam with 261-269@050 and 104 lsa but iam running 11.5 cr and i havent got the car ready for it yet so iam glad to see you managed some 10s with yours.


i bought my cam from jack ferris, a member here who ran it in his gto and was in the tens, i think he said he was at 10.0 cr.

i would have thought yours would be a bit sluggish with a cam that size at 8.5 cr but in any case your moving along pretty good, just makes me wonder how well you would run with a touch more cr.

guess i set my sights to low with my car seeing how i was aiming for hi tens with a 310 ehead

[Tom Vaught]

255-260 @ .050 .590 lift 106 (Intake at 102) 10-1 cr (This was in 1978)

Not very fast, 11.55 at 117 dragging on old full weight 64 GTO Vert with a 6 point.

Tom Vaught

[TIN TIGR]

wilkie is my daddy..lol..or my son..its all in the in close/ex open, and overlap flows..

the "really old faithful" 290b6, and its bro, the 300b6, both work great with tight lobes..for a store bought cam, i never found better..

the stock eliminator cam i worked out was on 104 lobes, but i cant give ya the specs, becuz i'd have to kill you..

[imlgnd]

TIN TIGR you speakin greek? cuz I dont understand any of that!

[quicksilver97ta]

Originally Posted by imlgnd

I would love to hear from experience. I have a 455 260int/210ex cfm d-port heads 6.8 rods 8.5:1 compression and 278int 280ex @.050 .700 lift lsa 104 icl 102 Best time 10.81 at 128.8 over a mile high on the worst track in America=)

Who's done something like this? Results/thoughts

No experience with something like but what rpm do you shift at with that combo?

[TIN TIGR]

read wilkies post, 2 up..we agree on many items about cammys..people get too intense on weird "checking" numbers, and lose sight of what the cam is really doing..

as a nitrous guru, i can tell you, there is no secret cam for anything..its all very subtle..change any part of your combination, and the valve timing is off..a bigger carb, or headers..all you are doing is hunting for the point where the highest power is made..it is a great game..

so dont get bent about this or that cam timing thats advertised..figure out what your combo wants(you think it does), and start there..
those who search for optimum valve events are the true pioneers..intake manifolds, heads, exhaust...these items do not move..but the valves do, and they control the whole engine..learn what wilkie speaks of in his post, and you will go farther..

[BruceWilkie]

Originally Posted by TIN TIGR

wilkie is my daddy..lol..or my son..its all in the in close/ex open, and overlap flows..

the "really old faithful" 290b6, and its bro, the 300b6, both work great with tight lobes..for a store bought cam, i never found better..

the stock eliminator cam i worked out was on 104 lobes, but i cant give ya the specs, becuz i'd have to kill you..

Maybe brothers? I'm 57 with 6 kids from 8 to 37. Last one was a gift I never expected, oops.


The cam stuff?
Be very careful and dont get hung up on numbers. Either EXACTLY copy someone elses success or seek the opininion of a cam designer familiar with non Chevy belly button trick of the month cams. If one of the first few things stated are "you are going to need x lobes on x LSA" before getting into all the details of your combo and how you plan to use it, just hang up the phone.

Harold Brookshire's writings are my inspiration. Learned from Harvey Crane and Isky's writings as well. However Scooter at Comp and Tim over at Bullet can help you get the right cam if you have the right info they need from you. Forgot to mention Eric at Erson as well. Dont forget our Pontiac vendors either. Many of them can put you into a great cam for your application.

[imlgnd]

I try n shift between 5000 and 5200 cuz all my power is below that point.

Tin Tigr... I agree carb intake exhuast and ignition timing is where the minor details sit. I started this thread cuz I feel large duration and narrow lsa and early intake events have made the most power for me. Ive tried a few cams in search of a better combo but I feel I am on to something not well known in the pontiac community. So before I light the world on fire (read a big ha!ha!) I thought I'd see who has already been there done that and try n learn a little somthin somthin

Bruce... I understand (kinda) I think Im asking for you to be more specific. For example tell me about a specific combo that I can follow. For instance you have a certain flow head with this or that intake and this or that stroke (4.21 or bigger would be nice) with this or that exhuast so you thought this or that cam would be good then you used another cam and the result was a little better or worse because the valve timing changed... now I can relate it to my combo cuz the example has more meat to it. I'm tryin to fully grasp valve timing in relation to the pistons position but it's not easy to imagine from scratch. If you give those 'measuring points' from the cam then I can do the math to determine (kinda) where the cam is in relation to the crank. To me this would make it soo much easier to see how a minor change affected your power band... right?

[screamingchief]

Originally Posted by imlgnd

I started this thread cuz I feel large duration and narrow lsa and early intake events have made the most power for me. Ive tried a few cams in search of a better combo but I feel I am on to something not well known in the pontiac community. So before I light the world on fire (read a big ha!ha!) I thought I'd see who has already been there done that and try n learn a little somthin somthin

There's nothing really "new" about cams like that,the pontiac super stockers have been using them for decades now.

Though a lotta guys these days would say that's "old school" cam thinking.

For the typical SS car/combo,the iron heads really can only go "so far" on their own merits,beyond the head's abilities,one often has to make the cam carry things as well.

Problem these days is some guys dont like that approach to cams,so it has fallen out of favor somewhat as of late,epecially when we have much better heads at our disposal these days,but the old timers know damn well it can work when one approaches it with the right mindset,and the proper combination of parts.

But it's not the sorta thing that's super easy to simplfy in these discussions,or distill down to a "formula" or such for all to follow.

Regardless,one always has to watch the valve timing/events to really see what's going on with any cam,as the "numbers" often wont tell the whole story.

Anyhow,I digress.

Bret P.

[TIN TIGR]

yep, i learnt frum an old guy..actually, i did the same deal..started doing the "opposite" of what hot rod and car craft and even pontiac literature said..then when it worked, i was allowed into the seamy sneaky world of stock and superstock racing..ya wont find smarter folks, or better cheaters..

[BruceWilkie]

Your missing my point. My combos would not be yours. Flow curves not just a flow number, stroke(rod length to a minor degree) exhaust intake compression etc. Car itself and same conditions(big difference running below 1000 ft vs 5000 ft.) all would have to be the same to say this LSA is better than that LSA. You cant make a strawberry from an apple and an orange. ITS ALL ABOUT TIMING AND VALVE LIFT RATES/CURVES, NOT LSA/ICL. All relative to stroke/piston position and flow curve/quanity, displacement, rpm range.

I havent fully restored my pc since the crash but I can give you some approximate numbers. A 400 with stock rod length reaches max piston speed approx 75 degrees bbdc. A 455 iirc approx 73 degrees. There would be no power to gain opening the exhaust much later (say 65). Opening sooner than peak piston speed it appears you could lose power because of combustion efficiency losses. Technically you do lose a bit but you can gain more than you lost through improved ve possibilities. A factory 2 barrel cam starts opening the ex valve by 75 deg bbdc. A performance cam may start cracking the valve at 85 bbdc (or sooner) but by 10 degrees later(75bbdc) its opened the valve much further. By bdc 75% or more of the combustion(exhaust) pressure in the cylinder can be relieved. The valve lift rate and exhaust flow curve affect how much. Stroke/rod length determine how long the piston stays at or near bdc dwell. As mentioned in previous post this affects pumping losses on the upsweep side of the exhaust stroke. This greatly affects overlap requirements because of presure differentials in the intake and ex ports vs pressures within the cylinder.

Your intake cant start flowing into the cylinder until atmospheric pressure becomes greater than the pressure remaining in the cylinder and chamber during overlap(even grannys 6 cyl) That big high pressure exhaust release way back before bdc is somewhere down the pipe and lower pressures are behind it. When those pressures go lower than atmosphere the intake charge begins moving into the chamber. You want to close the exhaust valve as soon as the purge has completed and the piston hasnt moved far enough out of tdc dwell to create a condition where the exhaust reflected pressure waves could reverse ex flow back into the cylinder.

Once past overlap the intake valve continues to open and ATMOSPHERIC PRESSURE fills the cylinder. The piston is still near tdc on the down stroke and the chamber and small space above the piston in the bore are already filled with mostly fresh charge. As the piston descends atmospheric pressure pushes the charge into the cylinder, its not sucked in. As rpm increases so does the charge velocity and the cylinder can continue to fill beyond bdc up to the point of cylinder pressure equalling the charge pressure.(more rpm later closing) Closing the intake valve earlier than that decreases the amount of charge available for power/rpm. Closing the the intake sooner after bdc also increases cylinder pressure because it increases the number of degrees the cylinder is in compression.

As you can see the exhaust opening and intake closing are very important. However the exact timing of these events are trade offs intended to take best advantage for the task at hand.

Your combo appears to stop making power early because the intake is closing early.

It has helped your low compression ratio high altitude condition obtain higher dynamic compression. Possibly alot of your intake charge is going out the exhaust as well. Hard to say how much power you may be losing elsewhere due to other timing events and header tuning and such without a full analysis.

Better cylinder fill(ve) could make a denser charge at a lower cylinder pressure and more power at same and higher rpm. A good 260 cfm head on a 455 should support power to 5700 rpm before it becomes a restriction to making power above that point. If the intake and exhaust arent restrictions too.(not that you cant make power above 5700, it just makes it harder to do so because of the ve's required) Average horsepower across your race rpm band width is more important to racing than power below that point. HP is how fast you do the work(torque). More rpm with your torque = more hp.

[Half-Inch Stud]

In agreement with Wilkie cam summary. I see LSA as a concluding observation. WE all agree that:

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

I implore that when a fella considers the effect of leaky Lifters on the Exhaust-only then the LSA is sooo last place in lobe-center spec'ing. Of course Leaky lifters provide an assured cooler Exhaust valve & seat.

More important consider the varied Intake Closing TOO because leakers were desired for their broader power-RPM product. Wrap your head around that and figure LSA is meaningless during that analysis. Enjoy. HIS