lsa vs intake length

[sjre]

Thinking about where to put lsa when you have to deal with a crossram manifold with long runners 12 in. + .Some specs 4.380x4.35 13 to 1 comp. heads around 3.8 csa. 2x 750 carbs.Thinking at first around 112 lsa but then trying to visualize getting the intake charge started early, (tighter lsa) or maybe wider lsa with 4+ deg. adv.Engine is a torque monster but power signs off at around 6000. Trying to get it to rpm more.Any thoughts would be appreciated.

[Walter R. Malik]

Thinking about where to put lsa when you have to deal with a crossram manifold with long runners 12 in. + .Some specs 4.380x4.35 13 to 1 comp. heads around 3.8 csa. 2x 750 carbs.Thinking at first around 112 lsa but then trying to visualize getting the intake charge started early, (tighter lsa) or maybe wider lsa with 4+ deg. adv.Engine is a torque monster but power signs off at around 6000. Trying to get it to rpm more.Any thoughts would be appreciated.

Lobe separation angle is merely the result of getting the correct amount of valve overlap for any given situation.
Different lobe duration amounts will alter the needed lobe separation on even the same combination.

[sjre]

Walter I realize about duration, but in this situation you what im thinking is for the amount of duration needed you either have too much or you close the valve too early.

[sjre]

Also for some reason I dont think overlap is your friend here.

[Baprace]

I think your engine is running out of air, years ago on tunnel ram bbc 4.28 x 3.76 I used 2 carbs 850 cfm ea and it pulled to 8400 with no issues, later I put a 4 port blower injector with 3 inch throttle plates on the same engine and it pulled harder on the top end with no issues, I didn't turn it any higher because I was afraid of dropping a valve. This was back in 1971 when valve springs were marginal at best, valve lift was .785 . JMHO

[LSP]

Changing LSA isn't going bring it to life up high.
What are duration @ .050" numbers?

[sjre]

Duration @050 is 276 int 282 exh currently on a 108. The main question is with the long runner crossram do you go with a wide lsa to control overlap or a tight lsa to start cyl fill early to start the column of air moving? Im basically looking for suggestions/experience with long runner flat crossrams and what worked in cam lsa.I hear heads are weak and maybe they are, however I still think change is possible with cam events.

[MadBill]

What do you have for valve size and head flow (estimated is better than nothing), preferably without and with manifold?

[F-BIRD'88]

A long length runner cross ram is in max resonance at a relative low rpm ( fequency) . It is very efficient at that point. But at higher rpm ( frequency) its resonance moves progressively out of phase, this becomes less efficient at higher rpm. Thus powr and air flow becomes limited by higher rpm.
A shorter tuned length runner ( maybe larger CSA too) becomes a BETTER cross ram at higher rpm. More power at 6000+ rpm... Chrysler had a modified Siameses short internal runner cross ram.. it looked like the long ram but the runners are siamesed inside to shorten their effective tuned length.
Thus more top end power.
Look at a pipe organ. Note each tuned pipe's length is tuned to the frequency of the note being produced. It is a narrow band of tune. And progresses from long at low notes to short at higher notes. Cross rams and tuned port EFI intakes work like that. Long runners give a BIG low to midrange torque boost. Shorter tuned runners make the torque boost at higher rpms. The cross esctional area of that tuned runner is a trade off between needed port resonance energy at the intended rpm and raw air flow. A tuned runner for higher rpm and high engine power needs a shorter runner and a bigger runner too to optimize it.

On a engine the exhaust headers have to "play along" too to create correct exhaust scaveging in phase at the needed rpm band.

Simply put you may have to play with the camshaft AND play with the length and CSA of the intake runners too to get more top end power. (Shorter bigger runners)

[F-BIRD'88]

What is the total length of the intake runner from the manifold plenum to the back of the intake valve...? ( intake runner length + length of the intake port of cylinder head.)
What is the intake valve diameter? Wedge head, Hemi head?
Current engine torque peak rpm? current engine horsepower peak rpm?


What is the
trend" of current camshaft adjustment? What happens at 6000 rpm if when valve lash is decreased, then increased? What happens at 6000 rpm if when the current cam is retarded?,, Then advanced? What does the engine tell you about this?

[rebelrouser]

my experience with a mopar cross ram, racing in Nostalgia super Stock, is that it makes lots of torque, but the engine runs out of wind about 6,000 rpm. four different cam grinds, 108 to 110 centerline, 250 duration to 270 duration, it was like it had a governor on it. Low 10's 3200 lb car was just where it would run. Used same carbs on an inline 2x4 intake and it ran a best of a 10.64 @ 140 mph. I love the looks of the old crossrams, but they are just old technology.