[Joe-Touring]

Originally Posted by JUDGE3

to explain where i'm at: posted my cam because several had stated my cam is a problem with the rare round port exhaust manifolds. specifically the 110 lobe separation. wanting to learn why?

The big reason why equal length headers are the hot ticket is due to exhaust scavenging.


A good set of headers that have a well designed collector(v important) will flow more CFM than the diameter of the tubing would suggest. This is because of the scavenging effect. When an exhaust valve opens and the gases move thru the primary tube and past the collector, the other 3 tubes will then be affected by the low pressure zone following the high pressure pulse. This helps draw out the exhaust gases from the next cylinder. I think this is a simplistic explanation, there’s other factors, resonance/heat etc.

Lobe separation comes into play because it’s directly related to overlap(intake/exhaust valves open at the same time). While both intake/exhaust valves are open, the scavenging effect moves into the intake, drawing more fresh air and fuel into the cylinder.

From what I understand, ram air manifolds like 112-114 degrees LSA.

Quad in post 6 linked a good reference:

Originally Posted by Quad

SD performance have a good tech article on this topic. RARE OS manifolds can make almost as much power as headers. OR a lot less. Link below explains.

https://www.sdperformance.com/moreTech.php?newsID=39

[Quad]

Dave Bisschop goes into detail on post 90 of following thread on the importance of overlap for when deciding if a cam is a good choice to run with the RARE manifolds.

http://forums.maxperformanceinc.com/...=775018&page=5

[JUDGE3]

Quad and Joe-Touring, thank you for the info. the links to learning the "whys" of reversion/scavenging/backpressure in relation to cam overlap was exactly what I wanted to understand better. using the wallace racing calculator i'm at 76.50 overlap that puts my cam on the top end-ish territory of horsepower loss with rare manifolds. with that however, also learned intake dual plane/single plane makes no difference.

this puts my particular combo to a great (to me) test. as stated I have the rare manifolds on now and will see how carb tuning & timing affects the engine.

the end game is that if I get the engine idle and exhaust tuned in running clean with the manifolds I don't mind giving up horsepower.

but if not, I certainly have no problem going back to headers. this engine was originally built to use headers so I realized using exhaust manifolds could have an adverse effect but thought only with horsepower loss not tuneability. will gladly go with headers if required definitely will not change cams over it. engine is a healthy 10 year old engine no way I want to tear into it, looking forward to seeing how many years it lasts with my heavy foot!

[JUDGE3]

Quad and Joe-Touring, thank you for the info. the links to learning the "whys" of reversion/scavenging/backpressure in relation to cam overlap was exactly what I wanted to understand better. using the wallace racing calculator i'm at 76.50 overlap that puts my cam on the top end-ish territory of horsepower loss with rare manifolds. with that however, also learned intake dual plane/single plane makes no difference.

this puts my particular combo to a great (to me) test. as stated I have the rare manifolds on now and will see how carb tuning & timing affects the engine.

the end game is that if I get the engine idle and exhaust tuned in running clean with the manifolds I don't mind giving up horsepower.

but if not, I certainly have no problem going back to headers. this engine was originally built to use headers so I realized using exhaust manifolds could have an adverse effect but thought only with horsepower loss not tuneability. will gladly go with headers if required definitely will not change cams over it. engine is a healthy 10 year old engine no way I want to tear into it, looking forward to seeing how many years it lasts with my heavy foot!

[JUDGE3]

I had a post somewhere in the street section I believe regarding carb tuning my mentioned hyd roller cam I had previously ran with dougs headers. bumped it to this thread.

DURATION @ .050 INTAKE 242
DURATION @ .050 EXHAUST 248
VALVE LIFT INTAKE .540
VALVE LIFT EXHAUST .562 LOBE SEPARATION 110.0
*ALSO ADD 1.65 ROCKERS*

just noting a follow up to that. now, I fully understand headers r best for the above cam and why. thats why I had headers to begin with. briefly, I wanted to move away from headers due to moving away from racing (as much) and the hassels of no pinion support mini starters.

I had chosen the 1,000 cfm holley rated dichromate carb in the pic. Tuned that as best I could with no success and was having way to rich black carbon fouled plug issues and corresponding foul exhaust. shaker455 has this carb reworking it. also swapping it to annular boosters among other things.

I'm planning another 455/6x heads/hyd roller cam/d port ra manifold build for another car so I need 2 carbs anyway so I purchased this black/red anodized quickfuel "race" 750 carb from summit racing.

I had done a lot of jet changes seeing what the engine liked prior, so before ever installing the quickfuel I set the transition slots, jetted it 84 primary 88 secondary, 4.5 pwr valve, set the float levels, and tuned the 4 corner idle using a vacumn gauge.

I am super happy to say this carb is awesome. I have clean spark plugs, clean exhaust, excellent idle. the engine loves it and is very responsive, it screams. the only other change i'll be doing is adding a front 50cc accl pump as it has a stumble from a stand still wot launch. I had read an article in motor trend about choosing the right carb where it stated "in almost every circumstance, a one size smaller carb than you think you need will achieve best tuning results". so I went with the 750. home run. my cam is not an issue whatsoever with my RA oversized manifolds. better than headers? no way. but no mixture tuning issues so i'm happy.

this qft has a billet vacumn ported baseplate, billet metering blocks, clear sight glass, secondary adjustment screw. comes with blocked rear power valve and jet extensions.

as good as it is, looking forward to comparing it with the shaker455 reworked holley. both carbs will be on healthy 455's.

for me this is a real world result from an article in hpp by my all time favorite pontiac guru Mr. Pete McCarthy addressing over-carburation.

[PAUL K]

Nice!

[Half-Inch Stud]

Try to imagine the iron manifolds with a COLLECTOR of decent length. You will thank me later.

[sdbob]

I remember Jim Miller had one of those metal sheds back in early 70s. Because of all the guys he worked for and knew went to headers.It had cast iron HO manifolds,both d port early, late and 1st gen 69 Round port. They all went to junk yard! I get sick now thinking about them