What is better for a cam with overlap, higher lift numbers or midrange lift numbers.

Is it that cams with overlap can overcome lower midrange lift CFM in exchange for higher lift CFM flow?

 

What do you mean by "overcome lower midrange lift CFM"? Overlap only occurs through a short time during the valve events, at low lifts. It has litttle to do with lift and is more a function of duration and lobe seperation angle. Also, better midrange lift numbers are always most desireable, the valve spends most of its time at midlift as opposed to max/low lift. Maybe try wording your question a little better?

 

"Overlap" has more of an effect on cylinder filling / rpms, than lift flow.
At lower rpms large overlap bleeds of cylinder pressure, hurting low rpm efficiency and torque. And even dillutes intake air with exhaust gases, kinda passive EGR. This also allows you to run higher static compression w/o detenation.
BUT as the rpms / piston speeds increase, the amount of time to fill cylinders decreases. Then larger overlap allows better cylinder filling b/c the out going exhaust flow will actually suction the chamber, pulling fresh air in through the already open intake valve(scavanging) before the piston really starts down its intake stroke cycle.(kinda like how a turbo has to spool up, the exhaust has to speed up for good scavanging)
So to a point, at "higher rpms", more overlap improves lift CFMs slightly, across the board.

"Lift flow" (CFM) is a function of valve and head design. Valve "shrouding" and "bowl shape" and a"valve's head" design, are the main factors in achieving good "Lift Flow"numbers. "Low-Mid Lift Flow" is where a good port job pays off big!

Any motor is going to make more power with heads that have bigger low-mid lift flow numbers, the higher lift cams will just allow more"time" for the valve to be open at those flow rates to fill cylinders. The actual time spent at max lift is pretty short, also the reaction time for the flow to start when the valve opens hurts Low flow, so the more flow you can get at middle lift is going to make bigger gains.

It's hard to explain in a short page or so, but I hope this helps a little?...

 

The reason I ask if because my buddy has 462s and I have Vortecs, we are both running the CC XE268H cam in our 350s and I noticed that the vortecs have much better flow from .3-.4 lift than the 462s, So even though the 462s have a decent .5 lift flow, the vortecs are much better in the middle.

 

The Vortechs have taller ports, better as-cast bowls under the valves, and swirl ports, all which boost mid-high flow numbers.

The 462's will need; a bowl port, good 3-angle valve job, under-cut-swirpolished valves, and a little grinding around the intake valve in the chamber(unshrouding)to really improve those numbers...
Excellent all around cam, BTW!...One of my personal fav's..

 

I'm hoping for high 13s....10.7:1 CR....I've got my fingers crossed...

 

Shouldn't be a problem..
I noticed your carb is a 670cfm, why don't you run a Torker or Vic. Jr. intake?? With the performer being a dual plane, each of your cyliners is only going to see 1/2 of that cfm rate. Driverside carb, feeds passenger side motor and Passenger side carb feeds drivers side motor...I'd either go 800cfm or a Mild Single Plane intake...
You'll do OK the way you are, but I think a small carb + single plane is Killer!..

 

I went with the performer and smaller carb thinking that it would help me down low. I've got a 1" carb spacer in there as well. It pulls real strong once I get to 2000RPM, but it's pretty sluggish down low.

I'm really in need of some carb tuning before I do anything else. I'm getting like 9 MPG (thats generous) and the idle is not very smooth at all. Engine is very hesitant until 1000RPM, even when fully warmed up it will sometimes idle at 500RPM or sometimes 750. It will idle smoothly for a little, then a very noticeable stutter and RPM drop then increase.

 

The spacer will help a lot, I've seen people getting good results with a similar set up...
Yeah, that's a good sign of running "fat", it'll run doggy down low and come on strong like at half track..
Have you reset the float level?..
Does your carb have the "sight glass" or the removeable screw??
Lower the float 'til fuel is at the bottom of the "hole/glass"but not quite high enough to drip out..
You can also change the "power valves" for the correct vacuum produced...I can't remeber just how it goes now...my 2-670's Vac/sec's are in the shed since I began really gettin into EFI..
I'll look through some of my old books for tips / tricks again...

 

What ya mean running "fat"? You mean rich?

I SWEAR it smells rich, but everyone I talk to says my exhaust smells normal. I have sight glasses, floats are set, fuel is JUST at the line where a tiny drip will come out with the sight glass out. That is how the holley manual said to set it, BARELY coming out.

I heard a few things about power valves but nothing for sure. The thing that bothers me is my buddy is running the exact same build (he's got 9.7:1 compared to my 10.7:1, and hes running the RPM performer) but other then that our 355s are the exact same pretty much. His car runs ALOT smoother, and he was getting 15-17MPG, niether of us have lockup connected. He has a 600CFM holley. He set his floats, then tuned.

I didn't tune my carb, my boss did it for me. He said he had to turned my idle screws out 2 and a half (its supposed to be like 1 and 1/4) I'm going to his house, he's got a rich/lean sensor so I can try and tune the car. Just don't know why my car is so bad under 1000RPM, and the gas milage is just terrible.

 

I know you're suposed to set the idle screws to obtain the highest idle possible, or where it pulls the most vacuum, then adjust the curb idle down.
I usually do this by going past where I should until the idle starts to fall, then come back in the other direction until the idle falls again, then go back to the middle...
I installed an Edlebrock RPM carb on a '65 Caddy about 2 months ago...SWEET car!..
I haven't played with my Holley's in almost a year...

 

I appreciate anything ya can tell me.

http://www.thirdgen.org/techbb2/show...hreadid=215462

Thats a post I put over at TGO, maybe it has more detail.

 

OK, let me get some data in the 'ole noggin..

How did you establish TDC??
How did you install the cam?Straight up,,or advanced??How did you know?
How did you reinstall the distributor??

I'm not saying anything about you're knowledge, just maybe some of my experience might catch something...
A good friend of mine who is knowledgeable, thought he had TDC, but I found it 180* out...He knew, that he knew it was right...but everyone makes mistakes..
Break it down in some detail....

 

TDC is marked on the damper. Original mark that was put on when the engine was built was wrong, so we had to pull #1 plug, and hand crank the engine over to find it, you know the drill. Original mark was like 12 degrees off.m The current one is good.

Cam was installed straight up, same guy who helped build my engine built my buddys 350. The cam was put in right.

When I put the distributor BACK in the 2nd time I pulled #1 plug, put a piece of paper towel in, cranked the engine til the paper blew out. Lined the damper marks up and put the distrib in with the rotor facing #1 plug.

The problem is definately ignition and carb related....that and compression. Guy over at TGO thinks my 10.7:1 is too high for pump gas....summit told me I couldnt' run 10.7:1 on pump gas when I ordered the pistons. A million other people told me that I could run 10.7, even the performance shop I had my machine work done at said you could do 10.7 on pump gas w/o a problem.

 

I don't think high compression will cause the symptoms your saying..10.7 is doable on pump gas...93 octane..
The cam you have will help bleed off some of that low speed cylider pressure b/c of the 110*LSA of the CC cams.
If you think it the carb, try your buddy's 600, if he's a good bud...(hint: you may have to buy him a case of Bud,,)

I was gonna say replace the damper, my damper spun about 20* on me too, and getting everything EXACTLY on TDC, ON THE COMPRESSION STROKE is beyond important!..
It still sounds like something isn't lined up quite right...if it were me I'd verify everything 'til "I know that I know it's right!"...