Overall overlap, or lobe center understanding?? ¿.

-
Abodybomber

Abodybomber

Breaking street machines , since 1983.....:)
Legendary Member
Joined
May 21, 2011
Messages
30,853
Reaction score
7,188
Location
Arroyo grande ca
My high school buddy,had a 350/327 cam ,in a 10:1 350 Camaro. 3:36 gears out back.The Crane blueprint grind,78 overlap. Gas mileage ,was bad.(imagine that!). Does anyone,pay attention anymore?
 
i gues he has a cam designed for the 350hp 327?
is it realy a 10:1compression engine or does it just have "10:1" pistons? the gears sure seem like a bad choice for this combination.
however cams with lots of overlap doesnt need to be all that bad on fuel.. i used to have a fairly mean solidroller in a previous 340 and i miss it alot it had fairly good gasmilage despite being hot enough to realy turn alot of heads on cruisenights,it was over 260degrees".050 and that adds up to a fairly big overlap nomather how you look at it..
 
I've seen this happen a lot. Especially when I was younger. I remember a kid I went to school with showed up one day with a really radical sounding mid 70's Camaro. He popped the hood to reveal a 307 with it's original dinky 2 barrel. It had the same cam as your buddy's car, the 350 hp 327 cam. Sounded mean, wouldn't outrun a kids bicycle in a block.
 
No, I don't look at "overlap" at all. It's close to meaningless when it comes to picking a camshaft. kinda like looking at a few numbers in a lobe chart and trying to pic something useful.
 
I admit I'm "low" on this kind of theory. My strategy is to pick a cam that "someone else" has good luck with.

I still remember the old factory Chev 350 hp 327 cam, sold by the tons to guys over the counter, was a "damn good" street cam "back then" That thing went into countless 283, 301's 327's and even 350s.
 
no that was the 375hp 327 solid grind, if my memory is correct. The 350hp 327 grind was 224/224 114lsa ish Around 285 seat and 470 lift? Nothing special.
 
no they are talkingabout the 327-350 hydro cam came in novas maliboats and camaroaches....lol
 
The 327/350 HP cam was .447 lift 222 duration @ .050 with 306 advertised ground on a 114. The same cam was also used in the 350/350 HP engine.

Overlap is actually a very good tool to used when choosing a camshaft, especially one for a pump gas street motor. While generally nowadays other figures are looked at more, any old schooler will tell you overlap was once pretty dominant for cam choice. There are many ways to pick a camshaft. I try to look at everything.
 
I'd like to know how overlap is used in picking a camshaft. Can you give some examples? Do you use "advertised" duration or at a set lobe lift to figure overlap? where in the engine cycle is overlap helpful? Where is it not? Please help.
 
Sure. Lets say we gotta 440 with a single plane intake. Not that it matters, but lets say we got 9.5 compression. We gotta cam in it now with say 60* overlap. Overlap, when both intake and exhaust valves are open at the same time. We take that out and replace it with a cam that has 108* overlap. Why did our power go to hell? Because both valves are being held open for a longer period of time. This makes the engine think it has a hole in the intake. A vacuum leak if you will. The cylinder adjacent to the overlap cylinder has the intake valve open while the two valves are still open on the other cylinder. As the piston moves down in the cylinder with the intake valve open, it pulls in exhaust from the adjacent cylinder where those valves are hangin open, INSTEAD of through the intake and carburetor. Since the piston in our "overlap" cylinder is on the way UP, and the piston in the adjacent cylinder is on the way DOWN, the pressure drop is greater there than through the carburetor and intake, so the adjacent cylinder pulls in the exhaust from the overlap cylinder. Not good for power. I gave the example with a single plane because it's easier to see. It can happen with a dual plane as well, just not as prevalent. This is why large amounts of overlap and a single plane intake on the street usually don't work out too well.
 
How in the heck does one piston pull in exhaust from the adjacent cylinder???
RustyRatRod said:
Sure. Lets say we gotta 440 with a single plane intake. Not that it matters, but lets say we got 9.5 compression. We gotta cam in it now with say 60* overlap. Overlap, when both intake and exhaust valves are open at the same time. We take that out and replace it with a cam that has 108* overlap. Why did our power go to hell? Because both valves are being held open for a longer period of time. This makes the engine think it has a hole in the intake. A vacuum leak if you will. The cylinder adjacent to the overlap cylinder has the intake valve open while the two valves are still open on the other cylinder. As the piston moves down in the cylinder with the intake valve open, it pulls in exhaust from the adjacent cylinder where those valves are hangin open, INSTEAD of through the intake and carburetor. Since the piston in our "overlap" cylinder is on the way UP, and the piston in the adjacent cylinder is on the way DOWN, the pressure drop is greater there than through the carburetor and intake, so the adjacent cylinder pulls in the exhaust from the overlap cylinder. Not good for power. I gave the example with a single plane because it's easier to see. It can happen with a dual plane as well, just not as prevalent. This is why large amounts of overlap and a single plane intake on the street usually don't work out too well.
Click to expand...
 
RustyRatRod said:
Sure. Lets say we gotta 440 with a single plane intake. Not that it matters, but lets say we got 9.5 compression. We gotta cam in it now with say 60* overlap. Overlap, when both intake and exhaust valves are open at the same time. We take that out and replace it with a cam that has 108* overlap. Why did our power go to hell? Because both valves are being held open for a longer period of time. This makes the engine think it has a hole in the intake. A vacuum leak if you will. The cylinder adjacent to the overlap cylinder has the intake valve open while the two valves are still open on the other cylinder. As the piston moves down in the cylinder with the intake valve open, it pulls in exhaust from the adjacent cylinder where those valves are hangin open, INSTEAD of through the intake and carburetor. Since the piston in our "overlap" cylinder is on the way UP, and the piston in the adjacent cylinder is on the way DOWN, the pressure drop is greater there than through the carburetor and intake, so the adjacent cylinder pulls in the exhaust from the overlap cylinder. Not good for power. I gave the example with a single plane because it's easier to see. It can happen with a dual plane as well, just not as prevalent. This is why large amounts of overlap and a single plane intake on the street usually don't work out too well.
Click to expand...
Why would a SS 9.5 to1 engine make more power with large overlap than say a mild street cam with low overlap? So large overlap does not reduce HP (if that's your goal?,you did say that you need low overlap with low comp.) LOL How does overlap effect cam choice? That was my original question. Just waiting on an answer. LOL I'm sure this will be good.
 
from my very limited understanding large overlap is ok for high RPM because there will be enough airspeed to cram fuel/air into the cylinder from the intake without the cylinder feeling the need to steal its neighbour's exhaust. I recently chose a cam for my low-ish comp propane 318, and overlap was something that i definitely considered because the propane converter relies on a vacuum signal to draw the fuel through. no vacuum = undercarbed at low RPM.
 
To expound on Rusty's excellent description...

It's basically a shorter path with less resistance from the intake of the "adjacent" cyl to the intake of the "power" cyl, while the valves on both cylinder's are open.
 
I'm with Goldmember on this one... His question remains unanswered.

Personally - Overlap is what "has to be" as a result of the lobe lift and LSA choice. I really don't care what the overlap is so long as the lobes are shaped they way I want them and placed right.
 
My thumpr cam has 74 degrees of overlap and works fine in my 10:1 318 with 3.23 gears out back as a street car.

Yes, gas mileage is bad.

To me overlap is just a result of a desired lobe duration and lobe separation.

because of this you can easily have two cams with the same overlap which are completely different

IMO overlap specs should often be a good indicator of how an engine might idle.
 
You must log in or register to reply here.
-

Back
Top