What will this cam change do ?

[prairiehotrodder]

BBC 555 12.9 CR good heads (AFR 385) tunnel ram

Cam #1
erson 280 / 288 @ .050
.765 int / .731 lift ( had 1.8 rockers on intake valves so lift was more like .810)
112 LSA advanced to +4 IC (108)

Cam #2 (just installed this cam)
erson 286 / 296 @ .050
.808 int / .765 exh lift ( don't have any PV clearance to use the 1.8 rockers anymore so stick with the 1.7)
112 LSA advanced to +4 IC (108)

[blykins]

What do you call good heads?

[prairiehotrodder]

best heads i could afford. AFR 385

[blykins]

Well, since no one else is answering, I'll hazard a guess. I'm not a Chevy builder and have zero experience with that combination.

I would have to see the advertised durations so I could calculate overlap. A larger cam doesn't always make more power and a smaller cam doesn't always lose power. An engine needs a specific amount of camshaft overlap. Modern heads need less. Older, factory heads need more.

But my guess is that you may not see much difference at all. At some point, you can keep adding duration and it will do nothing because the heads are a choke point. You may have also added too much overlap for the combination. On engines this size (in Ford land anyway), a lot of the camshafts I use with durations in the 280 range at .050" duration will have a fairly long LSA, like 117-120°, because the advertised durations are up in the 3-teens and 320's and the heads just don't need a lot of overlap.

You *may* gain a few ponies due to the extra duration split and more lift, but I don't expect it to be a groundbreaking change. I could be wrong.

[Walter R. Malik]

BBC 555 12.9 CR good heads (AFR 385) tunnel ram

Cam #1
erson 280 / 288 @ .050
.765 int / .731 lift ( had 1.8 rockers on intake valves so lift was more like .810)
112 LSA advanced to +4 IC (108)

Cam #2 (just installed this cam)
erson 286 / 296 @ .050
.808 int / .765 exh lift ( don't have any PV clearance to use the 1.8 rockers anymore so stick with the 1.7)
112 LSA advanced to +4 IC (108)

I don't think you will see much difference at all; maybe only a couple hundred more usable RPM up top.

[ClassicComp]

Post the combo,weight time slip shift points and DA you race in.

[lefty o]

a few hp, and maybe another 100rpm.

[Old School]

Well, since no one else is answering, I'll hazard a guess. I'm not a Chevy builder and have zero experience with that combination.

I would have to see the advertised durations so I could calculate overlap. A larger cam doesn't always make more power and a smaller cam doesn't always lose power. An engine needs a specific amount of camshaft overlap. Modern heads need less. Older, factory heads need more.

Could you explain your thoughts on why different heads need different overlap? Why do the older factory heads need more overlap compared to newer heads? Thanks

[prairiehotrodder]

Thanks for replying friends, A couple hp up top is about what we expected. I wasn't expecting anything big. One question : Does anyone know if removing the 1.8 rockers and going back to the 1.7 but still having the same valve lift because of the bigger cam will free up any hp due to reduced frictional losses ?
Brian

[canada1]

What was peak HP RPM with the old cam?
How high are you spinning the 555?
Is the tunnel ram modified?

I have followed this deal for a long time!
Good luck.

[prairiehotrodder]

What was peak HP RPM with the old cam?
How high are you spinning the 555?
Is the tunnel ram modified?

I have followed this deal for a long time!
Good luck.

The motor has never been on a dyno. It will spin up to 7500 really quick though I'm not sure if there is any benefit up there. I was using the weiand street t-ram from the 80's but last year i switched to the slightly bigger weiand t-ram thats got more of an air-gap under the plenum. It helped so i know it wanted more flow. I would like to try the Edelbrock 7085 t-ram but i don't want to switch to dominators.

[ClassicComp]

Post the combo,weight time slip shift points and DA you race in.

[prairiehotrodder]

3100 pounds race ready
4.10 gears, 5500 stall
29 x 10 slicks
powerglide trans

Best time :

1.39 60 foot (nowhere near my best)
5.87 in the 1/8 at 119.75
9.166 at 148.01 1/4 mile

shift around 7100 - 7200

1500 feet elevation, air isn't usually very good.

[canada1]

Is that Weiand tunnel ram OTB? Not ported?
I would be measuring the intake plenum CSA and port exit CSA.
Some of those older tunnel rams need a fair bit of help with a grinder to make them work with large cubic inches and rpm.
Some had smaller CSA in the plenum than at port exit.
Measuring the CSA at different points will help determine what may or may not be required.

[blykins]

Well, since no one else is answering, I'll hazard a guess. I'm not a Chevy builder and have zero experience with that combination.

I would have to see the advertised durations so I could calculate overlap. A larger cam doesn't always make more power and a smaller cam doesn't always lose power. An engine needs a specific amount of camshaft overlap. Modern heads need less. Older, factory heads need more.

Could you explain your thoughts on why different heads need different overlap? Why do the older factory heads need more overlap compared to newer heads? Thanks

Usually just due to a poor, less-flowing, inefficient port. Overlap lets the exhaust port scavenge the intake port.

This tends to be a mistake that I see a lot of guys make with newer style heads. They spec a cam with way too much overlap, which just puts the horsepower out the exhaust instead of converting it to power in the cylinder.