RPM Air Gap Runner Length (SBC)

[skinny z]

Edelbrock's RPM Air Gap for Gen 1 SBC.
Nosed around the internet looking for a number but I could find nothing.
Speier's website has some listings but they're all single plane.
Any direction would be appreciated.
Thanks.

[af2]

Why? they are around 8"

[skinny z]

Why? they are around 8"

Refining an engine spec. Cam selection primarily.
That 8" length seems excessive. I was under the impression that it's closer to 6". Single plane look to be anywhere from 4.5" to 6.5".

http://speierracingheads.com/manifold-lengths.html

[GARY C]

Why? they are around 8"

Refining an engine spec. Cam selection primarily.
That 8" length seems excessive. I was under the impression that it's closer to 6". Single plane look to be anywhere from 4.5" to 6.5".

http://speierracingheads.com/manifold-lengths.html

I would say 6 to 6 1/4 but they are hard to measure, the other thing to note is that unlike a single plane intake they have little to no tapper from plenum to exit.

[6.50camaro]

Can't really remember exact numbers ,forgot to right them down a few years ago and dont have the manifold any longer . But it was in the 5.5" to 6" range . Thats is measuring to where the two runners merge , not to under the carb . The avg. Cross section with the flange gasket matched to a 1205 was around 1.46 sq.in. hope this helps.

[skinny z]

I would say 6 to 6 1/4 but they are hard to measure, the other thing to note is that unlike a single plane intake they have little to no tapper from plenum to exit.

Thanks Gary. Sounds about right. I've emailed Edelbrock seeing if they can verify that.

[randy331]

Refining an engine spec. Cam selection primarily.

So,.. how will different runner lengths change your cam selection?

Say 5" vs 6" vs 7" runner length ?
How will you cam those differently ?
And, why will you cam them differently for those runner length differences ?

Randy

[skinny z]

So,.. how will different runner lengths change your cam selection?

Say 5" vs 6" vs 7" runner length ?
How will you cam those differently ?
And, why will you cam them differently for those runner length differences ?

Randy

It won't. I'm experimenting with software. I'm looking at the correlation between induction length and RPM peaks for TQ and HP and am playing around with inputs.

[randy331]

Sorry, the "refining an engine spec. Cam selection primarily" part of your post lead me to believe you'd change cam based on runner length.
I just wanted to know how you would change one of them vs the other ??

Randy

[skinny z]

Can't really remember exact numbers ,forgot to right them down a few years ago and dont have the manifold any longer . But it was in the 5.5" to 6" range . Thats is measuring to where the two runners merge , not to under the carb . The avg. Cross section with the flange gasket matched to a 1205 was around 1.46 sq.in. hope this helps.

Thanks for that. I'll venture a guess and say that the plenum wouldn't be a part of the overall length. But I've been wrong before.

[skinny z]

Sorry, the "refining an engine spec. Cam selection primarily" part of your post lead me to believe you'd change cam based on runner length.
I just wanted to know how you would change one of them vs the other ??

Randy

I see my reply was a little misleading. At this point the cam spec is in place. But since I have your attention...
If we build a header spec that focuses peak power at a certain RPM range, (as we do with primary diameter and length as well as collector sizing), is there a benefit to tuning the intake to the same RPM range?
The reason I'm asking (at the risk of this thread going off the rails) is that I can see how the induction length changes peak HP and TQ RPM.
Something as simple as a 1" spacer could pull that peak RPM down to more closely match the approximated peak RPM that the cam predicts.
The difference between an 11.5" vs 12.5" length is about 500 RPM (IIRC).

[skinny z]

If we build a header spec that focuses peak power at a certain RPM range, (as we do with primary diameter and length as well as collector sizing), is there a benefit to tuning the intake to the same RPM range?

Found an answer to that.
And as usual, it all depends.

[6.50camaro]

Can't really remember exact numbers ,forgot to right them down a few years ago and dont have the manifold any longer . But it was in the 5.5" to 6" range . Thats is measuring to where the two runners merge , not to under the carb . The avg. Cross section with the flange gasket matched to a 1205 was around 1.46 sq.in. hope this helps.

Thanks for that. I'll venture a guess and say that the plenum wouldn't be a part of the overall length. But I've been wrong before.

Thats my take on it also . Where the two runners merge is not directly under the carb pad ,but recessed back creating a plenum thats about 2 to 2.5 inches longer than the opening in the carb pad . That is why I stated "where they merge " , some wrongly take a straight line down to the floor at the edge of the carb pad .Just my take I could be mistaken it would not be the first time . Dan

[skinny z]

Yep. Same principle as measuring primary header length. It's where the pipes end that counts.

[skinny z]

Here's the answer from Edelbrock in it's entirety:

Hello Kevin, No Sir, Sorry Edelbrock does not have this information anywhere, as I have been asked this before, and after working here for 28+ years, the Engineer's tell me why do they need to know. Even so, it is really hard to measure the runner length, especially in a dual-plane, where as a single-plane is a lot easier, sticking a tape in each runner. ...there is cross sectional area to deal with and the longer the runner the more torque it will generate...

It ends there.