At one point the discussion of "dbl fire swap cams" on a BBC with dual plane intake has came up, a respected member and friend of mine, CFM I think seemed to feel it wouldn't work well as the paired cylinders may do funky things. More recently, there was a discussion on carb spacers and how they affect idle quality, vacuum, etc, especially on a dual plane. I recently dynoed a customers 509 w afr 305 heads, 9.7-1, dbl fire swap cam, performer rpm intake, moderate cam with duration in mid 230s to low 240s (exhaust) at .050. I did NOT build or spec this motor, I simply dynoed it and tried to get tune best it was going to be. Since reading Scotts statement on the fire swap cams vs dual plane I have been curious to see what would happen and how afrs cylinder to cylinder would be. On My dyno I have 10 o2 sensors so I do see whats happening in each cylinder. My dyno partner Norm DID go thru this customers 850 carb ahead of time and mill body flat, pre-cailbrate it etc BUT did NOT do anything to increase overall air flow. For coming discussion we will just stick to OBSERVED tq/hp numbers as the dry density calculated altitude bounced around 4500 to 4700 feet during this 2 days of dynoing (90 to 110 degree intake air, dew point at 70 to 80 degrees, relative humdity around 75 to 80%) which yield a CF using J607 of 11.5 to 13%. with correction factors that high, there is room for error. For example, I had a pull that corrected to around 615 hp (13.5%), made a change, 15 minutes later made a corrected number of like 607 with 11.2% BUT observed was measurably HIGHER.
So: what happens when you run a spacer on dual plane, how bad does it kill the idle and fuel signal, especially a TALL spacer??
how bad does a double fire swap cam screw up cylinder balance on a dual plane NOT intended for it??
How much tq will a big spacer give up "down low" vs any hp gains up high??
Love to hear your answers, Ill post data and results tonight or tomorrow after I review the dyno data and take notes for a follow up on this, Smitty

Why today ? Lolololol I’m too tired fir this today I can’t even figure out if 1+2=4 . Too much boating and sun last three days. Way too much heat today working 120f+ on my gravel work surface. Ugggh

My brain is fried.

Maybe later today after a swin.

lololol.

Don't have any answers, but here to learn...

I feel like I'm stepping of a cliff here. The big spacer makes the intake look like a single plane so the carb won't see the intake pulse at idle which will make it somewhat unresponsive to adjustment. The firing order swap with a dual plane will totally screw with the port velocity at the low end for the swapped cylinders and possibly for the following cylinders as well. I'm going to guess those cylinders will be lean.

Good work. Almost seems some of the 4/7 swap benefits would cancel out with anything more than a 1” spacer and potentially comprise low mid gains. On the other hand would this help balance afr’s throughout. Fk I don’t know but fill us in and much appreciated once again. I’m with SB on this one. Makes my brain hurt after all the alcohol consumption over the past 4 MONTHS.

Any gains up top I’d guess minimal however nothing ventured nothing gained. Sometimes we spend $$$ on spacers and see no real gains at all on the water. They look cool and will always experiment with them as every build/combo reacts differently. Just my experience and opinion as always.

Right on time with this Smitty. My new Comp cams arrived last week. Lol When discussing the swap with Comp they said the biggest benefit from the dual swap was cooling the engine. The swap displaces the heat more evenly throughout the block. Valako and Chris (Comp) were talking about the benefits and that was the biggest reason plus the minor gains in power and balance on the crank.
Very curious on the spacer affect.
Decent read.
https://www.hotrod.com/articles/lunati-cams/

I have the cam swap in my 496s with Victor Jr intakes on the dyno pulled 561hp and 563hp. I forgot I had 2 2” super sucker spacers I put them on a couple months after the dyno the only thing I can see is they idled a little better.

Okay.

In terms of camshaft firing order change......Remember a dual plane is designed around the iriginal firing order so each next cylinder in firing order takes it’s air/fuel from the opposite side of the intake. Completely separated from each other. This gives each small plenum time to refill for the next cylinder that draws from it.

As far as carbuetors and dual plane intakes......
Each cylinder draws air from half the carb. This is why a dual plane intake can use and usually likes a larger carb.
Also, the plenums (storage rooms) are much smaller than single planes. The high side of a dual plane has a tiny plenum.

Adding an open spacer not only increases the size of the plenums but now makes the carb appear much larger as each cyl is now able to draw air from each throttle bore. So, biggest gains will be from an engine that doesn’t have enough carb cfm.

So, throughout what you know for single plane intakes when talking carb spacers, carb cfm, and firing order changes.

Niw where is that damn cfm troublemaker Smitty speaks of. . Lol.

my 1st post is about I’m so beat I don’t want to spend the 5-10 minutes today seeing if the double swap will join put any next firing cyl right after each other on same intake plenum.

So if you can hold out from sharing results until tomorrow aft/night I’ll do the simple leg work.