Selecting a manifold for AFR 265's - single versus dual plane - thoughts anyone?
 

Getting close to putting the finishing touches on a 489 build. Engine is running AFR 265 heads, about 9.2:1 compression, Lightning headers with silent choice built into the collectors, and a hyd roller cam with 226*/230*, .596/.612, 114* LSA for the specs, with the 4/7 - 2/3 firing order swap (LS style). I have an HP500 Holley 800 carb to run on it - although I realize that more power is probably there with a larger one. I don't plan to spin it past 5400 RPM, and I spend 80% of my time running between 3000 - 4000 RPM. I have at my disposal a Dart single plane manifold and an Edelbrock Performer RPM Air Gap - both oval port. Here are the pro's and cons that I see with each manifold. Looking for feedback from others with experience.

Dart oval port

1. Will probably make more power from 4800 on up.
2. Will probably work better with an "undersized" carb, i.e. single plenum will "see" the carb as larger.
3. I might need to have the power higher in the RPM range to spin a larger prop at that range.
4. Probably best setup for max speed.
5. Looks cool! :)

Air Gap

1. Makes more power from 2000 - 4500 RPM
2. Divided plenum might force me into a larger carb
3. Will make a lot of torque right inn the middle of the power band, but not sure how a boat takes advantage of that without a bigger wheel.
4. Dual plane design might act as a hedge against reversion at idle.
5. Has provisions for 4-corner cooling if I should need it.

Quick question: I see some people running the 4-corner cooling to help with more even cooling in the heads, especially in the back cylinders. Is that more of an issue with engines that are running a crossover instead of a circ pump? Also wondering if the LS firing order will help me out with more even heat distribution.

I need to figure it out this week and move on to port matching for the "roval" ports in the heads. Appreciate opinions and experience anyone wants to offer.

Based on your RPM band, I think you carb is spot on and the dual plane would likely provide a broader torque curve with better acceleration. You also might feel a bigger kick in the pants in the midrange with the dual plane intake.

:food-smiley-007:here we go.

I have a similar combo with my 509 and looked at a lot of data on this very subject....... Around 5,200 rpm the advantage starts to slowly go to the single plane. After 5,400 on up its single plane all the way?..... 30 ft/Lbs more torque at 3,800 rpm with air gap..... This is with a 500 CID engine with good heads and exhaust. 540 and bigger engines do better with the single plane and seem to be restricted by the air gap. My vote is air gap all the way for your combo...... TBF

I also have a 1" phenolic spacer under my demon 850 to add a little plenum volume to try and keep the power up a little higher in the rpm range.....

Mike, I hope it doesn't degenerate like some of the other threads have. Just looking for some honest opinions, and i realize that it will probably be a toss-up for my combo at the end of the day. Let's just stick to everyone expressing their experience without downing the others, and let it shake out.

Besides - it's still winter, it's damned cold outside, and most of us don't have anything better to do than talk about motors.

http://cdn.memegenerator.net/instanc...x/44649048.jpg

TBF, I was thinking about that too. Planning to try out a spacer. I also saw where notching the plenum further down will also make it act more like a single plane. Similar concept. Thanks for the reply.

Budman II,

Just a week or so I posed the same question for a HP 500. You responded with several posts.

You are in the perfect position to solve the problem and answer the question for all of us. If you are going to dyno then run both and see how they compare. If not dynoing pick one to use as a baseline and do some test runs mid range and top end. Then as soon as you can switch to the other intake and compare. It would be ideal if this could be done back to back.

I am still undecided at this point. If you do the above please post the results. I still wonder if the Air Gap had been out when the HP 500 was designed if the Dart would still have gotten the nod.

Both Scoggin-Dickey and Gilbert/Sallee Chevrolet have done 502-509s rated at 600 to 630 hp using the Air Gap.

Mark

I don't see you fealing a diff on the lake with either of them. While im no expert, i dont think marine cams have enough duration to take advantage of a nice big single plane. I did the same thing with a sbc setup and I could not feel any diff other than it got on plane a little better with the air gap duel plane. The top speed was the same. I have an air gap with the 1 inch spacer. There was a big diff with the spacer opposed to no spacer!!

Before the aftermarket came up with a good flowing air gap type dual plane, the answer was obvious .A dart single plane .,or equivalent. Now with stuff like the edelbrock rpm air gap ...it's different .If you are only going to be spinning 5000+ - or less .you'll be happier with the dual plane..On a higher rpm ,or more air thirsty motor(bigger)as John Lingenfelter told me years ago" A 496" has so much torque anyway ,I'll sacrifice a little in the middle ,for more on top" .Back when he was alive and I dealt with him there wasn't a really good flowing dual plane made ...So I'm not sure what his answer would be today .