Jetting for a dual plane intake

[dodge freak 2]

Have a 318 with 360 MP high swirl heads,  switch my intake from a Holley Street Dominator to a Eledbrock RPM-hoping to get a little better mileage. Just had it on for 2 days now.

Well it idles smoother, very noticeable when cold or luke  warm but it not as quick. Just does not go out as far as quick as it used too. Part throttle, full throttle. Mileage is at least as good as before, maybe 1-3 miles better, still early.

For jetting I always kept both sides the same but since 1 side is high, the other much lower, should the lower side be 1-2 sizes richer? I haven't used a dual plane intake in 10 years. Thats all I can think of what could be it. Motor like revs quick to 5500 rpms, before it go to 6000 rpms before it stop pulling as nice. My cam is not too big, unlike the RPM cam Eddy has for it, its just a comp HE 268.

I am keeping it on till spring but if I can't get it to pull as good, IDK. It sounds great, feels great but I can tell it does not go as far and the other cars on the road seem a little quicker now.   Carbs a DP so its not the spring for the secondary. I have a 3/8 open spacer on to keep the carb a little cool, exhaust heat crossover is open, little I alway keep it with all intakes-Michigan weather- My air cleaner just clears so I can't put even a 1/2 spacer on it.

[dodge freak 2]

BTW, I have not played with the jets yet, I did go down 2 sizes on the secondarys tonight and will see hopefully-if no snow-tomorrow if it seems better.

But I would think stager jetting is the way to go to fine tune it, or is it no big deal the hight difference of the two planes ?   

[Chryco Psycho]

most dual planes will run lean on # 7 , check you plugs , typically I have jetted both side the same to get decent average color , dams inside the plenum may help when only 1 cylinder is lean or 4 corner idle

[dodge freak 2]

Well I was able to get some time on it today. Low end power is differently better. Mid and top range down a bit. Idle much smoother and stronger.

Its like my cam got smaller.

Jetting it down was not good, went back to what it was. Funny jetting seems to be the same for both intakes.

Gas mileage was right @ 10 MPG, maybe 10.5 which is a bit better. With the 360 heads my mpg went from 10 mpg to no better than 9 mpg.

Here it is mid winter and theres no snow or salt up here in Michigan, not sure if it make it for a few more days, if so I know where its at.

Still runs cool and can take care of itself up here where I am at. Just have to see if I can move the jets a bit and maybe the timing but everything is very close and the stronger idle is nice. It does rev as fast as before just IDK, after 6000 its all done for sure before it could be pushed a little more--which could kill it--318 bottom end. So its like safer and better for the motor. I have a manual valve body, so if I hold off shifting for a second the motor stays at the same rpms.

My carb is still the 750 Annular Style Booster type, might see how the down leg booster is now, before the annular was better, maybe its the other way now....we see. I am way ahead for spring time tuning for sure 
 

[MEK-Dangerfield]

I really don't think changing intake manifolds will have a drastic effect on gas mileage. Before you play with the jetting, look at your spark plugs. If white-ish, then you are lean, and black-ish it's rich.

It's tough to check the secondary jetting. You pretty much have to floor it down a quarter mile pass, then pull off right away and check the plugs. I also think the 750 carb might be too large for you. Just my opinion.


  Mike

[dodge freak 2]

Maybe, I do have a 625 Carter afb and a avs for it too, but a 650 holley dp might be the best for it, power wise. With the Carter for mileage--I like to have 1 Carter jetted just right for it, just to have and if I get a few more mpg, great since I guess $4 gas is coming.

With good old E-bay, what I don't want will sell on there during the late spring time. Paid $120 for a like new rpm intake--guy had it listed wrong as a old performer with a BIN of $100.

Found my heads that way too, you need to look at the newly listed items all the time, the good buys go fast-in a few hours.

[dodge freak 2]

I am also using a MSD 7al box and hvc coil. Works great but its very hard to color the plugs. They seemed to get white most of the time even as its getting too rich. It wears the plugs out sooner too. I can feel the difference thou power wise when I jetted the back down 2 sizes.

The best is temp. sensors in each header but I am not rich---if I was I could careless about my MPG.

[dodge freak 2]

Update, I removed the 3/8 open spacer and bolted the carb right to the intake with only a paper thin gasket. Motor seems to pick up power at all speeds  The air temp is the same today, same gas in the tank, all that changed was getting rid of a little 3/8 of an inch open spacer. Rained most of the day so could only drive i once but its better, maybe much better.

[Chryco Psycho]

every combination seems to react differently, worth testing for sure

[dodge freak 2]

Update...I switch carbs from a holley 750 dp but with annular  boosters to a holley 950 dp with down leg boosters. Made a noticeable improvement in power. I believe now its better than ever, pulls really nice to 6000, maybe more. Just got to drive for an hour, darn Michigan weather.

I have hear that with a dual plane intake you can not over carb, unlike a single plane intake. So maybe it was not getting enough A/F from the 750 carb, those annular boosters makes the carb smaller.

Yes, 950 dp on a 318 sounds too big but with holley, its just a 750 carb with a 1 3/4 base plate. 

[MEK-Dangerfield]

Wow...

   And here I thought your 750 was too big.    Whatever works. 

[dodge freak 2]

Yeah, I also picked up a 650 dp Demon carb for it. I had a 1 3/4 base plate here and on E-bay I found a new holley 950 main body-super charge type too for $115 so I got it and thought I E-bay the whole carb this spring. Just for the heck of it, I ran it today. 76 jets in front, 86 jets in back. Idles good, seems fine around town.

Was going to run the 650 this weekend but we are to get more snow tomorrow.

Each time it rains and cleans the streets, 1-3 days later we get more snow. Must be from these big lakes around MI.

I am ready for spring.

[dodge freak 2]

Again, don't let dumb Holley # fool you. A Holley 850 flows more than a Holley 950.

Both the 750 and 950 has 1 3/8 venturis but the 750 uses a 1 11/16 base plate and the 950 uses the 1 3/4 base plate.

My 650 Demon I found on E-bay is the race model and I notice the idle holes in the main body are just like the 1 3/4 base plate. So with a little grinding it could use the larger base plate too but those venturis are 1.28 as to 1.375, might be just what it really needs.

Maybe the dual plane intake likes the down leg boosters better. I find out one day.