Small port heads, big port intake. How bad?

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DionR

DionR

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I've got a 71 Dart with a 318 and an dual plane M-1 intake. Popped a vc off tonight and confirmed that it has the stock 318 heads.

So, unless I am mistaken about the M-1 intake, I have a large port intake breathing into small port heads. I've always heard that was a bad combination, but how bad?

Trying to improve the mpg on this thing, got 14 last time I checked. Thinking I should swap a 2bbl intake and fix the problem. Any thoughts?

Thanks!
 
DionR said:
I've got a 71 Dart with a 318 and an dual plane M-1 intake. Popped a vc off tonight and confirmed that it has the stock 318 heads.

So, unless I am mistaken about the M-1 intake, I have a large port intake breathing into small port heads. I've always heard that was a bad combination, but how bad?

Trying to improve the mpg on this thing, got 14 last time I checked. Thinking I should swap a 2bbl intake and fix the problem. Any thoughts?

Thanks!
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M1 is a single plain right? To large (air speed to slow for the stock flow) won't get better mileage

2bbl is the best mileage manifold..........Because, your lead foot, can only pull two barrels:-D
 
Having the larger ports on the intake will just make the incoming fuel/air mixture slam into the ridge created by the smaller port heads and cause turbulence. It's more of an issue on high performance applications. Changing to a 2 barrel or a different 4 barrel intake will help the mileage.
 
DionR said:
Trying to improve the mpg on this thing, got 14 last time I checked. Thinking I should swap a 2bbl intake and fix the problem. Any thoughts?

Thanks!
Click to expand...

If you put a mechanical secondary 4-bbl on it with primary's that are smaller than a comparable 2-bbl you are looking at, you should be able to get better gas mileage as long as you keep your foot out of the carb.

Then you have the 4'bbl whey you want to 'get it' and can be miser when trying to doso.

Just my .02.
 
chewy said:
If you put a mechanical secondary 4-bbl on it with primary's that are smaller than a comparable 2-bbl you are looking at, you should be able to get better gas mileage as long as you keep your foot out of the carb.

Then you have the 4'bbl whey you want to 'get it' and can be miser when trying to doso.

Just my .02.
Click to expand...
Yeah, and you can disconnect the secondaries completely if they're mech.
 
DionR said:
I've got a 71 Dart with a 318 and an dual plane M-1 intake. Popped a vc off tonight and confirmed that it has the stock 318 heads.

So, unless I am mistaken about the M-1 intake, I have a large port intake breathing into small port heads. I've always heard that was a bad combination, but how bad?

Trying to improve the mpg on this thing, got 14 last time I checked. Thinking I should swap a 2bbl intake and fix the problem. Any thoughts?

Thanks!
Click to expand...

If you need a stock 2 barrel intake I've got one I could part with. You could probably even talk me out of a 2v carb, worked before I pulled it, although of course it could probably use a refresh.
 
chewy said:
If you put a mechanical secondary 4-bbl on it with primary's that are smaller than a comparable 2-bbl you are looking at, you should be able to get better gas mileage as long as you keep your foot out of the carb.

Then you have the 4'bbl whey you want to 'get it' and can be miser when trying to doso.

Just my .02.
Click to expand...

But how much does the reversion at the head to intake transition hurt me?

It has a Carter Composition Series carb, no idea what size.
 
72bluNblu said:
If you need a stock 2 barrel intake I've got one I could part with. You could probably even talk me out of a 2v carb, worked before I pulled it, although of course it could probably use a refresh.
Click to expand...

I've got an 2bbl intake already, but it's pretty crusty. Had some standing water in the ports, time will tell how it cleans up.

I will keep you in mind if it doesn't work. Thanks!
 
The 'stand-in' 318 I put in my car came out of a 75 Truck. With the truck 2-barrel it was getting about 17mpg in the dart. On the truck it got 12. It had large venturi for a 2bbl.

When I switched it over to an LD4B and the '65 4-bbl, it went from 17 to about 22 mpg as long as I kept my foot out of the carb.

The Carter Competition Carb is probably sizeably bigger than the '65 AFB on the primary's.

If you do go with a 2bbl, it should be the smaller style 2bbl. JMO.
 
chewy said:
The 'stand-in' 318 I put in my car came out of a 75 Truck. With the truck 2-barrel it was getting about 17mpg in the dart. On the truck it got 12. It had large venturi for a 2bbl.

When I switched it over to an LD4B and the '65 4-bbl, it went from 17 to about 22 mpg as long as I kept my foot out of the carb.

The Carter Competition Carb is probably sizeably bigger than the '65 AFB on the primary's.

If you do go with a 2bbl, it should be the smaller style 2bbl. JMO.
Click to expand...

Does the LD4B have small ports?
 
The reversion isn't helping, that is for sure.

LD4B (yes it is small port) or a regular Edelbrock Preformer (also small port) would be better.
 
The port mismatch isn't really any big deal. Much to do about nothing is made over port matching. Chrysler sold a rectangular port dual plane intake for the oval port W2 heads for years. Not many people wanted it because of the mismatch but Larry Shephard (MP engineer) swore up and down it didn't make a difference. I've seen magazine articles where they tested unmatched ports against matched ports and found no difference. The performance gains came mainly in the bowl and the cut leading into the valve seat.
 
Guitar Jones said:
The port mismatch isn't really any big deal. Much to do about nothing is made over port matching. Chrysler sold a rectangular port dual plane intake for the oval port W2 heads for years. Not many people wanted it because of the mismatch but Larry Shephard (MP engineer) swore up and down it didn't make a difference. I've seen magazine articles where they tested unmatched ports against matched ports and found no difference. The performance gains came mainly in the bowl and the cut leading into the valve seat.
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Just to be clear, I'm not talking about the port matching, I'm talking about the intake ports on the head being something like .97x1.95 inches while the port exit on the intake is something like 1.01x2.17. Is that what you are saying is not a big deal?

BTW, looks like the carb is a Carter #9636, a 625 cfm unit. Don't know if it is a spread bore or not.
 
DionR said:
Just to be clear, I'm not talking about the port matching, I'm talking about the intake ports on the head being something like .97x1.95 inches while the port exit on the intake is something like 1.01x2.17. Is that what you are saying is not a big deal?
Click to expand...

Yes that's what I'm saying.
 
btw...with acception to the corners, with a square port w2 intake to a w2 oval is more like the other way around, as in the intake hangs in the heads port window, so less detriment.
 
1wild&crazyguy said:
btw...with acception to the corners, with a square port w2 intake to a w2 oval is more like the other way around, as in the intake hangs in the heads port window, so less detriment.
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No not really, the square port intake was closer to a W5 intake port.

Even Chrysler used big port cast iron intakes from the 360 on the small valve 318 4BBl package. There were two 318 4bbl packages. One had the small port and 1.78 valve while the other had the big port and 1.88 valve. The same intake was used on both packages.
 
Guitar Jones said:
No not really, the square port intake was closer to a W5 intake port.

Even Chrysler used big port cast iron intakes from the 360 on the small valve 318 4BBl package. There were two 318 4bbl packages. One had the small port and 1.78 valve while the other had the big port and 1.88 valve. The same intake was used on both packages.
Click to expand...

raised?
last one i looked at width wise hung into the oval port window...but hey if you have'em ..I won't question what you say.

btw it costs money to cast a new small port window'd 4 barrel intake so thats how it ended up being used that way.

what came 1st?
the big port intake on the small port head?
or the matched big port intake/big port heads?
 
I don't care if one guy says it doesn't matter. I like my intake and head ports to at least be the same size and shape and that is how they will be on my engines.
 
who knows maybe the turbulence helps keep the fuel atomized. anyway i have a big port intake im probaly going to run on my small port heads but thats because its what i have laying around.
 
Here's a picture of the difference.

113_0304_junk02_z.jpg
 
I think there is a lot of good opinions on here. I have to agree that what is hurting you most is the fact that the plenum is too big and the runners are to big(volume wise). I think the mismatch in port shape(Or cross sectional area) is the least of your concerns. Even if you reshaped the ports in your heads to match the intake, it would still be too big an intake. IMO you should get a small port intake and a good 600 cfm range 4 barrel that is in proper operating condition and then tuned to your setup. That should net you the best mileage and all around powerband.
 
1968FormulaS340 said:
I don't care if one guy says it doesn't matter. I like my intake and head ports to at least be the same size and shape and that is how they will be on my engines.
Click to expand...

I agree fully. Mis-matched between 340/360 intakes to small port 273/318 heads just never sat well with me either. Aftermarket manifolds sold now dont port match to 318/273, which is why I bought a LD4B years ago and keep it. If I sell a car, I'll spend the $$ to get an Edelbrock performer which the new ones dont match or a Cast iron 360 4bbl manifold to go with the car and keep mine for my next project.

I also have the original Cast Iron manifold for my 273, and recently picked up a second one on a spare engine I bought for parts. Thats the only Small Port manifold I like better than the LD4B. But I am a bit biased and set in my ways. I havent had my hands on a '64/65 Offy 273 Dual Quad yet, I think that would be my new fav if one ever fell into my lap.
 
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