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Old 11-07-2013, 09:45 PM
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Which carb to use for pontiac 400

Have 400 pontiac, headers, 342 posi, XE268-280, 2000 stall and have
a 600 and 750 edlebrock performer that im considering to replace a 750 holley 3310 for better tuneability and gas mileage and automatic choke.

This is 99% a street car that my 18 year old grandson drives.

Using the formula 400ci x 5,500rpm x 85% eff. divided by 3,456, all I need is the 600 I think. this calculates to 541 cfm maximum required. That's why I'm leaning to the 600. The lope is not bad and i think i can stay out of the transition circuit or could drill holes in the primary throttle blades if needed.

Do the experts agree or can you sway my thinking with what is better?
Thanks

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Old 11-07-2013, 10:40 PM
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You are fine with a 650. Even a 750 wouldn't be too bad. I would recommend a Quick Fuel, 650 if you want a DP, 680 if vac. One other thing... I put a couple of Percy adjust-a-jets on my 650 DP. Best thing I ever did. The only casualty was the air cleaner, because the carb got 1" longer.
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Old 11-07-2013, 10:46 PM
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use the 750 you have. That flow formula is what will pass through the carb at 85% efficiency at 5500,,,
If you used that same formula for the heads,the engine would be a dog
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Old 11-08-2013, 05:02 AM
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For any performance application, an Edelbrock carb on a Pontiac larger than 350 CID is a recipe for tuning nightmares and low performance. NOT good carbs, IMO.

What intake manifold do you have? Carb choice could be determined by that. If still a Q-Jet (stock), it's a good one. A Q-Jet built by a good Q-Jet guy (or gal, I suppose) will out-preform ALL the mentioned carbs under 6,000. The "small" Q-Jets are 730 CFM and the "good" ones are 800. As they have a vacuum secondary, they "self-adjust' to engine demand. Cliff's High Performance in Mt. Vernon, OH is the best source for parts (Q-Jet). The Carb Shop is also known to do a GREAT job of rebuiding Q-Jets for performance.

A 400 needs a 750 CFM carb to properly perform. A 650 will fall flat on its face at 5,000... It's much more difficult to "over carb" or "over cam" a Pontiac 400 than most other engine families.

What year is the 400? The head castings? XE268H is a decent grind, but more of a "red-headed stepchild" for Pontiacs (very popular among the small block crowd). It should make power to around 5,400 RPM. The XE262H is our most popular grind for "stockers" and XE274H for street performance. 262 mimics parameters of the 068 and 744 (Ram Air III) factory performance grinds, and 274 is more similar to 041 (Ram Air IV). The XE grinds are much better in lower compression engines (under 9.5:1) than most other cams.

FWIW

Jim
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Old 11-08-2013, 06:22 AM
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The cylinder heads are 6x with 8.5 static compression , intake manifold is an edlebrock and its a 79 400 motor. Headers are headman, cranking compression is 160-170 psi.
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Old 11-08-2013, 08:14 AM
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Keep the 750 holley and convert it to an electric choke or teach the youngster how to start with the manual.
FWIW the quadrajet is an air valve secondary, controlled by spring tension (adjustable) and not a vacuum secondary.
Just sayin'!
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Old 11-08-2013, 08:29 AM
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After seeing the motor info, I think an 8.5:1 purely street driven '79 400 is going to be way overcarbed with a squarebore 750, even with a square 650 like I had suggested. For this engine IMO you'll get the best throttle response with a spreadbore qjet. The tiny primaries will have good velocity for metering, and you can adjust the air valves as mentioned so it won't bog. It'll never use the 780cfm, but what it can use will be smooth. This motor wants a base-price spreadbore dual-plane manifold anyway. I'm guessing at 8.5:1 it's not running a huge cam.
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Old 11-08-2013, 01:54 PM
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Im running 34degrees total, 12 in the dist and 22 initial with no hard start issues also.

It will go to the drag strip but will take victims on the street too and be easier on the wallet.

Any more out there that will support the #1406- 600 cfm vs the 1407- 750cfm?

Are there any more poncho guys out there that have run these carbs?
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Old 11-08-2013, 06:03 PM
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An "air valve" carb IS a vacuum secondary. Carter, Rochester, Motorcraft, whatever... The spring is dependent on venturi vacuum to operate. Carters use a weight. It's just different methods than a Holley or other diaphragm type. Semantics...

Walt,

It's a bad idea to ask people to tell you what you want to hear. Pontiac people, in general, don't use E-carbs, and for the stated reasons.

Cranking compression indicates either the cam is "advanced" (more than the provided 4 degrees) or static compression is higher than you think. Were the heads actually measured? Or was "published data" used? In general, an engine as described should make about 140-150 lbs. of compression. At 170, you're still okay for 93, but getting close to the "edge".

According to my simulator (Performance Trends Engine Analyzer Pro), 6X-4s (94 CCs) would yield 8.22:1 on a standard 400. With Performer, a 730 Q-Jet and 1 5/8" headers, using XE268H, threorhetical cranking compression is 138 PSI. Dynamic compression is 6.11:1. There's something "amiss" in your calculations or in the parts you list.

Jim
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Old 11-08-2013, 08:15 PM
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I get a lot of bad ideas at the age of 73 but keep on trying anyway. LOL

Adding a little more info, the heads were cc'd/measured for all the variables, head gasket as well as piston in hole by .008, and the measurement comes out to 8.5+ a little. Its bored .030 over also.

I installed the cam 2 degrees advanced to close the intake valve earlier to capture more compression. I wanted to take 200-300rpm off the top end powerband so stay around 5500 and this is a city car and stoplight to stoplight. I put a rollmaster .005 under and with a little stretch maybe I'll get back to the original 4 degrees advance. The intake centerline was never measured to see if its at 106 so I just used the 2* mark. Maybe in wrong here but that was my thought process. I figure the intake closing is around 64 ABDC stock so maybe now its 62.
As an aside, the engnine has about 10k miles on it but had a stretched chain that started the teardown that had me put the rollmaster on. The cam was pulling real hard at 5000-5500 before so that was why I advanced it to keep it in the midrange more for the street and keep my grandson out of trouble. Pontiacs aren't Chevys either and need to stay around 5200-5500 instead at 5800as the cam card said it made power at.

The number I got when I used the wallace calculator was not I was seeing on my old compression gauge as far as cranking compression either. I can't figure it out either.

The motor is assembled but probably I need to loosen the rockers on #1 and try to see what the intake lobe centerline is at instead of guessing. I've never done it on an assembled engine and don't know if there are any tricks but I have the dial indicator, degree wheel, etc.

It runs strong and maybe I should just set the cam at 0 instead of +2* adv. but that will decrease compression.

We havent been to the dragstrip yet to see what the engine like as far as total timing but 34-38 is the area i will play with to get the best mph. I was running 24* initial, 15* in dist for total of 39 but that was with a stretched timing chain. Folks told me to stay around 34-36. I need to find out the top mph at what total advance and then see the timing when the motor turns over hard to see what distributor needs. Maybe 10* instead of 15*

I wanted to use the e carbs so I could teach him how to set it up as the motor is in a '75 TA and will remain in the family while I'm pushing daysies. LOL That why i am leaning toward these carbs as I have both with out of the box settings that i've picked up and rebuilt.

I'm experimenting to find the best setup also so I'm open to all suggestion but I gather your saying to stay with the holley 3310. I don't know the jetting and don't know if I've got the metering plate or a block with jets for the secondaries.

Any suggestions are helpful.
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Old 11-08-2013, 10:30 PM
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I think a 3310 will have a plate on the secondaries.

If you want to tune the primaries, get an adjust-a-jet. I did and I love it. I did have to ball-grind a bit of plastic off the middle of the thing to allow the power valve to close fully (mentioned in the instructions). Not a big deal. It makes tuning for power and drivability a snap.

I'm amazed that you have a motor with the kind of tlc and specs you mention (you have a 108lsa cam?) running at 8.5:1. Guess Pontiacs are different from sbcs! You wouldn't think so with their parallel histories, and the fact that their various cam grinds are identical. I know Poncho heads were a popular mod for the sbc until the aftermarket caught up.
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Old 11-09-2013, 07:03 AM
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The cam xe 268-280 has a 110 LSA with 4* advance with the ICL at 106 and ECL at 110 but i'm running it 2* advanced for now. supposed to make power at 1800-5800 per CompCams.
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Old 11-09-2013, 08:30 AM
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JUst remember if you go with one of the eddy carbs that they are fuel pressure limited. Edelbrock says 6 or 6.5 psi but real world shoot for 4.5 to 5 psi. A small cheap regulator will work.
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Old 11-09-2013, 08:33 AM
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The 750 eddy will out perform the 600 and require less tuning too.
it will be very close out of the box.
The holley 750 will perform a lot better if you replace the sec metering plate with a new metering plate that accepts holley jets or a sec metering block. Use a #80 to 83 sec jet. Be sure the vac sec is opening correctly. A purple spring is usually very good.

No throttle drilling is required. EVER Put the drill away.

A 750 is not too big EVER.

Edelbrock carb are critical on fuel pressure 5 to 5.5PSI is good.

carb cfm formula:

That CFM formula does not tell you which carb will work best.
Or which carb is the right size. The 750 Eddy and Holley carbs always out perform the 600's and 650's.
ALWAYS. That CFM formula is BS and ALWAYS gives the WRONG carb size for a performance car. ALWAYS.

if you want a bit better fuel mileage replace the holley 750 pri jet with a #70 jet. (72 is stock)
or on the eddy 750 (1407) dial in the primary jet and rod using a AFR gauge.
Try the stock pri metering rod and a .110" pri jet. (.113" is stock)
Fuel mileage will have more to do with how you drive the car rather than the jetting or the carb choice.

If you want to launch hard and ET hard at the track get a 10" high stall torque converter for it. 3000+ stall
Nice wide band AFR gauge for carb tuning.
http://www.ngksparkplugs.com/product...rmance/afx.asp

Althou you can do everything you need to do to dial in the carb using a low cost narrow band type gauge/O2sensor.

Last edited by F-BIRD'88; 11-09-2013 at 09:02 AM.
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Old 11-09-2013, 08:46 AM
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Quote:
Originally Posted by walt03 View Post
I get a lot of bad ideas at the age of 73 but keep on trying anyway. LOL

Adding a little more info, the heads were cc'd/measured for all the variables, head gasket as well as piston in hole by .008, and the measurement comes out to 8.5+ a little. Its bored .030 over also.

I installed the cam 2 degrees advanced to close the intake valve earlier to capture more compression. I wanted to take 200-300rpm off the top end powerband so stay around 5500 and this is a city car and stoplight to stoplight. I put a rollmaster .005 under and with a little stretch maybe I'll get back to the original 4 degrees advance. The intake centerline was never measured to see if its at 106 so I just used the 2* mark. Maybe in wrong here but that was my thought process. I figure the intake closing is around 64 ABDC stock so maybe now its 62.
As an aside, the engnine has about 10k miles on it but had a stretched chain that started the teardown that had me put the rollmaster on. The cam was pulling real hard at 5000-5500 before so that was why I advanced it to keep it in the midrange more for the street and keep my grandson out of trouble. Pontiacs aren't Chevys either and need to stay around 5200-5500 instead at 5800as the cam card said it made power at.

The number I got when I used the wallace calculator was not I was seeing on my old compression gauge as far as cranking compression either. I can't figure it out either.

The motor is assembled but probably I need to loosen the rockers on #1 and try to see what the intake lobe centerline is at instead of guessing. I've never done it on an assembled engine and don't know if there are any tricks but I have the dial indicator, degree wheel, etc.

It runs strong and maybe I should just set the cam at 0 instead of +2* adv. but that will decrease compression.

We havent been to the dragstrip yet to see what the engine like as far as total timing but 34-38 is the area i will play with to get the best mph. I was running 24* initial, 15* in dist for total of 39 but that was with a stretched timing chain. Folks told me to stay around 34-36. I need to find out the top mph at what total advance and then see the timing when the motor turns over hard to see what distributor needs. Maybe 10* instead of 15*

I wanted to use the e carbs so I could teach him how to set it up as the motor is in a '75 TA and will remain in the family while I'm pushing daysies. LOL That why i am leaning toward these carbs as I have both with out of the box settings that i've picked up and rebuilt.

I'm experimenting to find the best setup also so I'm open to all suggestion but I gather your saying to stay with the holley 3310. I don't know the jetting and don't know if I've got the metering plate or a block with jets for the secondaries.

Any suggestions are helpful.
It should be obvious from looking at the outside of the carb if it has the metering plate or not. You wont see the thick metering block on the rear bowl if it only has a plate.
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