[Speargun]

I just started dating a girl who has a '75 Formula Firebird with a 350/350TH and she wants to rebuild the engine for a daily driver with a little attitude.
She has a set of 6X-4 & 15 heads to work with and plans on installing a 200r4 tranny. I'm not sure on the gear right now, but 2.56:1 looks like a common ratio behind an automatic.

I know there's a lot more to it, but assuming stock or +.030 bore, close to or at zero deck, and basic machine work, how would you build this?

[PAUL K]

Recently there was a thread about this same deal. It has a lot of good info. Search for it or maybe someone will post the link. Ironically it was for a 75 Firebird. Nothing wrong with a good 350 build but personally I'd only favor it over a 400 if it was the original block for the car.

[Burningbird]

I' m currently building 2 low comp Pontiac 350s. I used Mopar 318-pistons with no valve relifes (065 over stock bore) and 1.741" compression height. The weight of the Mopar pistons are close to the stock pistons, actually closer than Pontiac replacement pistons. The price for a set is under 100$ and are available in many bore sizes. The only drawback is that the rods have to be honed 0.003" (stock .980"/Mopar .983"). Sealed Power 030 over replacement pistons and 6X heads will have a SCR around 7.5:1, Mopar 318 pistons with the same bore size bump that SCR to 8.3:1. I will also use a cheap egr Q-jet intake with rebuilt Quadrajet and a Summit 2800 cam, or maybe a Voodoo 700.

[65CatCoupe]

If you go with that overdrive tranny you can be a lot more aggressive with gearing unless she is gunning for a land speed record. A 4.11 rear ratio would make that 350 fell strong but still make for quiet cruising.

[77 TRASHCAN]

To me, the biggest obstacle in building a good 350 is getting the compression ratio over 9:1. I don't know the chamber volume of the #15 heads, but I bet it's less than the 6X-4's

Whichever head you use, it will not require a lot of grinding to help them out, a 350 has less air flow through the engine. A good clean up and knocking off edges will help.

zero deck
stock iron intake
Q-Jet
Maybe Ram Air manifolds or headers...
068 cam would work for this

[Speargun]

Originally Posted by Burningbird

I' m currently building 2 low comp Pontiac 350s. I used Mopar 318-pistons with no valve relifes (065 over stock bore) and 1.741" compression height. The weight of the Mopar pistons are close to the stock pistons, actually closer than Pontiac replacement pistons. The price for a set is under 100$ and are available in many bore sizes. The only drawback is that the rods have to be honed 0.003" (stock .980"/Mopar .983"). Sealed Power 030 over replacement pistons and 6X heads will have a SCR around 7.5:1, Mopar 318 pistons with the same bore size bump that SCR to 8.3:1. I will also use a cheap egr Q-jet intake with rebuilt Quadrajet and a Summit 2800 cam, or maybe a Voodoo 700.

That's good info! I didn't know about the mopar pistons. I was considering milling the deck & heads down to get around 8.5:1, but a piston swap sounds simpler & cheaper.

[Burningbird]

Originally Posted by Speargun

That's good info! I didn't know about the mopar pistons. I was considering milling the deck & heads down to get around 8.5:1, but a piston swap sounds simpler & cheaper.

If you shave the 6X heads 0.030" and use the Mopar pistons with the 3.91" bore the SCR will be around 8.8:1. With the same pistons and #15 heads the SCR will be close to 9.9:1.

Correction from my previous post, the stock piston pin is 0.981" and 318 piston pin is 0.984".

Link to the pistons:
https://www.ebay.com/itm/303509628684

[72LuxuryLeMansLa.]

Originally Posted by Burningbird

If you shave the 6X heads 0.030" and use the Mopar pistons with the 3.91" bore the SCR will be around 8.8:1. With the same pistons and #15 heads the SCR will be close to 9.9:1.

Correction from my previous post, the stock piston pin is 0.981" and 318 piston pin is 0.984".

Link to the pistons:
https://www.ebay.com/itm/303509628684

I'm no expert but with that small bore, I'd like that!

[pmd400]

Originally Posted by Burningbird

I' m currently building 2 low comp Pontiac 350s. I used Mopar 318-pistons with no valve relifes (065 over stock bore) and 1.741" compression height. The weight of the Mopar pistons are close to the stock pistons, actually closer than Pontiac replacement pistons. The price for a set is under 100$ and are available in many bore sizes. The only drawback is that the rods have to be honed 0.003" (stock .980"/Mopar .983"). Sealed Power 030 over replacement pistons and 6X heads will have a SCR around 7.5:1, Mopar 318 pistons with the same bore size bump that SCR to 8.3:1. I will also use a cheap egr Q-jet intake with rebuilt Quadrajet and a Summit 2800 cam, or maybe a Voodoo 700.

Did the dodge pistons end up down the hole at tdc?

[Burningbird]

No, they will be 0.005"-0.010" out of the hole if i remember correctly. The 350s are not finished yet. I will update when they are ready for start up on the test stand.

[tom s]

I built a 366 years ago for my 63 tempest wagon.I put a 2801 summit in it and it ran great with both a single 4bbl and a tripower.It had the 2 speed tempest TQ(powerglide).Tom

[formula kid]

I have a 1974 Formula it has 4C heads WA engine code. Factory rated at 170 Net 7.6 compression. The 4C heads are air heads. The car has a 4spd trans. I have headers and a HO crossover exhaust. I recited the distributor and stripped off the emissions from the car. It always passed NJ emissions testing. If you change the heads on the 350 to any of the listed you will pick up a lot of HP. The first 67 # 670, 68 #37,68 # 16, 70 #12, 70 #13 these are all 72 cc heads get you about 9.5 compression Ratio. The # 48 heads from 69 are 68 cc heads and what came on the 69 350 HO cars and the 69 Ram air III 400 this will get you the highest compression. If the car is a 4 spd put in a 744 cam and it will rip you head off. If it is a auto car run the 068 cam and it will work fine. Your final compression will be about 9.to 1. Remember all is the heads that I listed are 2.11, 1.77 except for the 74 and your 75 heads they are 7.6 to 1

[Formulas]

Even if you go with any 68-70 2bbl. 350 head you will pick up power in normal RPM range on a modest build
Only Caveat with that is if you NEED the extra accessory bolt hole on some later heads

The slightly smaller valves won't be noticed especially with a good port and runner cleanup and good valve job

[formula kid]

The heads that I gave #'s on are all 4bbl heads 2.11 intake 1.77 exhaust. The one that should give you the highest compression are the 1969 # 48 heads they cc @ 68 verses the others that are 72 cc.

[Formulas]

Formula Kid with respect the heads you listed are not all 72 cc heads as alot of online charts show them to be

A 067 cam automatic 400 has a larger (approx 77cc) chamber than a 068 cam 4speed 400 (approx 72) chamber head

Does the 75 bird utilize the added accessory bolt hole? that might be a non starter for any early head anyways if Speargun doesn't want to mess around with changing that

[TheGrudge]

My Ma had a ‘72 350/350 Firebird in the 80’s. IIRC #12 heads, 068 cam, 2.56 rear.
Pulled like a boy of fourteen.

[Will]

You can live without the extra accessory hole. I've run both #13s and #061s in my Formula. They don't have it but the '72 alternator bracketry is designed for it. Just tack weld a nut onto the back of the alternator bracket then grind it a little bit to clear the head and you'll be fine. I also use an extra large diameter, thick washer for holding the power steering bracket to the head. Never had a problem with this.

[Jim Moshier]

The 2801 Summit cam does run well with the Pontiac 350 car was the 1971 Lemans TH350 and 2.56 gears I think.

[77 TRASHCAN]

Originally Posted by Will

You can live without the extra accessory hole. I've run both #13s and #061s in my Formula. They don't have it but the '72 alternator bracketry is designed for it. Just tack weld a nut onto the back of the alternator bracket then grind it a little bit to clear the head and you'll be fine. I also use an extra large diameter, thick washer for holding the power steering bracket to the head. Never had a problem with this.

I think Jim Hand performed same mod when using 64 heads on his 455.

[hgiv]

Use the 15 heads. No question. The others will have way lower compression. Gonna cost a couple bucks to have them done at shop but they will be nice as is compression wise.

Change cam for sure. Consider Lunati Voodo 256. Not too big. Good upgrade. Worth every penny.

Overbore. Pistons are an issue as discussed because the ain't making 350Ps anymore unless you go custom and that's big $. But you aren't building big HPs so don't be afraid of a cast piston. They are way better than they used to be. Others will argue but... its your money.

I don't remember what you said was on it for an intake but you'll want a 4bbl. Can probably pick up an Edelbrock for not too much $. Carb of choice.

I love long branch exhaust mani but they are big bucks and it doesn't sound like you want to spend that so stock will be fine. But add dual exhaust if you don't have it.

You won't be happy with those gears but there are a zillion 2nd gen TA rear ends around with posi and gears that will make you happy. Probably 3:08-3:23ish You can get one cheap and just swap. Be great on highway still and will be HUGE change.

You do all that and you... ah SHE, will be real happy. It will be huge jump in gitty up. Not a rocket by any means and very driveable.