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Have question ? My Trans-am has a Pontiac 400 engine I am swapping the transmission for a th350 street fighter, and a Holley 750 cfm 4160, with a eldebrock 2156 performance how much horsepower will I get ?
 

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Hard to tell without knowing which 400 is in there. If it's from 1967, it's likely a high compression engine. If it's from 1978, it's a low compression smog wheezer. Everything has to match, so any information you can provide will help.

There should be a two-letter engine code stamped on the front of the block, passenger side, just above the water pump/timing cover. There will also be a block casting number and date code on the back of the top of the block near the distributor. Then also post the head casting number from the center exhaust ports. Post those so we know what we're starting with.

Head Product Azure Sleeve Font

Motor vehicle Automotive tire Bicycle part Automotive design Rim

Wood Motor vehicle Auto part Automotive wheel system Metal

White Light Automotive tire Black Font

Or, some heads look more like this:

Wood Motor vehicle Font Wood stain Gas
 

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Yep more compression, more cam. On the surface this is going to be milling the existing head’s to replacing them. The cam is tell us how much power you want. A deeper dive is pistons and rods. The 400 has a cast nodular of CGI iron rod that Pontiac calls Arma Steel, nevertheless less it’s cast not forged so in the end it limits where you can go power wise my cut line would be about 400.

Heads and pistons are best thought of as a together decision, but most people don’t.

Tranny buys nothing though a TH350 absorbs less power than a TH400.

Bogie
 

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I did a 400 once for a friend a out of his TA and just like you, he wanted more power, so as mentioned above, he got new pistons, and used an Edelbrock performer cam and intake and had mild head work done.
I never got a chance to hear the motor run but he said it ran pretty decent.
So long story short, compression, cam and head flow are some power gainers, just like metoined above.
 

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I'm a bit of a Pontiac lover, but honestly.... one thing I don't like quite as much are the engines. They're heavy, long stroke, small bore, and can't rev anyway without aftermarket rods. But if you're just looking for a little more oomph from what you have, do it.

Get the head flow sorted, pick a good compression ratio and a cam to match. RPM-style intake, keep the Qjet. There is not a more accurately metering carb on the planet. Properly tuned, that thing will be as close to EFI as you can get without a mile of wires.
 

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If you put the Quadrajet back on like Curtis recommends, and I second, and it is correct for your car, I would consider the following. Shave 1/4" off the bottom of the phenolic bowl insert to free up space for more fuel. Use a smaller sized nitrophyl float from the '75 and later Q-Jets Napa/Echlin #2-442 to free up more space for fuel. Set float level at 1/4" for starters. Try .0135 needle and seat. Get a good quality high volume mechanical fuel pump like RobbMc and utilize the return line. Set total timing at 34 degrees and experiment from there. I use 36 degrees all in by 2800 starting at 800. Set secondary air flap wind-up spring at 3/4 to start. Try and adjust choke pull-off timing for 1.5 to 2 seconds and see how carb responds to your car. I have a very mild street/strip car with hydraulic roller that runs 12.9 at 105mph so far. This set-up has worked very well for me and I have a heavy Pontiac just like yours with a 3:08 gear and 13 inch torque converter that flashes to 2800 when I launch off idle. Try installing the longer style fuel filter housing and use the 2" paper filter. They are not as restrictive as you might think on a 12 second car or slower. Fabricate yourself a metal fuel line from pump to fuel filter housing for safety and to minimize leaks. Copper-Nickel alloy line from your local auto parts works great, is easy to bend, and is safe for gasoline. These ideas allow me trouble free passes all day long while enjoying visiting all my friends at the track and driving home afterwards. Hopefully some of this might work for you if you decide to go back to the Q-Jet as Curtis recommends. Good Luck!
 
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