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Pontiac of the Month

4 bucket 67's 1967 GTO

2019 March
of the Month

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I'm new to the forum and also to the build world.  Just want some good respectful, opinions, and ideas, how how to build my 73 grand prix 455. First like to start with what size carb and flowmasters?

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Welcome to the forum G118. Let's start with the basics. What are your plans for your '73 Grand Prix? Stock? Mild build up? Wild build up? Power adder? How much can you afford to spend? The wallet often dictates what you can do.

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G118 welcome bro. What are you wanting to do with it. Stock, reliable daily driver, track beast or?

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I do want to upgrade the motor. Not gonna take it to the track so it will be a street car that has some power. Ideas are mild cam, headers, exhaust,  etc. I know the more a person builds, the more they want to add. It's a running car so I will be driving with the build.  Start small and work my way up. HP 500-600

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On 3/14/2019 at 11:12 AM, Wrongway said:

G118 welcome bro. What are you wanting to do with it. Stock, reliable daily driver, track beast or?

Will be a car that I drive on the weekends if weather good. Want a little bit more power but not take to the track.  Enough power to be respected in the streets but cruise on the highway

Also I have been searching but I can not find floor pan patches for a 73 grand prix.  If a site has them, they are out of stock. 

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Hey bro, OPGI has the floor pan quarters in stock for $172.99. I'm bored over .30 with a small cam kit from COMP (part # K51-222-4), a set of Hooker headers (PART# 4902HKR) and she sounds great. I'll most likely be going with the Flowmaster super 44 mufflers (PART# 942545) and the exhaust set up that Last Indian recommended. I wont know the horse power numbers till I put it on a dyno (I'm sure there's some formula but I don't know it lol) but all I was looking for was kinda like you said, a little over stock, good sound, reliable, fun around town be interstate capable. Hope this helps some bro.

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Thanks, will use your advice.  Edelbrock or Holley? I know everyone has their fav

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Im running an Edelbrock 1411 750cfm bro

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Posted (edited)

Yes welcome! In “73” the 455 was down to 250 hp, 315 if it was a super duty. In “70”,”71” it was at its peak 335 hp. So improving those numbers are pretty easy, but to get to 500 or 600 hp, that’s a different animal. 

You can easily get to 400-425 hp by improving induction, (mild cam, intake manifold,  heads) exhaust, ( heads, headers, exhaust) and the ignition system,(hei, duel point or Magneto, wires and plugs). Anything over the 400-425 hp is going to escalate the cost dramatically. Plus the car will become more of a beast to drive and handle. It will take better gas quality, more maintenance and more attention to subtle details. 

Edited by Last Indian

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8 minutes ago, Last Indian said:

Yes welcome! In “73” the 455 was down to 250 hp, 315 if it was a super duty. In “70”,”71” it was at its peak 335 hp. So improving those numbers are pretty easy, but to get to 500 or 600 hp, that’s a different animal. 

You can easily get to 400-425 hp by improving induction, (mild cam, intake manifold,  heads) exhaust, ( heads, headers, exhaust) and the ignition system,(hei, duel point or Magneto, wires and plugs). Anything over the 400-425 hp is going to escalate the cost dramatically. Plus the car will become more of a beast to drive and handle. It will take better gas quality, more maintenance and more attention to subtle details. 

I agree with Last Indian.

Other costs you need to consider once you start pushing into this realm of added horsepower is definitely upgrading your brakes (front discs, drilled and slotted rotors, upgraded pads, and perhaps multi-piston calipers), a better transmission and torque converter to handle the increase in power, and a complete re-do of the rear end is probably warranted with either hardened gears and axles and if your car does not have it, a posi unit. Rear disc brake conversion might not be a bad idea either. Better rubber is also another consideration.

I would also inspect the state of your cooling system. More power equals more heat. Is the current radiator good enough or do you need to upgrade (talk to an engine builder for a recommendation)? I would definitely add a catch can/return bottle if you car does not have one.

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Posted (edited)

Guys when referring to the Horsepower numbers on these cars you also have to consider that measurement was also changed during that same time from SAE to net Horsepower. I know that compression dropped and cam profiles changed on the engines but the way the manufacturers advertised horsepower ratings had changed as well.

Here is an excellent read on the subject, enjoy.

https://www.hagerty.com/articles-videos/articles/2013/08/13/horsepower

Edited by 360Rocket
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1 minute ago, 360Rocket said:

Guys when referring to the Horsepower numbers on these cars you also have to consider that measurement was also changed during that same time from SAE to net Horsepower. I know that compression dropped and cam profiles changed on the engines but the way the manufacturers advertised horsepower ratings had changed as well.

Quite true 360. Gross (a.k.a SAE) horsepower and torque meant what the engine could produce at the flywheel with no air cleaner or filter and no parasitic drag (save an alternator). Net horsepower and torque meant a stock air cleaner and air filter were in place, the engine was run with an alternator, power steering pump, an AC pump and a smog pump, as necessary.

Also the introduction of unleaded fuel meant that manufacturers had to hardened their valve seats a few years prior to the introduction on unleaded fuel. Lower compression ratios soon followed which help drop net HP even further.

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Posted (edited)
16 hours ago, Frosty said:

Quite true 360. Gross (a.k.a SAE) horsepower and torque meant what the engine could produce at the flywheel with no air cleaner or filter and no parasitic drag (save an alternator). Net horsepower and torque meant a stock air cleaner and air filter were in place, the engine was run with an alternator, power steering pump, an AC pump and a smog pump, as necessary.

Also the introduction of unleaded fuel meant that manufacturers had to hardened their valve seats a few years prior to the introduction on unleaded fuel. Lower compression ratios soon followed which help drop net HP even further.

G118, all of this is very true, but power in the cars of that era were down as much as anything because of the oil embargo! The gas crisis was at it’s peak and the long term look was grim.

Still that has nothing to do with today. So the 400-425 IMO is a fair reach. Reasonably attainable, reasonably affordable and still comfortable to drive. The “73”  is a beautiful car! It’s a cruzer, not a handler, but it can be improved upon for handling. So you may want to consider improvements there and as Frosty mentioned the brakes as well. That combination of power handling and braking are worth doing! If you achieve that I think you’ll have a smile from ear to ear!

Edited by Last Indian
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3 minutes ago, Last Indian said:

G118, all of this is very true, but power in the cars of that era were down as much as anything because of the oil embargo! The gas crisis was at it’s peak and the long term look was grim.

Still that has nothing to do with today. So the 400-425 IMO is a fair reach. Reasonably attainable, reasonably affordable and still comfortable to drive. The “73”  is a beautiful car! It’s a cruzer, not a handler, but it can be improved upon for handling. So you may want to consider improvements there and as Frosty mentioned the brakes as well. That combination of power handling and braking are worth doing! If you achieve that I think you’ll have a smile from ear to ear!

I agree with Last Indian. Modest horsepower gains around 400-425, along with some well thought out brake and handling parts will dramatically change how your '73 performs without breaking the piggy bank.

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