Question about how to get better horsepower? - Pontiac GTO Forum
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post #1 of 11 (permalink) Old 11-22-2009, 11:14 PM Thread Starter
 
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Question about how to get better horsepower?

OK Guys, I got one for you and need some advice and opinions.

As some of you know. I have a 1968 Pontiac GTO. The car comes with the original matching-number automatic transmission.

The rearend and rearaxle was replaced to a 12 bolt rearend with a 2.73 ratio (non-posi)

The Block is out of a 1973 Pontiac (unsure of what body) (X4) and is rated at 185 HP and originally equipped with a 2 brrl (Currently a Holly 4 Brrl 600 cfm sits on it)

The heads are a Sept 02, 1971 "7K3" Heads with double spring setup and the vales are 2.11/1.77 96cc. (I think but am not sure but think we are running at 9:1 compression ratio) The heads are rated at 250HP

The car has an old edlebrock intake manifold that says pontiac performance on it and the EGR valve port has a plate bolted on it where the EGR wold sit. So No EGR system. The Carb is a Holely 4160 double pumper rated at 600 cfm.

The ignition is a standard ignition. Nothing performance.

And last, the fuel pump is some sort of Holley performance pump (mechanical)

With this data, assuming everything works properly with the timing set at spec) what HP do you think I am currently running at? I know they dropped off the horsepower in 1971 (I think they went from engine HP to Rear wheel HP but again, not sure) and how is this compareable to 1968 specs (rated around 330-350HP) Are they alike or way off? Also my dad is thinking about replacing the heads. She he go with a higher cc rating or lower? any advice or opinions are greatly appreciated. Thanks for the input and if you need any additional info, i can provide it!

Thanks,
Lukeduk1980

Last edited by Lukeduk1980; 11-22-2009 at 11:47 PM.
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post #2 of 11 (permalink) Old 11-23-2009, 07:54 AM
 
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All that stuff in the engine is OK, but A LOT depends on the camshaft that you are using. If you are gonna pull the heads you might want to invest in a good cam/lifter set. Eric



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post #3 of 11 (permalink) Old 11-23-2009, 12:01 PM
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Also, if you want seat of the pants performance, lose the 2.73 rear gear and put in a shorter one, like a 3.36 or a 3.55. (I put a 2.56 gear in my '67 recently, and it is noticeably slower off the line---but it's terror on the highway!)
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post #4 of 11 (permalink) Old 11-23-2009, 01:37 PM Thread Starter
 
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All great ideas, keep em coming. need more ideas
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post #5 of 11 (permalink) Old 11-24-2009, 07:51 AM
 
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Put a Pontiac 4 bbl intake manifold on it with a Quadrojet carb



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post #6 of 11 (permalink) Old 11-24-2009, 09:57 PM
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Changing the cc of the heads depends entirely on what pistons are in the block, whether the block was decked and/or/the heads have been milled. Getting a good baseline cylinder compression check will be a good indication of where you want to go. A pressure of 175# or more would indicate the heads are a good match now. I think the carb is a little small for it and as Eric stated, the cam has a lot to do with torque/hp and the 2.73 axle ratio is a performance killer in itself. Personally, if the engine runs and performs decently now, I would concentrate on getting a better gear set for the intended purpose first and see how much of a difference that makes on it's own.

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1967 GTO; Gulf Turquoise on Black, 4 speed, buckets SOLD
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post #7 of 11 (permalink) Old 12-13-2009, 02:05 PM Thread Starter
 
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Currently, the intake manifold is a Edlebrock pontiac performance setup with no EGR system in it. The motor runs very weird. We think it is dumping too much gad into the intake as it is. The timing way way off. I think it is way advanced (way above the timing marks) and when we adjust for the timing, the car dies immediately. We are wondering if the timing gears are off from the advancement. There is no EGR valve system on the car. The Edlebrock Pontiac Performance intake manifold has the EGR valve port capped off.

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Last edited by Lukeduk1980; 12-13-2009 at 04:09 PM.
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post #8 of 11 (permalink) Old 12-13-2009, 05:47 PM
 
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the egr thing you keep repeating is a non issue. turning the timing down will make it idle slower. you will have to compensate by using the idle screw on the carb. make sure you dont have any vacuum leaks.
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post #9 of 11 (permalink) Old 12-13-2009, 05:59 PM
 
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the timing light reading has nothing to do with the gears. you are reading the ignition timing not the cam timing.
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post #10 of 11 (permalink) Old 12-14-2009, 06:43 PM
 
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for just an occasional hp blast the cheapest and easiest way to go is nos.
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