# 66 GTO lacking power



## nmiller5000 (Dec 5, 2009)

I bought my first muscle car(66 gto) about a year and a half ago and have invested quite a bit of money in it with little results. It has a 72 400 in it, but has been bored .40 over, i've had hedman shorty headers put on, and had a high energy comp cam installed. It has a 750 edlebrock carb, magnaflow 2.5 full exhaust. It has been checked over with a diagnostic machine, and said to be running great. I'm just not impressed with its power at all. I guess to put into perspective, I feel my wifes 3.8 litre V6 has more get up and go. I would love to have 350-400 horsepower and feel with this set up, it is possible. Am I wasting my time with this engine? If anybody has any ideas or similar set up with advice with what they have done, I would love to hear from you. Thanks, Neil


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## Instg8ter (Sep 28, 2010)

what heads are you running with it? what is the compression ratio? , what is your timing set at? cam specs? 72' was the start of detuning for emissions standards did it mainly with larger chamber volume, could be as simple as a head swap to get you to around 9.5:1 and a tune from a good PONTIAC mechanic. more details are needed.


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## nmiller5000 (Dec 5, 2009)

As far as I know the heads are stock. Now this is Japanese to me but the info I got on cam is, range(1500 to 5500) valve timing (0.006) duration (268) duration @ .050" (218)
valve lift ( 0.454) lobe lift (0.302). I'm not sure about the compression. I think the mechanic who ran the diagnostics told me it's around 16 on the timing, If I remember right. I know he said I had 20" of vacumn and steady(if that helps). I live in central Indiana. Do you happen to know of any good pontiac mechanics in this area?


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## dimitri (Feb 13, 2009)

Buy the book How to Build Max-Performance Pontiac V-8s from Jim Hand. I suggest reading this book cover to cover before you throw any more money into this engine.


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## 66tempestGT (Nov 28, 2009)

you arent wasting time with a 400 but you must have a poorly tuned or mismatched combo. sounds like whoever is tuning/recommending parts purchases may not be real knowledgeable about pontiacs. they are kind of tweaky.


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## Instg8ter (Sep 28, 2010)

best advise for mechanic is find a local pontiac club, join and ask the members who does theirs, or talk to people on cruise night. maybe someone on here is in your area and has "gotta guy". would think it could take more advance than that but i am no mechanic. i sent PM with a suggestion on help


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## bullet20cc (Apr 4, 2009)

*Heads*

They started dropping the compression in 71 to just a little over 8 pounds of compression Check the number at the center exhaust ports. Probably 4X-6X on 73 to79 cars.Some later cars had big valves but large chamber CC's. Or numbers starting with nines or sevens If so you need to find some 10:5 are 10.75 heads. Pontiac made heads starting in 67 with larger valves. 2.11/1.77. Made them 67 thru 70. The good heads are #16--216--61--62--31 are RA-1--37--96 is round port RA-2--48--12 auto trans--13 auto trans--12 RA-3-614 round port. RA-5 is a tunnel port #44
722-614 RA-IV,forget these you'll never find them and if you do very costly.
In 71 the most horsepower you could get was a 455 with 335 hp. In 72 the most horsepower you could get was a 455 with 300 hp.
All round port heads have to have round port exhaust or round port headers.
I have to stop writing this is to much information and confusing.LOL
I have a really nice set of 61 heads if anyone can use them. Can't put them on any of my 455's they will kick the compression over 11 and can't run pump gas.


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## bullet20cc (Apr 4, 2009)

*Heads*

Best heads for 66 is 092--093. Neither have the big valves.


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## Instg8ter (Sep 28, 2010)

wonder where Bear's been he usually weighs in on engine problems....hmmmm


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## nmiller5000 (Dec 5, 2009)

If I am reading the correct numbers on center exhaust port, im reading 1 on left side and 7 on right.


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## BearGFR (Aug 25, 2008)

Instg8ter said:


> wonder where Bear's been he usually weighs in on engine problems....hmmmm


I'm here --- or at least back. Thanks for noticing. Short version of the story: Day after Christmas, helping #1 son put new speakers in his ride, got to feeling "funny", went to the E.R. - the next afternoon I had an angiogram and 3 stents in 2 cardiac arteries, and scared out of my gourd.
Doc says I'm fixed and have no damage ---- but I've been focused on getting my act together in terms of diet and exercise. I didn't have a heart attack, or at least not enough of one to sustain any permanent damage, but that's as close as I *EVER* want to get.

On to the business at hand. If that cam is making a solid 20" of vacuum then it's not real radical, but that might be a good thing. We also need to know what transmission and rear gear ratio you're running. Look on your cylinder heads and you should see a 2 or 3 character code cast into the metal, usually (but not always) over the center two exhaust ports and usually (but not always) numeric. If that exhaust is 2.5" from the manifolds all the way back then there's a very good chance that's what is killing your low end torque because they're too big, especially with that cam, and especially if those are small valve low compression heads. Check out this link - it will help you with a positive ID on which engine you really have. It can be difficult to see sometimes but getting the date code from the rear of the block next to the distributor is important because all the other engine codes were re-used over the life of the Pontiac V8 and meant different things in different years, so to correctly interpret the rest of the codes you have to first be sure which year block you're dealing with.

In general, Pontiacs are high torque motors and they make it all at low rpm, they aren't high rpm motors. There are exceptions of course, but that's generally true. If you try to set one up with a "formula" similar to the zombie army of small block (cough) Chevys, the results are guaranteed to disappoint.

First things first though, we need to figure out for sure which heads you have, which cam you have (Comp's part number would be helpful), carb, intake manifold, transmission, and rear gear ratio. 

Bear


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## nmiller5000 (Dec 5, 2009)

Bear,
Im glad to hear that you are feeling better.
Ok I will do my best with numbers.
Center exhaust port numbers have 1 on left and 7 on right (Just to the right above ports reads Passenger side-F178, Driver side-F288)
Transmission is a turbo 400(can't remember brand)
Engine block=0655029 YX
Casting number=481988
Edlebrock Performer Carb 1411
Pontiac performer intake manifold(I think air gap)
I'm unsure of gear ratio
I hope this helps a little bit, I'm definately not a mechanic, and having trouble finding that right pontiac guy to tune this thing correctly. Thanks you so much for your help.
Neil


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## Instg8ter (Sep 28, 2010)

good to have you back Bear, keep that engine of yours tuned as well as you keep the one in your goat, need it to be able to race when you hit the streets this summer....feel better bud:cheers


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## 66tempestGT (Nov 28, 2009)

glad to hear youre doing ok now bear!


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## geeteeohguy (Feb 2, 2008)

Bear, I noticed you were absent, but thought you were just out in the garage. Good thing you dodged the bullet and got through your ordeal. I am Thinking positive thoughts for you!!!
nmiller, no Pontiac 400 should run worse than a 3.8 Buick......I suspect somebody unfamiliar with Pontiacs "upgraded" your engine with mis-matched parts. Keep plugging away, you'll get it. Even bone stock, Pontiac 400's put out amazing torque and power.
Jeff


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## BearGFR (Aug 25, 2008)

Thanks for the kind words, guys. I'm back at work and feeling fine.

Neil, there's a guy about 9 hours or so east of you in Burkeville Virginia who "wrote the book" on Pontiac performance. Seriously --- he wrote the "Building Your Shortblock" chapter in Jim Hand's famous book on How to Build a Max Performance Pontiac V8. His name is Jim Lehart, and he runs Central Virginia Machine Service. If you're either not familiar with doing your own engine work, or just don't want to do it, the smartest move would be to get in touch with him and arrange for him to work your car over. He's the best I've ever found at being able to get the "max" out of a Pontiac that's possible while staying within whatever budget you might have. He has customers overseas who send their engines to him (think about THOSE shipping costs for a moment). He's a great guy and will help you even if you don't spend a nickel with him, and he'll tell you the truth.

Ok, so Casting number=481988 identifies your block as being at least a 1971 block. This casting number doesn't have as much strength in the main bearing webs as the earlier 400's, but that shouldn't be a problem unless you start getting over 600 HP. To nail it down to an exact year we'd need the 4 character date code on the top rear of the block on the drivers side of the distributor hole. YX was used in 71, 72, 73, and 74 and all of them differed. In 1974 YX was a 455, for example. I'm assuming the two casting digits on your heads are 17? If that's the case, then that means those heads are off a 1968 Pontiac motor because as far as I've found, 1968 is the only year that 17 was used as a head code. There are discrepancies in the details on these heads depending on which web site/reference you look at. Some list them as being unique to the 1968 350 but as having the larger valves, others list them as being used on both the 350 and on the 2bbl 400 but having the smaller valves. Given what you've said about the engine's lack of "grunt", I'm inclined to think they probably have smaller valves and probably also have pressed in rocker studs, but the only way to tell for sure would be to remove one and start measuring. If they do have smaller valves, then that lends more evidence to the notion that the big exhaust pipes are hurting you - probably a lot. So are the heads themselves. If that's the case you could wake this motor up a lot by swapping on some better heads - even a pair of "good" factory D-ports with the larger valves, ports sized for a 400, and screw-in rocker studs. You'd want to be mindful of combustion chamber volume in order to keep compression below 9.5:1, 9.3:1 would be safer.

For a budget build, that would be the first area I'd address.

That intake manifold isn't doing you any favors either. (BTW, all Pontiac intake manifolds, even the factory cast iron ones, are "air gap".) There's lots of evidence out there from many different sources that says until you get up into the serious HP numbers (500 - 550+) *and* rpm above 5500, none of the aftermarket "performance" intakes will make as much torque and power as the factory cast iron manifold, especially down low where it counts the most.


Bear


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