1970#12 heads, 9790071 block, '68 intake - Pontiac GTO Forum
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post #1 of 10 (permalink) Old 06-20-2019, 09:46 PM Thread Starter
 
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1970#12 heads, 9790071 block, '68 intake

1970#12 heads J179 , 9790071 YA block, 9790140 '68 intake.
This is whats in my '68 GTO.
I have no idea what cam it has
turbo 400 AT
what HP am i looking at?
Thanks
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post #2 of 10 (permalink) Old 06-21-2019, 07:00 PM
 
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Originally Posted by John Schutt View Post
1970#12 heads J179 , 9790071 YA block, 9790140 '68 intake.
This is whats in my '68 GTO.
I have no idea what cam it has
turbo 400 AT
what HP am i looking at?
Thanks
How are we supposed to know? Cam changed? Q-jet? Stock, Ram Air, or headers? Pipe diameter of the exhaust. Stock pistons or aftermarket? Flat top or dished?

Looks to be a 1968 block. Service Manual says the YA code is 2 Barrel, 265HP 8.6 compression engine used on full size Pontiacs. 1970 #12 heads came in 2 flavors. Small valve, press-in rocker arm studs or Ram Air III with large valves and screw-in studs. However there seems to be a number of differing specs/info on the #12 head depending on your source.

The Service Manual for '68 says all 400CI pistons are flat top. If the '70 heads are screw-in studs/big valves, they have 72 cc's which would put your engine near 10.0-10.5 compression. At that compression, you should be running 95-100 octane. If you can run 89 or 91 octane, then you don't have the 72 cc heads.

The cam, if stock, is of no power. Adding the higher compression heads won't gain a lot. So my guess as to a stock engine/cam with the higher compression heads might be 285HP.
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post #3 of 10 (permalink) Old 06-23-2019, 12:12 PM Thread Starter
 
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Thanks for the response and info PontiacJim.
The heads appear to be RAIII, screw in studs, but these could have been installed at any time.
Someone put roller rockers on it at one point too.
The car currently has long tube headers, to be replaced with RA exhaust manifolds this winter.
I know nothing about the cam, and won't until this winter when it get into the engine.
I don't know if the exhaust seats are hardened so i'm going to run lead additive this season.
I could measure the lift using an indicator on the rocker.
I did perform a compression check on a COLD engine this am with the following results:
1 168 psi
2 177 psi
3 178 psi
4 170 psi
5 175 psi
6 175 psi
7 184 psi
8 179 psi
Average of 175 psi
Any one have a chart or rough estimate what the compression ratio might be?
I used a bore scope to look at the pistons, i'll try and upload a couple of pics, I love technology!
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post #4 of 10 (permalink) Old 06-23-2019, 12:14 PM Thread Starter
 
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Bore scope images of the pistons
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post #5 of 10 (permalink) Old 06-23-2019, 07:05 PM
 
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Originally Posted by John Schutt View Post
Bore scope images of the pistons
Looks to have the "double" valve notches which from my experience is an aftermarket cast piston. I have not read anywhere that Pontiac used any style piston other than flat tops in all the 400CI engines and the chamber size is what is altered to get the assorted compression ratio's. I read that some 428 engines used a dished piston so they could run the GTO heads.

Service Manual states 8.6 and 9.2 compression will be 150-170 PSI at a cranking RPM of 155-165 RPM's. 10.5-10.75 compression is 185-210 PSI. So with the #12 72cc heads and the aftermarket pistons (assuming), you may be in the 9.3-9.5 compression range. So the engine may have been rebuilt at some point.
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post #6 of 10 (permalink) Old 06-23-2019, 08:08 PM Thread Starter
 
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Thanks for the service book info.
The first test I didnít have the carb opened up.
Second test with WOT 183 psi, no outliers
Third test, WOT, 20cc 10W40 in each cylinder, 187 psi, not outliers
Appears I need to find 100 octane since the engine is somewhere between 10.25-10.5.

On another note I installed a Summit Aluminum radiator and twin cooling fans with temp kit, keeps it real cool.
The full UMI suspension kit seems real firm, still needs alignment.
Iím disappointed on one choice, I found a thread on here that stated to use a 67-74 Camaro steering box, my 3/4 ton crew cab short box turns tighter than this. DONíT Do It!
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post #7 of 10 (permalink) Old 06-25-2019, 07:18 PM
 
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Originally Posted by John Schutt View Post
Thanks for the service book info.
The first test I didnít have the carb opened up.
Second test with WOT 183 psi, no outliers
Third test, WOT, 20cc 10W40 in each cylinder, 187 psi, not outliers
Appears I need to find 100 octane since the engine is somewhere between 10.25-10.5.

On another note I installed a Summit Aluminum radiator and twin cooling fans with temp kit, keeps it real cool.
The full UMI suspension kit seems real firm, still needs alignment.
Iím disappointed on one choice, I found a thread on here that stated to use a 67-74 Camaro steering box, my 3/4 ton crew cab short box turns tighter than this. DONíT Do It!

The upgrade is the Jeep Grand Cherokee, not Camaro. 2 wheel drive, not 4-wheel as it is different. Got mine at a Pull-A-Part and if you look around, you will see the differences in the two types. If you do a search of the forum you will find a few discussions on it. The internet also has it. I installed one on my '68. Bolted right up. Uses your Pontiac pitman arm. You need to get the correct rag-joint based on shaft diameter and spline count from the Jeep box. I used a "Lars Steering Coupler" (maybe the "201" is the correct number) from Advance Auto, and not a Dorman made unit. Hose is metric on the box versus the AN hose end on your PS pump. You can use an adapter or have a hydraulics shop fab up a hose having the ends you need. Can't give you any opinions as it is only installed and the car is nowhere close to road ready.
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post #8 of 10 (permalink) Old 06-29-2019, 06:03 PM
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Hate to be the bearer of bad news, but those are the dreaded 'eight eyebrow' pistons, which are cheap, fit-all jobber pistons with poor quench and they fit well down in the hole. They cause lower compression than a flat top, AND increased detonation/pinging due to their configuration. With those pistons, and those heads, you are looking at running 100 octane fuel to keep out of the danger zone. The reason for the 8 valve reliefs is so that they can be used in 389's as well as later 400 engines, which have different valve angles than the 389. The engine was definitely rebuilt at some point.
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post #9 of 10 (permalink) Old 07-01-2019, 11:00 AM Thread Starter
 
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I'm not looking at it as bad news, its informative news, much needed. This will put a rebuild on top of the list this winter.
As for the steering box i followed this thread, maybe i have the wrong steering box.
https://www.gtoforum.com/f178/steeri...tml#post885857
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post #10 of 10 (permalink) Old 07-03-2019, 06:31 PM
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Great news, John. You can rebuild the engine with dished pistons for about 9:1 or 8.5:1 compression and use your current #12 heads. I have a set I used to run, and they are perhaps the best D port head Pontiac came out with for power.
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