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Discussion Starter #1 (Edited)
I bought ny 68 GTO about 2 years ago. The person that sold me the car had only owned it a short time and did not know much about it. Well, I got lucky. The car is in excellent shape. Engine was recently rebuilt. You can see new gaskets everwhere on engine. Seller only knew that engine had been built, but did not know any details. Engine runs great, no clicking or tapping,does not smoke and runs really smooth. It has the numbers matching 360HP H.O. engine with 4-speed and 3:55 rear gear. Told you I got lucky, the seller didn't know it and I don't think he would have known what he had if he had know it. I sent off for the PHS papers and everthing checks out. It is the real deal. My question is this. The engine has a very smooth even idle. I would think that the H.O. would have a bit of a choppy idle. I am wondering what type of cam rebuilder used. Is there anyway to find out? I am tempted to pull top end down and inspect cam, but engine runs so good I am scared to mess with it. I took it to drag:party: strip a while back. Made 3 runs. First run I bogged real bad off line and just aborted that run. Next run with pretty good off line traction and granny shifting at 5,000 she turned a 14:26, next run was a 14;44. Both runs were around 98 MPH. What do yall think, is that what a stock 68 H.O. is expected to turn? It also has a set of hedman headers. I sent Qjet to Cliff's High Performance Qjet for rebuilding and sent the HEI dist to Sun Tuned for set-up. If I decided to replace cam and lifters, what would ya'll recommend. I would want Rhoads lifters. Thanks
 

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I think the cam should sound a "little lopey" mid 14 secs aren't that great. BEFORE you tear the motor apart, make sure every thing is tuned and hooked up properly.....it's a lotta money to see what cam you have! I would think you should turn mid 13 with that car.....E
 

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Discussion Starter #3 (Edited)
I agree. I think that the correct cam was not put in engine during rebuild. I think it is one of these 'RV" cams that are supposed to run smooth and get better gas mileage. I could have probably knocked a tenth or two off if I was more aggressive on the gear stick, but I didn't want to hurt car. I wish you could hear it idle. Everything on the engine is set correctly. One thing I have noticed, the car has never "spark knocked". I have run 87 octane gas and it still runs good. I don't think a H.O. would do that unless cam or pistons had been changed for unleaded gas and lower compression. One other thing, I put a bore scope down one of the spark plug holes and you can clearly see two big round valve reliefs side by side on the pistons.
 

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If it's a smooth idle, thats a good time. Did you drive the car like you hate it, for drag racing it's drive on kill or you'll never get the true potential out of the car. Do you want the car faster? Stupid question, lol.. Not knowing what you have limits what cam you can replace it with. You can read some of Mr. Pbodies threads and see the cams he recommends, that is what I am going to do if I need one. How radical do you want it, you have a 4 speed, you can run however radical you want. Sounds like a really nice car, whatever you do, decide what you want to do with the car before you go to far and the car isn't fun anymore. Also, did you have the car tuned to kill, or just running good?
 

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A stock 400 HO tuned to factory specs should detonate on pump gas, even premium. Sounds like somebody replaced the pistons to lower the compression. There is an excellent explanation on Pump Gas Compression Ratios on Rock and Roll Engineering's website under FAQ's at Pontiac Engines : Rock and Roll Engineering I think your et's are representative of a stock 400 HO
driven conservatively. Even with matching numbers cars there is no real way to know what is inside an engine.
 

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Discussion Starter #7
It has the number 16 heads. One head has a date code of January 15,1968 and the other is January 16, 1968. About the only option it has is the Rally Instument Cluster with indash tach ,H.O. engine,posi, and 4-speed. It has no power steering or brakes, no console, no hideaway headlights. So, it appears whoever ordered it new , ordered it to run.
 

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If you can run 87 without pinging it is down on compression, or the timing is retarded. If the pistons are flat tops with valve reliefs, the compression would still be up, probably 9-1 or better. With no power brakes you can run a bigger cam, as you don't need the vacuum for brakes. You can run a compression test and that will give you an idea of the compression ratio or engine condition. I would advance the timing til it pings lightly then back it up a smidge. Also run premium gas with the advanced timing, then you'll see the cars true potential.
 

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:agree.....87 Octane pump wee wee should make a "correctly built and tuned" HO motor rattle "like 2 skeletons makin' love on a tin roof during a hail storm"...... :eek:
 

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Discussion Starter #10
There is no question about the valve reliefs. They were very visible on top of the pistons. I think I will start advancing the timing until I get a ping and then back it down a little. Will let you know. One thing I had forgot is when the 87 octane was put in, the tank also had some 91 octane with some octane boost in it also. My gas gauge does not work and I am scared of running out of gas, so I stop and put in gas practically every time I drive it. It may have had enough 91 octane and octane boost to cancel out the 87 octane.
 

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A stock 1968 HO 400 will have a smooth idle. The 068 HO or Tripower grind cam is not a lopey cam. I've run one for years in my '67, and it's a smooth idle. Pontiacdude, mid 14's for what you have is totally respectable. Like others have said, if you're running pump gas without it pinging, the engine was rebuilt to a lower compression. Over the years, I have seen countless soilid, good running engines torn apart by well- meaning but misguided folks who want to "make improvements". Usually, the no-longer-running car is sold as a basket case after a few years as interest fades and the car languishes in the yard. Not saying you're that guy, but why mess with success? Why tear into a leak free, solid running engine? If you want to know if the compression has been lowered, do a simple compression test. If it's about 150-170 psi, you have 8.9-9.5 compression. If you're at 190-210 psi, you have the stock "high" compression. A lot of guys I have met up with over the years have changed out camshafts for "more lopey" sound and installed "high performance" manifolds to "improve the output of their engine". Engines built on this logic always perform poorly compared to an unmolested factory-engineered unit. JMHO.....
 

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Discussion Starter #12
Makes sense. I'll go to AutoZone and buy a cheap compression checker and see what I get. What would be the procedure. Do I take all plugs out at one time or do one cylinder and then move to the next one. Should I squirt a little oil into cylinder? Thanks, Mike
 

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Compression test: warm up engine. Remove all spark plugs. Disable ignition. Make SURE ignition is disabled (easy to start a fire here). Block throttle most or all of the way open. Screw tester into spark plug hole. Crank engine over about 5-6 times. The gauge will build most of the pressure on the first two cranks, but go 5 or 6. Write down the compression and move on to the next. No oil in the cylinders. That's for a "wet" compression test, which is used to check for bad rings if you have a tired old engine...not your scenario. Good luck, and let us know what you have.
 

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Discussion Starter #15
If I block throttle all way open, will it flood oil pan with gas? Just asking if I will need to change oil or not. Just changed last week.
 

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Discussion Starter #17
I was looking in my Service Manual for 68 Tempest & GTO and found the following paragraph:Low compression pressures on Ram Air,400H.O.,and 428 H.O. engines are not a valid indication of engine condition. Compression readings at cranking speeds as low as 120psi are considered normal.
 

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I was looking in my Service Manual for 68 Tempest & GTO and found the following paragraph:Low compression pressures on Ram Air,400H.O.,and 428 H.O. engines are not a valid indication of engine condition. Compression readings at cranking speeds as low as 120psi are considered normal.
Yep, you know why, right? The common denominator in those engines is a long duration cam shaft. With the intake valve closing "later" in terms of crankshaft degrees, cranking pressure is lower at cranking speeds because pressure can't start building until that intake valve closes. That's why the nastier the cam, the more static compression ratio is needed "in general" in order for the motor to build good cylinder pressure. I say "in general" because in a running engine the dynamics of intake flow velocity and inertia of the moving gas column come into play and things change across the RPM band.

Bear
 

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Bear's explanation is spot on. Pontiac Dude, your readings of 180 psi indicate a CR of about 10:1. From what I've read, on today's fuel, the maximum recommended cranking compression with iron cylinder heads is about 175 psi. This puts you really close to optimum. In my opinion, with careful tune (not too lean, a good cooling system, and the right ignition curve), you should be able to run fine on 93 octane pump gas. A careful tune by a competent person should yield excellent performance gains.
 

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Discussion Starter #20
Thanks Guys. I put new radiator and water pump in last year. It never runs above 180 on hottest days. Like I said in my first post, I was really granny shifting on the 4-speed. I just don't want to abuse this car , although I'm sure its had its share of speed-shifting in years gone by. If I had been more aggressive with the car I probably could have shaved close to a second off the 1/4 times. I now believe I will just drive car and enjoy it. I feel very good about the compression numbers I got and will definitely leave the engine alone. Thanks, Mike
 
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