[67drake]

What was the actual CR on a ‘69 400 with 10.5 advertised? Am I chasing a ghost trying to get it to stop detonating on 91 octane?
Just thinking out loud, but after all the tuning and screwing around, it still detonates, at WOT,and I don’t want to add cans of expensive octane boost everywhere, as I put lots of miles on this car now.
I don’t have any problems running my ‘71 GTO on the local gas, and it’s true CR is 9.6 now.
Considering pulling the cam and seeing what is in there, but based on the previous owners info, it’s a CompCams stick, so probably on a 110 LSA. Just trying to figure if it’s worth going with a factory type wide LSA cam, or just switch to a more reasonable CR head.

[Stan Weiss]

What psi does the engine show on a cranking compression test?

Stan

[67drake]

Originally Posted by Stan Weiss

What psi does the engine show on a cranking compression test?

Stan

Great question and something I didn’t think of. I’ll check it this weekend.

[ID67goat]

Also what is the ignition timing?

[67drake]

It’s at 12 initial right now. Mechanical putting in 20 maximum at crankshaft,So 32 total. Dialing it back just makes the engine lazy and no throttle response. But obviously the more advanced, the more detonation at WOT.
I was just on a 430 mile cruise with it this weekend, and I bought my timing light and distributor wrench to play with different settings, and 12 initial is where it ran best.

[Cliff R]

Comp grinds everything in their arsenal on a tight LSA. That NEVER works out well until you start getting into some of their grinds with a lot of seat timing and even then there are a LOT better choices out there for these engines, IMHO.

Anyhow, if you have a 400 that pings just about everyplace and you have to yank all the timing out to make it stop it may have multiple issues, some will be beyond the camshaft. BUTTLOADS of 400's were build with 8 valve relief cast pistons that ended up .030-.050" below the deck at TDC.

Even the early TRW/Speed Pro forgings were pretty short causing the same issue. Combine that with higher compression heads and a short seat timing cam and you're in trouble. I've had butt-loads of ill fated engines brought to my shop over the years for custom tuning. The very WORST of them were built with a lot of quench area and cams with early intake closing and tight LSA. Can't scream "turd" here loud enough and I've tried over the years (decades now) to keep folks from ending up in that scenario, but more times than not I'm asked to help AFTER they get in big trouble and not before..........FWIW.......

[Jay S]

It is hard to say what all has been done inside of the engine after 50+ years. Could have several thing hurting it.

These screen shots are pretty close to the actual compression on a 69 400, with #62 heads (75cc), adding 1cc for the cylinder scallops, 6.7cc VR’s, .042” head gasket, .016” piston to deck height (assuming the original cast pistons were .005” taller than L2262Fs. Forged replacement pistons are usually .005” shorter than OEM cast. Would have had an 067 camshaft.

[67drake]

Hmmm….Wallace site lists my ‘69 YD at only 10:1 CR ! I was going by my old Chiltons manual which has it listed at 10.5.
Yeah, like you mentioned Jay,who knows what’s been done to this engine, but I shouldn’t be detonating at a potentially only 9.5 CR.
My heads are the 46 castings.

[hurryinhoosier62]

Originally Posted by 67drake

What was the actual CR on a ‘69 400 with 10.5 advertised? Am I chasing a ghost trying to get it to stop detonating on 91 octane?
Just thinking out loud, but after all the tuning and screwing around, it still detonates, at WOT,and I don’t want to add cans of expensive octane boost everywhere, as I put lots of miles on this car now.
I don’t have any problems running my ‘71 GTO on the local gas, and it’s true CR is 9.6 now.
Considering pulling the cam and seeing what is in there, but based on the previous owners info, it’s a CompCams stick, so probably on a 110 LSA. Just trying to figure if it’s worth going with a factory type wide LSA cam, or just switch to a more reasonable CR head.

According to the late Roger Huntington ( 60s automotive journalist) Pontiac “fudged” its advertised compression ratios by as much as 3/4 of a point. 10.5 minus .75 equals 9.75 actual compression ratio.

[chiphead]

What cam? How fast does the ignition curve come in? Cranking compression is not a 100% predictive but good figure of merit. Near 200 psi or more indicates early intake closing point with 10:1 static.

My 69 400 with 62 heads runs perfect on 93. I'm at about 9.8 stock compression and 2802 cam. 35 degrees all in by 3300 rpm.

[Formulajones]

I've taken apart a hand full of virgin performance engines for rebuild and cc'd heads, checked piston to deck height, stroke, etc... including Pontiac, Ford, and Chevrolets, and none of them actually had their advertised compression ratios. They varied between brands but all fell short about 1/2 a point give or take

[67drake]

Originally Posted by chiphead

What cam? How fast does the ignition curve come in? Cranking compression is not a 100% predictive but good figure of merit. Near 200 psi or more indicates early intake closing point with 10:1 static.

My 69 400 with 62 heads runs perfect on 93. I'm at about 9.8 stock compression and 2802 cam. 35 degrees all in by 3300 rpm.

I don’t know what cam, as it was installed in the engine when I bought the car. Previous owner didn’t know the grind, just that it was a CompCams camshaft.
My catch 22-I plan on pulling the engine in the fall to do some other work on the engine over the winter , so I figured I’d do the cam swap then. My fear is that I’ve read there was a shortage of cam cores, so would rather figure out some of this ahead of time to get a cam ordered, BUT guess I can’t really do that until I know what’s in there to start with. My distributor curve-