Hey fellas, I have more of a curiosity question vs a help question. The 68 bird I bought last year unfortunately, doesn't have the original engine. It does however, from what I can tell, have a complete 68 gto 400, with #16 heads. It runs really well but needs freshened up. It smokes a little and starts a little hard when warm...... Maybe a timing issue? Question is, if I remember correct those #16s were like a 10.5:1 compression, so I would think it is safe to say someone has adjusted the compression to meet today's fuel supply? I can't see that the head gaskets are any thicker than normal. I started out at a rolling 20 mph in 4th gear, and gave it all she had and could hear no detonation. Is it possible being that old that it doesn't carry such a high compression due to worn rings, and is doing ok? It is a 4 speed car with 3:55 rear, so that would maybe explain the awesome pull it gets. Then again, it's been a long time since driving a high torque pontiac 4 speed. Thanks guys.

Possibly someone has changed pistons during a rebuild. Check cranking compression in the cylinders while engine is still warm. That will help show the condition. If burning oil it's most likely the valve guides.

They may not have made any changes on the rebuild...

It's possible if it was rebuilt with replacement cast Pistons. The ones I had for a year were quite a ways down in the hole. Never did try to figure the CR but probably in the 9s:1 somewhere.

Yeah, and if they used those dreaded 8-eyebrow cast pistons, that would also lower compression slightly. There are small scopes that can be used to go thru a spark plug hole, and look around inside, at the piston and valves. I've seen 'em used when teching dirt track engines, to check for oversize valves and illegal pistons. You might be able to borrow one of these from a professional mechanic, if you know one. Or there may be places that rent 'em. Or hey, maybe Auto Zone loans 'em.

There are all kinds, including some that work with your smart phone. But you'll need one that can look both at the piston and at the valves. Some are not that flexible, and do not have that capability.

https://www.google.com/webhp?sourceid=ch...automotive+use+

http://www.carsdirect.com/car-repair/an-automotive-borescope-buyers-guide

https://www.google.com/webhp?sourceid=ch...se&start=10

If there were no changes made if it was rebuilt, it would have detonation issues right?

Ha! That's funny that you mention the 8-eyebrow pistons. When I tore apart my 1969 400 engine, I had seen reciepts that it was rebuilt back in 1992 and saw that the guy who did the work replaced two pistons and once I removed the heads, I noticed that he replaced the the two original pistons with two 8-eyebrow pistons. I thought that was odd, while I also had always thought the engine was lacking some power, I always chalked it up to needing a rebuild because the odometer had not worked prior to the engine rebuild so I never knew how many miles was on that last rebuild. I did still have detonation issues and I have #16 heads also.

Well, that depends on exactly what the term "rebuilt" means. Back in the old days, they would do what was called a "ring job". Some would say they "put rings and inserts in it". The "inserts", of course, meaning bearings. I've done a few myself. You use a ridge reamer to remove the portion of the cylinder wall above the ring wear line, then use a flex hone to scratch up the cylinders, and put it back together, with new rings and bearings. It's been done millions of times.

OR, "rebuild" can mean a cylinder overbore, new pistons, rings, bearings, cam, lifters, oil pump, etc, as well as a complete valve job, with new valves. It can also include the resizing of the cast rods, and using new ARP bolts. This resizing will shorten the rods slightly, which will reduce the CR slightly, unless enuff is milled off the block deck to make up for it. Some builders mill for zero deck height, or maybe .005, to allow for a slight cut on a future build.

So, all these factors, and more, can come into play when trying to determine the CR. For example: if the rods were resized and the 8-eyebrow or dish pistons were used, and the block was not decked, then you will have lower than stock CR. And the cam can help prevent, or help cause detonation. If the engine already has around 10:1 CR and you try to use one of the steep ramp cams, like the CC XE or Lunati Voodoo series cams, then the increase in cylinder pressure these cams cause, could easily cause detonation or make it worse.

These type cams produce more of what is called "dynamic compression" (DCR). Whereas a cam with more advertised duration, (one that begins to open the valves sooner, and closes 'em later) will "bleed off" some cylinder pressure at lower rpm, thus making detonation less likely, than with the steep ramp cam. These longer duration cams are said to produce less DCR.

For an example: in a 10:1 CR engine, a CC XE or Voodoo 262 cam would produce a much higher DCR than a RA4 grind cam, and therefore would be much more likely to cause detonation. That may be a crude explanation. But that's my understanding of how it works. So, you see, without knowing all the exact specs of your engine, nobody can say for sure exactly what the CR is or how likely it is to experience detonation.

Also don't go by the advertised 72cc for all these cylinder heads, I have run a few sets of 16 casting and none were 72cc. In fact the set that is on my engine now are at 77cc even with a clean up mill and brand new valves.

Sounds like a good explanation to me. once driving season is over I had planned on pulling it for a freshen up. We will see how the funds look