I'm going to be building my 454 soon and was pretty comfortable lately (the last two years!)with all the talk about dynamic compression and what you can run for static on pump gas and read Pat Kelly's articles a few times and even more articles on the web then all of a sudden I came across a comment you made a few days ago in a post on your thoughts about dynamic compression and now I'm not feeling so comfortable.You used a 13:1 engine as an example and went on to say as the engine gains efficiency while speeding up even with a big cam bleeding off cylinder pressure it might still reach that 13:1 compression and detonate on pump gas.Or even an engine with less static compression.I hope I worded that all right.I can't remember the post you said that in.I guess I was always under the impression that detonation besides being caused by heat was also influenced by a certain amount of TIME.At higher speeds is there enough "time" for detonation to occur? Do you have real specs of your own from dynoing engines just how much a cast iron headed engine can have static compression wise running around 180-190 degrees temperature on pump gas regardless of what the dynamic compression is?I know other things like chamber shape and quench height are all factors in this detonation war but you really got me thinking.Dynos have better ways of "hearing" detonation than our own ears can hear or what we think we see on the sparkplugs.Sorry so long a post.Al

Al,
Now here's a loooong post :D

First let me say I think Pat Kelley's DCR calculator is an excellent tool.
My own opinions are that as a tool it needs to be used properly & as I said before, with a bit of common sense.
My example of a 13.0 engine with a huge cam is what I am speaking of.
If the engine is even fairly efficient that cam is actually going to help fill the cylinders at some point & this is where you can get yourself into trouble if the fuel you are using is not capable of sustaining an efficient burn.

Now,,,,
Unfortunately I feel there just is no set compression for either aluminum or cast heads.
As you stated there are way too many varibles.
It would be nice if engines could read all the theory & behave accordingly but it doesn't always seem to work out that way :D

I don't really care what RPM you are at, detonation can & will occur if the fuel doesn't have the octane or is to lean for the circumstances at the time.
This is true of gas, alcohol or nitro.
The heavier the load & the lower the rpm the more likely it is to detonate but it can happen at 8000 or 3000 if things ain't right.

To your question I prefer no more than 9.5-10.0 for a cast head & 10.5-11.0 for a aluminum head with 91-93 octane pump fuel on a carbureted engine.
There are combos that can & do run higher & some that will not tolerate this much but I feel the risks far outweigh the advantages of pushing the limits with compression.

As for hearing it on the dyno, you usually will not unless it is really bad.
You will probably see it if you happen to be measuring crankcase pressure & you will see it in the torque curve when the power goes to hell again if it is bad.
The exhaust temps may or may not go up & sometimes they will go down.
The best deal is the in cylinder pressure measurement stuff that is available but it starts around $25-30,000 for one cylinder :eek:

Short of that, it pays to keep an eye on plugs & start on the rich side with conservative timing just like at the track.

I used to drive a blown alcohol dragster & you would not hear detonation but you would hear & feel the engine "lay down" & it was a good idea to consider getting out of the throttle at this point if it wasn't a race ;)
This was usually on the top end so the rpm was definitely on the high side as this thing went through the lights well over 10,000 rpm

Just for info here is what I posted that you are referring to

"Gene,
I realize what the L88 cam does in regards to cranking pressure but I have my own opinions about DCR.
While I don't post it much, here are a few thoughts for you to ponder.

First let me say I think that Pat's DCR calculator is an excellent tool when used with a bit of common sense.

Now on to my thoughts about DCR & high compression.

With a big cam you will see lower cranking compression but you also have to use a little common sense with the slam a big cam & lower the DCR syndrome
It may have low cylinder pressure at cranking & low rpm's but somewhere it is going to start getting efficient & the engine is going to see whatever compression you have.
If the fuel isn't good enough when this happens ugly things can & eventually will happen.

If the plan is to use & lean on your engine with pump gas most of the time I would prefer to limit the static compression with aluminum heads at 10.-10.5 for 91 octane & 10.5-11 for 93 & change the cam accordingly.

You are right in that cranking compression which is part of the DCR is determined from when the intake valve closes.
The higher the static compression the higher the cranking compression will be the sooner you close the intake valve.
This I think we all agree on.

Now there is a lot of other things that will effect cylinder pressure to a certain extent, like Barometric pressure, cam intensity, engine temp, air temp, humidity & probably a lot of other little things I missed as well.

Here's another thought to screw up the works,,
You can build an engine with say 13.0 & overcam the crap out of it & on paper the DCR will tell you it will run on 91 octane.
The problem comes when that engine gets up in the rpm & becomes efficient & starts actually seeing that 13.0. Now that DCR becomes less of an issue as you are starting to keep pressure in the cylinder & with enough load without proper fuel, it will detonate.

The load part is why lighter low gear cars will usually tolerate more compression than a heavier or higher geared car.

It also works the other way.
We do restricted engines you just could not put a load on at low rpm in most cases.
Small cam hi compression deals like 13.5+ with a cam of say 240 or so @ .050 & at low speeds they will detonate themselves to death if you ain't careful but from say 3500-4000 up they will never see that 13.5 compression as the intake is too small to let enough air in to fill the cylinders :(
Is this enough confusion yet??

Anyway, you are only compressing what is in the cylinder from the time the intake valve closes & this can be completely different depending on a number of things but at cranking speed most of them are listed above I think :confused:

Anyway,,, these are just a few of my uneducated thoughts on the merits of DCR in regards to just using when the intake valve closes.

Someone else can pick up the quench, chamber efficiency & timing issues :D "

good stuff Mike, glad you are here to share your knowledge. I remember reading an old magazine article when Joe Sherman built a 12.5 to 406 that ran on pump gas but if I remember the total timing was like 28 degrees or it detonated like crazy, made excellent power but I bet efficiency would have gone way up with better fuel. Running on the edge can be frustrating I bet when the weather changes :rolleyes:

Thanks Mike.It's great to have guys like you to talk to on this board.Eases the mind alittle better.Al