So, I suppose I’m becoming famous for bone head questions by now...
I am wondering how much effect there would be on spooling, when the compression ratio is dropped from 9.2:1 down to 7.8:1?
But wait, there’s more...
I understand the effect on the same engine... reduced temperature = slower spool.
Now, what if the lower compression is coupled with a far superior flowing head?
Does this have any effect?

Am I daft to think that a better flowing engine (with the lower cr) will spool a larger turbo with similar effect?

Original system was a bone stock engine. Now, larger valves, port work, cam, carb.
If I use the same turbo on both engines, will it spool the same?
Does the turbo spool based on temperature alone? Does the exhaust velocity come into play at all?

Sorry if this is a really dumb line of questions.... I’m quite tired and can’t get my head right on it.

 

Head flow equals hp potential at a given air density, compression is more combustion efficiency. Compression really isn't worth large amounts of hp, but does really improve off boost response, and tq, especially on a small cid engine. There's really no reason to go that low on cr these days.

 

Not dumb questions by any means.
First, the compression. More is better....until it isn't. When you run out of ability to keep the high pressure charge from detonating without pulling a bunch of timing, it's too much. When fuel availability is an issue, it's a problem. Higher ratios improve efficiency and certainly make the engine more responsive out of boost, which helps it spool and get into boost faster.

The bigger heads raise your overall potential, but may hurt responsiveness at lower rpm. Spooling could be slower with the lazy airflow in the larger port. Old assumptions still hold true.

 

Here is a dyno test on a LS engine comparing what you are talking about. The 706 heads have the smaller combustion chamber, valves and runners of the cathedral port heads. And the 317 has the largest chamber, runners and valves (I know 799 and 243 have the same runner and valves but this is about flow AND compression). Same principles will apply no matter the engine.

 

Well, stink!
What I’m taking from this is that I’m just hurt myself with the lower compression... which wasn’t by choice.
I bought a used short block that had a roots blower on it. Since I didn’t have the money for a whole new setup, this seemed a reasonable way to go.
I think it best to run the smaller of the two turbos I have, which is near identical to the one I was running on the stock engine. If the lower compression is going to cause a bit of lag, the smaller turbo will still be full boost below 3000 RPM.
I can always up the boost.

And, I start saving my pennies (not really... we don’t have them anymore) and buy some custom pistons for a higher cr.

Thank you for your help!

 

It will still run at the lower CR. The big losses are off-boost response, horsepower out of the boost curve, more emissions and less fuel economy. The big gain is you have a much wider tuning window and you can probably get away with more boost on lower quality fuels. Make it run, drive it, decide what you like and don't, make changes accordingly.

Builds are ever evolving, very few hot rodders "get it right" the first time, most tinker and change parts on a project until they are dead or the project dies, then there is a new project and the cycle repeats.