69 383 hp comp ratio

[northerndave]

Hey all, I've been thinking about a rebuild for my 383 in my 69 RR. Pretty sure the stock comp ratio is 10-ish:1?

Well, my rebuild (running fine now actually) but my rebuild would probably involve some headwork such as hardened seats, stainless valves with chrome stems, some porting... Thinking about the pistons & the comp ratio... would it be sacrilige to pull some huff out of the comp ratio? like say knock it down to 9:1?

Go after power gains with some flow improvements (porting, headers, intake, carb etc)

Just to take some of the fuel sensitivity out of the picture?

My RR is a 4 speed car, guess I'm looking for a nice sound like I currently have (slightly lumpy idle) Power in the 400 hp range.... & well manored for town driving.

thoughts?

 

[696pack]

These engines LOVE compression. If you build a quench engine you can still have comprssion and run it on pump gas premium.

 

[northerndave]

lets pretend I don't understand "quench engine" :grin:

 

[Meep-Meep]

Quench is an area in the combustion chamber that has a very tight clearance to the piston top (~.040"-.050") when the piston is at TDC. What this does is swirl the fuel mixture and cram it toward the plug for a more complete burn. The more efficient the chamber (I.E. fast burn) the less advance you need to get the job done. As mentioned, it also helps you run a slightly higher compression for a given grade of fuel. Pop up pistons usually make things worse because the flame front has to climb a mountain to consume all the available fuel, and you have to start the spark earlier to do it. A 10:1 engine with a good chamber design may only require 91 R+M/2 but the same CR with a poorly designed large pop up piston may require more to keep it from detonating. Also, bandaiding a high CR engine by not giving it its full advance so it will run on pump gas is not a win-win situation.

I just got a 440 running that I put together and it has an honest 10.1:1 CR. I used a flat top piston (with valve reliefs) that are at zero deck and the 915 closed chamber head. My quench clearance is about .050" and with the cam I'm running I get 190 PSI of cylinder pressure. It runs on pump 91 but has a slight spark knock probably because of 50+ deg vac advance I get as I tip into the throttle. I'm still in the tuning phase and should easily make that issue go away. I'll see how this combo works as time goes on and hopefully I don't put a hole in a piston finding out!!

Watch your cam and compression ratio. In my experience 160 PSI of cranking pressure will work on 87 R+M/2.

 

[northerndave]

Alright thanks!

Are you going to try a tunable vac advance? I had one on a GM dist once on a 327 that I had fits with getting the timing perfect. It had a screw inside of it that was accessable with a small allen wrench through the vac line port on the diaphram. You could limit the diaphram travel distance. It was nice because I needed to limit the vac advance for the bottom end & I could let the mechanical adv come in where I needed it with weight/spring combination.

 

[Meep-Meep]

Yeah, either I'll look to see if it has a screw or modify the stop at the arm. One way or another I need to limit it.