Static vs dynamic compression ratio

[Alphabettispaghetti]

So, I've been doing some maths.

A genuine 10.5:1 compression ratio, with my rod combo and cam choice gives me a dynamic of 8.2:1

which is in the ball park for the cam and should run nicely on pump fuel with carbs etc.

At this point, I could get the compression ratio higher to 11.0:1, which gives about 8.6 dynamic.

What have others gone for and with what cam specs? how is it on regular fuel(anything upto 99ron).

 

[Alphabettispaghetti]

ETA, I think the largest stock cam is about 256deg? which if I put in my engine gives 8.81 dynamic, based on running 10.5:1 static

 

[johnymd]

This will depend on the squish area of your head. More squish = more turbulence therefore less chance of pinking as you go over 10:1 compression. You can stop the pinking by pulling ignition but then you lose all your power. This is one of the limiting factors of the open chamber heads like the N42.

I must add that I'm no expert on these things but it does interest me and I'm happy to be corrected by those that know better.

 

[Alphabettispaghetti]

Squish is what’s regarded as in the optimal range.

I guess when you say ‘more squish’, you actually mean a lower number?

 

[johnymd]

In an open chamber head (n42) there is no squish area, unless you skim the head to create one. The p90 on the other hand has a large flat area of the head that does not make up the combustion chamber with the piston at TDC. This is the squish area and as the combustion mixture is forced from this as the piston approaches TDC, turbulence is create which creates a faster burn and help prevent pinking. To improve this effect you want to make this gap as small as possible but not too close so the piston hits this flat part of the head when the moving parts have stretched to their maximum.

 

[jonbills]

In an open chamber head (n42) there is no squish area, unless you skim the head to create one. The p90 on the other hand has a large flat area of the head that does not make up the combustion chamber with the piston at TDC. This is the squish area and as the combustion mixture is forced from this as the piston approaches TDC, turbulence is create which creates a faster burn and help prevent pinking. To improve this effect you want to make this gap as small as possible but not too close so the piston hits this flat part of the head when the moving parts have stretched to their maximum.

In my experience, about 0.55mm is as close as you can get.
Which is what I've got on mine.
I don't know what the duration of my cam is though, so couldn't reply to the original Q.

 

[johnymd]

I've seen people talk about 18thou and others mention 38thou. From that I've read and not what I've tested the general consensus seams to be 0.55mm/18thou.

 

[Alphabettispaghetti]

In my experience, about 0.55mm is as close as you can get.
Which is what I've got on mine.
I don't know what the duration of my cam is though, so couldn't reply to the original Q.

I guess the question is, given a blank sheet, how would you do it. There's lots of cars that run a 12.0:1 static and run fine on 91(us) fuel, i'd hazard that there's just been a bit more thought put in? Or just more cam anyway.

Where do your car run out of steam? RPM wise?

 

[jonbills]

As John says, I think squish is key to avoiding detonation. my E88 head pinked at 11.0:1 with a 284/294 cam on 98 RON. My P90 at 11.2:1 doesn't pink with 95 RON and I don't imagine this cam is a bigger duration. Pretty sure it's good for > 12:1 too.

 

[johnymd]

My stroker engine should have a CR of 11:1 with an n42 head but will be running an ECU with knock control and more advanced timing control than your average ECU. I'm hoping this will mean I can get away with taking less timing out and still be on the safe side of knock.

As I'm not building a race engine I'm not overly fussed about high CR so when I cc the chambers to get the exact CR, if the chambers are bigger than standard then I'll be happier with a lower CR.

 

[Alphabettispaghetti]

As John says, I think squish is key to avoiding detonation. my E88 head pinked at 11.0:1 with a 284/294 cam on 98 RON. My P90 at 11.2:1 doesn't pink with 95 RON and I don't imagine this cam is a bigger duration. Pretty sure it's good for > 12:1 too.

Thats what I was saying, I've got good squish. All things being equal, would you up the static to get the dynamic in the sweet spot for your combination? I'm not building a race engine, just one that will perform as it should do and properly thought out/built as a package.

 

[Alphabettispaghetti]

My stroker engine should have a CR of 11:1 with an n42 head but will be running an ECU with knock control and more advanced timing control than your average ECU. I'm hoping this will mean I can get away with taking less timing out and still be on the safe side of knock.

As I'm not building a race engine I'm not overly fussed about high CR so when I cc the chambers to get the exact CR, if the chambers are bigger than standard then I'll be happier with a lower CR.

I'd like to try and aim on this for what the cool school with carbs do, as you say it gets much easier with EFI.

 

[jonbills]

Thats what I was saying, I've got good squish. All things being equal, would you up the static to get the dynamic in the sweet spot for your combination? I'm not building a race engine, just one that will perform as it should do and properly thought out/built as a package.

I'd just aim for 12:1 static, or do what I did last time, which was follow the advice of Pete, Steve and others [emoji4]
If you're happy you've got good Squish, and you know what top dynamic compression is for good Squish engines, then seems logical to pick a setup with that peak dynamic compression (everything else considered too, like valve to piston clearance!)

 

[Alphabettispaghetti]

I'd just aim for 12:1 static, or do what I did last time, which was follow the advice of Pete, Steve and others [emoji4]
If you're happy you've got good Squish, and you know what top dynamic compression is for good Squish engines, then seems logical to pick a setup with that peak dynamic compression (everything else considered too, like valve to piston clearance!)

It makes sense from an engineering standpoint, you can backwards build from a dynamic based on cam and make sure all the bits inbetween match