More friday nite noodling

[Blown 472]

Hey Fiat can you splain the diff tween static and dynamic comp? My buddy has a 455 ponitac that he redid and had a roller cam supposedly spec'd for what he was doing and we played with this car and it runs like it is a pipy two stroke.
Well they ran the numbers and it has a dynamic comp ratio of 6 to 1, they are going to mill the heads to get the static up and try to help that too. How does that all work?

[Fiat48]

You'll have to find somebody sharper than me on the "dynamic" compression ratio. I see the term used and I gather it's more what we used to call cylinder pressure.
On all out race motors I build as much static compression as I can. To me, fuel quality and flame travel are my only two limitations. Always seems to be more HP with more compression as long as you don't screw up the flame travel and stay away from detonation.
Lately I have heard some of the 10,000 rpm stuff is actually lowering the static compression ratio a bit. But I don't turn that high.
That high static compression covers up the large camshaft we put in the motor and allows the motor to leave hard. Sometimes I'll sacrifice some camshaft duration so I can get more cylinder pressure down low to make the motor leave. Just depends on what kind of racing the motor is going to do. Sometimes we will change camshaft phasing or valve lash to make the motor leave.
Theres always been a couple shcools of thought on the blown stuff. Some guys want to build for low static and drive the blower higher. Others want to do higher static and less blower drive.
I have always subscribed to the higher static and less blower drive. And no more camshaft than I have to. Reliablity seems to be better for me.

[058]

'scuse me but I'd like to take a stab at this dynamic c/r thing. As I understand dynamic is the total of the static and the volumetric added together. A 10 to 1 engine that produces a V/E of 105% might have a dynamic c/r of 10.5 to 1 Or a supercharged engine that makes 15 lbs of boost should be double of what the static c/r is of that engine. If that blown engine has a 8 to 1 static c/r the dynamic c/r would have app. 16 to 1 based on the fact that 15 lbs is about one atmosphere at sea level. On the other side of things a closed throttled engine that is producing high vacume then that same 10 to 1 engine may have only a 4 to 1 dynamic c/r because the cylinders are not being filled. Dynamic c/r is just another form of volumetric efficency but with a $.50 name. If ya can't dazzle them with brilliance...baffle them with bullshit.

[Fiat48]

Dang there 058! Thanks for the explanantion and education.
I don't know all the terms but still the old bottom line is to treat an engine as an air pump. Cause that's all it is.
Taking my air pump out to the lake today. Seemed it worked so good last time I need a confirmation pass.

[Blown 472]

Originally posted by 058
'scuse me but I'd like to take a stab at this dynamic c/r thing. As I understand dynamic is the total of the static and the volumetric added together. A 10 to 1 engine that produces a V/E of 105% might have a dynamic c/r of 10.5 to 1 Or a supercharged engine that makes 15 lbs of boost should be double of what the static c/r is of that engine. If that blown engine has a 8 to 1 static c/r the dynamic c/r would have app. 16 to 1 based on the fact that 15 lbs is about one atmosphere at sea level. On the other side of things a closed throttled engine that is producing high vacume then that same 10 to 1 engine may have only a 4 to 1 dynamic c/r because the cylinders are not being filled. Dynamic c/r is just another form of volumetric efficency but with a $.50 name. If ya can't dazzle them with brilliance...baffle them with bullshit.
Ok, then would that explain why the car leaves hard off idle, pulls then lays downs a bit then comes back on strong?

[058]

Blown....in a word, No...there is something thats not working properly and I don't believe it has anything to do with static or dynamic C/R What is the static C/R of that 455 and how did you arrive at the 6 to 1 D-C/R?

[058]

Originally posted by Fiat48
Dang there 058! Thanks for the explanantion and education.
I don't know all the terms but still the old bottom line is to treat an engine as an air pump. Cause that's all it is.
Taking my air pump out to the lake today. Seemed it worked so good last time I need a confirmation pass. Bob, sounds like you got it sorted out. What did you find wrong? Have fun and be safe.

[Blown 472]

Originally posted by 058
Blown....in a word, No...there is something thats not working properly and I don't believe it has anything to do with static or dynamic C/R What is the static C/R of that 455 and how did you arrive at the 6 to 1 D-C/R?
I believe it is 9.5 to 1 and they used some online calculator deal that you put the cam profile and heads and stuff into it.
We checked everything on the car, advance curve, fuel psi, etc so it is not tune up related, I think it might be the over zelous porting and the big roller not working right.

[058]

Originally posted by Blown 472
I believe it is 9.5 to 1 and they used some online calculator deal that you put the cam profile and heads and stuff into it.
We checked everything on the car, advance curve, fuel psi, etc so it is not tune up related, I think it might be the over zelous porting and the big roller not working right. What induction is on this 455? My 1st thought would be fuel curve is not right, if this is a carbed delio then I would look at power enrichment circuit, power valve or metering rod selection and/or where it is opening. If it were port/cam being too big then it would be lazy on the bottom end and not make much power until the cam starts to work.

[LakesOnly]

I recently read this big long paper about D/C/R but am unable to locate it. All the crap about cam timing affecting atmospheric c/r, etc. 058 touches on stuff that makes my reading seem inclomplete.
I tried to do a search on the subject and came across some fancy software programs that ask for every minute detail about a build and then spits out your D/C/R. No access, though.
Here is a really simple one that puts you in the ballpark:
Basic Dynamic Compression Ratio Calculator (http://www.rbracing-rsr.com/comprAdvHD.htm)
I remember using a much fancier program to evaluate my engine, and this program above gave me pretty much the same answer.
LO