Going to an overbore increases your compression ratio by increasing your displacement at bdc. Your chamber remains the same size at tdc. Bigger cylinder being compressed into the same size chamber= higher ratio
:Word: ... with the +.030" bores you raise compression.
C o m p r e s s i o n -is- c o m p r e ssing the air & fuel mixture from the bottom of the cylinder stroke up to the top of the stroke and cramming that increased cylinder volume (.628 liter vs .618 liters) into the same combustion chamber volume (58cc).
5spd ... Total combination plays a roll as well ...
Static compression is only one factor .. cam timing and port volumes are a big factor in whether or not you fill the cylinders well (raising compression).
I run 93oc in my 10.5 331 with relieved flat tops ...
I went with 185's and the RPMII not for top end, but to run larger ports/valves and decrease the cylinder filling and chances of pre-det on the static 10.5 ratio. I could probably even get away with 11 if I wanted too .. but I played it safe and it will scorch the meat all I want ... so who cares.
ok so that whole concept is making some sense now but w/ a .030 over bore exactly how much more compression will that give u? u said urself its about .5 more ci a cylinder.
ok so that whole concept is making some sense now but w/ a .030 over bore exactly how much more compression will that give u? u said urself its about .5 more ci a cylinder.
About .1 lol. on a 302 to a 306 everything else being equal crapola
Just trying to get the facts straight and clear things up. Not the best way to gain compression on a 302 for sure and really not a concern when selecting pistons for the new overbore as the pistons/heads themselves will be the determining factor. Just trying to clarify what static (mechanical)compression ratio is and how engine size plays into it.
Problem is that it changes with rpm. Thats the actual compression/pressure that the air fuel is subjected to before ignition. Thats what you would see on a pressure gauge when looking at cranking pressure ......... valve event timing, duration , lsa all has an effect but that effect changes with engine rpm, intake and exhaust capabilities and ring seal. You would read different cylinder pressures at different rpms.
This is where the popular misconception started about too little back pressure in an exhaust system hurting performance. It was true in alot of oem applications, the exhaust events were desinged with the backpressure in mind. They had no choice as they needed to meet strict guidelines for emmisions AND economy in a short time. Losing the restrictive exhaust (early cat designs) killed cylinder pressure and performance and increased emmisions. Nothing wrong with this setup as long as you remember the goals were economy and emmisions and not overall power output.
I guess I know just enough to understand why one of the respected cam grinders can actually get you more with less.
Static compression ratio is simply the volume of the area above the piston when it's at bdc divided by the volume above the piston when it's at tdc. Dynamic compression attempts to take into account the impact on static CR of camshaft overlap -- that period of time at the end of the exhaust stroke and the beginning of the intake stroke when both the intake and exhaust valves are open simultaneously. Their both being open and the timing of that relative to piston position impacts cylinder pressures -- especially at lower rpm.
General rule of thumb is that you can usually get by with more static CR without having timing/detonation limitations if you run more overlap. But as mentioned above -- it's hard to generalize about any of this.
That's why I mentioned earlier having Ed tell you the impacts of running his cam with higher static CR. He can tell you what the results will be.
I havent had any compression related problems is what I meant. I had a starter issue twice, but once was a used starter, the second time was due to some wiring.
mike.