commpression ratio

[getstroked]

on my fathers 383 stroker he has flat top pistons and 76cc heads wut is the compression ratio? what would the ratio be if he went to a 67 or a64cc head?

[dmbrisket 51]

to much for pump gas, pushing to high for track gas... and i know this place is for help, but b4 you go messing around to much more with this moter, do a google, or a www.howstuffworks.com, and learn more about what your worken with, you seam pretty confused on a small block chevy... for example, is this a 350 turned 383 or a 400 turned 383

[getstroked]

it is a 350 turned 383. What will the ratio be though?

[dmbrisket 51]

so its a true stroked moter, long throw, already has high compression, you would most likely blow head gaskets with 64 cc chambers...

[maxwedge]

Originally Posted by getstroked

it is a 350 turned 383. What will the ratio be though?

Google compression ratio formula, first hit was a tech article from Hot Rod using a small block Chevy.

[DCarr511]

Originally Posted by dmbrisket 51

so its a true stroked moter, long throw, already has high compression, you would most likely blow head gaskets with 64 cc chambers...

Why woud that combo cause him to blow head gaskets ?

I'm about to build a similar combo and am very curious.

[dmbrisket 51]

you already have high compression with a stroker, raising it even more is bad, not only will 110 octain be pushing detonation, but the weakest link (head gasket) cannot take prolong useage of that... when building a 383 you go one of two ways, 1 is 350 stroked, with stock 350 heads and simple flat tops w/valve releief you get HIGH compression, smaller heads raises this compression even more, to high of compression and you can blow a head gasket, watched it time n' time again when (no offence to anybody, im trying to teach not preach) the less experenced think any combo works and power has no sacrafice, they wind up in drivability issues because they can't keep gaskets and cams in the car... the other way is take a 400 and lessen the displacement, and make that a 383, this is the 'new' way that the dirt track drivers build them, this is a lower compression way wheres use of smaller heads is ok, but no where needed because either 383 method will make some power.

[maxwedge]

Stroker or not the compression is gaged by displacement of the bore no matter how that is achieved, cyl head volume and pistone dome configuration, deck height all figure in the final ratio. 9.5 would be the max for street driving and fuel availability with steel heads, maybe 10.5 with aluminum, I don't understand the 100 octane pushing detonation, it would prevent detonation as opposed to lower octane. There are head gaskets availble now that will withstand 14:1 compression with a level deck and head surface.

[getstroked]

i think i understand what everyone is saying but my Question still isnt answered to an extent. What is the compression ratio now and what would it be if he switched to 67 or 64 cc heads? Like i said b4 76 cc heads now and flat tops w/ no decking done

[maxwedge]

Like I said google the formula and do your math, input all the known specs in the calculation and you will get the figures.

[getstroked]

sry i didnt see the message on that one

[richtazz]

what dmbrisket was getting at has more to do with piston speed than compression ratio. A long stroke motor's piston travels faster than a short stroke motor at a given rpm. This will contribute to detonation, and is multiplied as compression ratio increases. A 64cc chamber head is out of the question on your motor, as it will push compression over 13:1.

[getstroked]

i might be confused here but how does piston speed affect compression? With the online calculators the motor should be running about 10.5:1. If so how come it runs fine on 87octane here is SD? I have always been told with the 76 cc heads that the ratio is about 9.2:1. Are the calculaors and what i am told be the difference between static and dynamic compression ratio?

[richtazz]

Piston speed doesn't affect compression, but it has a huge affect on detonation. The faster piston speed will cause the air/fuel mixture to be more violently tumbled around in the cylinder as it is drawn in and compressed. This is what leads to more prevalent detonation in a long stroke engine at higher RPM. 2 engines with identical compression ratios and displacement (1 long stroke/smaller bore, one short stroke/larger bore) will perform much differently, and their torque/hp peaks will be at much different RPM. A long stroke engine will have a lower rpm torque peak vs a short stroke engine. However, since the reciprocating assembly is more compact, a short stroke engine is capable of higher RPM, all other factors being equal.

[jveik]

as for static versus dynamic... dont quote me on this... this is a guess maybe one of you can verify this... i would hazard to guess that static compression is the actual volume difference between tdc and bdc and then dynamic compression takes into account other variables such as heat and stuff like that and is much higher than static? is that anywhere close lol?