cranking compresion ?
 

got my car back together. had a reciever groove put in the heads to match the oring in the block. they had to weld up the water holes and mill them. they went from 75cc to 70cc. the head gasket i went from an .062 to a .043. the cranking compression has gone up from 90# to 150#. does this seem correct going from 8.25:1 to 8.86:1 ??

compression
 

No, it's not right................something else must have changed.

I have 12:1 compression and my cranking compression is 150psi

Tim C

really depends on cam -- but 5 cc and .020 change on gasket should make more than .6 diff in comp. in the calculator i just used it shows over one point of increase--

The heads usually give you about .005 per CC of cut.

So, they were probably cut around .025. With that cut + the .019 of the gasket, then you are around 10cc smaller compressed volume.......much more than they are telling you increased compression.

Bryan

Hydraulic cam? lifters bleed down? might make your numbers go up.

ok, the numbers i come up with still say only .6 increase in compression. the combo is-- 4" stroke, 4.35 bore, .004 deck hieght, .043 x 4.38 head gasket. 43.8cc dish on pistons, 70cc heads. only change= 4.38 x .062 head gasket and 75c heads. my calculations go from 8.25:1 to 8.86:1. if i am wrong somebody please give me the write formula., thanks brad.

How do you calculate an engines CR by doing a compression test? Ive never heard of this being done.

Did the 90 PSI compression test involve any leaks?

my 11.5:1 cranks 225.

JDMERK, you are correct, you can't compute compression ratio by knowing the cranking compression. the only way is math, or computers and know all of your specs. i was just surprised by the difference in my car. by all my numbers i only rased compression from 8.25:1 to 8.86:1. the cranking compression went from 90# to 150#. the only other change is the lash setting, tightened it up from .030 to .024, this might be what changed things the most. pontiac jack the cylinders that only had 2.5% leakdown before were 90#, so no leaks. cgeis, yes on a flat top piston 10cc total volume change(head and gasket) will change from 11.6 to 12.93, but when you start with a 43.8cc dish piston, 8.52 it only goes to 8.86 with the same 10cc diferrence.

My vote is the valve lash adjustment changed the numbers.

Tom V.

8.86 should make the engine much happier and be worth a few hp too.

Tom V.

Long time no see George, how you been?

OK
 

Tom,
I'm just doing great. Getting ready to put my engine back in my car...............made a few small changes to it.

I've owned 455s (+.030 or +.060) that have cranked

150-180,
180-210,
240-265, and
265-300+.

All with #48 D-Port heads, and all because of the Flat-HYD-Lifter&Cam personality (leakers, cam advance, large-Duration, and rocker ratio).

I think the 240-265PSI ran the fastest (if that means anything) and required high octane fuel (100 Octane). Important to point-out that the 280-300+PSI engine did not ping much and could run street fuels. Still in the "go figure" mode. :ranger: HIS

did some more testing today just to see if there was a diference. at .024lash the cranking compression was 145# at .030 it was 150# i guess with more lash there is less overlap. so it seems the compression ratio alone made it go from 90# to 145# go figure?? goerge hope you get your car back out soon. hoped to take
mine tommorow, but it looks like rain.

Engine
 

Ponchospidel,
Engine is in the car, I still need to redo the headers.....................they won't work on my new heads. Got some pretty DARN impressive Dyno numbers! KRE beat it up pretty good on the Dyno.............CAN'T WAIT!