Pat 69BigBlock

What should the compression readings be on a newly rebuilt engine with
10.25 to 1 compression. Should it read 160lbs? How about 9.75 to 1?

john's '81 mouse

Can't simply say, based on compression ratio only, another variable in the equation is what camshaft you are using and the amount of both lift, but more importantly, overlap/duration and lobe separation it has.

Most important thing is that all cylinders test out close to one another, like no more than 10% difference between all 8.

Larry B.

160 sounds pretty good. You really can't come up with a definate number for different ratio's as there are lots of variables such as cam timming and combustion chamber shape. Intsead you should look for uniformity between cylinders. I guess there are formula 's but they would be complex and really mean nothing. :)

LT1driver

my small block has 11to1 factory chevy and runs 190. :cheers:

ORVette

smokemup.com has a new calculator to help ballpark cranking compression.

Nomad78SA

I have wondered about mine for a while. I am in a frame off so I have not drove the car but the motor runs and I fire it up every now and then. The mods are in my signature but I have 235 in 7 cylinders and 215 in one. I checked this compression after letting the motor warm up, pulling all the plugs and opening the carb throttle. The secondaries were not wide open for they are vacuum. Would that cause my readings to be high? I have 17 hg at idle in my 388ci. Anyone want to guess what my compression will be running at? It was suppose to be around 10-10:5:1 with my components..go figure..It should run like a vette ought to run :smash: The short block is a stroked Speed-O-Motive StreetMaster version designed for High performance street use. Runs on pump gas and does not require a high stall torque converter. Expect over 400ft./lbs. torque from this daily driver engine build.
BORE = 4.060 CRANK =3.750
ROD LENGTH = 5.7 ROD RATIO - 1.52
DECK HT. = 9.0 COMP. RATIO =10.25-1
PEAK TORQUE =437 @ 4,500 PEAK H.P. = 423 @ 5,500
.004-.009 piston in the hole per cylinder
64 CC heads Comp cam 218/454 dual pattern
Neil in Tenn


[Modified by Nomad78SA, 7:33 PM 1/15/2002]

1979toy

78Nomad - Do you have the throttle open when you take these readings? Throttle should be wide open.

Gameshow Bob

I just took my compression readings tonight. . . I didn't let the engine warm up (I dont think i could hook up the compression checker with hot headers right there) so this may have affected the results. . . 1:180 2:181 3:190 4:175 5:179 6:172 7:172 8:181 Cylinder 3 had a bit of oil in it. That probably gave for the higher compression reading. . . but it also seems that the valve stem seals need to be replaced. . .

BAD-L88

I just checked smokemup.com's new calculator and unless bartometric pressure is a huge factor I don't think their calculator is very accurate. My readings before my motor came apart were all around 220. According to smokemup my compression is almost 16:1. That does not make sense.

Nomad78SA

1979toy,
78Nomad - Do you have the throttle open when you take these readings? Throttle should be wide open.

I had the linkage at wot and the primaries were open but not the secondaries for they are vacuum controlled. Do I need to manually open them up also?
Neil in Tenn

Flareside

My L36 10.25:1 shows between 155-165 during cranking depending on the cylinder I test.