View Full Version : Compression test

04-13-2001, 12:44 PM
Alright guys, this shouldn't be too hard. I rented a compression gauge so I could check my engine again, and decide if it's time to get a new 350. So, I got all the plugs out, engine is warm, throttle wide open, and ignition disconnected. Do I crank the engine for just 2 rotations, or until the pressure doesn't climb any more. The reason I ask is because I have between 185 and 200 psi on all 8, and I know the motor is tired. It is a 305 with a shade over 125,000 miles, that seems worn out and uses a lot of oil. My highschool shop teacher told me to crank till it stopped climbning. It just seems too good to be true. Thanks for the prompt responses guys, I only got the gauge for 12 hours. Appreciate it.


mike reeh
04-13-2001, 08:52 PM
definately crank until it stops climbing.


04-13-2001, 09:03 PM
The compression ration is 9.1 to 1 I believe. Does 185 to 200 psi sound a little high per cylinder? I guess this is a good thing. Now I just have to figure out why I'm using so much oil.

mike reeh
04-13-2001, 09:43 PM
compression test PSI is not an indication of compression ratio, but i was thinking that 200 did sound a bit high.. I think the last time I tested my '77 400sb I got about 165 across all eight.. the most imporant thing is not necessarily the #'s, but that they are all reasonably close to each other, i think 10-15% would be good. maybe carbon buildup in the combustion chamber?? not 100% sure on that one.. my engine uses a fair amount of oil too, not sure if I would call it excessive or normal, though.. I drive it a lot and drive it hard and I might add a quart a week, give or take. piston rings, valve guides/seals and even intake gaskets are a few things that might affect oil consumption.. maybe it just needs a major overhaul? 125,000 miles isnt really that much then again it may have lead a hard life.


04-13-2001, 09:52 PM
Yeah, it's had a fairly hard life, I'd say. Hauled a pretty big boat for most of it, along with fire wood in the fall, and rock and other gardening supplies every summer. And that was all before I turned 16. It's what I learned to drive in, so it served as this sixteen year olds race car and off road toy. I'll go through the gaskets, and also adjust the mixture and timing, to see if that doesn't help the tired feeling she's been having. Thanks for the help.


04-14-2001, 02:33 AM
How much oil are you using. I wondered about my 350 once and a mechanic thold me it wasn't out of spec as long as it wasn't consuming more then a quart every 1000 miles. Seems like a lot to me but that's what he told me.

04-14-2001, 12:03 PM

You want to run the engine over at least four revolutions due to the fact that it is a four stroke engine. This makes sure that you have actually had a compression stroke in there. As for poundage like Mike posted what you are looking for are any abnormally high or low cylinders indicating a very bad or very good cylinder. If they are all just about even and you are not using too much oil you should be ok.
Another thing to remember is that if you rebuild just the upper end. Such as doing the valves you put more strain on an already weaker bottom end. If you do just the lower end the opposite is true. If you freshen it up you should do the whole thing for the best results.

Just my $.02 worth


04-14-2001, 01:12 PM
Thanks guys, appreciate the input.

04-14-2001, 07:26 PM
Chevy 350's were supposed to have around 150?

04-14-2001, 10:48 PM
Wasn't trying to mislead you. I'm sure there is a criteria although I'm not sure what that is. I just meant that if you are in the range you should be then you are looking for any abnormallities.


04-17-2001, 06:41 AM
Another area to look at is the rod bearings.
What is your oil pressure like?
As the bearing wear (from years of pounding)
The volume of oil increases going past the bearing and is
slung up into the cylinder bores. It overwhelms the rings and their ability to control oil. Had this happen once.
hope this helps

04-18-2001, 02:07 AM
Yeah, oil pressure is not what it should be. About 50 psi on cold start, 35 when cruising warm, and about 15 idling warm. Oh, and that's with 20w50. Yeah, I don't know if the little 305 is worth my time to try to figure that mess out.

Jeff's Classics
04-18-2001, 12:21 PM
The thing about compression testing is that the results aren't always what you would intuitively expect. For instance, a low performance engine, such as a 305, has a mild camshaft with very little or no valve overlap. It will show more compression than a high performance 350 with a more aggressive camshaft, because the aggressive cam has more valve overlap and therefore bleeds down more cylinder pressure during the compression test.
As someone else mentioned, it is much more important that all your cylinders be within about 10% or so of each other. Oil consumption could be due to valve guide seals. Does your engine puff smoke at startup after sitting for awhile (overnight)? That's valve guide seals. Of course, the rings could be shot and if you're not showing much oil pressure that is most likely bearing clearances, indicating wear. If I were you I'd save the money from any 305 work you have planned and start preparing a nice 350 to replace it soon.

04-29-2001, 10:29 PM

Just a quick thought. A compression test will indicate if a given cylinder is capable of making compression. The ammount (PSI)isnt so important in the test. IF you find a cylinder that is beyond 10% lower than the rest you should start looking for problems there. However, a compression test doesnt measure the percentage of pressure going by the rings or valves. You may have 180 PSI on the compression tester but you may be losing 20 or more pounds past the rings and or valves.

A leakdown tester is a much more advanced test for the condition of the engine internals. The tester has two gauges, one for line in pressure and the other is for actual cylinder pressure. You bring the piston to TDC on the compression stroke, thread the adapter into the sparkplug hole and fill the cylinder with compressed air. Read the difference on the two gauges and you have a good idea of the blow-by for that cylinder.

Oh, one note, leave the other sparkplugs in place while you test each cylinder. IF the Piston isnt at TDC it will try to spin over!! Kinda surprised me when the wrench on the crank nut came up and whacked me! Good luck,


04-30-2001, 04:38 AM
Good point, Andy!
Don't you just love life's little surprises.