I'm in the process of finally finishing up a twin turbo blow thru building on my car. Its a 5.3 with 317 heads. I purchased the motor from a person I thought was a friend but I wont go into details on that. I have a question on compression. I've been planning on running the car on methanol and I read that you want the compression to be around 10.5:1 or possible a little lower. Over the weekend I checked the compression on each cylinder and they all showed to be 110 psi. This leads me to believe I have a really low compression ratio motor. Without jerking the heads off and doing a calculation is there a good rule of thumb compares cylinder pressure to engine ratio? I'm not sure if the pistons are flat top or dished and I have no idea what the cam specs are at this time.

Thanks
Chris

 

Flat top 5.3 with 317 heads is about 9:1 compression. Dish piston 5.3 with 317 heads is about 8.5:1 compression. This is assuming the heads aren't milled, but even if they are it still would be less compression than you're looking for.

 

110 is extremely low. That's down at like 8:1 static or so and won't run well. If I remember right the last 9:1 motor I did a compression check on was 140 psi cranking psi.

I'd be curious if the cam is off timing or it may just be that the rings are dry.

Put a little oil in each cylinder and turn the motor over several times and do another compression test. Also try another compression tester, I've had a compression tester bite me before.

A 5.3 with stock bore, flat tops, stock head gaskets and stock cc 317 heads gives 9.3:1 compression. I'd go for higher

With dish pistons it will be around 8.7

Slap some 706/862 truck heads on it or some 243/799's

The cam profile and cam timing plays a part in static/cranking compression. But if you're close to 10:1 you should have around 170 or so, hard to nail that down since so many factors play into that.

 

 

You can't accurately tell what the static compression ratio is by doing a compression test.

There are other factors at play here, Valve events alone can cause low compression test readings and so will ring gap.

My 12:1 compression ratio engine would only pump to 140-150psi and my stock 9.5:1 street engine would pump to 170-180psi.
For the most part the numbers are meaningless in most cases, We always broke in an engine and then ran a compression and leak down test to get a baseline so as the engine aged we could compare numbers cylinder to cylinder to keep track of it's condition.

 

Another reason compression can read low is the lifters can stay pumped up and not relieve fast enough just at cranking speeds. You can pull the rockers off one cylinder and try again if you suspect this is happening.

 

If that was true, the engine would never start. Lifters pump up at high speeds, not very low. They have all the time in the world to bleed down at cranking speed.

 

I will never do another 5.3 with 317 heads again lol.... dog down low . There is no need to drop compression that much tbh

 

This is the correct answer. It's an algebraic equation, so you could calculate it out if you wanted to. You'll need to know the intake valve closing point, altitude, and a couple of other data points. Do a search on "Dynamic Compression Ratio" to find the formula. Since it's an unknown engine, you'll want to measure the cam anyway, just to see what you've got.

I would just pull the heads. It's not much work. And the MLS gaskets can be re-used 2-3 times; so it won't cost you anything but time.

 

Sticking lifters holding the valve open would just equal pretty much zero compression, and it wouldn't be 110 psi on every single cylinder. The chances of all the lifters holding all the valves just lightly enough against the seat to hold exactly 110 psi would be like winning the power ball. Or higher odds.

 

Well, you never know; if you gave 'em a "happy ending," that might raise the numbers a little...

Seriously though, just so we're all on the same page, when you're doing your compression test, how many needle pulses are you going for? Because if you're just cranking it over one revolution, it will give you a really low number.
I generally go five pulses.

Barring some major mechanical problem, if you still have compression that low, I'd suspect that the cam might be installed retarded. (out of sync with the crank, not stupidly)