effect of compression pressure on HP? - PeachParts Mercedes-Benz Forum

t walgamuth · #1 06-07-2009, 11:38 AM

ON my Miata, two weeks ago I ran a compression test. While its supposed to have about 190 psi it had from 125 to 159. A bit of oil raised that to from 150 to 180.

I am wondering if anybody has any dyno information on the loss of power this might translate to?

Emmerich · #2 06-07-2009, 12:24 PM

A bunch.

t walgamuth · #3 06-07-2009, 12:26 PM

Yeah, that's what I am thinking.

OMEGAMAN · #4 06-07-2009, 06:01 PM

I'm wondering if you could get a more accurate look at ring seal by measurung crank case pressure under load. I have no idea where to find that spec for a miata you might have to compare with some other cars.

pawoSD · #5 06-07-2009, 08:17 PM

Why have a miata with a....miata engine.....when you could get a 320SLK like this?

I'm sure the 320 V6 has plenty of power to move that little car around.

tankdriver · #6 06-07-2009, 09:36 PM

Quote:

Originally Posted by pawoSD

Why have a miata with a....miata engine.....when you could get a 320SLK like this?

Cause the Miata handles better, is faster, and is cheaper.

t walgamuth · #7 06-07-2009, 09:40 PM

I bought my Miata for $4k.

What do you suppose that slk sells for?

Besides the way the classes are in scca no Mercedes that I am aware of has a chance to win anything.

The Miata is the ideal instrument for autocrossing. Light, rugged and classed so that any year of Miata is competitive in whatever class it falls in.

Most important, whatever part you can imagine you might want for it is available by mail order from at least three different vendors.

Chas H · #8 06-07-2009, 09:59 PM

I doubt it has that much effect, but it has some. When the engine is making power things are happening a whole faster than when you do a compression test. Usually if the engine starts easily and idles on all cylinders the compression isn't that bad.

JonL · #9 06-07-2009, 11:10 PM

Quote:

Originally Posted by Chas H

I doubt it has that much effect, but it has some. When the engine is making power things are happening a whole faster than when you do a compression test. Usually if the engine starts easily and idles on all cylinders the compression isn't that bad.

^+1.

Cranking speed and inlet restriction have a large effect on compression measurements. So does engine temperature, and I'd think ambient temperature would have some effect as well. Did you do the test on as hot an engine as possible, all plugs removed, throttle held wide open throughout? I've also seen compression gauges with small leaks around the spark plug seal, it can lead to a large error.

t walgamuth · #10 06-08-2009, 04:18 AM

Quote:

Originally Posted by JonL

^+1.

Cranking speed and inlet restriction have a large effect on compression measurements. So does engine temperature, and I'd think ambient temperature would have some effect as well. Did you do the test on as hot an engine as possible, all plugs removed, throttle held wide open throughout? I've also seen compression gauges with small leaks around the spark plug seal, it can lead to a large error.

The engine was warm but not hot. I had all plugs out and cranked ten compressions per hole. On the first hole I removed the compression tester twice and replaced it thinking I was getting a false reading.

I did not hold the throttle open.

This engine has the shims under the valve buckets for adjustment.

The reason I ran the compression test was that I was out in the country one day and opened it up. It struggled to reach a hundred mph in fourth gear. That seemed to indicate a lack of power.

The engine ran and started just fine and held excellent oil pressure, and even on the hottest day never got the least bit warm when autocrossing.

Chas H · #11 06-08-2009, 09:45 AM

Quote:

Originally Posted by t walgamuth

I did not hold the throttle open.

Try again holding the throttle open.

E150GT · #12 06-08-2009, 09:57 AM

I see that putting a teaspoon of oil in the cylinder raised compression. Is this a test or a small cure? Just curious.

JonL · #13 06-07-2009, 11:16 PM

Does the Miata have hydraulic tappets, or can you adjust the valve clearances? Your results suggest the possible need for a valve adjustment. IMO, the numbers aren't low enough to be leaking valves, nor did the oil correct the compression to the extent that would suggest worn rings. Plus which I haven't seen worn rings on a gasoline automobile engine in about a million years unless there was no air filter for a long time. (Marine engines have other problems.) Materials and lubricants are pretty darn good these days. Well, I see you autocross it, so maybe you thrash it good enough to wear it out, but I doubt it. How many miles?

tonkovich · #14 06-07-2009, 11:40 PM

off thread, but.. what is autocrossing. (sorry for my ignorance)

sounds kinda fun.

t walgamuth · #15 06-08-2009, 04:24 AM

Quote:

Originally Posted by tonkovich

off thread, but.. what is autocrossing. (sorry for my ignorance)

sounds kinda fun.

Check out scca website.

Its running a set course through traffic cones against the clock. There are classes for any vehicle except ones that have high centers of gravity. At an event you typically get five or six attempts which will last from forty to seventy seconds per run, with maybe twenty turns involved.

The emphasis is on precision driving and of course trying to get through it as quickly as possible without knocking over a cone. If you knock over a cone its two seconds added. If you miss a gate the run is annulled and you lose that turn.

Its the cheapest form of organized racing there is.

The great thing about it is that it gets the desire to go fast out of your system so driving fast on the street loses all its stimulation since you are getting all you want on the autocross course.

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