effect of compression pressure on HP? - Page 2 - PeachParts Mercedes-Benz Forum

pawoSD · #1 06-08-2009, 11:19 AM

Quote:

Originally Posted by t walgamuth

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.

I need to start doing that! Sounds like a blast.....my wife would never let me use the 300E there though, so I'd be doing it W126 diesel style!

PaulC · #2 06-09-2009, 06:38 PM

Quote:

Originally Posted by pawoSD

I need to start doing that! Sounds like a blast.....my wife would never let me use the 300E there though, so I'd be doing it W126 diesel style!

I'm there: http://www.youtube.com/watch?v=4kBwIZEF9L8

pawoSD · #3 06-08-2009, 01:49 AM

I suppose for autocrossing the miata would be better.....

or.....

this!

t walgamuth · #4 06-10-2009, 07:00 AM

Quote:

Originally Posted by pawoSD

I suppose for autocrossing the miata would be better.....

or.....

this!

That would do well on a race track but on the autocross I bet I could whip it with my Miata...especailly with the new lump.

JonL · #5 06-08-2009, 11:47 AM

You must do it with the throttle wide open. If you do it with the throttle closed, you are starting the compression stroke at a lower pressure (and with less mass in the cylinder), so the pressure at the end of compression will be lower.

ramonajim · #6 06-09-2009, 12:29 PM

What year miata are we talking about here? How many miles? Running top up or down (top down is mandatory, of course, but can cost you several MPH at the top end)? Manual or auto tranny (promise not to laugh at you (too terribly much) if it is a slush box)?

A tired early NA (90-93, 1.6L engine) topping out at ~ 105 mph or so wouldn't surprise me at all. Aerodynamics are NOT one of the miata's strong suits (neither is raw horsepower, but that is easily enough changed).

A few things to look at.....

What size wheels & tires are on your car? Stock set up results in an optimistic speedometer reading from the factory (typically around 10%); if I remember the math right, bigger wheels/tires (i.e. larger diameter) results in a lower speedo reading - so if you're running something bigger than the stock 14" wheels, you'll be reading actual or lower speed. Not to mention the effect that extra unsprung weight has on acceleration..... rolling dub style bling does come at a price.

As others have mentioned - re-check compression with the throttle wide open. Any other set up renders your readings meaningless. "Conventional wisdom" says anything more than 10% variance from one cylinder to another means you've got trouble.

Timing - what is yours set at? Is a pretty common "tweak" to bump up the timing for a bit more launching power - trade off is a lower top end (doesn't apply to the NB's (1999 and later US, 1998 later elsewhere)).

With the engine idling, look at the crankshaft pulley (biggest one, low and centered). If it's wobbling, you may have a potentially big issue - crankshaft bolt failure (search miata.net for short nose crank). Mostly a concern on the earliest 1.6L engines, but possible throughout the range. Typically first noticed as a lack of power from a dead stop, but also kills high RPM power.

How old are your plug wires? What color are they (seriously

)? Yadas eat plug wires about every 30k miles. Blue (NGK) is the right answer for color.

What do your plugs look like? Any hints there?

Any engine codes thrown?

When was the fuel filter last changed?

Plugged catalytic converter is a potential suspect.

Any hint of pinging?

Vacuum leaks?

Shift into 5th gear. These little buggers are running at ~ 4k RPM in fifth cruising down the highway at 80 mph or so. What was the tach telling you when you ran out of top end?

These cars ain't drag racers - they'll get outrun by more than a view stock minivans in a straight line. That said, I went from 2001 CLK55 (bought new) to a Miata - and haven't looked back - sure, torque is WONDERFUL, but the sense of actually being engaged in the driving is several orders of magnitude greater in the Miata. Would love to have another speed demon in the fleet someday, but if I had to pick ONE car to drive for the next ten years, it'd be an early Miata.

aklim · #7 06-09-2009, 06:59 PM

Quote:

Originally Posted by t walgamuth

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?

I don't know about the dyno information but the way I see this, there is a rather large difference between highest and lowest and even from spec to highest. I think the engine is on the way out. Also, with the difference, I wonder if one cylinder might be pushing it more than the other and cause further wear.

johnjzjz · #8 06-09-2009, 07:33 PM

what you are doing is a static compression and adding oil is at best inconsistent with a real test - the real test is a leak down - what it does is pressurize the cylinder and it takes a real measurement of the amount of loss each cylinder has like 10 % 11% or even 25 % and so on, and as it is doing it you can by removing the oil cap if the rings are at fault than you can hear the air blowing out, or the tail pipe if its an exhaust valve and so on, the test you are doing takes away a real test, meaning the battery condition at the start and the end will be at a different speed sooo, also the total amount of oil added is between a foot and the glove approach, but from what you have written my guess is the motor is tired and worn and a junk yard unit is the best way to go -- just my take we use a leak down on race cars almost every week end to keep tabs on engine condition - jz

JonL · #9 06-10-2009, 12:16 AM

All the talk here about the engine being worn out is off the mark until a compression test is performed with a wide open throttle. The engine may or may not be worn, but a closed throttle test is not valid.