JE Pistons, to 10.5:1 or not to 10.5:1 at elevation
 

I'm sure that this topic has been beaten to death but here is a new twist. I do not want to twin plug my 3.0 SC motor but replacing 9.3:1 Mahles with 9.5:1 JE piston seems like a waste other than for camshaft clearance. I live at 4500' feet and drive mostly up from there. As anyone who attended the Porsche Parade this year would know we have a pretty good loss of power at elevation. Could this possibly give me a free pass to 10.5:1 on pump gas?

I guess the discussion needs to go at whether or not air density has a direct effect on compression ratio. I know that it has an effect on static compression.
Example: an 800cc 2-stroke snowmobile motor at 4500' feet has about 120 psi compression give or take when new, conversly at 0' feet elevation it has roughly 145 psi. The difference is not so great in a small 4-stroke motor but it is still pretty close. I have tested this on several different small motors.

So, could this mean higher comp ratio is safer on the same octane fuel at elevation? Any motor builders at elevation have any experience with this? I need some input from someone better at this than me.

Also I talked with JE and they said with one more cylinder base gasket I could probably put the comp ratio down to about 10.1:1, calculated? Cam housings off center to much? Can of worms?

I am unable to answer your specific question, but I will say that you need to cc your heads and cc your dome volume to perform an overall static compression calculation. I wouldn't give anything JE said regarding advice on base gaskets or configurations in a 911 the time of day. The only way to really know what you have when you get into the build is to measure everything...JE has no idea what your heads measure. If you're talking tenths of compression, 1-2 cc can swing you significantly one way or the other.

I do not mean to be condescending in any way if you are an experienced builder.

I am an experienced builder of small powersports engines but not of flat six horizontally opposed air cooled engines. An engine is an engine right?;) (That's a joke)

I agree with BG that you have to take JE's advice with a grain of salt. Base gaskets do indeed affect compression ratio but the base gasket is intended to primarliy do two things:

1. Seal the base of the cylinder to the engine case spigot/bore. That's a given.

2. Set your deck height.

Setting compression is really not supposed to be done with base gaskets. You do that with your pistons and heads. For a given engine configuration, lets say a 3.0L SC-based engine for this discussion, the head volume is a given. So the piston sets your compression.

That being said, BG is also right that you really do have to measure your head volume to confirm the compression ratio static calculation. When I measured my head volume I got just over 91cc. Typical SC head is 90cc. My heads are twin plugged and the perimeter chamfered to clear 98mm pistons. Those modifications to the head are what increased the head volume. As a result of that volume increase, my "advertised" JE 10.5 pistons came in around 9.9 if I recall correctly.

My deck height was already at the recommended 1mm minimum. How I got my deck was by having a 0.25mm and a 0.50mm stack of base gaskets to get me where I needed to be. Point being is I was not willing to cheat on the deck to get my lost compression back. That's asking for clearance troubles.

No. Basically, the compression ratio is the volume ratio between max volume and the minimum volume of the combustion chamber. Thin air or thick air, the ratio does not change.

Scott

It can affect volumetric efficiency, however, so altitude can affect detonation (and carb jetting requirements).

I ran my nominal 10.3/1 Mahle 90mm race pistons with pump premium. Ditto with my 2.8 with 92mm J&E 10.5/1s. Ran them at Aspen, ran them at College Station and Brainerd. Big altitude differences, but no detonation. And I used an air density gauge to adjust the main jets. But both are twin plugged. I don't think it would be prudent to take the risk of anything over 9.8/1 on pump gas without twin plugging.

I googled octane requirement and compression ratio and found this Gasoline FAQ (part 3 of 4):
Gasoline FAQ - Part 3 of 4Section - 7. What parameters determine octane requirement?

I have the Kremer 9.8 P/C's in my 911 (which I believe are just the standard Mahle upgrade) and have run for years on pump gas. I recently installed a J&S Safeguard (great device by the way) and have the display built into my clock. Other than the odd red light just off of idle, which I think is a little bit of an artifact of how the advance is set up in the J&S, it never shows any detonation at all.

So....since I am planning on doing the 98 mm 10.3:1 Mahle P/C's in my new car with twin plug (and ITB, EMS and the lot) I am supposing that the extra 0.5 compression ought to be taken care of by the twin plug and backstopped by the knock sensor in the EMS. It seems sensible to me although I hear various threads saying that 10.3 is too high even with twin plug.

I guess I readily admit that my experience in this area is low.....but I guess I struggle to understand why it would be a problem.

D.

It would be interesting to compare the knock retard on a twin plug car when, as a test, you disable one of the ignition coils so you are running single plug.

Thanks John, there is some interesting info in that goggle search article. I copied the part about elevation and here it is:

The effect of increasing altitude may be nonlinear, with one study reporting
a decrease of the octane requirement of 1.4 RON/300m from sea level to 1800m
and 2.5 RON/300m from 1800m to 3600m [27]. Other studies report the octane
number requirement decreased by 1.0 - 1.9 RON/300m without specifying
altitude [38]. Modern engine management systems can accommodate this
adjustment, and in some recent studies, the octane number requirement was
reduced by 0.2 - 0.5 (R+M)/2 per 300m increase in altitude.
The larger reduction on older engines was due to:-
- reduced air density provides lower combustion temperature and pressure.
- fuel is metered according to air volume, consequently as density decreases
the stoichiometry moves to rich, with a lower octane number requirement.
- manifold vacuum controlled spark advance, and reduced manifold vacuum
results in less spark advance

Compression ratio is a fixed number regardless of altitude, but the density of the air being compressed is less at altitude.

Let's look at the simple static cylinder pressures, not including barometric changes, cam overlap, etc at 9.3:1 compression.

P-TDC (sea level) = 14.7 psi * 9.3^1.4 = 333.6 psi
P-TDC (5000 ft) = 12.2 psi * 9.3^1.4 = 276.8 psi

If we look at an equivalent CR that will give us the same pressure

P-TDC (5000 ft) = 12.2 psi * 10.6^1.4 = 332.5 psi

So a 10.6:1 CR at 5000 ft gives us roughly the same cylinder pressure as 9.3:1 CR at sea level

But there are other things to consider. Our engines are air/oil cooled, therefore, we are now trying to cool the heads and cylinders with air of less density, which doesn't cool as well.

The oil manufacturers knew this because the Premium pumps in areas of higher altitude contained less octane than equivalent pumps in lower altitude areas.

Drag racers would sometimes build higher compression 'high altitude' engines for the higher tracks by raising the CR 0.5 to 1 point

Some things to think about.

Zombie bump of an old thread.

I have been looking at this exact effect - looking at all the engine building info from the sunny sea level shores of California where one can enjoy 1 atmosphere of air pressure - where I am at, the corrected air pressure is more around 0.88 atmospheres. So the rule of thumb of keep static CR below 10.0:1 on single plug (assumed at sea level) goes up as elevation goes up (all other things being equal). VW engine builders use much higher compression rations (up to 11.0:1) up here.

The notes about volumetric efficiency and cooling are good (but one of the side benefits of living at altitude is that temperatures are rarely above 85*F). However has anyone got further insights into static CR at higher elevations?

An engine that knocks at sea level may not knock at 20k', so if you build it not to knock at sea level it won't knock at altitude.

An article on bikes from a quick google search.
https://www.cycleworld.com/2015/06/25/technical-update-effect-of-altitude-on-engine-performance-project-156-at-pikes-peak-hill-climb-race

The problem is, are you ever going to drive to a lower elevation? Or sell the car to someone in LA?

Couldn't you stay around 9.8 to 1 CR then advance the timing to make up the difference from the higher elevation?

Yes, No and maybe.

By my calcs, 9.8:1 here is equivalent to 8.6:1 at sea level. Not exactly spirited. I agree that I can play with cam timing and some other factors to alter dynamic compression ratio, but those aren't going to be "adjustable" at sea level the way that timing (using a 123Igntion digital distributor) and car jetting would be.

A point higher at 5,000 ft helps the power a bit (but nowhere near enough to make it all up). But the whole altitude thing is akin to setting the throttle to only open 3/4 of the way.

And correct me if I'm wrong, but doesn't high altitude pump gas have a lower octane rating than sea level fuel?

In nascar, the officials would use a tester called a "Whistler" to measure the compression ratio without teardown and they claimed it was accurate to within a tenth. My experience with it was always in that range, too.

A turbo would fix your driveability issue at altitude

The compression ratio for my 3.2 to 3.4 conversion using Mahle Max Moritz pistons/cylinders was measured at 10.0:1 by the volumetric method, and living between 5000 - 9500 ft, I have had no issues with my single plug setup and 91 octane premium. Premium gas is nearly all 91 octane at higher elevations in Colorado.

Keith, as a data point from someone also living in Calgary, my 3.2 Carrera is a Euro market car with the factory 10.3 compression ratio (which accuracy may be somewhat suspect as I understand might not be quite that high in actuality) and single plugs. I have no audible detonation issue running the 91 octane non-ethanol Shell gas at our elevation. However, when I'm driving out in BC where I might get down closer to sea level, I use Chevron's 94 gas. In either scenario I haven't experienced any problems.