930s: Boost vs. Compression Ratio
 

Scenario:

1) 8.0:1 compression + .8 bar of boost

2) 7.5:1 compression + .8 *or* .9 bar boost

Merits of either? Fair comparison?

Any and all opinions - especially informed experiences wanted -
THX

David posted this on Rennlist:

http://forums.rennlist.com/rennforum...d=123920&stc=1

Great stuff -

is this a efi motor? if you have accurate control over fueling and timing you can get away with allot

Nope, not yet. Right now were on damage control rebuilding a motor that isn't that old.

CIS, with the Imagine Auto fuel head ($500)

Craig,
Checkout the post titled "PSI to Bar conversion"
Looks like 7.5:1 with .8 bar boost gets you 13.5:1 compression.
8.0 at .9 bar gets you 15.2:1 compression.
I start getting nervous over 12:1.

Off boost with the higher compression will yield a more responsive motor....if you can control fuel and timing at WOT you will be good....I guess I am suggesting EFI

Yasin

No arguments intended, Doug, but that formula doesn't play out to completion in the real world. There is measured compression and realized compression. There are several guys running substantially beyond this point without error.

Craig, we all love Pelican. BUT Rennlist is THE source for this type of question. Geoff on there is a wealth of info as are many others. You also get direct answers from some of the better known 930 tuners.

Now, my opinion and it just happens to be the route I've taken is to go with 8:1 compression and initially less boost. After converting to digital ignition and EFI you'll be able to turn the boost back up without going back in the engine. You then have both hp for the track and responsiveness for the street and off the corners.

It really depends on just so much. Timing curves, Boost threshold, Camshaft (big here), Weight, Gearing, Driving Style, Octane, Use, IC (also critical for decision), Fuel Delivery, Single or Dual Plugs, Piston Design, Bore Size, and even method of Turbo Control. I know you know all of this, but for others who might come across this thread it all needs to be considered.

Luke,
No question you can go pretty high with boost and the subsequent compression real or just a mathmatical number.
Of course how long such a motor stays together can be problematical.
These aren't my numbers. I learned them from a couple guy's you may have heard of. They are referred to in the post I mentioned.
For those who do not wish to look at the post, the names are Bruce Anderson and Henry Schmidt.
Again, no question there are others boosting beyond numbers some of us are comfortable with.
They do however, seem to spend more time rebuilding. Now that's OK. How else do we learn?

Don't forget Boost brings along more air and fuel with it, so it is always going to make more than just mechanically squeezing the mixture a bit more.

You are assuming VE of 100%, which is not the case. Especially not at high revs.

Modern cars run 1.2 bar of boost on 9.5:1 C/R, in factory tune. Craig has twin plugs so I believe his problem is not directly boost/compression related. I believe his motor should withstand 0.8 bar on both 7.5:1 and 8:1 C/R if fueling is working properly.

We run 2 bar on 7.5:1 C/R It's Fun engine on VP and can run 1.2 bar all day long on street gas w/o hint of detonation.

Something is wrong with his engine and it isn't C/R vs. boost. Maybe a mix of flat spots in CIS and stuck distributor advance.

There is nothing magical EFI will do except keep good grip on fueling and ignition. Maybe it's all that is needed in this case.

Well put Goran & Luke.

Luke said:
"Craig, we all love Pelican. BUT Rennlist is THE source for this type of question. Geoff on there is a wealth of info as are many others. You also get direct answers from some of the better known 930 tuners."

I appreciate the knowledgeable answers.
I also frequently post on Rennlist ;)

The original question was

8:1 or 7.5:1 with more or less boost.

To start out with more compression and less boost, you will have more driveability and better low end torque. Heck the new 997 turbo is what, 9.4:1 compression.

To start out with less compression and add LOTS of boost will give you much more apperant turbo lag becuase the car will have less Hp off boost. Most of the factory race cars (at least the early ones) start out with enough compression to make the car run on its own (6.5:1) and then boost the heck out of it.

If you are only talking about the difference between 7.5 and 8:1, then I think you will notice only a marginal difference between the two. I would go with the 8:1 unless you are talking a track car that will never get below 4500 rpm, but this does not sound like the case.

Ed
76 2.15L turbo GT4s.
79 930 twin turbo street car.

The 930 doesn't run off boost - 3,200 is the magic point on this car......above that, is bliss. Ex-962 mechanic tells me that I won't notice a difference between 7.5 and 8.0.

He did volunteer a preference to the first equation in this example:

7.5 with .9 bar
vs
8.0 with .8 bar

Goran,
Thanks for your input. My first reply was simply to present the math for calculating compression ratio with boost.
I have never experimented and ran the boosted motors you folks are talking about.
It seems that there has been much discussion about the new Porsche models running over 11.5:1 compression, and the quality of fuel, (low octane) available in the US.
From what I have read on this site, the knock sensors and ECU usually retard the spark and probably other things like richer fuel mixture, which of course reduces the performance compared to what is available with European fuel.
So to see this discussion and your experience with these highly boosted motors is indeed educational.
Since I live in New Mexico at altitudes generally over 5,000 feet, we are aware of the benefits of turbo charged motors vs normally aspirated.
The rules we live by at higher altitudes simply aren't faced by most of the enthusiast population.
And yes we see even more turbo lag than someone close to sea level.
You stated:
"We run 2 bar on 7.5:1 C/R It's Fun engine on VP and can run 1.2 bar all day long on street gas w/o hint of detonation"

What is VP? What is the octane rating of the fuel you are referring to?
And for the rest of us, what is the altitude where you run in Sweden?
Thanks,

Hello Doug!

VP fuel is nothing but race-fuel. As I mentioned in this thread, boost is only half of story. Just mashing the air/fuel mixture into the cylinder won't make it detonate. It's the mixture of combustion chamber geometry, AFR's, fuel octane rating, air temperature and last but not least, ignition timing that dictates the onset of knock.

You could probably run 2 bar of boost on 930 engine w/o any detonation if you retarded your ignition in absurdum. Engine will run poorly, produce less power and your headers will glow, but hey...you'll be boosting 2 bar for bragging rights of it.

Remember, it's not the boost we are interested in but highest possible swep area under power curve. Actually, you would want to run as little boost as possible for target power.

Each installation has it's sweet spot, and many people overshoot that spot by far in their blind quest for more boost (which they believe is linearly equal to more power).

To put it in perspective:

You can make 300hp on particular engine by using 1 bar of boost and, say 23 degrees on ignition. Your engine will be slightly laggy as it takes time to spin the turbo. It will also be a bit less efficient as turbocharger will heat up the air more at that particular mass flow.

You can also make same amount of power by running 0.7 bar and more aggressive ignition curve, octane rate permiting. Engine will be more responssive, have more torque low down and even more efficient as turbo won't need to work in it's less efficient area (depending on which kind of turbo).

I hope you understand what I'm trying to say. Turbocharged engine is a hairball of different parts that have to be bungled together and made to work as good as possible.

A hi-comped engine will have to use less boost or more ignition retard. A low-comped engine will allow more boost (and thus more power w/o detonating) but it will be sluggish off-boost.

Using programmable EFI and dyno, you can optimize the engine to use most of situation available. You can advance the ignition untill torque starts dropping off or engine starts knocking. You can fiddle with AFR for optimal power (usually around 12.8:1). You can adjust the boost until you hit the sweet spot between boost/CR/ignition. You can use twin plugs to lighten the fire from two sides and thus combust the mixture in shorter time, lessening the need for more ignition advance as you don't want things burning in cylinder when piston is on it's way up.

OEM 930 engines cannot do this and are configured ex factory to stay "on the safe side".


New engines have water-cooled pentroof 4-valve heads, fully programmable EFI and knock sensors. Aircooled 911 engines have big combustion chambers, unfavourable combustion chambers, elevated head temperatures due to aircooling etc.

There are less margins there. That's why it's hard to compare ignition timing and C/R with modern engine.

When using MAP based EFI, you can program the engine for altitude correction. SAAB's for instance do altitude correction up to certain altitude, then cap the boost to avoid overspeeding the turbo.

To round it off:

The real challenge is to optimize the engine so it runs as efficient and well as possible from the material available. On older OEM 911's, it's very hard as there are no fine-tuning means available so one must default on the safe side. Having EFI makes it easier to achieve optimal setting.