I have a little project in the works . I have a european 300e we are going to lower compression ratio and mount a turbo kit on her. My friend in florida did his and runs about in the 12,s at the track. I have someone building my custom stainless exhaust and I plan on dropping the valve body out and modifying for 1st gear. It will probably take a few months since customers cars are first. I run a low 15 in the 1/4 now so I would like to see what this does. I will post pics and updates.

__________________
euro 287
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck

Sounds like fun, I can't wait to see some pics! you may want to consider running a higher rear end ratio. that would probobly knock off a second to begin with. I do have one question.
Why lower the compression when you could just lower the boost pressure? I'm not a turbo expert but it sounds logical.
Adam

__________________
Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320

Because then he won't be getting as much power if he lowers the boost .

euro 287, what kind of turbo will you be using?

To get the max. boost from any forced induction, it's best to lower the compression ratio anywhere between 8.0-9.0:1

If we were to use a turbo & supercharger on a M104 C36 engine (10.5:1) the most boost I could run safely with 91-octane would probably be 3-4 PSI.

If I were to lower the compression ratio to 8.5:1, I could go as high as 10-12 PSI, and even higher with race gasoline (100 or 104 octane).

:-) neil
1988 360TE AMG
1993 500E

Does that then mean that you get more power out of more boost then a higher compression ratio?

You're lowering one number to raise another number. Would they even out at a certain point? (ok, I'm learning here, help me out!)

Thanks.

-m

__________________
Now:
2018 Tesla Model X
1999 S500 Grand Edition 164k
1992 300D 2.5 Turbo 287k
2005 E320 4MATIC wagon
1991 Alfa Romeo 164L 99k (sleeping for a while)

Then: 96 Lincoln TC, 93 Lincoln TC, 87 560 SEL, 87 300 SDL, 80 300D, 89 560 SEC

My understanding is that unless you're running race gas, the combination of boost plus compression ratio, needs to be 14:1 or under (mostly 11:1 for everyday reliability).

A high-compression engine (e.g., 10:1) doesn't have much room for forced induction, hence the need to either lower the compression ratio with different pistons, a thicker headgasket, and/or a combination of timing and increased fuel to eliminate detonation.

Generally you want the engine to have at least a 8:1 or 8.5:1 compression ratio in order to have at least some power when there is no boost.

Someone please correct me if I'm wrong.

:-) neil
1988 360TE AMG
1993 500E

Hmm, ok here's what I'm thinking..... I think I'm wrong, but I'm not sure why.....

If you're at 10:1 and you provide enough boost to get to 11:1 (say 4psi) then you've acheived X horsepower.

If you're at 8:1 and you provide enough bost to get to 11:1 (say 12psi) then you've acheived X horsepower.

I'm sure my psi numbers aren't accurate, but you get my point.

So, my question is why bother getting down the 8:1 when if you're already at 10:1, boost it less, stress the engine less, and end up with the same horsepower?

Maybe there's torque differences in boosting then regular N/A....

Thanks for the info...

-m

__________________
Now:
2018 Tesla Model X
1999 S500 Grand Edition 164k
1992 300D 2.5 Turbo 287k
2005 E320 4MATIC wagon
1991 Alfa Romeo 164L 99k (sleeping for a while)

Then: 96 Lincoln TC, 93 Lincoln TC, 87 560 SEL, 87 300 SDL, 80 300D, 89 560 SEC

You get more power from lowering compression?

Well said Invisik,
My point exactly,,, Whats the difference?

Thanks, Adam. I've heard the same about having lowered compression to be able to boost more quite a bit. But if the results are the same (with similar wear factors), I can't justify the expense to rebuild the bottom end.

But, I do admit, it would be cool to say I was boosting 14psi on my 300E versus saying 4psi.

Maybe we should invite Speedtek to this thread..?!

I think SV was partially referring to the fact that you need a "decent" compression ratio to have reasonable performance before boot kicks in. Going lower then like 8:1 yeilds so little horsepower you'll have bad starts.

But the original question still stands..........

-m

__________________
Now:
2018 Tesla Model X
1999 S500 Grand Edition 164k
1992 300D 2.5 Turbo 287k
2005 E320 4MATIC wagon
1991 Alfa Romeo 164L 99k (sleeping for a while)

Then: 96 Lincoln TC, 93 Lincoln TC, 87 560 SEL, 87 300 SDL, 80 300D, 89 560 SEC

Thanks for the invite.

Figure what is better:

8 psi @ 180 cfm
8 psi @ 300 cfm

Which makes more HP?

Everyone always talks about boost psi but forgets the CFM......
(CUBIC FEET per MINUTE - AIR VOLUME)

I learned early on to read compressor maps. You can go to http://turboneticsinc.com and learn more about this. I was using hybrid turbos back in the early 80's. I built some Turbo Nissan 280ZX's that would run 12 seconds on 8 psi.

Of course I am a HI-BOOST FREAK!
My motto is "Life begins at 18 psi"

About compression ratios. On a 6 cylinder engine i would not worry too much about turbo lag. As long as you get the right turbo the lag is almost un-noticeable due to the torque of having 6 cylinders. On a 4 cylinder engine it is worst. Running a Hi-Comp engine is good but the heat generated is sometimes hard to deal with. On 6 cylinder cars with 4 valves per cylinder I prefer 9.5 to 1 comp ratio. 2v I recommend 8.5 or 9.0. If you plan to run over 15 psi then I recommend lower ratios. Plus we could go on and on about pump gas.....you play you pay!

GOOD LUCK!

Speedtek,
Your Awesome!
Now it makes sense, Better flow = More power! Okay...
Thanks!
Adam

Hey,

Ok, I'm still a little lost.

I can recognize 8psi with 300 cfm is better then 180cfm. More air is getting flowed into the engine at 300 cfm.
Are you saying you get 180cfm at a lower CR and 300 at a higher CR?

I'll check that Turbonetics link....

Thanks guys.

-m

__________________
Now:
2018 Tesla Model X
1999 S500 Grand Edition 164k
1992 300D 2.5 Turbo 287k
2005 E320 4MATIC wagon
1991 Alfa Romeo 164L 99k (sleeping for a while)

Then: 96 Lincoln TC, 93 Lincoln TC, 87 560 SEL, 87 300 SDL, 80 300D, 89 560 SEC

I'll take a crack at a simple explaination...

Power = (Volume of Fuel-Air Mixture Burned) X (Combustion Efficiency)

A higher compression ratio improves combustion efficiency, but only by a relatively small percentage (say 10-20% increase in power).

A higher boost pressure increases the volume of fuel-air that enters the engine... in effect, increasing the engines virtual displacement. If you run a boost of 14.5 psi, this doubles the fuel/air volume processed by the engine at a given speed. Note that atmospheric pressure is 14.5 psi so in this example inlet pressure = 14.5 psi boost + 14.5 psi atmospheric = double or 100% increase in power. Similarly, running 7.25 psi boost would increase power by 50%.

Complicating things a bit is the fact that adding boost also raises the effective compression ratio at the same proportion. So if you consider that most engines even on high octane fuel can only handle about 15:1 CR, then at a boost of 14.5 psi, the mechanical compression ratio of the engine could be no more than 7.5:1 and similarly at 7.25 psi boost the mechnical CR should be no more than 10:1.

So, as you can see the objective with boost is to increase the virtual displacement of the engine without exceeding the limits of compression ratio which would cause detonation.

For the purists out there, please excuse the gross approximations and simplifications in the above explaination. I am only attempting to show the greater influence of boost on power as compaired to compression ratio.

__________________
-Wayne

1994 E320 Coupe (120k)
1990 300SL (BBS,RENNtech adds)
1988 260E Sedan (180k)

I'll give this a crack.....,

The 300cfm is with a lower compression piston.

A engine is a air pump, that's it. The more air you can ram throught the thing, the more power you will make. Forced induction (i.e. turbo, supercharger) is a mechanical device which will ram more air into the cylinder then the piston is at its lowest point inside the engine. Remember the more air ( and fuel, the ratio must be balanced at 14.7:1) the more power.

Now, why would you want to mess with the pistons? If you lower the compression ratio with modified pistons..... lowering the compression ratio, you can force more air + fuel into the same engine = more power. The area inside the cylinder when the piston is at it's lowest point (BDC) can now have more air + fuel crammed into it = more power.

One more important thing to mention, when you compress air you get heat, a turbo will create a ton of heat into the incoming air. When you do this without cooling the air (i.e. intercooler), the engine is more likely to detonate or ping, severe detonation can put a hole threw a pistion, not good. Also, the more heat in the air, the less power you will make.

Heat is the enemy in this little picture. If we simpley cranked the boost up on a high compression engine to begin with, you would have big headaches with the heat ( detonation). With the resonable boost levels of 5-7 psi, a nice balance is being struck in regards to what amount of heat ( again detonation) could damage internal parts and the amount of heat is being put into the cylinder (more heat = robbing power).

I hope I didn't confuse anyone hear??

Wayne! I get it. Thanks for your explaination.

Wow, that's very interesting. So lowering the CR is a must-do on a serious turbocharged vehicle.

So how much and where do these pistons come from?

-m

__________________
Now:
2018 Tesla Model X
1999 S500 Grand Edition 164k
1992 300D 2.5 Turbo 287k
2005 E320 4MATIC wagon
1991 Alfa Romeo 164L 99k (sleeping for a while)

Then: 96 Lincoln TC, 93 Lincoln TC, 87 560 SEL, 87 300 SDL, 80 300D, 89 560 SEC