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  #1  
Old 03-07-2007, 12:11 AM
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How Do They Do It?

I was reading that the new Lexus 4.6 engine produces well over 300 horsepower and has 11.5:1 compression. What I'd like to know from experts is, how??? That compression is in muscle car, Sunoco racing gas territory. My 560SL has maybe 9.5:1 and requires premium as it is. My 380's compression is so low I'm surprised that the engine doesn't come with an emergency hand crank start in front.

It's amazing that they can do this with the worse gas and tougher pollution controls of today.

Basically, how are all of these newer cars doing it? I have some theories:

1. Computer-optimized heads and combustion chambers with minimized "hot spots"?
2. Super-smart engine controls (with really, really extensive sensor coverage and very large ROM maps)?
3. Magic???

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  #2  
Old 03-07-2007, 12:44 AM
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Points 1 and 2... Mostly point 2.
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  #3  
Old 03-11-2007, 08:48 AM
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Just for comparison, check out specs for the Lexus and other V8 engines:

Lexus:
http://autos.yahoo.com/lexus_ls_460_luxury_sedan-specs/?p=eng

Covette:
http://www.corvettemuseum.com/specs/2005/specs.shtml

Mustang:
http://www.automotive.com/2006/12/ford/mustang/specifications/index.html

The Lexus has 4 valves per cyl.. Both the Ford and Chevy are pushrod engines. The mustang has modest HP, but considerable torque, and it comes on at 3500 rpm, not 4500.

Just food for thought.

jp
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  #4  
Old 03-11-2007, 08:55 AM
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Quote:
Originally Posted by 300EVIL View Post
Points 1 and 2... Mostly point 2.
exactly.

tom w
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Old 03-11-2007, 09:36 AM
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I have an 06 Corvette C6 with an LS2 engine.
400HP/400LBFT torque.
Suggested to run on premium, but will run on regular with no harm to the engine.

ECU incorporates knock sensors, which will adjust the timing accordingly.
Lower octane will give reduced power, but not that noticeable.

Oddly enough the car will gets 26-30 MPG on the highway at a 65-75MPH cruise and 20-23 MPG all around driving mileage.

The importance of a higher octane and quality gas is primarily in the detergent additives that help the fuel injection system perform at optimum.
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Old 03-12-2007, 01:10 PM
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A late closing intake valve will reduce compression ratio as well. Your static 11 to 1 can drop below that ratio with a late intake close while remaining technically an 11 to 1 engine. A trick, if you will.

Don~
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Old 03-12-2007, 02:15 PM
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let's talk about fuel octane ratings. 87 verus 92. the truth is the higher the octane the slower the burn and the cooler the flame front is. so in a high compression engine the higher octane fuel burns slower and cooler to help with predetonation and hot spots. it has alwayy been a myth that higher octane fuel will give better performance....this is not true, lower octane gives better performance but higher combustion temps. so take it as you will, use lower octane fuel unless you have ping(predetonation) issues.
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  #8  
Old 03-12-2007, 07:15 PM
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Heptane ( a 7 carbon chain ) and Octane ( an 8 carbon chain ) are no longer the only ingredients in gasoline.

Used to be 87 Octane gas was: 87% Octane and 13% Heptane to make your 100%. Now there are more complex chains and other additives to the mix.

Funny how a 7 carbon chain like Heptane will ignite with such ease and the 8 carbon Octane is so difficult.
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  #9  
Old 03-31-2007, 11:38 PM
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Quote:
Originally Posted by tvpierce View Post
Just for comparison, check out specs for the Lexus and other V8 engines:
Mustang:
http://www.automotive.com/2006/12/ford/mustang/specifications/index.html
The mustang has modest HP, but considerable torque, and it comes on at 3500 rpm, not 4500.
Those are the specs for the 4.0 v6
4.6 v8 has 300 ponies and 320ft lbs *advertised*
Also its 3-valve SOHC

Last edited by Gen3Benz; 03-31-2007 at 11:45 PM.
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  #10  
Old 03-31-2007, 11:49 PM
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Quote:
Originally Posted by Don M View Post
A late closing intake valve will reduce compression ratio as well. Your static 11 to 1 can drop below that ratio with a late intake close while remaining technically an 11 to 1 engine. A trick, if you will.

Don~
That's true at low RPM. At high RPM with the right intake, you actually get more air into the engine with the late closing valve, increasing the cylinder pressure.
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  #11  
Old 04-08-2007, 09:56 PM
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My Kawasaki ZX14 has 12.0/1 ratio and I run it on 87oct pump gas, when it gets real hot (95+) here I will bump up a grade to 89/90 depending on the station (also for the record, it run 9.50 @ 146 in the 1/4 mile in street trim). My ZX12 that I sold ran 12.5/1 and I ran it on 87 pump gas (Best pass with a slick and bar, 9.22@145). Unless it knock when riding/driving it hard use the lowest oct rating that you can for the best performance out of the engine. If your using a higher oct than the engine needs your wasting money.
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  #12  
Old 04-09-2007, 05:01 PM
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Quote:
Originally Posted by slojoexxx View Post
Unless it knock when riding/driving it hard use the lowest oct rating that you can for the best performance out of the engine. If your using a higher oct than the engine needs your wasting money.
Absolutely agreed. But using a lower octane than your engine needs can waste money a whole lot faster.
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  #13  
Old 04-09-2007, 08:56 PM
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Quote:
Originally Posted by Don M View Post
A late closing intake valve will reduce compression ratio as well. Your static 11 to 1 can drop below that ratio with a late intake close while remaining technically an 11 to 1 engine. A trick, if you will.

Don~

I dunno,

Compression ratio is a mechanical ration and to me has nothing to do with valve timing or lift. The volume of the cylinder with the piston at TDC, divided by the volume of the cylinder with the piston at BDC is compression ratio.

BMEP changes greatly with valve timimg / lift. I believe you are referring to BMEP.
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  #14  
Old 04-10-2007, 12:19 PM
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Quote:
Originally Posted by waybomb View Post
I dunno,

Compression ratio is a mechanical ration and to me has nothing to do with valve timing or lift.

Yes, but you can bleed off some of your mechanical compression via cam timing thus giving you a lower effective compression ratio..making it possible to run a lower quality fuel w/o pre-ignition.

Also, Kawasaki is one of the kings in regards to combustion chamber design.. They have been able to get away w/ very high CR #'s on street bikes for years..in most cases their #'s a good bit higher then the other Big 3.

Jonathan
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  #15  
Old 04-14-2007, 10:00 PM
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Jaguar and Porsche have done the same for years. Check out a late '80s XJS.
Aluminum heads are tolerant of much higher compression levels than cast iron due to better heat dissipation.
Hemispherical, or semi-hemi heads also allow higher compression... something about flame front and burn patterns... beyond my depth.

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