Ok guys....1st of all, I'm posting this in general discussion also. 2nd, we all know that with a supercharger or turbo, lower compression pistions are used.
3rd...I'm an experienced engine builder/overhauler...however it has been aircraft engines, so I've been around fuel injection and turbos for 25 years. SOME aircraft use what we call "altitude compensating" turbos. In other words they provide NO boost at sea level or under normal operating perameters. They do provide boost (as necessary) at high altitudes. This makes the engine produce full rated HP when it normally would be considerably less due to the less dense air at high altitudes. These engine use standard, high compression pistons. But the turbo provides less boost than a regular turbocharged engine would. But, the resulting HP is the same. This set up allows the engine to produce high HP without using boost (due to the high compression) and saving the turbo and its related components. And then it provides full rated HP without overstressing the engine when needed. SO..........my question is.......has anyone ever put a turbo on a vette (or any other car) without changing pistons? Using the stock, high compression pistons? And set it up using less boost than other turbo set ups?It seems to me the results would be very similiar....high comp, pistons with low boost vs. low comp.pistons with high boost. I'm assuming the resulting combustion pressures would be about the same, and that is what increasing HP is all about. Intake mods, turbos, superchargers,cams, and heads all increase HP by increasing combustion pressure, either by increasing the compression or by forcing or allowing more fuel/air mixture into the combustion chamber. I'ld appreciate any thoughts here folks..

 

Most people here are running hyperutectic (sp?) stock pistons. LS1 guys either change heads to 6.0L heads to lower compression or keep the boost between 5-8 lbs.

 

Sure, you can get really good reliable power out of a completely stock engine running between 6-8psi of boost (400-500rwhp). That is enough to put a big smile on most peoples faces (for a while at least).

You will most likely need to run forged internals and lower compression once you want to start making insane power levels (like that of a 500+ci motor). There is a point where the relatively small combustion chamber cannot produce any more thermal energy without detonation taking place. This is why you lower compression to make more overall power on a FI motor. You are allowing more room for a/f and the resultant thermal energy on ignition. Even though the thermal efficiency goes down, you still make way more power due to the increased volume and decreased sensitivity to uncontrolled ignition (again, you have to have a compressor and fuel system capable of supplying some really good flow to make this happen). There are tradeoffs to this; the low compression setup is going to be much more sluggish and less fuel efficient when not running some amount of boost.

 

The difference is basically how you are going to use the car. If it's for the street, 99.9% of the time you will be at part throttle and the best situation there is high compression (as high as the fuel will allow) for best efficiency, power, mileage and smog (HC). You will be limited to naturally aspirated power, i.e. 14.7 PSI from mother nature. Most cars are designed like this from the factory, but with a safety factor to protect the manufactures warranty.
If you want more power available for the street most of our cars will tolerate 5-7 PSI boost, and give good efficiency, power, mileage and smog, but now you have lost your safety reserve! Witness some of us going through engines quickly!
If you want sustantial power increases, you put in much, much more air, i.e., 1 to 3 atmospheres, but you are working against the basic design parameters of that particular engine, so to avoid detonation from the high effective pressures you lower the compression ratio to 7.5 to 8.5:1 and you make 1000 to 1400 HP provided you have strengthened your engine appropriately. You will make power proportional to the amount of air you can flow through the engine.
However if you drive on the street 99.9% of the time with this engine you will get 6-8 MPG, gross polluting and leaving a trail of smoke!
So you find a middle ground as most of us are trying to do with our street machines. You can pretty much see where that is by looking at the signatures of the "big guys" on the forum indicating their HP.
In a nutshell, you can only get so much from increasing CR but the increases from turbo charging are many fold that of increasing CR, depending on the sacrifices you are willing to make!
The ricers have it easier cause their bores are smaller and hence the flame front has less distance to compress the unburned charge so they can run proprtionally more boost with a given CR!
Thats why we occasionally get beat by the itty bitty cars :sad: .

 

You are allowing more room for a/f and the resultant thermal energy on ignition. Even though the thermal efficiency goes down, you still make way more power due to the increased volume and decreased sensitivity to uncontrolled ignition

QuickSilver and Metamorphosis make excellent points about lowering C.R. Life is always a compromise isn't it? Another way of expressing their point is that an engine runing 8psi boost on 10:1 C.R. will NOT make as much power as engine runing 8psi boost on a 8:1 C.R. Part throttle OFF boost suffers somewhat but bring on MAX boost and hang on. Part throttle ON MINIMUM boost, ie 2psi, makes up for lowered C.R. Mileage is a tradeoff. Good Luck

:smash: :smash:

 

um, yes it will. the 10:1 motor will make more power. Higher static compression, with the same boost pressure, will result in higher dynamic compresion (i.e. cylinder pressure), which will make more power.

Yes, off-boost drivability and MPG suffers with lower compression. But, it buys you the ability to run MORE boost to make up for the lower compression. At 8psi, sure, the 10:1 motor makes more power. But, give the 8:1 motor 11psi of boost, and it will make more power than the 10:1 engine. But, try 11psi on a 10:1 engine and...POP!

With good, high octane gas (100+), you can run high boost on a high compression motor and have the best of both worlds.

That's why the good ol' DSM engines run 7.8:1 compression from the factory :) Can you say 30psi on a stock bottom end? And, the race engines that folks build are all at least 9:1 these days, and they run lots of boost with C16 at the track.

-Jake


[Modified by JakeL, 12:53 PM 11/5/2003]

 

JakeL-Well, I guess we will have to agree to disagree. Just so we are on common ground 99.9% of us use pump gas around 91-93 octane on the street for openers. Which C.R. engine packs in the most molecules of fuel/air? Therein lies the secret. Extra fuel/air burns LONGER on power stroke. I am not saying lower C.R. is the only way just that overall the optimum way with F.I. More fuel to burn longer on power stroke and not flirting with knock retard. Thanks for your input.

:smash: :smash:

 

I suppose that is true. I'm sure we both know how productive arguments on the internet can be ;), but let me take another shot at it, in a friendly way. I wish I was the one with "TT" after his handle! :(

Look at it this way: By your logic, the increased combustion chamber volume from an engine running 2:1 compression (at our aforementioned 7psi boost) would make more hp than an 8:1 engine. You'd barely be able to pull the skin off of pudding with a motor like that. Sure, you can put about 40psi through it to make up for the lack of static compression with boost, like the real hardcore Cosworth guys do (6:1 engines, I've been told, but over 30psi boost), but It could hardly even generate the pressure to spin the turbos to 7psi in the first place!

Lower compression is simply to help reduce a tendency towards detonation...that's it. You have to make up the difference with boost, or you don't make as much power, because the resulting cylinder pressures are lower. I lived and breathed turbo motors for two years - I'm not talking out of my Jockeys here :D

Reading through your post again, perhaps there is one more misunderstanding:

The answer is, neither: they both pack in the same amount. Regardless of the combustion chamber volume, or the dish on the pistons, two otherwise identical 345cid LS1's will get the same amount of air in.

On the intake stroke (suck), the intake valves (er, valve, I'm used to 4g63's)are open, and air is coming into the cylinder. Somewhere near BDC, the intake valves close. Those events are the same for the two engines. Now we have some X amount of air/fuel in the cylinder, and X is the same for both motors.

By the time the piston is at TDC, when the air that has been intaken is being compressed (squish) (to whatever ratio), the intake valves have been closed for a long time. In the lower compression motor (Engine A), yes, there is more space in the combustion chamber, but the same air/fuel mass is occupying a greater volume than in the higher compression engine, Engine B.

Therefore, the same air/fuel mixture in Engine A is at a lower pressure than in Engine B at, say, 10:1. When the spark touches it off (bang), the mixture at higher pressure generates more horsepower/tq. In other words, Engine B makes a bigger bang. You only get up to (and beyond) Engine B's pressures in Engine A, by adding more boost.

*shrug*

agreeing to disagree, :smash:

-Jake


[Modified by JakeL, 6:29 PM 11/5/2003]

 

Thanks for the input guys!!.. basically what I'm looking for is a adding a few ponies..maybe get up to 450-475 without going "internal" on the engine. I want to maintain daily driveability also. Also...by keeping the system "external"..I could put it back to OEM stock without too much trouble.
Thanks again!!

 

No, the resulting power potential is not the same. We obviously have a static compression threshold to work with here (whether you're using 105 unleaded or 93... assume both motors will use like fuel). Dropping the CR and thus allowing yourself to run more boost without running into detonation will produce more power than high CR/less boost. This is b/c the volume of air getting into and out of the motor will be greater the more boost you can safely run.

As has been correctly mentioned before, there is a trade-off, especially when considering emissions-compliance and fuel efficiency.

It's all about airflow. :)

 

Jake,
I recognize your points as also described in "Maximum Boost". More people should buy and read that book.

Sack
I would agree that for 91-93 octane 8.5-9:1 CR would be optimum with 80% I/C efficiency and running about 1.82-2.00 bar (12-15lbs of boost).

"The two largest influences on compression ratio are boost pressure desired and intercooler efficiency. Fuel octane certainly plays a big part, but we are usually limited to using commercially available pump gasoline"

rule: A turbo engine must never be reduced to a low compression slug.

 

Agreed!

I'd agree with that one too. The convenience of using the 6.0L heads to get down to the mid nines (9.4?) seems like a good way to go, to me.

To pick a nit, since we seem to have gotten technical in this thread, you're looking to maximize air mass, not air volume. 1 liter worth of air at 300* is much less mass than 1 liter of air at 32*. The whole ideal gas law, and all that. (which, I will admit, is probably second only to Bernoulli's laws in Internet Gospel misquotings...)

Same reason that boost pressure can be misleading: I'd much rather have 1psi flowing 50lb/min, than 20psi because I can only flow 10lb/min. That's why you can install a better cam on your engine, increase the VE, and see less boost pressure because the restriction of the intake tract has gone down.

Forced induction rocks.

To answer the original question for the original poster:

Yes, you can get 450-500whp out of your stock LS1. A company called TurboTechnology puts out a kit that can be bought over-the-counter, or the nice folks at Xtreme Motorsports or Detroit Speedworks (and, I'm sure others), will install it if you dont' feel qualified. It's reportedly good for around 450whp and 500rwtq on a stock engine, and around 500/500 with a small cam upgrade and lower compression 6.0L truck heads.

Hope that helps,

-Jake


[Modified by JakeL, 6:41 PM 11/5/2003]

 

Thanks again for all your trouble guys. As I stated in my original post..I am familiar with turbos and F.I. engines. However...in dealing with aircraft engines..I'm pretty much limited in what I can do. (i.e. no mods or experimenting). The feds..FAA ...really frown on changing the OEM configuration of certified aircraft engines. And you think the FAA is bad..just try dealing with aviation insurance companies!!!
Again....Thanks a lot!!
:seeya