I'm going to be building an LS3. My plan is to bore it out .010"and use a 4.1" stroke crank for 428 cubic inches. What is the maximum safe compression ratio? I'm an old cast iron guy where we tried to keep it under 9.5:1, 10.5 with aluminum heads. These were old, non computerized motors. I know that the aluminum head with an aluminum Block and computerized control, you can go much higher. How high is high. I've searched and not found what I'm looking for. I'm trying to figure out what dish I need on the pistons.

Thanks in advance.

 

What octane/ type of fuel?

Intended use/ vehicle?

Cam specs?

All of that stuff can make a drastic difference on what is "safe."

 

A STOCK LS3 is 10.7:1
But like mentioned above there are lots of things that will determine what you can do as for MAX compression.

 

We have 93 octane available. I'd like to run on 87, but i don't think that will be possible. I will not run E85.

My goals are to have a very fast, mild mannered, sleeper. The car will be an 81 to 87 Pontiac Grand Prix. The car should be able to handle daily driving, although that probably won't happen. It will probably be a weekends in the summer fun car, but it should be capable of daily driving. It must be capable of long drives. I'm in Maryland and will be spending a month or two in Florida in the winter, next winter being the first. I want to be able to drive the car down there, should I choose to, without worrying if it will make it. I would like 500 hp. As you can see, I'm not trying to get every last hp that I can out of it. I don't want a lopey cam anymore. In my younger days, I did. Stock is 430 hp. With the added cubes from the stroker kit and a mild cam upgrade, I'm thinking/hoping I can hit my goals while keeping the heads, intake, throttle body, etc. stock. I plan on using a 4L80e trans and a Terminator X Max controller to control the motor and trans. I will be using Wheel Vintiques Pontiac Rallye II's, 17x9 with 275x40x17's in the back and 16x8 with 225x50x16's up front. They will be very close to the stock tire diameter so the car will have a stock appearing profile.

The Sloppy Mechanics Stage 2 cam was recommended to hit my goals. The specs on the cam are, 228/230, .585/.585, 112 LSA.

Everything is negotiable at this point, but this is where I am in the planning stage. I was asking about the max static compression ratio to help determine the pistons to use. My math on the stroker kit tells me that with the pistons in the kit that I'm looking at (1.050 compression height) and a 6.125" long rod, the pistons will be .015" in the hole. With that, it gives me an 11.3 static compression ratio. As an old cast iron, carburetor guy (yes, I'm old), that scares the hell out of me. But with aluminum, fuel injection and computer controlled motors, I've read where some say 11.4:1 is OK.

 

I plugged your data above into the Wallace Racing dynamic compression ratio calculator at "http://mobile.wallaceracing.com/dynamic-cr.php".

I assumed your cam has 2 degrees of advance ground in, so the ICL is at 110. I assumed your cam has very aggressive ramps, so the delta between .006" lift and .0050" lift is only 25 degrees. This gives an intake valve closing point (at .006") of 69 ABDC, which allows a bit less static compression than softer ramps would. The 25 degree assumption is conservative in that regard.

I *assumed* you'd be at sea level ... you can get away with a bit more static compression at altitude, but you can't take road trips to the coast.

Using that as input, your dynamic compression ratio comes out to 8.76:1. Most people say 8.7:1 is about as much as you want to run on a street motor using 92 octane fuel. My engine, with 11.8:1 static compression, has a dynamic compression ratio of 8.85:1. I run it on 92 octane on the street, and when I go to a track day I add a bit of Octane booster. My car is very light, so doesn't load the engine as much. I'm also not towing with it.

I'd say you'd probably be fine with that static compression ratio and that camshaft. If it was me, I'd run it.

 

If you close the intake valve just a **touch** later (2 degrees), your dynamic compression goes to 8.61:1, which is a bit more margin. The engine would rev out a bit better with the later IVC. For an engine that large, that's by no means a late IVC.

That cam would be 230/230 112.5+1.5 LSA ... Cam Motion could grind you that. This has 5 degrees of overlap, like the Sloppy Stage 2.

Given that your car has an auto trans, it might run nicer on the street with a bit less overlap, especially if you don't plan to upgrade the converter.

As an example, a zero overlap cam that would give 8.61:1 DCR is a 228/228 114+2. I'm pretty sure that's a vanilla shelf cam from TSP.

 

Thanks, that's great information.