I have a LS3 engine in my 2010 Grand Sport Corvette, my current setup is Ti trim supercharger, Texas speed blower cam, long tube headers, meth and I plan on adding nitrous to the setup.
I'm hopping to get a 800rwhp car. I recently ported my heads and I'm concerned about maybe I've taken too much off. I took the stock gasket on the exhaust port
scribed the opening to the back of my heads and ported them out to the same size as the gasket. My question is what are the repercussions for porting it the same size as the gasket port.
I thought that porting them to that size would help me get the exhaust out of the car quicker thus helping to make it run cooler. I didn't think about backpressure. or port velocity. are these port sizes
going to kill my performance or daily driving.

 

"Didnt think" statement is a red flag. I dont see why you'd cut into a set of heads without at least a rudimentary understanding. At this point I'd be more concerned with thinning out the runners too much and pushing water into the motor than performance implications

 

Oh god another troll

 

In for answers.

FWIW I too may have done what the OP did, blue print the head port and exh/ header manifold port to match the gasket.
eg: That good ole' hotrod procedure.
Perhaps the only way to know is a dyno w/ air fuel report/ graphs?

 

Template of header manifold shapes to the current heads, what does it look like.
Are the header exhaust ports smaller than the header ports you over sized via the gasket, and now just blowing exhaust out of the head into a flats on the header flange now?

Hence most of the time, the exhaust gasket is never shaped correctly for the head flange ports to start with, so you need to make a template of the header flange ports to start with, then you can transfer that over to the heads for your scribe line, and then leave the header ports a touch smaller that your scribe line markings for any slight off set when the headers are mounted to the head (so your not flowing exhaust into the the header flange flat .

Now at this point, before you use the exhaust gasket, make sure to trim it slightly larger than the header flange ports for slight off fit set as well.
Hence head exhaust ports slightly smaller than header flange ports so the heads exhaust ports will flow cleanly to the header ports.

Note, if the case was the above gasket was the exhaust port size template, then grinding is need on the header flange to match them with the head exhaust ports instead.

So again, template of the the header flange or head exhaust ports to start with for both sides, then rework the template back and forth with trimming to get your ports sized for both the heads and header flange, then go to town on both; and leaving the head exhaust ports slightly smaller all the way around, so when the headers are mounted and do have a slight shift, the exhaust ports are not flowing into the wall of the header flange.

As for 860'ish crank hp on a LS3, lets just say that you better have the bottom end build for that much HP to start with, and the piston rings gap'd correctly for the extra NOS heat as well. Myself, would say screw it to the Nos, and just rework the motor to rev 8k plus to pick up the HP that way instead.

Hence LS built to high rev, made 867 horsepower peaking at 8,640 rpm, and 563 lb-ft of torque at 6,540 rpm without a blower.
Simply, you can make all the HP you want, but it being in a small rev range makes the power harder to use instead.

 

Here the head and gasket i mentioned. I also placed it on my long tube headers and its a perfect match
Here is a pic of the heads they are a perfect match for the header and heads

 

Compared to gasket, you over sized the ports just a touch.
Do you have any room on the header flange port to slightly enlarge them now?

Hence since you know that your going to have a slight step between the head ports and header flange ports when the two are bolted together with any off set, you want the header flange ports slightly larger than the head ports; so you don't have a step from the off set in the other direction isntead.

 

How far into the runners did you go? I assume you're running LT's? What size primaries?

 

Yes, I'm running LT headers and they are 1 7/8 on the primary.

 

Whoever told you to do that stop listening to them; You may not have hurt anything if thats all you did provided you have no exhaust leaks.
Most that give DIY advice shouldnt be. (Not discouraging you just sayin)...

 

Since you have high HP goals, those heads need to be CNC'd anyways. I'd pull them and send them of to WCCH, AI, TSP, etc. TSP will CNC your heads for $750, and I bet they can tell you if you've screwed something up or not. Probably can do flow bench testing...

 

yes there maybe some wiggle room. Not sure if I need to take anymore off as the primaries, header gasket holes and the cylinder port are almost a perfect match. You probably cant see it I the photo but the exhaust port is just a hair smaller than the gasket