What would make more power on a 421 stroker, the 2.5 5.3 heads with a ported fast intake, or ls3 heads and intake unported?

 

I'd say LS3 heads. They're better sized for over 400ci. The smaller ports of the cathedral heads would choke it out early, I think.

 

Stock LS3 heads vs ported/polished 5.3's?

 

stock ls3 heads... the p/p flow 310 at .600. stock ls3 will flow 310 at .500

 

I would go with the LS3's on that big of a bore. Plus, you could always pull them latter to be ported for even better flow.

 

LS3. The stock intake for the LS3 is as good as the FAST LS3 version and the LS3 doesn't gain a ton by porting (maybe 20rwhp). So even stock/stock you're looking at well over 500rwhp with the right cam.

 

Alot of people on the Corvette fourms say the LS3 intake isnt even worth porting. There was an article where one didnt gain any power. Seems like everything on the LS3 is beast out of the factory lol

 

5.3 chambers would have to be opened up to match the larger bore.

 

I'm gonna guess that the ported 5.3's with a ported fast will perform a tad better than stock LS3's with unported intake.

 

I would disagree with you on that one.

 

You could be right. I'm just going off what I've heard on this one. Guess it would depend on how much those 5.3 heads were worked over. Work the LS3 heads and obviously there is no comparison though

 

Yeah, I agree. To me it's a no brainer. You can put on the already ported 5.3's which leaves you really no more room for more, or opt for the LS3's, which are going to be right there with the ported 5.3's, but you have room to grow my having them ported down the road. Plus, you don't have to pay a kings ransom for a FAST intake/TB setup because the stock LS3 intake is already pretty killer.

 

Agreed^^

 

Yeah I was thinking the LS3's might be better,I just already have the ported fast and the prc 5.3 heads. I guess my follow up question would be how much better would the LS3's be?

 

You could sell both of them rather easily on the classified section on here and have a lot of money leftover when you buy the LS3 heads and LS3 intake.

In regards to how much better the LS3 setup will be, my guess would be 5 to 10 maybe, but the main selling point should be for future power. Like I said, CNC porting the LS3 heads would be worth a lot more in the future.

 

Depending on your future goals I might just consider using the 5.3 setup since you already have it. With the money saved you could add some nitrous or do some suspension or appearance mods, or whatever

 

I just went from Stage 2.5 5.3 heads to LS3 heads. In between I went 9 inch rear and 3.5 inch driveshaft (+2lb rotational weight).

Pre 9 inch rear: 372ft/lb 419 HP

9 inch rear (-torque/hp) + LS3 top end: 405/430.

If you consider the 9inch to eat up 3-5% more power, the gains are a lot more than what the numbers indicate.

 

LS3 heads - $600
Porting - $1000
LS3 intake with rails/injectors $400

Sell your old stuff for $1800 easily (more if it includes rails and injectors) and you will have more power for next to nothing.

 

Same dyno before and after? No other changes in compression or anything at all?

 

Exactly. Even if you don't get the LS3 heads CNC ported, you're still going to be better off and have about $800 in your pocket for future mods.