Assman

iTrader: (2)

So I ordered a 408 from Thompson Motorsports last week and am having a tough time deciding on a top end. I was planning on reusing my PRC 5.3 heads but that would put me at 12.36:1 compression which is to high imo. Now I'm trying to pick heads or just go for a ls3 top end. I have a ls6 intake that I'll be forced to use for now but will upgrade to a fast setup next winter. I had my eye on the PRC as cast 225cc head.

On the other hand I could get a ls3 top end swap and possibly save some money. I know the ls3 intake is damn near as good as a fast 90 so that's a huge plus, but this swap is still intimidating cause I don't know all the little things it'd take to make it work.

What are the pros and cons to these options? Stick with cathedral heads or go square port? Decisions decisions...

cr250r

iTrader: (4)

just do the ls3 its more power unless you invest in some serious cathedral port heads like afr ported ls3 heads flow about 360 cfm and cost as much as the afrs. + the huge flow of the ls3 will require a less radical cam to make the same hp

HISS

iTrader: (52)

Go square port. It will support the 408s air flow needs, and by selling the cathedral stuff you'll come out even or ahead on money.

whatsa347

iTrader: (6)

I don't have any valuable input on anything, but you have the best name on this site.

Tuskyz28

Is this a street car or a race car ? Automatic or stick car ?

HCI2000SS

iTrader: (2)

Are you positive about that compression? I'm running a 370 from Thompson Motorsports and I'm at 11.5:1 with my PRC 5.3 heads

Assman

iTrader: (2)

Ya I used several calculators to verify, I looked up the 370 after I realized how high it was and wished I would've gone that route.

This is a t-56 street car first and foremost that sees the track about twice a month.

Tuskyz28

Definitely go cathedral head!! This sounds more of a real street car to me. No need in putting on a LS3 head that's only good in high rpm applications. On the street you want low end grunt..... I say get with Tony Mamo on the 408 ci for a set of his new heads he has developed. Also let him spec the cam to take full advantage of the heads.

dr_whigham

iTrader: (49)

I'd absolutely go with an LS3 based head. That motor needs to breathe, and you're not looking to break the bank I'd imagine.

LS3 heads are great on a stock 376 LS3. What makes anyone think they only flow in the upper RPM. It's a 408. I think he's got all the torque he needs.

Sell what you have and recoup some money, and pick up some LS3 heads and intake. Even get them ported if you'd like. Rehr Morrison (Darin Morgan) and Frankenstein both have good programs for those heads.

NAVYBLUE210

^^^AGREE with TUSKY^^^
MMS 235s and 11.5:1 Comp with medium HR 235*/239* - 239*/243*
.620"/.600" LSA 114*-115* +3*-4* can make 540+RWHP with all the
Fix- ins (attention to ALL the details) call Tony,it ain't cheap but
Do it once, Do it right! Mild DD that will rip your head off! LOL

Tuskyz28

I can't argue with the LS3 head been the best BUDGET head out there. That's just the fact!! On the other hand if you want to have tire smoking torque anywhere in the power band go cathedral!! Also in the quarter mile it'll be quicker PERIOD. I see a bunch of guys running LS3 headed 408ci motors that put down great rwhp numbers but be stuck in the 11s..... While a serious cathedral headed run 10s at the track. Seen it over and over like watching a movie they show all the time on TV.

ragtopz28

iTrader: (57)

^^^I, on the other hand, agree with Dr Whigham^^^
I just picked up some P&P L92 heads for $800 and a LS3 intake for $180. You can sell the LS6 for $350-400 all day. If the L92 heads only make high end power, why do the put them on trucks

Tuskyz28

Good point you make there.... too bad you can't put a Truck intake underneath a F-body cowl which that intake was designed for torque unlike a LS6 style intake. Richard Holdener has proven that on the engine stand a couple of times....
Great guy that does real tests on cams, heads, intakes, etc...
He also tested a 408ci motor on the dyno using both a LS3 head vs a Out the Box AFR head. The AFR head not only made more torque but it also made more horsepower.

HISS

iTrader: (52)

Here we go with the square vs cathedral. My cathedral headed 364 makes 366 ftlbs......not exactly a torque monster. These are ported stock castings that flow in the 300 cfm @.600 area. Bad thing is I was talked out of square ports by almost everyone.......so I spent more to make less. Smh

ragtopz28

iTrader: (57)

I think we both make good points...I'll be alright, I'm gonna spin it and hit it w/300 shot!

Tuskyz28

Good deal!! Just get a good cam to really take advantage of the juice and a nitrous exhaust port would benefit the LS3 heads a lot.

dr_whigham

iTrader: (49)

I'd surely hope an aftermarket head of any sort would make more power... Especially against a STOCK GM head.

Let's take that LS3 head and shoot it out to one of the two aforementioned companies, or even Richard at WCCH and compare again.

OP you can't go "wrong" either way. There's a plethora of options out there. I think the real question is how deep are your pockets?

Tuskyz28

Maybe we can get Richard Holdener to do a test and find out. It's ton of info on his website..... Most people don't even know about it

Assman

iTrader: (2)

I talked to Thompson Motorsports yesterday and they recommended some rhs 232cc heads with one of their custom grinds. They said a customer did this exact build with an ls6 intake and made over 520 at the wheels. This is definitely not the cheapest option but it sure sounds good. I do like the idea of the ls3 saving some money though, this build has more than doubled since I started...

HISS

iTrader: (52)

The square port norm on a 408 is over 500 rwhp, that's not the cathedral norm. Yes some are doing it but it cost more for the same results so why spend more money. Save your cash and set the car up right with supporting mods, your gonna need it with the new found power.