I have spent the last few months reading almost every thread on this subject that I can, however, I am still undecided which is best for my combo. Heads are the most important part of an engine build which will ultimately be the single most expensive part on the motor. The motor is a new stock displacement LS3 with -3.2CC pistons and forged rods and will be N/A. It will eventually reside between the fenders of a 1990 RS mated to a T56/Magnum a. The car will be a full weight street car and should tip the scales around 3300 to 3500 lbs with the goal of 450 to 480 rwhp.
I am torn on which head profile to go with. My main curiosity is focused on aftermarket offerings of the two and how they differ. Meaning, do the smaller intake ports of aftermarket rectangular heads close the gap in low/mid-range power compared to an aftermarket cathedral head? There are pro’s and cons to both but with the same price tag, I cannot decide which direction to go. Here are a couple factors that weigh into the decision, along with the heads I am considering.

Rectangle Heads – TFS GenX 255 or PRC 255

Pro’s

• LS3 intake manifold assembly is a great performer and very cost effective, and is turn-key with TB, Injectors, etc.
• Make great top end power, reduced low/mid, and could help with a traction limited car.

Con’s
• Not as responsive off idle, doesn’t have that “feel” despite dyno numbers.

Cathedral – AFR 230, TFS 225, or PRC 237

Pro’s
• Fat power-band from idle to redline
• Great street manors with responsive power off idle
• Tremendous amount of aftermarket knowledge
• Heads typically are available with chamber size I want (65CC to obtain 11.1 ish CR)

Con’s
• Would require expensive FAST intake set-up along with ala-carte parts to complete the intake set-up.
• Recommended to have an expert make them even better (Mamo, Tick, BTR, Frankenstein Etc)

My Pro’s and Con’s are based on all of the reading I have done on this subject. However, it is based on discussions around aftermarket cathedral heads versus ported OEM LS3/L92 heads. As mentioned before, does the smaller port of the aftermarket rectangle heads give them the “feel” and responsiveness of an aftermarket cathedral head? Or are the two still worlds apart in how a combo performs? I am leaning towards rectangle port heads, merely do to the cost effectiveness of the LS3 intake (could be $1k savings in total on this motor).

 

rectangle over cathedral, ls7 style over ls3

ls7>ls3>cathedral

 

With your power goals, I would use the factory LS3 topend and swap camshafts.

I have a factory LS3 topend on a 404, and it is plenty responsive.

 

What compression are you running and are they untouched stock heads? I was leaning towards aftermarket since ported stock heads almost are the same cost as aftermarket castings.

 

11.5:1, and the heads were only milled .030". The ports, chambers, and intake are untouched. With a 231/239 cam, the 404 put down 490/470 at the wheels through a T56.

People don't give the factory LS3 head enough credit some times. It has its limits, but your goals are well within those limits.

 

Very interesting. I appreciate the feedback. I assume you are running a 6.0 block? I just read all these stories/reviews how the stock LS3 is sluggish off the line and that a cathedral headed car is explosive and faster than square ports at the track (with the same power levels). Then again, power is power and 490rwhp is a lot.

 

Yes, it is a 6.0 block, bored .010" over, and a 4" stroke.

I don't know where the "sluggish" comments came from, but there is a lot more to a "sluggish" feeling than cylinder head selection. So long as you properly match the cam to your heads and displacement, and gear accordingly, your car will have plenty of snap off the line.

Just remember that the rectangle heads absolutely hate overlap when you are picking a camshaft.

 

Agreed 100% on the cam selection. I suppose it is easy to be seduced by the reading material provided by the likes of Tony Mamo and folks.

 

http://www.superchevy.com/how-to/ght...-head-to-head/

They make some excuses about needing more lift but who is going to use a .700 lift hydraulic for the modest steer build you are looking at.

Something to consider, do you think it coincidence that when GM give a vehicle the larger displacement 6.2l with rectangle ports they also give it a tranny with deeper first gear?

The stock gearing(first) in the SS is equivalent to a 4l60E car with 4.10s, slightly deeper even.
First gears
TH400/4l80 2.48
700R4/4L60E 3.06
6l80 4.03

 

The addition of two extra gears is what allowed them to go with a steeper first gear. First gear in a 6L80E doesn't need to cover as wide of a range as first gear in a TH400 or 4L60E. It has absolutely nothing to do with whether or not the LS3 heads make less torque.

 

At your power goals eithier head will get you there. Take a coin, make a flip, and enjoy. I honestly think a oem head sent to Chris Frank will be the cheapest way out.....

 

Then name an application where GM used the 4l80e with the rectangle head.

 

Silverado Vortec Max
L76
4L60E

 

Not finding much info on it other than it makes 5ft.lbs less than a LQ9 and 20 more HP and needs VVT to do that. Had a bit less compression than a LQ9. I would have hoped a cylinder head flowing 20+% more with VVT could do better than the rectangle port heads introduced half a decade earlier.

Do you have a source for more specs on the truck? Friend who has a 2007 Sierra 1500HD has 4L80, maybe they used the 4L60 behind the rectangle port to get it more gearing.

I genuinely want to know more about the truck, if you have a link please share.

Comparing tranny/axle gearing combos is of particular interest.

 

Well when you put 400ci under it, it's not as sluggish. It's usually the stock stroke guys that complain the LS3 stuff feels sluggish.

 

I don't have a link. My friend bought one when they were new. It had the L76, 4L60E, and his was optioned with 3.73 gears. He went through two or three transmissions before getting rid of it.

Even when I had the heads on a 6.0 with a 227/235 cam, it didn't feel sluggish to me, but again, it is all perspective, I suppose. There are people who will put a 241/251 camshaft in a LS1 and still claim stock drivability.

 

Thanks for the insight guys. I know either head will support my peak power goals, but was more concerned about power curve and overall driveability and feel for a street car with full accessories.

 

I wouldn't say that they "hate" overlap, but they certainly do not require nearly the amount that a smaller valve cathedral port head requires.

 

Yeah, I should have specified and said, "they hate tons of overlap."

 

I prefer the newer style stuff because you can simply use factory intakes and injectors which are cheap.

Go Cathedral and you need to spring for a FAST and you need to buy bigger aftermarket injectors (or adapt factory ones).

I've never really driven any LS stock displacement car that didn't feel a little sluggish at low rpms, it's kind of the nature of these motors. I had high compression 243 light *** FRC and it still didn't really get moving until 4k rpms.