Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

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Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by Shockwave » Sun Oct 08, 2017 10:58 pm

Hey guys, so I've been doing a ton of research on here regarding induction for my next build, and I'm wanting to see what your opinions are on this subject.

1st off, I'm a SAM grad who went into Aerospace (CNC repair tech) instead of the race industy, so I have some basic knowledge, but I haven't been able to keep up with the trends and I'm always looking to learn.

The question is, for a 4.030 bore 408ci iron block motor, solid roller, shooting for 8000-8500rpm, do you guys prefer a cathedral port head, or the LS3/L92?

I know conventional wisdom says the larger L92 would be better for that kind of RPM, but I've also seen guys like Greg Good and Brian Tooley saying they really don't like them compared to the larger cathedral port stuff, for a few reasons... seems like the intake valve size and placement have a lot to do with it. But they usually seemed to be talking about more moderate RPM ranges.

I have a set of both heads, so I'm trying to decide if I should stay with my cathedrals and sell my L92s, or dump the caths and rock the big rectangle ports.

My cathedrals are Dart 225s that I've ported myself (no access to a bench, 2.050 valve, throat set at 88%), and I run them with a ported super victor and 4150 TB,..... currently on my 11.25:1 stock bottom end LS1, and I spin it roughly 7300rpm. Car is a 4th gen F body, 4.56 gears, T56, 3200lbs.

The L92s are CNC ported from one of my former SAM instructors. I have a set of Jesel Pro shaft rockers for this head already. IIRC, they flowed right around 355cfm on a 4.030 bore.

I appreciate any info and thoughts.

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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by Shockwave » Mon Oct 09, 2017 3:34 pm

Oh, and If I posted this to the wrong board, sorry. Feel free to move. :|

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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by BILL-C » Tue Oct 10, 2017 9:57 pm

What is your power goal? Or is it an rpm goal?
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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by Shockwave » Tue Oct 10, 2017 10:15 pm

BILL-C wrote:
Tue Oct 10, 2017 9:57 pm
What is your power goal? Or is it an rpm goal?
I am looking to turn the motor as much RPM as my heads can feed and/or I can afford and see how much HP I get out of it.
8500 is about the maximum RPM I can realistically afford to maintain.

So my conundrum, is whether to run the ported L92s I already have, or sell off my L92 stuff and possibly do some more work to the Darts. Maybe send them into Dart for their 250cc CNC program.

Im also concerned about running a big solid roller on the GM L92 casting, with the thin spring pockets. I was looking at the PAC 1335 spring.

Thanks

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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by A_VAS » Wed Oct 11, 2017 12:34 pm

what do you plan to run for valves, in either of them ?
the L92 stock intake valves are very heavy...won't serve you well for this.

I'm interested to see what others say, but my vote would be with the bigger rect port head, and titanium valves so you can keep the spring loads down.
assumption is that the cnc work on your L92s was done with thought and quality. Not like what you would get from say the GM performance cnc head.
too lazy to make power w/o boost

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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by Shockwave » Wed Oct 11, 2017 1:46 pm

A_VAS wrote:
Wed Oct 11, 2017 12:34 pm
what do you plan to run for valves, in either of them ?
the L92 stock intake valves are very heavy...won't serve you well for this.

I'm interested to see what others say, but my vote would be with the bigger rect port head, and titanium valves so you can keep the spring loads down.
assumption is that the cnc work on your L92s was done with thought and quality. Not like what you would get from say the GM performance cnc head.
Im looking at either something like Ferreas hollow stems, or titanium. Im not sure how much lighter Ferreas are than GMs, but Ive hear people like them, and theyre significantly stronger.

The port is legit. The original hand port came from one of my old School of Automotive Machinist instructors.

The darts have a 2.05 valve now, and I can go up to a 2.080 if I went to the Dart 250 CNC program. So I could get a lot lighter valve with a significantly stronger spring pocket and rocker boss, but im giving up the big flow #s from the rectangle ports.

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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by BILL-C » Wed Oct 11, 2017 1:49 pm

We've been disappointed with the performance of the cnc gm head. The intake ports are already a little too big. It doesn't make sense to make them even bigger unless you care more about how pretty they are than their velocity and port energy. Less than half the area in the runner should be touched if you are serious about perf. The rest is just cosmetics, but that is what sells.
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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by Shockwave » Wed Oct 11, 2017 3:16 pm

BILL-C wrote:
Wed Oct 11, 2017 1:49 pm
We've been disappointed with the performance of the cnc gm head. The intake ports are already a little too big. It doesn't make sense to make them even bigger unless you care more about how pretty they are than their velocity and port energy. Less than half the area in the runner should be touched if you are serious about perf. The rest is just cosmetics, but that is what sells.
These are definitely not the generic GM CNC, and the port was not done to sell or look pretty.

It was done by a reputable head porter for in house use at SAM, and since I helped digitize it when I was there, we got to cut it into my heads as a side project.

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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by Shockwave » Wed Oct 11, 2017 5:26 pm

BILL-C wrote:
Wed Oct 11, 2017 1:49 pm
We've been disappointed with the performance of the cnc gm head. The intake ports are already a little too big. It doesn't make sense to make them even bigger unless you care more about how pretty they are than their velocity and port energy. Less than half the area in the runner should be touched if you are serious about perf. The rest is just cosmetics, but that is what sells.
Question.... on what size motors and general rpm ranges are you speaking of?

Doesnt the port size and velocity directly relate to the displacement of the motor your feeding, piston speed, etc?

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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by BILL-C » Wed Oct 11, 2017 9:59 pm

You are going to have to whip that stock block 408 real hard N/A to need more air than those DART cathedral ports can supply. Looking for just peak numbers on dyno, or to accelerate a vehicle? Yes, big cubes and alot of rpm requires bigger ports, valves, etc than something less. What is performance goal?
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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by Shockwave » Wed Oct 11, 2017 11:48 pm

BILL-C wrote:
Wed Oct 11, 2017 9:59 pm
You are going to have to whip that stock block 408 real hard N/A to need more air than those DART cathedral ports can supply. Looking for just peak numbers on dyno, or to accelerate a vehicle? Yes, big cubes and alot of rpm requires bigger ports, valves, etc than something less. What is performance goal?
So, the goal is to accelerate my 3200lb 4th gen Firebird as fast as possible. Its a stick, and I'll run as much gear as needed.
Im trying to do this by turning as much RPM as my heads can feed.

I guess the root of the question is, will the Darts be able to adequately feed a 408 at an upper limit of 8500rpm, or will I need to step up to the larger L92 port, and will the bore shrouding the larger intake valve negate the positives of going to a taller, better flowing port?

It seems most info on the L92s is generally regarding 6500-7500rpm stuff. I dont see alot about their suitability for higher RPM zones, and the few things I do see, tend to point to them having structural issues.

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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by Shockwave » Wed Oct 11, 2017 11:55 pm

BILL-C wrote:
Wed Oct 11, 2017 9:59 pm
You are going to have to whip that stock block 408 real hard N/A to need more air than those DART cathedral ports can supply. Looking for just peak numbers on dyno, or to accelerate a vehicle? Yes, big cubes and alot of rpm requires bigger ports, valves, etc than something less. What is performance goal?
So, the goal is to accelerate my 3200lb 4th gen Firebird as fast as possible. Its a stick, and I'll run as much gear as needed.
Im trying to do this by turning as much RPM as my heads can feed. 8500rpm is about the number I can afford parts for.

I guess the bottom line is, On a 4.030 bored 408, at 8500rpm, which head do you guys prefer and why?

Personally, Id like to stick with the Dart heads if they'll be able to feed the 408 at 8500, but Im concerned they wont be able to keep up.

It seems most info on the L92s is generally regarding 6500-7500rpm stuff. I dont see alot about their suitability for higher RPM zones, and the few things I do see, tend to point to them having structural issues.

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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by BILL-C » Thu Oct 12, 2017 7:16 am

I don't understand the fixation on 8500 rpm. Please explain. Considering the rest of the parts required for an engine to produce peak power at 8500 like intake man and headers and oil system, you sure you have the budget? If yes, then the rectangular port AFTERMARKET heads are the best choice. Budget is usually the most influential factor in a design. Peak power rpm is the result of the combination of parts used. If the peak power occurred at only 7200 rpm on your best effort, would you still be happy?
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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by Shockwave » Thu Oct 12, 2017 9:45 am

BILL-C wrote:
Thu Oct 12, 2017 7:16 am
I don't understand the fixation on 8500 rpm. Please explain. Considering the rest of the parts required for an engine to produce peak power at 8500 like intake man and headers and oil system, you sure you have the budget? If yes, then the rectangular port AFTERMARKET heads are the best choice. Budget is usually the most influential factor in a design. Peak power rpm is the result of the combination of parts used. If the peak power occurred at only 7200 rpm on your best effort, would you still be happy?
I understand that.

Im trying to build my 408 combo around one of these 2 heads to turn the most possible RPM it will support, therefore producing the most horsepower.

The 8500 number isnt a fixation, its just a budget ceiling for what I can afford to maintain re:valvetrain/parts.

No, I would not be happy with peak hp at 7200rpm. Im not building a bolt on street engine with a FAST intake.
If thats all the RPM either one of these heads would support, id throw them both in the dumpster.

But I know thats not true, since theres all kinds of 370-400ci LS motors turning 8000+ rpm with ported 243s or aftermarket cathedrals.

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Re: Thoughts on L92/LS3 vs Cathedral port for 8500rpm 408ci

Post by Shockwave » Thu Oct 12, 2017 9:47 am

BILL-C wrote:
Thu Oct 12, 2017 7:16 am
I don't understand the fixation on 8500 rpm. Please explain. Considering the rest of the parts required for an engine to produce peak power at 8500 like intake man and headers and oil system, you sure you have the budget? If yes, then the rectangular port AFTERMARKET heads are the best choice. Budget is usually the most influential factor in a design. Peak power rpm is the result of the combination of parts used. If the peak power occurred at only 7200 rpm on your best effort, would you still be happy?
I understand that.

Im trying to build my 408 combo around one of these 2 heads to turn the most RPM its airflow will support, therefore producing the most horsepower.

The 8500 number isnt a fixation, its just a budget ceiling for what I can afford to maintain re:valvetrain/parts.

No, I would not be happy with peak hp at 7200rpm. Im not building a bolt on street engine with a FAST intake.
If thats all the RPM either one of these heads would support, id throw them both in the dumpster.

But I know thats not true, since theres all kinds of 370-400ci LS motors turning 8000+ rpm with ported 243s or aftermarket cathedrals.

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