Silly me, I thought they were round.

And, I thought they were pretty big, seeing how the Hawks/Spohn/SS swap headers were 1.75" primaries. Putting the calipers to them, I see they're only 1.5" wide and 1.25" tall, and the factory exhaust manifolds match them.

So, dumb question (probably about #72 on my list so far) - what happens when the exhaust steps from the stock port to 1.75" primary headers? Is there a big velocity drop? I know going from small to large is the "right" thing with exhaust, but that seems pretty substantial. Are they that big to allow for future growth? (Perhaps assuming "most" people who do this swap will opt for upgrades.)

Speaking of future growth, what do the exhaust ports on professionally-ported heads, aftermarket, etc., look like? Are they full round, perhaps larger?

I have been assuming that I'd do headers for this swap, but now I'm wondering if, with hardly any mods to the engine (LS6 intake, cat/EGR/A.I.R. delete), maybe the stock manifolds wouldn't be that bad - or, to put it a different way, headers wouldn't be that much gain. I'd still do 2.5" duals (oh, guess I haven't said much about that to date).

(That puts me up to 75 dumb questions now. . . )

 

Well, it can't be too awful bad, as the 1.75" headers seem to be right at the "sweet spot" with regard to most of the LS1 engines I've seen. I know that you very likely have a pretty good base of knowledge on the Gen-I SBC, so I'll just say this -

LS1Tech recently had a thread where 1-3/4" headers were compared to 1-7/8" headers. The consensus was that you didn't gain anything with the larger headers, so I'm guessing that the 1-3/4" headers are about the equivalent of 1-5/8" headers on your average Mouse. Given the fact that the LS-series engines are a fair bit more efficient than the Gen-I engines, that size differential sounds about right to me...

Yeah, up to a point...

Sizing the headers for the typical "bolt-ons & cammed" car would seem to be a reasonable way to do it (given that a smaller percentage of owners would go beyond that point), so I'd think that you're on the right track with that.

As far as the ported heads, I remember seeing before/after shots on a set of heads with D-shaped exh ports not too long ago. Of course the flat portion of the port was on the bottom, parallel to the ground, & after porting, it had been opened up & rounded a little bit - not as much as I would have expected, & it wasn't opened up enough so that it became a round port. I seem to recall reading that much of the benefit comes with freeing up the last inch or so of the port, but to be honest, I'd double-check that before doing any porting or ordering any work done..

Thankfully there's no "quota" on dumb questions, or I'd have been banned by now!

Again, I seem to recall reading (on LS1Tech?) that the headers (on avg) are worth about 20-25 hp, but I don't know if that's worth the price that they're asking... I've sort of thought about seeing whether CTS-V or "Blue Devil" exh manifolds might work on my car, rather than paying that much. Worse comes to worst, the regular F-body manifolds are a lot better than the ones that came on our cars from the factory!

EDIT: BTW, I just responded to your PM!

 

The reason bigger header primaries work well on so many heads is to reduce
reversion.

With the step from primary tube to port, expansion pulses which bounce
around in the exhaust system are restricted from reverting into the chamber.

One thing you have to ensure is that the primary doesn't cover any of the
port, but going too big normally moves the torque peak higher and/or results
in poor performance.

Ideally with the correct port shape and cam timing, 1/8" bigger diameter than
the largest point of the port is ideal.

On some high end race heads, the port is matched to the primary pipe.

 

I see Hedman makes 1-1/2 - 1-5/8" step headers for 4th gens.

I hear the 2001-2002 manifolds were a better design. No EGR, either, I believe. Perhaps find a cheap set of those to install.

 

More science on exhaust than I can absorb. Specifically the "exhaust science demystified" post

http://ls1tech.com/forums/showthread.php?t=520845

 

Yeah, I've seen that. Didn't really answer the question about "mostly stock" engine.

I'll probably go with the headers, just another budget increase request I'm not looking forward to.

 

I've seen some posts where LT's alone gained 30 rwhp on a stock motor with a tune mind you. On a stock motor with manifolds I've seen posts where 15rwhp was gained with a tune. I'd only run the manifolds for a bone stock swap. The mod bug starts running rampant with these motors. Its just sooooo cheap and easy to gain more. I barely have one season on mine and I'm already looking at new heads and cam LOL. The price is too good to pass on

 

I too resemble that commment

The LSx exhaust ports aren't physically huge, but they flow like crazy - thats why they like the bigger headers. Stock LS1 heads flow something like 225cfm on the exhaust side - untouched.

 

So, is this a "log", or "as good as shorties"?

Engine came from a 2000 Camaro SS.

 

Do both sides look like that? I know the older style are very different left vs right but the newer versions are very similar. probably due to the lack of EGR on the newer ones

**edit** but yes they look like the newer "good as shorties" kind to me

 

I haven't taken the driver's side off yet - can't see much behind the shield. I'll go take it off and post of pic of what it looks like.

 

Okay, the driver's side.

Unwelcome discovery - an exhaust manifold bolt broken off in the head. Looking forward to getting that out - NOT! All of the bolts were pretty tight coming out, no antiseize, apparently.

 

Newer style definitely. As for the bolt... yikes...

 

At least the engine is out for the extraction process. Far rear driver's side, would be a bear to get it out with the engine in the car.

Another dumb question: I thought each bank had an O2 sensor. There's only one in the manifold, but there's a connector for another sensor down by the starter. Plus two back on the tranny (after cat sensors, I presume). Is the passenger side O2 sensor in the pipe? The seller included three loose O2 sensors, plus the one in the driver's side exhaust manifold.

 

Yeah the LS engines use four o2's. Two before the cats that the PCM uses for operations and two after the cats just to monitor emissions these are typically deleted and not required to run. They will give you codes though so you must either have them tuned out or use some O2 sims that plug in place of the sensors and trick the PCM into thinking they are there. All of the O2 sensors themselves are the same though so dont worry about mixing them up

 

The rear O2's were tuned out by the seller (part of the service he provides when you buy an engine from him - got a bunch of stuff done already). I was just thinking that if I ended up using the manifolds, I'd have to make sure I had a bung welded into the passenger side pipe for that sensor (if I'm understanding this correctly).

 

2 things -

1st, yep, those look like the newer, "better" style to me - definitely not the "log" type.

2nd, unless I'm misunderstanding how the engine mgmt system works on the Gen-III/IV engines, you'll need one cat on each exhaust bank. (Of course, it could be that I'm misunderstanding what you're saying, too...)

Sorry to hear about the broken bolt, THAT would have me grumbling a bit...

 

Can you say, "collector" plates?

Comes with the territory, I guess. If I use the manifolds, I have 4 broken off studs to get out as well.

 

Yeah, that works! (I'm not jealous - REALLY!)

Ya mean that JB Weld won't hold 'em??!?

Oh well, at least the engine isn't in the car...

 

Correctomundo you need two O2's one for each side of the engine so if only one of your manifolds has a bung then you will have to weld a bung on the other side in the pipe close to the manifold. Full Throttle performance sells stainless bungs with a plug for 8 bucks iirc