LS1 Swap

[BigAls87Z28]

Alright quick and easy one...hopefully.

1 3/4 headers come in either 2.5 or 3 inch collectors. Im feeling 2.5 for clearance issues. Will I see that much of a difference with the extra half inch?

[BonzoHansen]

are you doing 3" exhaust?

[BigAls87Z28]

It has a 3 inch exhaust on it already. But there is still a whole y pipe between the two

[KirkEvil]

I think you should stay with the 3". Choking the motor and adding the hassle of adapting to the 2.5" to 3" is not worth gained clearance. It will have to be custom fabricated anyway right? That should get you clearance.

Also, 2.5" and 3" dont seem that different but if you look at cross sectional area its 4.91 sq. in versus 7.07 sq. in. Thats a gain of 43% cross sectional area even though its only 0.5" larger diameter! Much larger area if you multiply that by the length of pipe...

[BonzoHansen]

I'd go 3 as well. if no other reason going to a 3" y-pipe later would be easier.

[BigAls87Z28]

Well, the Y can be made of 2x2.5's into a 3 inch section that attaches to the cat, and then into the I pipe.

The y will make up any difference in size from the LT's to the cat,

[sweetbmxrider]

its a 1/4" of clearance.....

[PolarBear]

Originally Posted by sweetbmxrider

its a 1/4" of clearance.....

IMO while the above statement is true, he isnt going to gain ANYTHING from going to 3" collectors. Dual 2.5" into a 3" is more than most people need for the street, if you are looking for EVERY horse you can possibly get on the track it would be beneficial.

[BigAls87Z28]

It does have a bit of work done to it, so maybe I should go with the 3 inch. I just figured that 3 inch collectors going into 1 3 inch pipe would cause soem sort of bottle neck into the cat.

[LTb1ow]

Originally Posted by BigAls87Z28

It does have a bit of work done to it, so maybe I should go with the 3 inch. I just figured that 3 inch collectors going into 1 3 inch pipe would cause soem sort of bottle neck into the cat.

And hence, !cat.

But in all seriousness, you plan on sticking one cat in this mess?

Yea... good luck, if it doesn't clog, its gonna do nothing but be a hindrance to flow.

Get two three inch cats and stick in in the Y pipe. Win.

[BonzoHansen]

why would it clog?

[BigAls87Z28]

I dont see why high flow CC's would clog flow. Its not like its lard. Its gas.

[WildBillyT]

Originally Posted by BigAls87Z28

I dont see why high flow CC's would clog flow. Its not like its lard. Its gas.

Would one 3" cat have enough cfm for two banks of LS1 cyl's?

[BigAls87Z28]

Hrmmm...have thought about that too.

[BonzoHansen]

Originally Posted by WildBillyT

Would one 3" cat have enough cfm for two banks of LS1 cyl's?

Are they different from SBC cylinders? There are a lot of fast SBC powered cars out there blowing through a single 3” cat.

[LTb1ow]

Originally Posted by BonzoHansen

why would it clog?

A single cat after the Y?

Seems like it would never get hot and never do as it should and clean the exhaust gases, clog may have been an overstatement.

Granted that thought was all from research I did when I had a dream to run cats as well, and from what I read a single cat after an ORY will never get warm enough to properly scrub the exhaust gas.

So Al will prob line his catback with catalytic converters. Back pressure!

[WildBillyT]

If the rule of thumb is 2.2 cfm per 1 hp (best guess I found doing a few searches) you would need 770 cfm for a 350hp engine to free flow. That would be two cats, since one 3" unit (even the least restrictive ones) seem to top out in the 500s.

I guess if there is backpressure from the mufflers I guess it does not matter? And I do not doubt that there are fast cars out there running on a single cat. But is that optimal or just not robbing enough to matter?

http://www.eng-tips.com/viewthread.cfm?qid=104735
http://www.superchevy.com/technical/...xh/index1.html

Originally Posted by Lts1ow

Granted that thought was all from research I did when I had a dream to run cats as well, and from what I read a single cat after an ORY will never get warm enough to properly scrub the exhaust gas.

This is a different problem.

[BonzoHansen]

Isn't the cat after the Y in 3rd gens already? Or is this y going to push the cat even further away? What about nice & simple 3"collectors to 3" cats to y?

It appears Al's budget is tight which is what made me think single cat like the OE setup. Is he building a fast street car or a race car? If he leaves 10hp on the table in a street car because his budget does not allow more, so what? You can't always chase the theoretical best when the budget is limited. He can always go back and change it later as funds allow.

[LTb1ow]

Originally Posted by BonzoHansen

Isn't the cat after the Y in 3rd gens already? Or is this y going to push the cat even further away? What about nice & simple 3"collectors to 3" cats to y?

It appears Al's budget is tight which is what made me think single cat like the OE setup. Is he building a fast street car or a race car? If he leaves 10hp on the table in a street car because his budget does not allow more, so what? You can't always chase the theoretical best when the budget is limited. He can always go back and change it later as funds allow.

But he is not doing an OE setup, he is doing long tubes. I don't wanna be argumentative, but LTs will push the stock location of the cat way past where it was, and since you can no longer have the cat straight after the merge, the merge of the ORY will stick the cat pretty far back. I just would hate to see Al throw all this together and then have a cat that does nothing but cause a flow blockage.

An OE Y pipe allows for both manifolds to dump together relatively close to the general area after both manifolds, LTs will not allow that, so to place the cat an equal distance from each header it will need to be far back, or you run two.

I'll shut up now.

[BonzoHansen]

I'm just asking/thinking out loud. That is why I was asking. How bad is the room if you do cats right off the collectors? Will that make the y-pipe impossible?

[WildBillyT]

Originally Posted by BonzoHansen

Isn't the cat after the Y in 3rd gens already? Or is this y going to push the cat even further away? What about nice & simple 3"collectors to 3" cats to y?

It appears Al's budget is tight which is what made me think single cat like the OE setup. Is he building a fast street car or a race car? If he leaves 10hp on the table in a street car because his budget does not allow more, so what? You can't always chase the theoretical best when the budget is limited. He can always go back and change it later as funds allow.

I dunno. I just saw going from two cats from the factory to a single cat (even a high-flow) as a downgrade from stock, flow wise. A very tight budget trumps it all though.

[BonzoHansen]

Originally Posted by WildBillyT

I dunno. I just saw going from two cats from the factory to a single cat (even a high-flow) as a downgrade from stock, flow wise. A very tight budget trumps it all though.

You ever see a stock 4th gen y-pipe? It's ugly.

[WildBillyT]

Originally Posted by BonzoHansen

You ever see a stock 4th gen y-pipe? It's ugly.

Yup. But as crappy as it is, each half of the y only has to deal with 4 cyls. Again, if the budget is very tight then I am off target. A single 3" high flow will be fine.

[Frosty]

Cost vs. benefit IMO. Lets say your theory is correct...you figure..what? A 5-8hp difference? Would that be worth the amount of $$$ he'd need to spend? I can't imagine having that custom stuff made being cheap...not to mention a decent cat is $150-200.

[LTb1ow]

Originally Posted by BonzoHansen

I'm just asking/thinking out loud. That is why I was asking. How bad is the room if you do cats right off the collectors? Will that make the y-pipe impossible?

I know with 4th gens it is possible to stick a pair of cats in the Y pipe with no issue, but 3rd gens are a mystery to me.



Those are Hawks LS1 swap headers... WOW... does not look fun at all Al...

But on these, it looks doable to cut out a section of the ORY and weld in some cats...