Confused about exhaust size.
 

So ive done my research here and I see over and over, "1.5 inch primarys for the headers into 3 inch DP. Then reducer going into 2.5 inch..."

What would be the point in the 3 inch downpipe if youre reducing the size to 2.5 anyway?

Wouldnt it be better to just go 3" until the Y pipe? Then 1.5 to the mufflers?

Im asking in regards to lowering KR.

If youve seen any of my recent topics youll notice im planning to mod my 05 and just want to be absolutely sure about everything.

Im ultimately planning on just going with the speed daddy headers and want to know if the crossover pipe is sufficient.

Edit: btw im running catless.

Is this for a supercharged setup or NA? 1.5" primaries would be fine for an NA setup but for anything supercharged I'd at least do 1 5/8".

Certainly you could run 3" exhaust from the headers back, but you aren't going to gain any real power on a basic build. The stock catback isn't particularly restrictive until you get into a pretty serious setup.

IMO get some 1 5/8" headers, adapt that down the the stock catback and just run that.

Also not sure why you'd be worried about the crossover, it should be designed to work with whatever headers you buy. It's not like you're going to spend the money building a new crossover pipe as its size is dictated by the collector outlet from the front 3 primaries.

it is always specific to what you want or your doing
I had one failed donor motor for top swap, but failure gave me ideas

I also agree with bumpin. The restriction in our rexhaust is the manifolds and downpipe. The cat back is good for 300whp maybe more. People go 3" because if your doing it might as well go big, then if you max the catback change that later instead of needing to change downpipe again with it.

Yea, I should have mentioned its supercharged and I already have u-bend, cat, res, delete.

Concerning my plans, all I think im gonna do is rockers, headers, t-body, then drop to the lowest pulley size I can.. Im already getting almost zero KR but want to take all the precautions plus the small gains are worth it.

Later on (if i can bring myself to spend the money) ill swap the supercharger and put on an intercooler but that wont be for some time so.. Plus I guess I can justify doing all this cause whatever I upgrade and make obsolete can be put on my bonneville later on.

So from what im saying you think that 1 5/8 would be optimal? Im now thinking of making them myself but still need to read up a little more on it. Dont see any reason why I couldnt tho. I have decent metal working skills.

As far as off the shelf headers go, I'd run any 1 5/8" or 1 3/4" header. If you're doing them yourself, you have the freedom to pick what works best with your overall design.

Note sure how the current cheapo ebay brand are, but the old SLP knockoff style used to be pretty good for about $300. IMO it would be hard to beat that with custom made headers when you figure how much labor youd have into it.

I definitely get custom headers on an application where there is nothing available off the shelf, but there are a lot of 3800 header options between new and used that would really cause me to think a bit before making a set.

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X2 what John said...

You threw me you wanted to terminate the pipe run with 1.5"

​​​​​​I think what he was doing was taking the 3" diameter and dividing that by 2 for 2 pipes after the y. To the OP- unfortunately it doesn't work like that. The cross sectional area is very different. A 3" diameter pipe has an area of about 7.1 in2. A 1.5" pipe has an area of about 1.8 in2. So two 1.5" pipes combined have about 1/2 the cross sectional area of a single 3" pipe.

What makes it worse is when you get into the gas flow through the tube. The flow along the wall is extremely slow compared to flow down the middle. So the 3" tube having a greater cross sectional area open down the center is even a bigger benefit to flow in that aspect.

Just an update if yous were curious but im def gonna go with them speed daddy headers off ebay..

$210.00 is just too good of a deal to pass up..

Eventually when I feel like it im gonna make a set myself and put the knockoffs on my bonneville.