2.25" H-pipe Restriction?

[Brown_Muscle]

There seems to be a lot of misinformation online... I currently have a stock 2.25" catted mustang h-pipe on my vehicle. My 351w should be pushing 375-400hp at the crank (trying to be conservative). How restrictive is my h-pipe compared to a 2.5 catted pipe? I do feel its lacking in the upper top end (above 5000rpm), but the cam should still have plenty of power up that high, so that's why i'm wondering if the pipe is holding me back.

[sly]

Check the bends. Typically the restriction is in the over axle bends that really crimp down the pipes to as small as 1.5 inch in diameter in the middle of the bend. That may be your biggest issue. If everything is mandrel bends, you should be fine and need to check cat/muffler flow next. You may need to go 2.5 inch to get more out of the top end, but I imagine there's other spots to look first. The H-pipe shouldn't be a restriction point unless it's made wrong.

[gadget73]

yes, that. The corkscrew over the back axle is likely way more of a choke point than another 1/4" of H pipe would be.

if you want to know for sure, a pressure gauge plumbed into one of the O2 bungs would tell you. If the exhaust is a choke point it will show on the gauge. Won't tell you where the restriction is, just that there is one.

[DerekTheGreat]

I'd step up to a 3". I believe Engine Masters did an episode on exhausts and if I remember right, even the turd engine benefitted from a 3" exhaust. The ole "You want some restriction or you lose bottom end" argument was invalidated.

[gadget73]

the more practical concern is just one of space and ability to bend pipes. Most places can't do 3", and if they kink-bend it, fair chance the actual flow ability is going to be less than a mandrel bent pipe of a smaller diameter.

[Brown_Muscle]

the bends on the h-pipe are extremely good, I don't think they were madrel bent, but there's really no kinking/downsizing from the bends. The mufflers are super-turbo's, which are the freest flowing quietest mufflers you can get (engine masters did a segment with them as well, and they flowed as well as a straight pipe). I'd also prefer not to go 3" because I'm trying to keep it as quiet as possible, so if 2.5" will do it, that's fine by me. Perhaps that cats are the restriction at these power levels, The pressure gauge would definitely provide some interesting data

[sly]

How are the bends over the axle or do you have the exhaust dumped at the mufflers? the 2.25 inch pipes on my 93 are crinkle bent over the axle and pinch down to just over 1.5 inches at one point.

[Brown_Muscle]

The rest of the exhaust is all 2.5", save for the h-pipe. The Bends over the exhaust are mandrel bent, impala tailpipes, so basically no restrictions there, although they are getting up there in age at this point. I remember my stock tailpipes were so tiny, bent and restrictive it was laughable

[sly]

hmm... Yeah... then the only thing I can think is intake, cats, headers, or maybe just needing the 2.5 treatment on the H-pipe. I know the tails and manifolds are the restriction on mine. If I put headers and mandrel tails, I would probably need 24# injectors to keep from leaning stuff out. My intake may also be a restriction. It's not much larger than stock but the MAF is now matched to the intake tube instead of being half an inch smaller.

[jaywish]

Here is a random table which makes me feel good cause I thought I remembered anything near 400 hp needing a larger exhaust. Mostly i remember when deciding what exhaust to put in old wagon my research showed 2.25 was plenty for my goals as I never even conceived of getting into the high 300's.

So 2.5" it is Also if I remember right diameter was more important at the front with the hotter gases having higher volume.

https://m.roadkillcustoms.com/determ...t-tubing-size/

[Brown_Muscle]

That makes sense Jason, its not a huge deal, but when it rots out, makes sense to go bigger. I fully support you upgrading the headers, tails, and injectors

That's a nice little chart Jay, according to the first, i'm undersized, according to the second I'm right at the limit of my exhaust.