tbi y pipe flow

[black dawg]

I keep reading that the y pipe on tbi 350s is really crappy, but looking at mine from the outside it looks like it would be ok. I am getting ready to do exhaust on this truck, and would do the y pipe at the same time....if I saw a reason to. Has anybody cut into one to see how they were built/how restrictive they could be?

 

[1-ton]

Here is something to read on TBI exhaust systems. http://tbichips.com/?page_id=73

2. Exhaust, You need to start there because doesn’t matter how much you improve the air going in, if you can’t get it out it will run poorly and the stock exhaust is terrible. Well the KEY issues with the exhaust is the Y pipe and the catalytic converter. GM intentionally made that Y pipe restrictive to increase back pressure and most have paid someone to spread a myth that back pressure in the exhaust is GOOD. Its NOT. Air velocity is good not back pressure. The reason for the back pressure was so that the EGR smog system would work better and the increased pressure would build up a lot of heat to keep the o2 sensor hot. A good free flowing exhaust does typically need a 3 wire heated o2 sensor conversion to maintain its temperature. To fix 90% of the exhaust issues, I recommend a Flowmaster Y250300 collector from your favorite vendor and replace that section of Y pipe where the 2 pipes merge. Dramatic improvement over that stock GM design. As for the cat, I know most say it should fall off and turn a blind eye but I personally don’t like the smell of the exhaust with no cat and want to do my part towards our environment as well. you can search ebay for THUNDERBOLT 3 CATALYTIC and those high flow cats are like $50. As for headers, not really recommended on engines under 300hp and for muffler, you want an aggressive sound that’s fine pick from several. I have been using a FULL BOAR SS muffler off ebay which are stainless and cheap and flow well but not going to be a huge gain in power. the Y pipe will add a noticeable amount though and will help support more modifications.

 

[dyeager535]

I find it hard to believe that the Y-pipe, coupled to poor flowing manifolds, bolted to poor flowing heads, with a small cam, is THE problem. On long tube headers, with a single wire O2 sensor FAR back from where stock was, I watched my setup go closed loop as soon as the throttle was touched, so there is no heating issue. EGR could be true, but GM also used two different methods of EGR operation, positive and negative, one did require exhaust pressure to operate correctly, so that's a variable, not a constant.

If you look at the variety of charts out there for X displacement + Y HP=Z pipe diameter, you'll find that one half of a TBI engine generally needs 2-2.25" pipe diameter. I'm guessing all the crossovers are right in that range.

From what I've seen on the one y-pipe I cut apart (perhaps a diesel only deal, I don't know) it was dual wall. The restriction wasn't any worse than any other old technology exhaust that was used at that time.

The old cats (pancake style) are definitely restrictive pieces of junk, but that was the technology back then, can't blame GM for that, but it's an easy fix. Plenty of aftermarket high flow converters that don't cost an arm and a leg (if you aren't in CA). I would be willing to bet that on a stock TBI rig, the converter, unless plugged, is no more a restriction than the heads or manifolds. Thus, probably well matched.

Long story short, with long tube headers being worth ~20-25HP on a stock SBC, my opinion, derived from the testing that has been done, is that the manifolds are the major problem. With TBI, head flow will be another issue, but until the manifolds are dealt with, you will deal with that bottleneck. Then you will get into cam, intake, throttle body, etc. You can put effort into a TBI engine and make some additional power, or you can do an LS and get a lot more of everything. Or just leave it alone and realize it's a low RPM torque motor, that's it.

 

[black dawg]

I guess I will cut the pipe right up close to the y and have a look for myself. Not going after any power with this thing, but looking for as much mpg as possible. I considered an ls in this suburban, but I really don't think the tbi motor is holding back mpg in a big square truck.

 

[dyeager535]

I guess I will cut the pipe right up close to the y and have a look for myself. Not going after any power with this thing, but looking for as much mpg as possible. I considered an ls in this suburban, but I really don't think the tbi motor is holding back mpg in a big square truck.

Why cut it? It's thin wall pipe (maybe double wall, maybe not). There isn't anything hidden in it, what you see on the outside is whats on the inside, minus the pipe wall thickness. Measure OD and you've got your answer.

You aren't going to see any measurable MPG gains modifying the TBI. I've got an L31 (Vortec) crate engine in my truck, TPI, and medium length headers. 33" tires, SM465, 3.42 gears. I make the same MPG numbers I did when I had a K5 with 305/700R4 and 31" tires and 3.08 gears, and when this truck was running an old carbed 350 and manifolds. 16-19MPG is the upper end of what you will see with a gas square body for highway MPG.

That doesn't mean you can't make more power by modifying things, but you aren't going to get around the MPG issue.

 

[black dawg]

Why cut it?

because now I want to know. I guess if the two pipes going into the y go too far into the 3" there could be some restriction, but I doubt it was made that poorly......going to be hacking on the exhaust anyway, might as well find out.

 

[runamok151]

You're wasting your time and $ if you don't do manifolds, too.

 

[sweetk30]

I can say I had a like new gm long block tbi engine in a c10 truck with 3.42 gears years ago . I took it to the exhaust shop and had them do a new system off the back off the stock gm y-pipe to a 3" in and 2x 2.50 out flowmaster delta 40 muffler . . . . all I can say is WOW after misplacing that pancake style cat on the shop floor she was a whole new animal .

 

[diesel4me]

I remember a old Buick in shop class we put an exhaust system on when I was in high school,a carpentry teachers car..he complained it felt gutless on the highway rather suddenly,but it still ran smooth .no misfiring,and the auto repair shop teacher had more or less given up trying to diagnose it ,as a tune up and carb rebuild changed nothing,and their ignition analyzer showed no bad plugs ,coil,wires or cap & rotor..

The car had a 455 with a Q-jet and normally the thing was a beast according to the owner..my shop teacher said he'd look at some things they might have overlooked..

First thing he did was hook a vacuum gauge up to the intake,and noticed it was high & steady while idling,but if you floored it and let off suddenly,it took the gauge's needle longer than usual to return to the normal reading..he said this indicated a restriction in the exhaust somewhere,maybe a baffle came loose in the muffler,etc..car was too old to have a catalitic converter too,I think it was a '72 Lesabre..

My teacher removed the y-pipe (and got to show us how to use a torch to heat up the nuts on the manifold and quench them to get them to come off without breaking the studs !)..

After he got the Y-pipe off he tried pushing a thick rope thru the y pipe on each side,and the passenger side wouldn't let the rope pass thru it..he got a hacksaw and cut the pipe there the rope stopped,and found it was double walled there,and only the inner pipe had collapsed almost completely shut about 4" from the "Y"..looked completely normal on the outside..pipe didn't even look old or rusty either..

The car ran much better after the new exhaust system was installed..back to being able to pass anything but the gas station again..

I know more than one guy who lucked out buying a vehicle with a "tired engine" ,that turned out to only need the catalitic converter removed and replaced ( or straight piped it!)--and they ran strong as a bull again..first thing my friend does when cars get towed in that died on the highway and he suspects a restricted converter,is to take one spark plug or the o2 sensor out--if they have a plugged converter it'll start right up then,now that the exhaust has somewhere to go..

 

[RubberFloorMat]

I will agree that over a non clogged up cat and a free-ish flowing single exhaust there won’t be a huge gain going to duals if you keep the manifolds, however a set of duals, with headers you will see a larger inprovment, perhaps only 20hp/20ft lbs on a stock engine, but there will be a seat of pants feel, and it will sound good.

I know on my 75, running some great headers off the mild 355 into some junk turbo mufflers and pipes there was a significant improvement once those old kinked and nasty pipes were replaced.