protestthehero

iTrader: (1)

Im rather confused at the piping size i should use. i have a 1991 bone stock 305 tpi. im on a budget so i figured i would go with the hedman #68460 and their y pipe #17478 which is 2.5". so what im really confused about is in the sticky they mention that the tubing should really be 3" instead of the 2.5 where they join. My question is if i run these headers, y pipe, a high flow cat and a decent muffler, would this be better than stock? or not by much.

thanks

torque_is_good

you will be absolutely fine with 2.5" piping. Take a look at the stock exhaust manifolds and then look at the headers. Now, what does your common sense tell you???????????? Do you think the headers will be better than stock?

blubirdformula

Everyone makes a big deal about the 2.5 tubing, it will not cause any back pressure unless you have 270+ hp. And with a 305 I don't think that's gonna happen. With the set up you mentioned you will probably gain 30+ hp and probably cost like $650

LS4GXP

iTrader: (2)

I had a slightly modded 305 with edeljunk headers and stock Y pipe and exhaust. I switched to a custom Y pipe and 3inch exhaust with a junk flowmaster and it was night and day on performance. I might have had 2 1/4 piping seeing the car originally had a v6, but either way if your going to replace the whole thing i would do 3 inch and spend the extra 100 if that.

blubirdformula

Well stock piping is 2 1/4 for 305s and about the 3" tubing , with a 305 you will lose low end torq with such a huge pipe. Unless it is modified and pumping out a lot of hp. Bigger isn't always better..

LS4GXP

iTrader: (2)

only thing modified about my 305 was a cam with about .505 lift on stock 350 heads, edeljunk 650 carb, untuned and unprofessionally built. It only ran 14.6 at 95 with a T5 and 3.73 gears, so theres not much bolt-ons at all. The 3 inch piping compared to the 2 1/4 like i said, was night and day, not just a seat in the pants feel. I bet if it was on a dyno you could see 10-20hp. If theres any such future mods going on the 305, your a moron not going with a 3inch exhaust.

First thing i told the mechanic after i got my car back was its too slow. He said lets put a 3inch exhaust on it and see how it is. I was that happy with the 3in that i wouldnt run anything less on a 305.

we could go back and forth about less torque, backpressure not being ment, yatta yatta. This is my experience between a 2 1/4 and a 3 inch exhaust system on a 305. i wouldnt run anything less then 3 inch

i would say mine wasnt pumping out any seriouse numbers what so ever, but the power gain was huge

weswisehart

I put 3inch exhaust on a stock 305 tbi and I've had no problems with back pressure. I don't even have a cat.

TA

iTrader: (1)

A cammed/650 Carb 305 is not on the same airflow category as a stone stock 305 TPI engine. A manual trans and 3:73 gears are also incredibly forgiving to an engine with decreased midrange (until the upper gears, which is where your spanking will happen).

Also, the stock accordioned, flattened and horribly merged 2 1/4 y-pipe will see more of a jump in flow to a well-made and merged mandrel 2 1/2 than from the 2 1/2 to the 3" and I agree, especially on a 305 TPI/Auto you could loose valuable torque with the 3". The only way to know for sure on your particular combo is to try them both and dyno them, but on the original posters' combo, I would bet on the 2 1/2 y as making more midrange and virtually the same top-end power. I've been wrong a lot before, but not lately

Going to a larger diameter tubing primary gives you a false sense of improved power. How? When you nail the throttle in first gear, the gears pretty much sing you past the mid-range, then as soon as the engine hits the top of the mid-range it suddenly lights the tires and screams. Faster, right? Wrong! You have so weakened the mid-range that now at the transition to high end you have a very sharp power spike into the high rpms (about the same peak power as before, maybe a touch more), that now makes the car seem like an animal. The car feels fast, makes all kinds of noise and smokes the tires, but is actually slower accelerating up through the gears, and a car you used to run fender to fender with now easily drives away through the mid-range, and your awesome top-end rush just holds the gap steady until he changes gears, whereup he proceeds to drive away again. You can see this on a dyno chart and it will also be obvious at the dragstrip, but just blasting around town you will think you are the hottest thing on 4 wheels!

blubirdformula

Very well said TA
But all performance aspects aside, unless you really need it, 3" y pipes are only available when you buy a package of both headers and y pipe (hooker 2055) , And the cheapest you can get the set is like $450 uncoated! ouch,,
and the cheapest 3" exhaust system you can get is like $300. So you are talking $750 and throw a cat on top of that and now $850!! once again ouch..
You can get a 2.5 header y pipe combo for like $300 and a catback for like $200. So that's an extra $200 for the 3" so unless you need it, it's really pricy

LS4GXP

iTrader: (2)

Then say down the road u actually do any performance upgrades your 2.5 is too small and now your out the money you spent on the 2.5in because you need 3. Ouch....

If your staying stock and not upgrading then stay 2.5.

protestthehero

iTrader: (1)

well it looks like id better go 3" when the time comes because i plan to do a 355 build. i appreciate the help it cleared a bunch up

LS4GXP

iTrader: (2)

Which was my point in the first place. 95% of fbody owners will not stay stock. They will always upgrade or put a bigger motor in. Good thinking on saving money in the long run and going 3 inch. Which has been my point from day one with this discussion

blubirdformula

Well sorry, I thought we were talking about a bone stock 305..

paul_huryk

Back in 1990, when I had an almost stock LG4, going from a stock exhaust withe the 2.25" tubing to a gale banks y-pipe, 3" iroc cat, and 3" cat back was worth 35 to 40hp. I still had the restriction of the cast manifolds, but it did not give up anything down low - it probably picked up some TQ (hard to prove without verification). Talking about taking a high 16 second car and dropping it into the low to mid 15's.

Friends that didn't want to listen replaced their stock Lg4/TBI exhaust with an aftermarket 2.25" were lucky to pick up 10hp, pissing them off in the process.

Another similar idea is how you don't need to use a 3.5" pipe until you hit 425hp or something - I picked up 20hp by going from a 3" to a 3.5" cat back on a 375hp engine (now 400hp).

LS4GXP

iTrader: (2)

As i would agree it picks up good power, i will throw a BS flag when you said high 16 too a low mid 15. It takes alot more HP to drop a 1/2 sec or even 1 second off your 1/4 mile times.

for example :
I used to have a 2005 GXP LS4 v8, pumped out 303hp, 249whp stock. I ran a stock 03 cobra at the strip. His time was 13.0, mine was 13.8. He had almost 2 1/2 - 3 cars on me. If am not mistaken the 03 cobras are around 350+ rwhp, some freaks pushing 400, thats well over 100 more whp then my old car, and it wasnt a full second faster.

protestthehero

iTrader: (1)

we still were talking about the stock 305 its just i was gonna see how well the 2.5 inch flows. if its gonna be too small for the 355, then i see no point in doing it until i build and drop it in. for a quick upgrade it would be nice, only wanna do it once.

TA

iTrader: (1)

I'm running 2.5" w/cats from front to back on a 383 making 390 HP on a Dynojet chassis dyno.

You will actually benefit from running a 3" after the cat because the cat super-heats the exhaust and it needs more area at that point until it cools back down. The big problem I have with all of these opinions and numbers thrown around is that I don't see any dyno development or testing to back up the opinions, and as I said before, I thought I had greatly improved power output a number of times only to find out on the dyno that it was an illusion.

paul_huryk

Not just HP at the peak, but power all under the curve including a decent bump in TQ and a lot less of a drop off above 4000rpm. In fact, I remember the exhaust letting the engine pull to 5K, where prior it was done at 4500 or so.

At the high 16 second level, it doesn't take much to strip 1/2 second off your times. 150 hp = high 16's and 190hp (like a 5.0L HO) is mid 15's with an auto.

A GXP to Cobra comparison is apples to oranges.

Seriously, the 3" exhaust is a very smart move on an LG4/TBI as GM choked the F out of those cars on purpose to keep the HP ratings low to sell more TPI cars. And that carries over to the single snorkel intake and the pitiful timing curve.

1killerolds

iTrader: (1)

I know this thread is 5 months old, but nobody pointed out that the 17478 y-pipe does not fit the 68460 headers. in fact, from what I have read, there are no y-pipes for 68460's. the 17478 is for use with the 68470 shorty headers, not the long tubes. I just wanted to post this for clarification in case someone else reads this.

Noel_z28

did the y-pipe (17478) actually bolt on and work with the (68460) headers? if so do you have any pictures?