Originally posted by MEANGREEN:
Not to argue, 88IROCs, but hell, if that's the case, then explain why if you were to take that theory, put the stock exhaust manifolds on a dyno and then put full tube headers on it, you would see literally no gain whatsoever in torque, but a horsepower increase in the upper RPM range.
Well to start-off with we were talking about cat-back systems, so to argue the relative merits of headers vs stock manifolds is a whole new subject. Now if you are arguing that putting headers on an engine with a stock cat and catback system, I would tend to agree with you partly. The stock cat and catback would create enough backpressure to negate any gains made by the headers and might actually cause a loss. And that would tend to be the case at especially at higher rpm's where the stock pieces would pose more restriction to exhaust gas flow(where the increased gas pressure would cause more backpressure for the same amount of restriction). This is the reasoning for upgrading the cat and catback system before upgrading from manifolds to headers. Hopefully you are not arguing that: with a higher flowing cat and catback system, headers would make no difference in low rpm torque?

It isn't so much a matter of backpressure, it's a matter of too much of a good thing.

It is totally a matter of backpressure, and when it comes to removing backpressure, there is never "too much of a good thing". Backpressure is an impediment to exhaust gas flow, and will never improve output regardless of the rpm at which it occurs. Unfortunately, production based vehicles are required to have catalytic converters and mufflers, ensuring that you will never completely eliminate backpressure(and remain street-legal).

Just like my 1 3/4" headers wouldn't do jack on a basically stock 305 vice a stock LT1. Big difference here, kinda like the introduction of nitrous vice without.

So are you now trying to tell us that you put 1 3/4" headers on your LT1, yet left the stock(restrictive) cat and catback system intact, and claim you felt "big difference"? If that is not the case, what is your point?

True, the stock setup is restrictive, but if you're gonna direct someone to better their exhaust, it's only gonna flow as well as the cylinder heads regardless, so what the hell would he need a 3" dual setup on a 200 (possibly) horse 305?

Ah, so now you want to change the subject back to catbacks? Even though the 305 heads are known for their lacklustre flow characteristics, the stock thirdgen exhaust system poses more of a restriction to exhaust flow. Reducing the amount of restriction in the after header exhaust has a compounding effect. As less of the exhaust gases energy is lost to overcoming restrictions(and back pressure), the higher the velocity these gases will leave collector area with. This in turn helps create a stronger scavenging signal, which in turn aids in the removal of burnt combustion product from the cylinder, which means there is more room for the incoming intake charge. Improved scavenging also helps to draw(pull) the intake charge into the cylinder. Using the old analogy that an internal combustion engine is nothing more than a complicated air pump, anything you do to improve intake flow will improve output.

I would tend to agree that a dual 3" setup would be overkill on any motor displacing less than 600 cid. A 350 can breathe just fine on dual 2 1/4's, and a 305 needs even less. However you argued that dual 3" pipes would reduce torque, citing "no backpressure" and "larger pipes" as the cause. This is simply not accurate, as removing backpressure will increase torque.

He will see no gain, unless he's got a clogged cat, bent stock exhaust, or he's simply got a freak for a car.

If you have abnormal restrictions(clogged cat or bent pipes), anything you do to reduce or remove those restrictions will improve exhaust gas flow. And even if he has only the "normal" restrictions, anything he does to reduce or remove those restrictions will improve exhaust gas flow. What exactly is your point?

Perhaps 1 5/8" headers, 2 3/4" or 3" y-pipe, new cat or gut the old, 2 3/4" or 3" cat back will give him all the flow that engine will ever need. Anything bigger is a waste.

It would be a waste in terms of diminishing returns - you would spend more money and effort for increasingly smaller gains in exhaust flow. Once you have eliminated the exhaust system as the major source of flow restriction, you would be better off putting effort in to removing other, more offensive, sources of flow restriction.


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Originally posted by PONTIMANIAC 90:
Veeery interesting stuff. I mean that.
I have a 90-305 TPI w/5 speed.From what I understand the 90-92 305s with stick came w/up-graded cams. That being said, I bought my car w/stock dual cat set up going into a 3" Flowmaster cat back.
My questions are:
1.Was it a waste for the guy to put that up?
2.Whould I benefit from 1 5/8 headers, keeping the stock dual cat set-up?
Thanks in advance.