Five0hFox

I've got an 88 with a freshly rebuilt 302 with BBK LT's and o/r H. I was thinking about getting the Pypes Violators and maybe dumping them before the axle, but since their a flow through design, I was wondering if I'd have a problem with not enough back pressure? Also would I have any problems with popping/back firing?

projectresto83

I hope this question doesn't bring up the argument on exhaust and backpressure again.......Portmaster is no longer here so prolly not....

Anyway, you will be fine with the backpressure. Back firing is from improper timing, not from exhaust.

boonedj

back pressure is a mythe lol the way it was explained to me was with straws lol anyways engines do not need backpressure so ur fine

mjr46

you want your exhaust to flow freely not restrict it

Five0hFox

Just checkin. Thanks mjr, I'll take your word on it. you seem to know your stuff.

But the reason I asked was because a few years back I heard something about if your exhaust pipe is too big, you could harm your valves.

Joel5.0

Mythological hearsay........

67mustang302

There is definitely a 10 mile long list of "Mythological Hearsays" as Joel put it, when it comes to exhaust. Backpressure = bad, and just about everyone confuses backpressure with pulse waves. Backpressure is resistance in the system due to not enough flow(whatever the cause may be), the pulse waves are the tuning part that are energy waves which assist in exhaust scavenging.

Too large of an exhaust system won't hurt the engine, but will hurt performance, ironically enough by increasing flow resistance(or rather reducing negative pressure in the exhaust system). Higher velocity in the exhaust system causes a pressure drop that helps draw exhaust gas out of the cylinder, when the pipes are too big they can cause a reduction in flow velocity and a resultant INCREASE in pressure, which reduces flow efficiency(Bernoullis Principle in action). The trick is to get the system big enough to create 0 resistance but without getting so big that it causes a loss in flow efficiency.

As a general rule of thumb, most street V8 engines work well with dual 2.5" systems up to around 500hp(cat backs can actually run a bit bigger because of the extra heat the cat creates). Once you start getting up near 500hp you want to look at 2.75-3" pipes. A 3" setup on a mild 302 won't hurt the engine, but depending on how it's set up, might hurt performance. The flow of the mufflers also plays into it as well.

HACKGT350

you are very very right ^^^^^^

Joel5.0

This is an area we disagree on. .... what is considered as too large of an exhaust system (ie. 3" full pipes past the header collector) will not affect negatively engine operation, on the contrary.... it allows for better tuning. The exhaust system "tuner device" is at the collector/extender...... after which free flow (not velocity) characteristics should be ensured/provided. Why does a stock or mildly engine setup perform better with LT's open?..... why does such a condition allow for more aggressive A/F and spark tables/maps? ... and there's no "velocity increasing" setup when open headers are used.

By the same token, and using Bernoulli's principle, wouldn't an increase on the exhaust end velocity, dampen the exhaust pulses and their timing, used to enhance/increase scavenging at the exhaust port level? Also... since this is a stream of gases, wouldn't increasing velocity be the same as restricting its flow?...... think garden hose and nozzle..... isn't flow stream linear velocity increased when the nozzle restricts water flow? ... isn't that back pressure at the water faucet side? Same principle, increasing stream velocity at the end, or post the headers = a restriction = back pressure at the exhaust port side...... where the exhaust gasses are trying to exit the cylinder.

The exhaust setup cannot be "divorced" from engine power/performance production... it is also a mechanism (obviously neglected) to produce performance, not a setup tied to what the engine makes first.... and usually, performance gains due to the exhaust do not show in terms of HP or TQ.... even though it does affect how fast the setup will be.

Can you make good power with a 2.5" full exhaust?... yep, but it would be "easier" for the same engine, if it's setup with a full 3" system and tuned accordingly. It's the sum of the parts that define performance, not one single magical component though.

67mustang302

I was trying to speak more generally, rather than specifically to any particular engine combination. Though the primary tuning component in the exhaust is the header(primaries and collector), when attached to pipes after that, it becomes necessary to evacuate gasses from those pipes quickly enough to prevent them from backing up and having an effect on tuning at the collector. Larger diameter pipes may help or may hurt, it depends on the rest of the system and the engine itself(like you said, it's not divorced from the engine). If the pipes are so large that velocity drops off too radically, it can cause gasses to back up. Yes, increasing velocity comes from increasing restriction(in terms of pipe diameter), but like I said, the trick is to balance the best velocity with the best overall flow. Too small and you strangle the exhaust, too big and you can lose some efficiency in the overall system. There's 2 components at work, reduced restriction from larger pipes, and reduced restriction from higher velocity.

The pulse tuning is primarily affected by header design(primary diameter, length, collector size, length, how they blend etc). Gas velocity won't really effect pulse tuning other than determining the original strength of the pulse when it's generated at a diameter change in the system, mainly when the exhaust gas initially exists the cylinder and creates a pressure wave spike.

But like you said, it all comes down to how the whole system works together. That's why the F1 and Cup guys spend so much time on the dyno just to build an exhaust system...searching for the ideal diameter, ideal length, the most power. For us, unless we have a set of headers custom built, we're stuck with an off the shelf header design, that can only put power in the ballpark of where we want it.