Does anyone know if adding a quad exhaust will add more hp, assuming all the other piping back to the engine is stock.

 

Do you mean quad exhaust tips? If so then there will be zero difference.

If you do not change any of the other piping then the flow rate isn't going to get any better.

 

Even if you did "true" duals, the flow would be worse than merging into a single tube and splitting out from there.

 

Just qurious, how would the flow be worse?

 

The flow wouldn't get any worse, but the exhaust velocity would slow down considerably if you just slapped in the biggest pipes that will possibly fit. Think about a garden hose as a big pipe, and you putting your finger over the end will make it a smaller pipe and the water will squire out faster. If your exhuast piping diameter is too big, it causes your exhaust velocity to slow down...reducing low end power output. Too small of piping will cause a choke in the system. It's all about tradeoffs, Low end power or more high end power. The goal of a well engineered exhaust system is to find a good comprimise between the two extremes.

 

Basically a right size single exhaust all the way back from the headers would work best for power. Unfortunately this would cause the **** to look pretty bad and uneven. Check out this thread where pretty much all exhaust options for the ISx are discussed in detail. https://www.clublexus.com/forums/sho...hlight=exhaust

 

Jeff pretty much nailed it here. If you look inside the stock tailpipes, you'll notice that the exhaust tube inside the tips are actually very small. The tips are there merely for looks.

With the aftermarket exhausts, you're merely trading low end power for peak power gains. Frequently, you'll lose more power down low, aggregately v. the power gained up top. This may be good for road racing, where you're spending most of the time in the upper RPM's. However, the low end grunt for everyday driving will most likely suffer.

 

Not necessarily true for the IS350. Take a look at this dyno comparison I did. My IS350 with all cats in place vs no rear cats in place. He picked up quite a bit of down low power and still got better peak power.

 

You can't really compare 2 different cars like that. Waynetech's baseline could have been higher than your baseline to start with.

The only way to determine a gain of a specific mod is to run it on the same car (with and without) and preferably on the same dyno on the same day.

 

Yes, you are right in that we can't accurately determine a gain between different cars on the same dyno on the same day. Honestly though, do you really think the difference would be this much, esp down low? You would think that by reducing the backpressure his low end would have suffered slightly. We've seen plenty of IS350 dyno's and they are all pretty consistent with each other plus or minus 1-2% I'm fairly certain that with the secondary cats off reducing the back pressure slightly gave his car an advantage for some unknown reason. I guess once I get my rear cats configured to be removable I'll test it on the dyno again.

 

By low-end, I'm referring more to the rpms' between idle and 3 (perhaps 4K rpm). This is where I do 95+% of my driving and I'd wager most of the forum members as well.

In most things performance related, you usually have to give a little to get a little. Unless they come out with a variable exhaust system (as they've done with valve timing), this isn't exempt from the general rule above.

 

Yeah you are probably right with the power change in the low-end, but since we all have auto and any semi hard acceleration on the throttle between idle and 3-4k rpms the car will down shift. Any loss of power this low is not going to be noticeable/measurable in any way on the road, dyno or track as we are in this low-end for cruising and light throttle acceleration. Easy driving like we do 95% of the time we could possibly assume there would be less power there if the car couldn't down shift.