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Old 08-30-2016, 01:36 PM
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Ben@SpeedTech Ben@SpeedTech is offline
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Hey Ron, thanks for the input so far.

I have been trying to get some research done on exhaust flow in effort to maximize my exhaust in a tight package but actual data backed information has been hard to find. I have 3 main questions that seem to mostly only invoke uneducated opinions like "race cars run open headers so bigger is better" when I ask on muscle car based forums. I'd really like to know a professional's data based opinion.

About the car- It's a G body with single exhaust platform. I stuck with the single because less weight, components and cost. I currently run no cats, with 2 1/2" dual head pipes with the left bank crossing over behind the trans and into a Flowmaster Y pipe with 3" tube through the high flow muffler an continuing 3" over the axle and out the back under the bumper. My old .480 lift 406 SBC motor had roughly 325 hp and 400+ ft lbs net at the flywheel and it ran great, good midrange torque and decent power from about 2000-4500, but hp noticeably dropped off after that. Higher rpms were likely limited by the factory 400 smog heads and small 600 cfm carb. With this 406 the 3700lb w/me car on a stock convertered TH350 and 3.42 gears ran sea level 13.40s at 98 through the full exhaust. I now have a new 12.5:1 SBC 406 that should be roughly 500+ hp/500 + ft lbs net at the flywheel. The motor will see mostly street driving and autocross, and occasional drag racing. Realistically the engine will likely spend most of it's racing time between 3500 and 5500, despite the tq peak is around 5000 and hp peak 6500. Likely only in drag racing will it ever see above 6000. I'm hoping for around mid-low 12's with this new engine and a new 3000 stall converter combination.

1. In my own research reading internet posts, mostly supported by first hand info including time slips or dyno #s, there's some information indicating that smaller tube, (opposite of most people's "opinions"), has an advantage in full exhaust cars because it will create more velocity which improves lower to mid range power. Along with that, a single exhaust of an appropriate size will help scavenge as well if not better than an X pipe, the combination of which will create decent exhaust flow through the entire rpm range. Backing that up I've read of 650 rwhp small displacement turbo cars on 3" single exhaust.

Since Flowmaster markets big power gains through use of your Y pipe in a single exhaust system, I'm curious what your experience with single exhaust has been with an average 450-550 hp N/A V8? Will my current 2 1/2" dual to 3" system continue to work well or will it restrict the new engine's potential significantly enough (upwards of 20+ hp/tq loss) to warrant the expense of an entire new larger system?

2. I've read that a dual 2.5" system can support upwards of 500+ hp and 3" duals is better for 500+ up to ___ hp. At what horsepower level would I want to go to 3" dual into a 3.5" (or 4") single Y pipe and then follow through as single 3.5" (or 4") out the back? Since most of my racing will be revolving around 3200-5500 mid range autocross power range, would the larger pipe help across the whole rpm range or like swapping to a single plane intake mainly just shift the rpm range up, decreasing low to mid tq and increasing high rpm hp potential?

3. What about tail pipes? How much of a power increase or loss difference if any do they make? I have been considering running my current system with the 3" exiting out the pass. side in front of the rear tire to avoid the general internet assumed opinion of power robbing bends of the tailpipe. However I'm not excited about the increased noise level, the car is already pretty loud as it is. I've also read on occasion that as the exhaust cools it has less and less affect on hp, and by the time it hits the tail pipes they don't make a significant difference to hp figures whether on the car or eliminated with dumps in front of the axle.

Your/ Flowmaster's Thoughts?

Thanks in advance, trying to find quality info on single exhaust systems is a bit tough.

Pics of current exhaust...

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Last edited by Ben@SpeedTech; 08-30-2016 at 01:47 PM.
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