Quote:
Originally Posted by Daschicken
If you are going for low end torque, upping the exhaust size significantly is not the best decision.
|
I've thought about this a lot, and done a lot of searching on exhaust performance. I haven't found a lot of actual dyno support for the "small exhaust = better low end torque." Most of the low RPM scavenging effects are related to the exhaust headers, and the free flowing mufflers seem to have better low end torque and also better high end torque (and HP). Most dynos are done above 2500rpm though, so it's hard to find a good low end comparison that isn't a turbo-diesel chart. The one advantage of the C6 transmission is that it stalls around 2000rpm, pretty much regardless of vehicle loading. So I'm really trying to optimize the 2000-3500rpm range.
For example, here's a 2.7L Tacoma dyno with headers and stock muffler/2.4" pipes (blue) and with Magnaflow 12586 straight thru muffler and 2.25" pipes (red). No significant power difference, but the richer mixture on the Magnaflow implies less scavenging of exhaust gases for the same fuel input. My guess is this is because the stock pipes are slightly less than 2.5" (measured 2.4") and the replacement 2.25" pipes are more restrictive. There may be a slight advantage to the Magnaflow system above ~3500rpm, but otherwise they are basically the same. Run 007 with stock muffler, run 010 is with the Magnaflow in the attached dyno.
https://www.tacomaworld.com/threads/....334168/page-3
His later test compared a Flowmaster #842452 delta baffled (green) vs Magnaflow #12586 straight thru (blue) vs stock baffled (red), the one that makes more power up high also makes more torque down low. The Flowmaster is slightly better than the Magnaflow across the whole band, and generally shows slighly higher AFR = higher scavenging/lower backpressure. In this case, "better flow = better low end torque and better high end HP."
https://www.tacomaworld.com/threads/...334168/page-14
The theoretical flowrates for exhaust piping is:
2.25” exhaust flowrate = 408cfm / 185hp
2.5” = 509cfm / 232hp
2.75” = 622cfm / 283hp
3” = 757cfm / 339hp
And the displacement of 4.9L at various RPMS is:
4.9L @ 1000rpm = 69CFM
4.9L @ 2000rpm = 138CFM
4.9L @ 3000rpm = 208CFM
4.9L @ 4000rpm = 277CFM
The Tacoma 2.7L is a 2.25" exhaust, using an LCE header with what looks like 1 5/8" primary tubes. It could be 1.5" though, it's hard to tell. Based on AFRs, it looks like it would benefit from a slightly less restrictive exhaust. Since my engine is 1.8x as much displacement, it should theoretically need 1.8x as much exhaust CFM. So 2.25"CFM = 408 x 1.8 = 734CFM. Based on this, the "optimum" single tube exhaust for a 4.9L is probably around 2.75"-3".
The main reason for doing a cat-back exhaust is that the stock pipe is kinda rusty, and the exit is to the right behind the passenger rear wheel. That makes a good rear wheel flare impossible, so I'm changing it to a rear exit pipe. If I decide it's an improvement in midband HP then a larger diameter is a good idea. Otherwise I'll just stick with the cheapest pipe size I can buy (probably 2.25" or 2.5") and break out the MIG welder.
If I can figure out a way to do it, I'll bring two complete exhausts and go to the local dyno shop. That would be interesting for me anyway, especially for future projects!
Today
Other popular topics in this forum...
It should also help decrease the blazing sun factor in the lunch and evening drives, so hopefully I can use a lot less A/C. The two tanks when using A/C were at 13.79 and 13.92mpg, so I'm pretty confident it's around 13.85 with A/C. My last fillup was 14.36 without A/C, which is basically the same as what I saw before I fixed the A/C. I'm running through this tank to confirm, since in the last tank I did about 50 miles @ 80mph with no drafting while rushing to get to the airport. I'm sure that hurt MPG by a little bit, but I'm not sure how much.
