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Originally Posted by Joe Perez
(Post 749754)
All of the exhaust does not leave the cylinder head; not even on a naturally-aspirated motor.
While naturally aspirated motors have difficulty exhausting every last bit, with the correct valve overlap fi motors can easily begin to exhaust part of the intake charge. A lot of the time turboed and supercharged motors actually lower exhaust valve temps and combustion temps. In most miata applications there probably won't be very much of this because of relatively low boost and poor manifold/collector design but I thought it may be worth noting... |
http://www.eng-tips.com/viewthread.c...137022&page=63
Bear in mind that a legal cat has several psi of back pressure all by itself (there is a very expensive tradeoff involved). If I had to pick a number, I'd guess most production exhausts, with cats, have 4-6 psi back pressure at max power. First, about backpressure: as was pointed out above, bacckpressure has little effect on engine performance except at high rpm and WOT operation, and even then it is dependent on a number called the "valve overlap factor" (VOF) which is defined as the area under the valve lift vs crankshaft rotation plot in the region of exhaust valve closing/intake valve opening. That is, the greater the valve overlap factor, the greater the engine's backpressure sensitivity, or loss of power with increasing backpressure. I have stashed away somewhere a fascinating study done at Ford in the late '80's where power loss was plotted vs VOF for a variety of passenger car and light truck engines, done as part of the MN-12 (Thunderbird) program. It showed clearly that 4-valve engines (case in point was the 5.0L Porsche 928 S4 rated at 300HP) typically have very low valve overlap factors, compared to 2-valve engines, and very very little backpressure sensitivity. I later confirmed this in testing done on the Lotus-designed ZR-1 Corvette's LT-5 engine, for which I designed a variable-backpressure muffler. |
I like back pressure.
During a massage that is! |
1 Attachment(s)
I re-measured at 15 psi / 270 hp (dynapack). I got a disappointing 9-10 psi. Previously I got 5 psi at 220 hp/10 psi (dynapack).
I have a custom 2.5" exhaust tystem. I also plotted it vs power (looking at my dyno plot), and from 4000 RPM to power peak, it fits a quadratic curve pretty well (RPM^2). Suspicious, I then looked at the pressure drop across my test pipe with a louvered resonator. 40% was across it! I'd have never thought it would be worse than my metal cat. I'll re install my metal cat and re-test. When I had this bolt-in test-pipe made I knew a louvered resonator wasn't as good as a perforated straight-through resonator. However it never occurred to me that it might have more backpressure than a metal cat. Louver resonators lose about 1/2" diameter in the center flow diameter if you look through them. (comparison image below) Another disappointment was that my custom BEGI divorced downpipe showed ~1.5 psi, comparing just behind the turbine outlet, to just before the outlet flange before the cat / test pipe. Ian's old FM exhaust, with 260 whp, shows 5 psi at the bottom of the downpipe. More later. https://www.miataturbo.net/attachmen...ine=1323042657 |
Awesome data, thanks. I may check mine when I runs again so us AF guys can have some data. That's if I'm not on the yacht with VJ, Ross, Ron, and Christian, us AF guys are a pretty big deal.
Do you have pics of the adapter you made to measure? |
I have a 1/8 NPT hole for EGT at the turbine inlet which I use for TIP (turbine inlet pressure), and one at the top of my wastegate. I use a 1/8" NPT compression fitting with a ferrule and 2 ft of 1/8" copper tubing. Its' to isolate the heat from the vac hose, which leads to a boost gauge. For TIP I use a 0-60 psi gauge from the hardware store (a 0-30 might be better).
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How in the hell can a louvered resonator cause so much back pressure? Thats really depressing because that is what I have in my miata and I liked the sound of it so much I put one in my GTX exhaust as well.
Bummer. |
Originally Posted by hustler
(Post 802232)
us AF guys are a pretty big deal.
and Rummy called them unk-unk's |
Originally Posted by wayne_curr
(Post 804071)
How in the hell can a louvered resonator cause so much back pressure? Thats really depressing because that is what I have in my miata and I liked the sound of it so much I put one in my GTX exhaust as well.
Bummer. Now of course, with an increased i.d. there's less room for quieting packing material... |
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