87grandnational
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- May 29, 2004
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seems like it does,.. just wondering.
open downpipe hurts my performance?
- Thread starter 87grandnational
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turbobuickman
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My car seems to run better (smoother at least) through the free flowing exhaust. It may just seem that way, however, because the open dp has a very coarse sound by nature. I dont have times to verify yet though.
mazdatech78
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Before I put on my 3in single shot I drove my car around with an open 3in mease DP.
The car seemed to pull a little harder than when it had the 2 1/2in duals on it. My boost went from 17 to 19 with it open also.
James
The car seemed to pull a little harder than when it had the 2 1/2in duals on it. My boost went from 17 to 19 with it open also.
James
On the dyno, by simply opening the DP with the same tune I went from 388rwhp to 405rwhp. I have 2.5" duals with Dynomax muffs.
I think the reason you may feel like it does not have as much power is because its a lot louder and it sounds like you are stomping on it when in fact your not stomping on it as hard as when going thru the muffs. A restriction free exhaust will always benefit a turbo motor because the scavenging effects of pulsed backpressure is of no benefit because of the turbo.
I think the reason you may feel like it does not have as much power is because its a lot louder and it sounds like you are stomping on it when in fact your not stomping on it as hard as when going thru the muffs. A restriction free exhaust will always benefit a turbo motor because the scavenging effects of pulsed backpressure is of no benefit because of the turbo.
I've opened up the dump on my THDP 2 different times/days at the track. On back to back runs - after a 30 to 45 minute cool down my car, with the open dump, was 1mph slower the first time and 1/2 mph slower the 2nd time. 
I run the hooker cat back and after my 2 experiences, will probably not open the dump again. A friend at the track (Lance Young aka MileHiGN) experienced the same 1 mph loss when the dump was opened as opposed to running through his ATR 2 1/2 dual system.
I run the hooker cat back and after my 2 experiences, will probably not open the dump again. A friend at the track (Lance Young aka MileHiGN) experienced the same 1 mph loss when the dump was opened as opposed to running through his ATR 2 1/2 dual system. My car feels slower with the dump open too, but I think it's just the extra noise. I think it's just more impressive when a quiet car pins you to the seat.
I have a Mease 3" DP with a Hooker 2.5" Cat back. The car will only benefit by opening the DP if it is running rich. Last time I was at the dyno, my car only gained 5 peak HP by uncorking the pipe. I'm sure with the right tune I would have gained more.
If you're losing mph with the pipe open, I suggest bumping the fuel pressure a little and see what happens.
I have a Mease 3" DP with a Hooker 2.5" Cat back. The car will only benefit by opening the DP if it is running rich. Last time I was at the dyno, my car only gained 5 peak HP by uncorking the pipe. I'm sure with the right tune I would have gained more.
If you're losing mph with the pipe open, I suggest bumping the fuel pressure a little and see what happens.
turbobuickman
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Makes perfect sense.
Thing you gotta watch out for is lack of backpressure with the dump open. Backpressure in the exhaust is necessary to maintain proper quenching of the cylinder. Too much backpressure and the cylinder doesn't quench enough, leaving exhaust gas in the next cylinder's mix. Not enough backpressure tends to suck the unburned mix out before the exhaust valve closes, causing a weaker AF mixture, and lack of power.
I see this happen all the time with these Honda guys who think that a loud free-flowing exhaust without cat or muffler can make the car go fast. Instead, it has just the opposite effect. All you do is upset the balance with that line of thought.
my .02 anyway.
-John Spina
www.casperselectronics.com
I see this happen all the time with these Honda guys who think that a loud free-flowing exhaust without cat or muffler can make the car go fast. Instead, it has just the opposite effect. All you do is upset the balance with that line of thought.
my .02 anyway.
-John Spina
www.casperselectronics.com
Yes they do. Which is why backpressure downstream of the turbo should in and ideal world be kept to a minumum (absolute zero would be perfect, but not practical in a "driver").
Backpressure is a N/A setup is in fact desirable and needed for all the reasons John spoke of.
But not in a turbo.
The only problem I see in 87grandnational's question was, no data. He only said it seems like it does, with no real hard data to back up his feelings.
There of course have been times when running an open pipe on one of these cars has hurt performance some. But in each case I saw the car was in a pretty bad state of tune to start with, and opening the dump just made a bad situation even worse.
If the T/R is tuned properly, any thing that will reduce post turbo backpressure will always result in more power and torque.
Not knowing much about flow dynamics, I'm kind of thinking this one out, and I gotta believe the stock cam requires a certain amount of backpressure in order to flow to the spec. I suppose it depends upon the turbo, some flow thru well, others (stock) are pretty restrictive. Something between absolutely no backpressure and too much backpressure would most likely be the ideal setup - with a stock cam.
Most likely, aftermarket cam timing and overlap events are designed with the likelyhood that you've upgraded the turbo and exhaust system to reduce backpressure.
By nature of turbo design, there's always going to be some backpressure potential. Maybe there is 'just' enough...
Just burnin' cobwebs from the brain.
-John Spina
www.casperselectronics.com
Most likely, aftermarket cam timing and overlap events are designed with the likelyhood that you've upgraded the turbo and exhaust system to reduce backpressure.
By nature of turbo design, there's always going to be some backpressure potential. Maybe there is 'just' enough...
Just burnin' cobwebs from the brain.
-John Spina
www.casperselectronics.com
John, you'd be amazed at the amount of backpressure the turbine housing produces!!
In a typical stock turbo it is close to a 2 to 1 ratio (for every pound of boost there's two of bacpressure) over boost, and a fairly efficient turbo will still produce something on the order of 1.5 to 1 backpressure to boost ratio. Some turbos have been seen making a 3 to 1 backpressure to boost ratio.
All this backpressure I'm refering to is in the exhaust manifolds/heads, pre turbo. That's why it's imparative to keep post turbo backpressure to as close to absolute zero as possible.
In a typical stock turbo it is close to a 2 to 1 ratio (for every pound of boost there's two of bacpressure) over boost, and a fairly efficient turbo will still produce something on the order of 1.5 to 1 backpressure to boost ratio. Some turbos have been seen making a 3 to 1 backpressure to boost ratio.
All this backpressure I'm refering to is in the exhaust manifolds/heads, pre turbo. That's why it's imparative to keep post turbo backpressure to as close to absolute zero as possible.
Also, not that it matters in the turbo world to much, but on cars that you are speaking of requireing backpressure, that is an old time idea. The truth is that you can go to big on your exhaust, but it is not lack of backpressure that is your problem. If you go to big on the exhaust it cant scavenge. With a properly sized exhaust on a NA application, each exhaust pulse will leave a slight vacuum behind it and help evacuate the next cylinder. If the exhaust is to big, the exhaust pulses tend to slow down to much in the pipe, and not allow this effect.
Ben
Ben
I would say that by opening the dump you are causing a disruption in the flow of gases through the pipe and actually causing a little more back pressure. That is the only reason i could think that you could get a reduction in power by opening the dump.
the change in A/F ratio would not be significant enough to cause a 1 mph loss.
the change in A/F ratio would not be significant enough to cause a 1 mph loss.
Originally posted by TurboDave
John, you'd be amazed at the amount of backpressure the turbine housing produces!!
In a typical stock turbo it is close to a 2 to 1 ratio (for every pound of boost there's two of bacpressure) over boost, and a fairly efficient turbo will still produce something on the order of 1.5 to 1 backpressure to boost ratio. Some turbos have been seen making a 3 to 1 backpressure to boost ratio.
All this backpressure I'm refering to is in the exhaust manifolds/heads, pre turbo. That's why it's imparative to keep post turbo backpressure to as close to absolute zero as possible.
*Frank* some years ago at DIY-EFI posted some pressures relative to his Sy. The stock turbo was over 2:1, and going to a TA-60 dropped it to just over 1:1.
Some of the compacts, are running at *cross-over*, that is less back pressure then boost. The penalty is alot of lag.
The oem Mopars are the worst at about 3:1 from what I've read.
As far a being slow with an open pipe, it's probably about being leaner.
In a N/A application, back pressure has ALOT to do with the self ERG'ing of the engine. Adding EGR means adding timing and fuel, reducing it means you HAVE to REDUCE the timing, and lean it out a little. AL things being equal. In the real world and Honda's fueling strategies, you'd probably have to add fuel to make up for the increase in in cylinder temps.. Honda is way past being at the level of being obvious. ie in the Marysville dyno areas, they don't even look at WB AFRs, but rather look at MANY gases to try and understand what's going on. So if you look at a data lof from a factory Mule car, it's nothing like what you'd expect to see. Or so I've been told....... <G>
