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Old 02-03-2003, 04:25 PM   #1
zoomfactor
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Default Scoobysport Downpipe Owners comments wanted -

I'm on the fence between a Perrin Downpipe and the Scoobysport (SS) -- leaning towards the SS. Please tell me why you chose the SS. I have searched and read till my eye's hurt about the benefits of divorced wastegate flows vs. bellmouths, etc.

I'm not a top end horsepower junkie -- just someone concerned with increased torque "under the curve".

Any comments greatly appreciated.
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Old 02-03-2003, 05:24 PM   #2
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http://www.rallyperformance.com/engi...port%20Exhaust

Just a lil bit of info from the link pertaining to your concerns...

"- Power: While peak horsepower is improved by +20 to 25bhp with the turbo back, +29 to 33bhp with the full system, the real difference is in driveability, because the power curve is so superb with no flat spots. Midrange torque peaks at an amazingly low 2900 RPM ! 93% of that torque is still available at 6000 RPMs.

- Torque delivery: Delivery is where the difference is. There may be a one or two systems that edge us out in max. horsepower, but the low and midrange torque afforded by the ScoobySport hybrid design (varying diameters) is perhaps unmatched in a turbo back / full exhaust. "
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Old 02-03-2003, 05:34 PM   #3
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I have to say if you plan on sticking with the stock turbo, you can't beat their downpipe for low end torque. That's why i bought it, I love mine but i'm planning on a bigger turbo in the near future so the SS might have to go . Oh well ,I may end up keeping the axle back to keep that sweet sound. -James
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Old 02-03-2003, 07:31 PM   #4
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The low end torque stuff is just marketing hype, they say they make more torque low down with their necked down restrictive design. I dont buy it. Its a fine exhaust but I'm not going to believe that it will provide more torque then a perrin pipe (or any other properly designed downpipe) matched to a same diameter catback unless some reputable tuner dynos it and provides the results for all to see. Until that day, this guys thoughts are what im going to believe to be true.

Quote:
The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on www.impreza.net regarding exhaust design and exhaust theory:

Howdy,

This thread was brought to my attention by a friend of mine in hopes of shedding some light on the issue of exhaust size selection for turbocharged vehicles. Most of the facts have been covered already. FWIW I'm an turbocharger development engineer for Garrett Engine Boosting Systems.

N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.

For turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end.

Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.

Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.

As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.

"As for the geometry of the exhaust at the turbine discharge, the most optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter-- via a straight conical diffuser of 7-12 included angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge. Many turbochargers found in diesels have this diffuser section cast right into the turbine housing. A hyperbolic increase in diameter (like a trumpet snorkus) is theoretically ideal but I've never seen one in use (and doubt it would be measurably superior to a straight diffuser). The wastegate flow would be via a completely divorced (separated from the main turbine discharge flow) dumptube. Due the realities of packaging, cost, and emissions compliance this config is rarely possible on street cars. You will, however, see this type of layout on dedicated race vehicles.

A large "bellmouth" config which combines the turbine discharge and wastegate flow (without a divider between the two) is certainly better than the compromised stock routing, but not as effective as the above.

If an integrated exhaust (non-divorced wastegate flow) is required, keep the wastegate flow separate from the main turbine discharge flow for ~12-18" before reintroducing it. This will minimize the impact on turbine efficiency-- the introduction of the wastegate flow disrupts the flow field of the main turbine discharge flow.

Necking the exhaust down to a suboptimal diameter is never a good idea, but if it is necessary, doing it further downstream is better than doing it close to the turbine discharge since it will minimize the exhaust's contribution to backpressure. Better yet: don't neck down the exhaust at all.

Also, the temperature of the exhaust coming out of a cat is higher than the inlet temperature, due to the exothermic oxidation of unburned hydrocarbons in the cat. So the total heat loss (and density increase) of the gases as it travels down the exhaust is not as prominent as it seems.

Another thing to keep in mind is that cylinder scavenging takes place where the flows from separate cylinders merge (i.e. in the collector). There is no such thing as cylinder scavenging downstream of the turbine, and hence, no reason to desire high exhaust velocity here. You will only introduce unwanted backpressure.

Other things you can do (in addition to choosing an appropriate diameter) to minimize exhaust backpressure in a turboback exhaust are: avoid crush-bent tubes (use mandrel bends); avoid tight-radius turns (keep it as straight as possible); avoid step changes in diameter; avoid "cheated" radii (cuts that are non-perpendicular); use a high flow cat; use a straight-thru perforated core muffler... etc.

"Comparing the two bellmouth designs, I've never seen either one so I can only speculate. But based on your description, and assuming neither of them have a divider wall/tongue between the turbine discharge and wg dump, I'd venture that you'd be hard pressed to measure a difference between the two. The more gradual taper intuitively appears more desirable, but it's likely that it's beyond the point of diminishing returns. Either one sounds like it will improve the wastegate's discharge coefficient over the stock config, which will constitute the single biggest difference. This will allow more control over boost creep. Neither is as optimal as the divorced wastegate flow arrangement, however.

There's more to it, though-- if a larger bellmouth is excessively large right at the turbine discharge (a large step diameter increase), there will be an unrecoverable dump loss that will contribute to backpressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge.

As for primary lengths on turbo headers, it is advantageous to use equal-length primaries to time the arrival of the pulses at the turbine equally and to keep cylinder reversion balanced across all cylinders. This will improve boost response and the engine's VE. Equal-length is often difficult to achieve due to tight packaging, fabrication difficulty, and the desire to have runners of the shortest possible length.

"Here's a worked example (simplified) of how larger exhausts help turbo cars:

Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:

(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure

So here, the turbine contributed 19.6 psig of backpressure to the total.

Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case).

So in the end, the engine saw a reduction in backpressure of 12.6 psig when you swapped turbobacks in this example. This reduction in backpressure is where all the engine's VE gains come from.

This is why larger exhausts make such big gains on nearly all stock turbo cars-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level.

As you can see, the backpressure penalty of running a too-small exhaust (like 2.5" for 350 hp) will vary depending on the match. At a given power level, a smaller turbo will generally be operating at a higher turbine pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbine than a larger turbine/turbo would. As for output temperatures, I'm not sure I understand the question. Are you referring to compressor outlet temperatures?

The advantage to the bellmouth setup from the wg's perspective is that it allows a less torturous path for the bypassed gases to escape. This makes it more effective in bypassing gases for a given pressure differential and wg valve position. Think of it as improving the VE of the wastegate. If you have a very compromised wg discharge routing, under some conditions the wg may not be able bypass enough flow to control boost, even when wide open. So the gases go through the turbine instead of the wg, and boost creeps up.

The downside to a bellmouth is that the wg flow still dumps right into the turbine discharge. A divider wall would be beneficial here. And, as mentioned earlier, if you go too big on the bellmouth and the turbine discharge flow sees a rapid area change (regardless of whether the wg flow is being introduced there or not), you will incur a backpressure penalty right at the site of the step. This is why you want gradual area changes in your exhaust."
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Old 02-03-2003, 07:38 PM   #5
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sorry dude, i'm just giving you my personal opinion, which is what the guy asked for. The DP does make a lot better power down low than others I've seen(which isn't many). I'm not going by marketing hype, i'm going by experience with the SS.-James
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Old 02-03-2003, 08:32 PM   #6
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I'm a SS owner, but I can't say I'm qualified to comment since I've only dealt with one exhaust on my car. I have been in a car with a Kakimoto turbo back exhaust and my car's power delivery was smoother. Power didn't feel too different between the two cars...sorry I can't tell ya more.
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Old 02-03-2003, 09:44 PM   #7
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I bought the SS because:

It works
Proven fit
Allowed me to keep the stock 3rd cat
Good feedback from other SS owners
Oh, and it works

I did not buy it for the bling bling factor. I didn't want a shinny uber crome piece that would have the 'man'questioning it. Right now, it's dis-colored just right, blends in with the rest of the bottom end of the engine IMHO.


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Old 02-04-2003, 10:45 AM   #8
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Thanks for everyone's response!
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Old 02-04-2003, 10:46 AM   #9
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I bought mine because of what Big Joe said, plus it is cheaper then some of the other polished ones. Who the hell cares what some piece of exhaust pipe underneath the car looks like; they will all look the same after one winter anyways. I will have to say that rally perfomance's customer service bites though. They need to hire some more help or something, and they have to be losing sales because of it. They could at least make a post now and then, or update their website, letting us customers know when products like the Ecutek Stage 2 remap will be available.

Last edited by MMastro; 02-04-2003 at 11:31 AM.
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Old 02-04-2003, 11:48 AM   #10
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I have a SS downpipe and 3rd cat elim. I've had it on with everything from the stock turbo up to a PE1820 and no complaints.

I bought it because of the SS modular design (replaces stock exhaust parts) and b/c there wasnt much else around when I started shopping.
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Old 02-04-2003, 01:34 PM   #11
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I've been running a SS downpipe and front midpipe for about 35K miles. No complaints at all.

Due to the power I'm making and the boost creep I've been experiencing, I've just ordered a Perrin catless downpipe. I need 3", and I need a divorced wastegate tube.

I'll post up a comparison in a couple days when the Perrin gets here.
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Old 02-05-2003, 12:53 PM   #12
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Austin - definitely give us an update on the perrin & your boost creep issue.
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Old 02-05-2003, 01:08 PM   #13
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I love my perrin downpipe. you can read my review on the car parts section. I cant pinpoint its exact gains however as i did the UP DP pulley and IC hoses all at once. The Perrin DP is by far the best constructed DP, more so than the SS one. Its 321 stainless and is just beautiful to behold and awesome to feel.

chris
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Old 02-05-2003, 03:27 PM   #14
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I have the Scoobysport down pipe with a Vishnu up -pipe. The up-pipe came first with noticable improvement. Then added the downpipe with REALY noticable improvement.

I highly reccomend both items. The down pipe made the most noticable change. Still have stock exhaust for now will be completing the system and will be using full Scoobysport.

Owen
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Old 02-05-2003, 05:40 PM   #15
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Quote:
Originally posted by Austin
I've been running a SS downpipe and front midpipe for about 35K miles. No complaints at all.

Due to the power I'm making and the boost creep I've been experiencing, I've just ordered a Perrin catless downpipe. I need 3", and I need a divorced wastegate tube.

I'll post up a comparison in a couple days when the Perrin gets here.
I'll be very interested.

Kevin
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Old 02-05-2003, 06:59 PM   #16
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First off, Alta Performance is awesome! I ordered a Perrin downpipe Monday at 3pm, and UPS delivered it to my door Tuesday at 4pm. That's quick shipping!

I threw it on last night.

I haven't done datalogs yet to post an exact comparison, but my car spools considerably quicker with the Perrin. With the VF34 and ScoobySport downpipe, I was spooling to 15psi at 2960rpm in 4th gear. With the VF34 and the Perrin downpipe, I'm spooling to 15psi in 4th gear between 2750 and 2800rpm. That's 150-200 rpm's quicker!

Low rpm/off boost torque felt identical between the Scoobysport and the Perrin... it's still a 2.0 liter 8.0:1 compression four cylinder.

High rpm/high boost response is much improved, as one would expect when making around 380hp and moving from a 2.5" pipe to a 3" pipe. The car is definitely faster.

As far as fit/finish...
The Perrin downpipe is a mandrel bent beauty. No welding slag anywhere in the pipe. Fit perfectly.
The Scoobysport downpipe is a crush bent ugly duckling. Of course no one cares about having a shiny downpipe, but the Scoobysport has ripples and bumps down the length of it. There was also quite a lot of welding slag inside the pipes. Fit perfectly.

I bought the Perrin because of the divorced wastegate tube - an attempt to cure my boost creep. Running off wastegate spring pressure, my car creeps up to 1.35 bar (20psi) boost around 5000-5500 rpm. I thought a divorced wastegate tube would be my answer... but I thought wrong. My boost creep is unchanged between the two pipes... Woe is me...



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Old 02-05-2003, 07:10 PM   #17
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sucks about your boost creep austin but great review of the perrin pipe! maybe this will help dispell the hype about low end torque and downpipe design.


btw...thats disgustingly fast spool for a vf34...
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Old 02-06-2003, 11:16 AM   #18
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Thanks for the pics and followup!
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Old 02-06-2003, 01:22 PM   #19
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I thought that comparison would do better in it's own thread...

http://forums.nasioc.com/forums/show...hreadid=311965
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Old 02-06-2003, 05:09 PM   #20
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Happy W/Scoobysport DP.
Stock 03 w/just that..definatly feel more power.
no idea in comparison w/perrin though
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Old 02-06-2003, 05:15 PM   #21
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I would think just the switching from 2.5" to 3" with you bigger turbo it would help out alot. It would be nice to see a comparison with the stock turbo(what the SS is designed around)-James
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Old 02-14-2003, 04:53 PM   #22
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Any you guys read this,

http://www.users.bigpond.net.au/msgo...stsystems.html

Where do you think APS got the idea from? They were asked if they tried this years ago and to reply that " there is no gains we found"

The twin pipes don't work as it has been pointed out the WG only just opens and flow is directed into the main flows path using a IHI turbo, Mr Garret was smarter and turned the pivot point of the WG 90 deg so it opens and gas is mixed with the main flow and doesn't create turbulence. With the second pipe on twin pipes the flow from a IHI turbo and no divider will see most of the gases not even use the second pipe, so fit a divider and then you increase flow restriction as the gases expand into the back of the turbo, around the valve and then are forced back into a small pipe which all in turns adds to backpressure and less than perfect WG operation.

As for an exhaust to be as big as you can isn't totally true, might be ok if you are after the last kw at redline, what happens is you loose to much fresh charge out the exhaust valves on cam over lap, maybe you should check with Prodrive on this and i am sure you will get the same answer as i got

Open mouth is best WITH a divider/splitter that fit ALL the way into the back of the turbo, The difference in torque will blow you away guaranteed!


ANY exhaust i have removed and fitted mine I have made gains, the biggest notice is low end power and mid range torque. I look at what some of your tune shops over there and what they sell makes me laugh

My V5 STI with nothing more than a cold air exhaust and small add ons makes 20 psi @3000rpm under full load with the std VF28 turbo and IC. I got gains of 66kw ATW with cold air , exhaust and retained the std air box, IC, engine and turbo.

http://www.users.bigpond.net.au/msgofast/STI.html

Don't take my word for it, copy it and make your own!

The TDO4 & 5 WG swings in the same direction as the VF, so yes there are big gains there too.

Splitter lengths are different between turbo's, TDO turbo's are only 22mm deep, VF turbo is 32mm deep bar the VF30/35 which is 27mm deep, you need to use 3mm flat plate or it WILL bend , it only protrudes into the dump pipe flange, so on a VF total length will be 44mm using a 12mm dump pipe flange plate.

The splitter needs to be in the edge of the step in the back of the turbo ( VF) with small 5mm radius on the corners and needs to be a reasonable snug fit.

The collector has to be 4" at the flange and down to 3" ideally over 12 " long in the tapper with a slow tapper, works well on a pre01 , but 01 on run a larger steering shaft and the tapper of the collector has to be made shorter, 5-6" , either way the tapper needs to be without a sharp reduction and 3" all the way to the tip. If you run a CAT it needs to be in the mid section, running them in the dump pipe kills low end power, look at the WRCar, they have there's at the end of the exhaust, deleted is even better

Some Jap tuners make systems that delete the rear S bend and places the muffler exit at an angle, this might be great for the last 4 kw in the top end , but you loose up to 15 kw in the mid range, if you have this type, place the muffler in the OEM position and add two 90 deg bends as per the OEM route and tell me how much better it pulls!

The V8 2.0lt STI uses a VF35 and is 27mm deep to the back, so the splitter length will be 39mm long with 27mm protruding and the turbo is 60mm high in the back, hopefully the flange plate is also 60mm high, It maybe be large , but just center the splitter and weld it in, remember to use 3mm plate, I buy a length of 50 X 3 mm black plate and cut it up.

I will give you some specs for the location of the splitter in the flange too.

If you are looking at the flange opening there are 5 bolts that hold it on, there is 1 above and below the Waste Gate, if you measure from the top hole to the right 19mm going from the hole center this will mark the position of the top spot for the splitter. Off the bottom hole measure 22mm from hole center to splitter plate center. This is measuring towards the turbine side.

http://www.scoobynet.co.uk/bbs/threa...=175694&Page=2

You will notice that the bottom hole under the WG is more to the left, more than 3mm and the plate doesn't look like it is square in the flange , but it does line up spot on with the step in the turbo.

I find that if i scribe a line to mark the protrusion length before i grind the radius on the corners on the splitter plate then you can use this line along with the lines you mark of the gasket face of the flange plate you can eye ball the line for protrusion , tack it in then make sure it sits right angle to the flange ( sits up straight) and then full welded with a nice fillet weld. This has been track tested in my own car for 3 yrs now and you need to use 3mm plate or it bends, first one was exhaust plate and it gets hot and bends.


The STI do have problems with over boost due to the mapping of the boost control in the ECU and there tinny exhaust. You would do good to Buy a Turbo XS MBC as these hold boost rock solid, they use a ball and spring and are more than just a shinny tap.

There are many guys that found they got more bottom end or mid range with 2.5" exhaust, this is mainly due to poor design in the dump pipe or the lack of the OEM S bends at the rear, You can still run 2.5 mid section , but now you will gain power right across the rpm with 3", how many people have fitted an exhaust and felt they lost power down low or at part throttle? This system makes GAINS EVERYWHERE, most are surprised at how much better it is off boost and how much quicker it comes on.


MSR
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Old 02-15-2003, 02:30 AM   #23
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One thing about scoobysport they've been doing scooby tuning long before Perrin was alive and no one has really talked about warranty's I know scoobysport has a 25 year not sure about Perrin......I should have my scooby any time now Ordered it directly from them....Just my 2 cents.......Ron
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Old 02-15-2003, 05:29 AM   #24
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Quote:
Originally posted by grnlantern1
One thing about scoobysport they've been doing scooby tuning long before Perrin was alive and no one has really talked about warranty's I know scoobysport has a 25 year not sure about Perrin......I should have my scooby any time now Ordered it directly from them....Just my 2 cents.......Ron
Nice load of BS there, scoobysport started in 1996. Mr Perrin isnt a 7yr old kid...your 2 cents are worthless.

Perrin stands behind all his products but its not like his high quality 321 stainless downpipe would ever have any issues. Its a much higher quality piece than the Hayward and Scott scoobysport...especially the welds. Some people like the sloppy hand made look (I even find it kinda appealing for some reason) but these two pieces arent even in the same league.
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Old 02-15-2003, 02:25 PM   #25
thebusiness999
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MSR- Thanks for that informative post. Do you think the gains you'd get by doing this to a ScoobySport downpipe justify all the work this mod requires?

I figure I'll do this when I upgrade my turbo and switch to a 3" exhaust system, but I'm curious as to whether or not this would give big enough gains in driveability to justify doing it now. What do you think?
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