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Boost question with stock muffler

629 Views 6 Replies 6 Participants Last post by  Rx7thHeaven
I am juat starting the mod process on my car and have a quick question. My car is an auto and stock with the exception of a RMM downpipe wastegate spring mod and hole in the front of stock airbox. I just put the greddy BCC and a manual boost controller on the car and turned the boost up. I know with the spring mod you should be able to hold 18psi to redline but my boost is falling to 15psi. I dont really know many people who have a downpipe and stock muffler so this might be a difficult question. Also today was about 90 degrees outside. Does this sound about normal or should I be getting more boost???


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That stinks for the car to breathe so well with a DP and then hit the restriction of the factory exhaust. I personally don’t think Id be attempting to run that much until you get a good 3"or larger cat back, and possibly a test pipe in place of the cat.

Id also consider maybe not running that much boost on the factory intercooler. I’m sure it can be done, but I'm also sure the efficiency won’t be great and the turbos are working harder than they need to, to push 18psi with a factory side mount in place.

Check out this link as well for info on the stock twins holding set boost to redline

The stock muffler isn't all that "bad". An after market setup would help of course. The stock sidemount is designed just fine. It may get heat soaked and become less efficient but part of the design of the SMIC is as a heatsoak. Just don't do too many 4th and 5th gear pulls in a row and it will be fine. I am your exact setup minus the spring mod. and my Hallman holds it up to 18 so I'd look for a leak.
I wouldn't do it too much on the stock exhaust. Alot of backpressure there. However, I would start looking for boostleaks, that sounds like the culprit to me!
...and possibly a test pipe in place of the cat...
The RMM downpipe replaces all the cats.

Over 7 years ago when I started moding mine, I did the RMM DP but left the stock catback in place. Even before I had a boost controller I did the bleeder tee mods, and with various bleeder sizes I had no trouble boosting to 17psi, maybe more I forget now.

I agree with bonz about the stock sidemount. It holds up just fine even with close to half hour of hard laps on a roadcourse during track day events. As far as boost capability, I ran my current small single for a couple months on the stock sidemount, and quite a few times hit over 25 psi and never had a problem with it.
"The stock intercooler does a decent job up till about 17psi on the stock turbos, after that you would probably notice a significant gain, especially in warmer temperatures, with a nice front mount intercooler."

I’m not sure what the pressure drop is on the stock side mount IC, but keep in mind the turbos have to push more than what your actually seeing to reach your desired boost level. There is something to be said for the efficiency
Thanks for all the replies, I will start out looking for a boost leak and then go from there. I have no prob hitting 17/18 lbs but it falls pretty rapidly to 15 by about 6k rpm and holds that to redline.
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