downfall of running 3" exhaust?

Joey's mx · Post by **Joey's mx** » February 5th, 2006, 1:08 pm

Ok i was wondering if there is any downfall on running 3" exhaust on a boosted DOHC b6? I will be running 8-10 psi! I am trying to figure out if i should go with 2.5" or 3" ?? Which do you think is best and why?

babyblueMX3 · Post by **babyblueMX3** » February 5th, 2006, 4:06 pm

Personnaly I'd go with 2.5 on your set-up

osargeant · Post by **osargeant** » February 5th, 2006, 4:20 pm

For your intended boost levels along with the fact that you are only 1600cc, you should be looking at no bigger than 2.5" for the exhaust. A 3'" exhaust will be way too big for your application and you will notice low end loss as a result.

Do some research on the matter and you will see what I am talking about.

Joey's mx · Post by **Joey's mx** » February 5th, 2006, 4:45 pm

I was leaning on the 2.5" exhaust myself but just wanted soem other opinions....thanks guys!!

osargeant · Post by **osargeant** » February 5th, 2006, 5:53 pm

BTW I hope you arent planning on using the 460cc injectors that I see in your sig, they are way too big to be controlled by the S-AFC, you want 310c, 330cc or 370cc injectors maximum unless you are going full standalone.

Joey's mx · Post by **Joey's mx** » February 5th, 2006, 6:06 pm

Really?? I hear of some poeple running the 460cc injectors in there B6-BP turbo set-ups!! So there is a limit to what the safc will control??

osargeant · Post by **osargeant** » February 5th, 2006, 10:17 pm

First of all, I cant say I know of any B6 or BP MX3s with a SAFC that are running 460cc injectors. Most of the fuel controllers out there like SAFC, Emanage, HKS SAFR etc only allow you to control injectors up to 150% over the stock size, the stock size on the MX3 is around 210cc, so that works out to be 210x1.5=315cc, therefore allowing you to run 310 or 330cc injectors effectively. Anything larger may still work but not very well and idle will probably be greatly affected.

ryanlindenberg · Post by **ryanlindenberg** » February 5th, 2006, 11:42 pm

yea i'd say go 2.5", that's what i have for my BPT. i dunno if 3" would necessarily hurt though. would 3" hurt his low end torque with the turbo providing backpressure? just curious, i havn't really studied this in depth or anything.

BuGS · Post by **BuGS** » February 5th, 2006, 11:51 pm

osargeant wrote:First of all, I cant say I know of any B6 or BP MX3s with a SAFC that are running 460cc injectors. Most of the fuel controllers out there like SAFC, Emanage, HKS SAFR etc only allow you to control injectors up to 150% over the stock size, the stock size on the MX3 is around 210cc, so that works out to be 210x1.5=315cc, therefore allowing you to run 310 or 330cc injectors effectively. Anything larger may still work but not very well and idle will probably be greatly affected.

Well ya know one now. I am running 460cc injectors with a SAFC II controller. I have a smooth idle, nice numbers, and very good tuning.

That is exactly what everyone else told me. I finally found one other guy that told me No one knows anything. He has the same setup as I do. So really, shut up man. You don't know what you are talking about.

Back to the topic, go with 2.5". If you ever need to go 3" you just do a 3" dp with an electronic cut out after the DP, saves weight and does better then a full 3" would. And usually cheaper too

osargeant · Post by **osargeant** » February 6th, 2006, 9:05 am

I dont want to take this topic off course but I feel the need to respond to Bugs comment. First off I do know what I am talking about with regards to the SAFC and large injectors, there was a guy around here with a turbo 1.6 DOHC with the SAFC 1 that was running 440cc injectors, car idled like piss and ran horribly in general despite tons of different settings, he shortly took them out and put in BPT 330cc injectors and that immediately solved the problem. I am not sure if the SAFC 2 is better or if it works better with the BP ECU but I am almost certain he will have problems with those injectors on the 1.6 DOHC MX3.

Aston Wards · Post by **Aston Wards** » February 6th, 2006, 2:56 pm

Surely whether or not the 460cc injectors work is largely irrelevant, unless you're pushing over 400hp, you're not gonna need them so why pay the extra?
same goes for the exhaust..., unless you've got the hp to back it up, 3" is just overkill!

this isn't stuff that i'm an expert on..., it just seems like commonsense to me

MazdaKiller · Post by **MazdaKiller** » February 6th, 2006, 11:13 pm

good point

Aston Wards wrote:Surely whether or not the 460cc injectors work is largely irrelevant, unless you're pushing over 400hp, you're not gonna need them so why pay the extra?
same goes for the exhaust..., unless you've got the hp to back it up, 3" is just overkill!

this isn't stuff that i'm an expert on..., it just seems like commonsense to me

BuGS · Post by **BuGS** » February 6th, 2006, 11:48 pm

First off 460cc RX7 injectors are like $75, cleaned.

2nd, I don't think your friend knew how to work the combination of a VAF and SAFC in tuning then. A Miata in the Dyno shop with the 1.6L had 550cc injectors running off a SAFC 1 and that car ran just fine.

Flashpoint2 · Post by **Flashpoint2** » February 7th, 2006, 12:37 am

I think something that hasn't been mentioned in this thread is the POINT of a bigger/smaller exhaust.. which is to maximize the velocity of the exhaust that is being expelled. If you have too large an exhaust, your flow rate is gone. With too small an exhaust, you end up bottlenecking. For our cars, unless you happen to be Babyblue a 2.5 should be enough.

nos92mx3 · Post by **nos92mx3** » February 7th, 2006, 8:54 am

Let me see if i can dig up a post i was reading on another forum.
When it comes to exhausts on a TURBO car, bigger is better but only to a certain extent. 2.5" will free up power, 3" will free up a little more and so on a so forth, until its overly large, (like 4-5 inch.)

Run a full 3 inch exhaust, dont bottle neck it from 3 ->2.5 youll lose power.
Again let me find the link with DYNO proven results.