downfall of running 3" exhaust?

Josh · Post by **Josh** » February 7th, 2006, 3:07 pm

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

so first off, 460cc good for 400HP. yah i dont think so. closer to 300WHP. im maxing my 310cc injectors out at 230. now if you knew about what Joey has in his car.

Joey-

you want the 460cc's, if you dont believe me and want to listen to thies guys then talk to cjthor . he is runnign 460cc on his B6 before he rebuilt his motor was running somewhere around 220 WHP. but with your turbo (16G) and cams (miata stage 2) i highly recomend 460cc injectors. they will give you alot to play around with. the only problem anyone has had with them on a B6 is at idle they run rich. but its nothing you cant fix with a decent coil, or MSD system, or just simply playing around with your idle adjustment. thats why its good to have a piggy back as well as a adjustable fuel pressure regulator.

now with the exhaust i highly reccomend a full 3", makes no difference with a turbo motor from 2.5" vs 3" other than at higher RPM's you will loose power with a 2.5" at high RPM's. its about the velosity of exhaust flow and NO backpressure is needed. you dont want backpressure. its a common misconception on turbo cars. like i said before with your turbo and cams i would go full 3" with a high flow cat, resionator, and nice muffler. other wise you will be kicking yourself later for not doing it. with that 16G you can Max thoes 460cc injectors.

Aston Wards · Post by **Aston Wards** » February 7th, 2006, 4:04 pm

Hey if he's gonna be getting 400hp+ (at the fly), then yes by all means get the big injectors...., same goes for the exhaust. I'm just not a fan of paying for stuff i don't need....., and chances are, he won't need it, having a list of (planned) mods in your sig doesn't make ANY hp!

Josh · Post by **Josh** » February 7th, 2006, 5:25 pm

Aston Wards wrote:Hey if he's gonna be getting 400hp+ (at the fly), then yes by all means get the big injectors...., same goes for the exhaust. I'm just not a fan of paying for stuff i don't need....., and chances are, he won't need it, having a list of (planned) mods in your sig doesn't make ANY hp!

im saying that 460cc injectors are not good for 400HP. for 400hp you will need at least 600cc to 700cc injectors. Now the reason to go with the 460cc injectors will be for tunning. they are alittle rich at idle BUT with the AFC you will not starve you engine of fuel under boost, and they are cheep as hell. now that 16G is a nice size turbo and he will need the fuel. im saying that with a VF10 im maxing out my 310 injectors @ 14psi. the 16G moves a hell of alot more air than the VF10. and thats the whole pourpose of having a piggyback system. so you get alittle oversized injectors, and tune it down with the AFC. its not too difficult to understand.

Joey's mx · Post by **Joey's mx** » February 7th, 2006, 5:33 pm

Hey if he's gonna be getting 400hp+ (at the fly), then yes by all means get the big injectors...., same goes for the exhaust. I'm just not a fan of paying for stuff i don't need....., and chances are, he won't need it, having a list of (planned) mods in your sig doesn't make ANY hp!

I have all those parts and they are currently being installed on my car!! Thanks Josh and nos92...

SO to clear things up I should run with 3" exhaust and 460cc injectors??

babyblueMX3 · Post by **babyblueMX3** » February 7th, 2006, 7:47 pm

Joey's mx wrote:
Hey if he's gonna be getting 400hp+ (at the fly), then yes by all means get the big injectors...., same goes for the exhaust. I'm just not a fan of paying for stuff i don't need....., and chances are, he won't need it, having a list of (planned) mods in your sig doesn't make ANY hp!

I have all those parts and they are currently being installed on my car!! Thanks Josh and nos92...

SO to clear things up I should run with 3" exhaust and 460cc injectors??

how much hp are you planning on ?

Joey's mx · Post by **Joey's mx** » February 7th, 2006, 7:59 pm

I honestly dont know! I am just setting things up so it is running safely and well with 8-10psi and then going from there!!! I want to end up around 14-15psi in the end!! So whatever that will get me!! I would atleast want 220-230

babyblueMX3 · Post by **babyblueMX3** » February 7th, 2006, 8:09 pm

that's the thing, IMO you set your exhaust size with your hp target

according to the exhaust size vs bhp graph in Maximum Boost,

2.5" is for 450+bhp
3" is for 700+bhp

sure it's not the same in real life but it gives a general idea.
it's an exponentiel scale too.

I chose 3" over 2.5 for a faster spool up time and I could care less about low end since I'm never under 3000rpm anyways.

jaydog5678 · Post by **jaydog5678** » February 8th, 2006, 8:33 am

babyblueMX3 wrote:that's the thing, IMO you set your exhaust size with your hp target

according to the exhaust size vs bhp graph in Maximum Boost,

2.5" is for 450+bhp
3" is for 700+bhp

sure it's not the same in real life but it gives a general idea.
it's an exponentiel scale too.

I chose 3" over 2.5 for a faster spool up time and I could care less about low end since I'm never under 3000rpm anyways.

Yeah I'm really not with Corky on his exhaust - to - piping diameter on this one. From expierience, I can say that a 3" is the way to go, no matter what size turbo you have under the hood. If the turbo is small, a 3" is going to spool pretty quick. It's also going to make more torque than a 2.5" exhaust - hands down. Torque is what we're really looking for anyway. I made over 20whp switching exhaust diameters and over 20ftlbs at the same time. I would have to say, "It makes a difference".

Franko · Post by **Franko** » February 9th, 2006, 2:46 pm

Ask any tuner.. 3 inch mandrel bent is the way to go with ANY turbo car. The key thing with a turbo car is to get the exhaust out of there as quickly as possible with little restirction.

projectmx · Post by **projectmx** » February 9th, 2006, 7:30 pm

when i started researching turbo for my car everyone said ready corky bells book on turbo.. can't recall the name of it i think its maximum boost.... i could have sworn in that book for a 4 cyl. turbo engine the smallest exhaust size for good over hp gains was 3in. i am currently running 2.25" on my na 1.6L dohc (done before i decided on engine swap and whatnot) and everyone said i'd loose power blah blah blah... i wouldnt go any smaller the gains was amazing so i can't imagine doing turbo on a pipe only .25" bigger you need less restriction for turbo... going higher than 3" unless you are going for over 400 hp would be dumb but if you are shooting for 200-300 ranger then go 3" if you want good injector advice either listen to poeple who have successfully turbo'd the same engine or go to forums.miata.net or http://www.msprotege.com and talk to any of the guys there who boost also

osargeant · Post by **osargeant** » February 12th, 2006, 10:49 pm

I am finding this post very interesting, I always thought that for our sized turbo engines 1.6 and 1.8 that 2.5" exhaust was the way to go and this was the route I took. Well i am always ready to admit when I am wrong and I will have to give serious thought to looking at a full 3" exhaust especially with the extra mods that I have done recently.

projectmx · Post by **projectmx** » February 12th, 2006, 11:09 pm

someone needs to freakin pull out corky bells' "maximum boost" and look up the answer cause that is suposedly the best book on boosting engines that way all us amatures and shut up and learn from an expert... if i had time i'd go down to the library to get it but i'm in the process of moving and looking for a place to live so i have little time... but hopefully someone will have the time to spare

nolig2278 · Post by **nolig2278** » July 16th, 2006, 11:37 am

projectmx wrote:someone needs to freakin pull out corky bells' "maximum boost" and look up the answer cause that is suposedly the best book on boosting engines that way all us amatures and shut up and learn from an expert... if i had time i'd go down to the library to get it but i'm in the process of moving and looking for a place to live so i have little time... but hopefully someone will have the time to spare

the book costs about $30, buy one, it helps, but it still will say just about everything that has been said in this forum and it can lead you to beleive 2.5 or 3 for reasons stated in forum. I have a 2.5 madrel on my bpt and i dont feel as if 3 would have been a help.

The smartest thing would be just do as mentioned, get a 2.5 system with and elctronic port for when you really need to let the air out. an electronic port costs under $200 installed, and allows you to have a daily driver with power under 4000 rpm.

Steeb · Post by **Steeb** » July 16th, 2006, 1:22 pm

a turbo pushes exhaust gasses out so the faster it can push it out, the earlier it will begin to make power. the more air it can flow will also result in more hp gained.

using a larger dp on any turbo'd car is always better because you will only loose power by going too small. too small can also result in boost creep/power not gained.

for turbo cars, more flow = better power and response throughout the whole rpm range. loosing power from having an exhaust diameter too large only applies to n/a cars. im not sure about s/c

here are a couple of dynos on a factory turbo'd car with only difference of downpipe size. even at lower hp levels, we can see the advantage of 3" vs 2.5" just imagine the difference it makes at higher hp levels.

Runs 1-3 2.5" Donwpipe
Runs 12-14 3" Downpipe

2.5" Downpipe - Runs 4, 5, 6
3" Downpipe - Runs 12, 13, 14

as you can see the only drawbacks in comparison of the 2.5 vs 3" downpipe is flow capability. the worst thing you can do to your turbo is put a restrictor on it to limit its flow.

turbo exhaust:
too small=

too big=

na exhaust
too small=

too big =

MechaManZero · Post by **MechaManZero** » July 16th, 2006, 8:30 pm

lol can we say PWNAGE