Q: Exhaust size and Cut-off valve

[cobrAA]

Just trying to make the most out of my actual bolt-on components, you often read that around 600whp its time to move on to 3.5 or 4inch.

So technicly, if you have a 70mm dump pipes matched with a 70mm downpipe, you would need a single 4 inch (100mm) to match the flow of the 2x70mm. With that said:

1) Did anyone had the chance to see back-to-back result coming from 3 inch to 3.5 or 4 inch on a ''mild-built aka 550-650whp'' turbo car ?

2) After reading this article, http://www.tercelreference.com/terce...st_theory.html ( who knows if this is the truth..?), it appears it would be better if the size would come up gradually. However, most Cut-off valve are beeing installed right after the downpipe. Whats your thought on this ? Would you still install the cut-off there and scrap the theory ?

Ideally, I would pay a exhaust shop to install a 4 inch cut-out valve right after the merging of the 2x70mm and reduce it to my 3 inch resonator and keep a very civil 3 inch exhaust.

[Skym]

The gradual size increase is the megaphone effect (what Apexi call it, who were the first in Japan to use it). The gradual increase is to stop turbulence, which can create a blockage of sorts. It should be max of 0.5 inch difference to prevent turbulence. Apexi explain more about it, so search the web using "Apexi Megaphone exhaust". From what I understand, the megaphone exhaust creates a suction effect that helps to remove exhaust gasses from engine. Also helps with response.

Exhaust size, type depends on what you want to achieve. Dual front pipe into single catback exhaust is for more torque. Dual exhaust from turbo's (one exhaust from each turbo) gives more mid to high hp. Veilside, Kakimoto, Apexi sold the dual exhaust's and Veilside used the dual exhaust on their R34 GTR with 1500hp (engine) RB26.

About exhaust size, can check via putting pressure probe in exhaust. If getting alot of back pressure, then change size of exhaust. Dyno, boost level should tell you as well (lower hp output, higher boost level for that output). Also there are calculations, search forum.

[Dragon Humper]

If you are going to do a cut out then stay with 3"/3". A 3" exhaust will be nice and civil and the 3" dump will flow as much as a 4"full exhaust when conbined with the 3" exhaust. Put the cut out where the CAT would be with a Ypipe (that usually comes with the kit). Do yourself a favour though and get the cutout that is a gate instead of a butterfly, the butterflies will rattlle and drive you nuts.


Jon.

[cobrAA]

I bet you mean these ? http://www.jegs.com/p/Dougs-Headers/...82659/10002/-1

and these are butterfly? http://www.quicktimeperformance.com/QTEC/

Then, all I need is a nice set of 70mm dump pipe OR... i keep my 58mm R34 oem dump paired with my 70mm downpipe and call it the megaphone theory lol! :P

[NoLimit0596]

You can check also race ready performance cut out it butterfly style but it doesn't rattle like other compagnie. I have it on my car and it works perfect

[Dragon Humper]

Hmmm...mine is a race ready and it rattles like crazy. However...mine is a 4" and i have it in line of my 4"exhaust to close it off and force all the exhaust through a 2" bypass pipe with resinator. That might be the reason.



Jon.

[NoLimit0596]

Oh first time i hear that from race ready personally i work in muscle car shop and we install this valve alot....

[cobrAA]

im curious how did you manage to avoid huge backfire under your car ? did you make the valve exit on the side ?

[Dragon Humper]

Most of the installs have them just dump under the car, so if you tend to shoot flames, that will look cool and scare the hell out of people.

This is what I did to give you an idea, the small pipe vents out the side and downward right under the corner of the bumper. Very quiet on the high-way.



Jon.

[cobrAA]

I do have backfire form time to time, i just dont want to damage under the car if that happens so I was wondering how you done it. thanks :P

[cobrAA]

You think this thing will fit before the cat ? http://www.youtube.com/watch?v=qi1VAtl6M9g

[cobrAA]

You can check also race ready performance cut out it butterfly style but it doesn't rattle like other compagnie. I have it on my car and it works perfect

where did you mount it ?

I think I will simply chop off a piece right after the merge of the 2 pipe into one and put a 2.5inch cutout there. This way I'm removing the bottleneck where it goes 2x70mm into 1x80mm.

[Dragon Humper]

Do you have to run a CAT in QB? If so, putting a cutout right before the CAT is just as illegal as running no CAT. You would be better off setting up the cutout to replace your CAT as a bolt in and just swap them out come inspection time...or when you get busted. Use the 3" Doug's Headers one from Summit Racing (or anywhere cheaper) with the cutout Ypipe and you have all you need for an exhaust shop to put it in.

If you type "exhaust cut out rattle" in any search engine you will find an assload of posts about the butterfly style....not one about the Doug's gate style. The only thing bad about them is they take up much more room so install is trickier.




Jon.

[cobrAA]

Do you have to run a CAT in QB? If so, putting a cutout right before the CAT is just as illegal as running no CAT. You would be better off setting up the cutout to replace your CAT as a bolt in and just swap them out come inspection time...or when you get busted. Use the 3" Doug's Headers one from Summit Racing (or anywhere cheaper) with the cutout Ypipe and you have all you need for an exhaust shop to put it in.

If you type "exhaust cut out rattle" in any search engine you will find an assload of posts about the butterfly style....not one about the Doug's gate style. The only thing bad about them is they take up much more room so install is trickier.




Jon.

I agree on your butterfly rattle comment.. However, I'm running a varex since 3 years now and never heard it rattle. Varex system dont rattle or beeing at the end of the car I can't heard it but either case, it worked great.

In QC, you need to run a cat, but I still got it replaced by an ULTRA-QUIET vibrant resonator. IMO, police care more about the sound db than the smell.

[Dragon Humper]

Im pretty sure the amount of rattling directly corresponds to the size of the valve...mine is 4"...rattles horribly. A tiny little 2" one, like in your muffler wouldnt have as much of an issue due to the much smaller surface area...and its in the muffler so that is hiding alot of it. I have nothing but pipe so the noise would be amplified. I may try and put a bigger O-ring into the seat and see if that helps.


Jon.