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3" vs 2.5" debate

3.8K views 38 replies 24 participants last post by  BDJ's Racing  
#1 · (Edited)
3" vs 2.5" depate

I just finish reading the 3" post and there seems to be alot of confusion on what works for you. Im going to try to done learn ya. :)



3" or 2.5"?

A 2.5" system will flow plenty. You can put down 400hp with a 2.5" system, this is a fact. Now at 400hp, if you where to swap out your system for a 3", then you WILL yeild more HP, 10+ hp from swapping to a 3" is not far fetched.

A 2.5" system will flow plenty, but the current catback systems out do not cut it though and this is why:

The american and european exhaust systems utilize slip fit setups where as the asian market uses flanges. I do not like the slip fit setups because they are difficult to disassemble being that the pipes are clamped to each other. It also makes it difficult to get a full upgraded exhaust that you can missmatch from different manufactures.

When you upgrade your exhaust, you will have a bottleneck in it. The great thing about the flange designed systems is that you can have a 3" catback bolt up to a 2.5" downpipe etc. For example, on an mkiii T supra, you can install a 3" Dp which will bolt on to your stock catalytic conveter. A racepipe will also bolt on to your stock exhaust system even though one is a 3" and the other is a 2.25". Now when you upgrade your catback to a 3", you will have a full 3" system with out having to modify it. This allows you to get X brand DP and mate it to a y catback brand.

The current catback systems currently out all have a 2.25" bottleneck. So your going from a 3" DP (assuming you have one) down to a 2.25" bottleneck that chokes your system then back into a 3" or 2.5".

Those of you who already have a Dp and purchase a catback, I suggest you take it to a muffler shop and have them cut the bottleneck. Have them take a 2.5" pipe and expand one end to a 3". This will alow you to get a smooth transition into a 2.5". Going from a 3" to a 2.25" then back to a 2.5" is a pretty poor design. But this is what the manufactures and customers have to deal with unless they purchase a complete 3" system from a manufacture.

What size exhaust you want sould be based on how much HP you want to yeild. A true 2.5" is plenty, but it will eventualy become restrictive. It all depends on what you want. Purchasing a 3" is purchasing insurance for future mods.

The more HP you make, the more HP you will get out of your exhaust system.

If you want to make the most HP, just take a straight 3" DP and leave it open. To loud for me and not practicle though. But that will yeild the most HP.

OH,

I remember an SP supra that had about 900+ HP to the wheels with a 3" exhaust. When they swapped it out for a 4", then gained about 100 HP with that alone!
 
#3 ·
hey hector, I do I get rid of that bottle neck??? I have the mopar Exhaust, and I have the stock downpipe, and cat.. how do I get rid of that bottle neck, what do I have to ask the muffler guys?
 
#6 ·
metfan2000_99 said:
hey hector, I do I get rid of that bottle neck??? I have the mopar Exhaust, and I have the stock downpipe, and cat.. how do I get rid of that bottle neck, what do I have to ask the muffler guys?
At this point, your bottle neck is the the whole exhaust from exhaust manifold to cat. You get the bottleneck that Hector is talking about if you have an exhaust like the Borla and install another company's 3" dp. Reason being that the Borla has comes to 2.25" before it meets up with the stock dp, so when you swap in a 3" dp it mounts up to the borla just fine, but without a bit of modification to the borla, you still have the small 2.25" piece of pipe at the end of the borla where it meets the dp. Does that make sense or am I just rambling?
 
#8 ·
Thanks for the lesson Hec.

I have a Mopar avail to me right now. Been debating on dynoing with 3 inch and Mopar just to get some good numbers.

The bottle neck is in the first part of any aftermarket cat back for the srt-4, made to fit the stock cat.

The stock cat has 2.25 inch pipe comming out the bottom. So for the aftermarket systems to hook up, it has to connect to the 2.25 inch cat, then step up to the 2.50 dia. Most aftermarket dp's are also made to hook up to the 2.25 inch bottle neck, so you end up going from 3 inch dp to 2.25, then back up to 2.5 for the rest of the system.

There is one aftermarket company that does offer a dp that hooks to the 2.5 inch section, but the bottle neck has to be cut off the cat back to install it.
 
#11 ·
maximus454 said:
well if you have to buy a 2.5 turboback, better to buy a 3 in.right away, the price difference is not that big i suppose ? or is it
I sell a modified Mopar cat back and DP as a package. The dp hooks straight to the 2.5 inch section of the Mopar with a flanged joint, eliminating the bottleneck.

With a stainless steel dp, the whole set up can be had for under $700, all stainless, bolt on application, no modifications needed.

PM me if interested.
 
#12 ·
everyone is forgetting about the biggest bottleneck in the system..............the outlet of the stock exhaust manifold...........it measures an eggshape size of approx just under 2 inches to just over 2 1/4 inches................just look at the pics of the stage turbo upgrades from the detroit auto show that have been posted on the forums here..................when i replaced my stock system with O2 housing downpipe race pipe and borla i took my die grinder and opened up the opening of the exhaust manifold and blended back as far as i could i opened up the opening to just under 2 1/4 inches to just over 2 1/2 inches................this bottleneck was told to us by ethan (PVO) at the detroit meet last summer.............and he explained that until this is removed the gain from larger systems will not see the full effect of the system........dont shoot me im just the messanger
 
#14 ·
I have the mopar exhaust right now, and later plan on getting a 3' dp. If I understood correctly, the 2.25 inch bottle neck comes from the cat outlet?

So if I get a 3' downpipe, couldn't I just get a new high flow cat with the down pipe? I'm assuming the hi-flow cat that would come with the 3' dp would have a 2.5 inch outlet, and then I'd have to modify the mopar exhaust to accept a 2.5' pipe instead of a 2.25 bottleneck pipe.
I'm no pro on this so please correct me if I'm wrong guys. Thanks
 
#15 ·
qwiksilvertrav said:
I have the mopar exhaust right now, and later plan on getting a 3' dp. If I understood correctly, the 2.25 inch bottle neck comes from the cat outlet?

So if I get a 3' downpipe, couldn't I just get a new high flow cat with the down pipe? I'm assuming the hi-flow cat that would come with the 3' dp would have a 2.5 inch outlet, and then I'd have to modify the mopar exhaust to accept a 2.5' pipe instead of a 2.25 bottleneck pipe.
I'm no pro on this so please correct me if I'm wrong guys. Thanks
Go here:

www.maxxfab.com

I'll make the perfect dp for you. If you can operate a hack saw, you can install my dp tha't installs to the 2.5 inch section of the Mopar cat back.

You can get the dp in aliminized or stainless, with or without cat.

Would love to do business with you.
 
#18 ·
Hector- I remember back from DSM day's it would actually hurt to have a 3" exhaust system if you did not push the boost to back it up. Is this true with the SRT's too?
 
#19 ·
Yep, You need to have a flange added to Borla where it slips onto stock DP. I don't think anyone makes a DP with that slip type fitting. With just the Mopar/Borla on stock O2 & DP you get between 5-10 hp, ditto for Tq.
 
#21 ·
srt4geezer said:
Yep, You need to have a flange added to Borla where it slips onto stock DP. I don't think anyone makes a DP with that slip type fitting. With just the Mopar/Borla on stock O2 & DP you get between 5-10 hp, ditto for Tq.
I make a dp with a 2.5 inch slip fitting to connect to the 2.5 inch part of the Mopar removing the bottle neck.

All you need to do is remove the intermediate pipe from the Borla, cut off the bottle neck (hack saw will work), reinstall it, install the dp, and install the band clamp.

Or I can make a dp with a 3 bolt flange joint. An exhaust shop has to install the supplied 3 bolt flange to the intermediate pipe of the Borla.
 
#23 ·
alterego said:
Keep in mind when you put a 3" exhaust (02 housing back ) on that you will run a lower EGT temp by a few hundred degrees at the turbo because of the exhaust gases leaving at a easier/quicker rate. This is a nice point that goes hand in hand with the extra HP.
I'm sure you gonna expect this from me:D ....got any #'s to go with this?
 
#24 ·
scottmech said:
I make a dp with a 2.5 inch slip fitting to connect to the 2.5 inch part of the Mopar removing the bottle neck.

All you need to do is remove the intermediate pipe from the Borla, cut off the bottle neck (hack saw will work), reinstall it, install the dp, and install the band clamp.

Or I can make a dp with a 3 bolt flange joint. An exhaust shop has to install the supplied 3 bolt flange to the intermediate pipe of the Borla.
Scott, are you fabricating intercooler pipes?
 
#25 ·
warriorbob said:
everyone is forgetting about the biggest bottleneck in the system..............the outlet of the stock exhaust manifold...........it measures an eggshape size of approx just under 2 inches to just over 2 1/4 inches................just look at the pics of the stage turbo upgrades from the detroit auto show that have been posted on the forums here..................when i replaced my stock system with O2 housing downpipe race pipe and borla i took my die grinder and opened up the opening of the exhaust manifold and blended back as far as i could i opened up the opening to just under 2 1/4 inches to just over 2 1/2 inches................this bottleneck was told to us by ethan (PVO) at the detroit meet last summer.............and he explained that until this is removed the gain from larger systems will not see the full effect of the system........dont shoot me im just the messanger
Good post and you hit the nail on the head about the exhaust manifold sizing. The same rule applies with a CAI system pipe size. You can only get so many CFM of air through the throttle body not matter what the size of the pipe.
 
#26 ·
PTC01 said:
Good post and you hit the nail on the head about the exhaust manifold sizing. The same rule applies with a CAI system pipe size. You can only get so many CFM of air through the throttle body not matter what the size of the pipe.
Yeah, what happened to that guy who's making a bigger TB for our car?