I have the opportunity to purchase a 2.5 inch Racing Beat downpipe for $80.

My question is: Will this make it much harder to reach my goal of 300whp than a 3inch DP if I run a 3 inch cat back after this 2.5 inch DP, will that affect performance at all?

 

all depends if ur turbo'd or not!! if ur turbo'd go with the 3", if ur not stick with the 2.5".

 

I see it's for an '89TII. Don't bother with the 2.5". Go with the 3" and you'll find it will be easier to obtain your goal. The addition of a 2.5" dp will have only a very minimal, at best, effect on hp.

 

Bollocks. A 2.5" DP will have a very noticable effect on performance compared to stock. I know, I've just fitted one. For mild mods a 2.5" DP is fine, but a 3" one will always be better. That's just logical.
However if you're aiming for 300rwhp then you're going to have to change the entire turbo anyway, so even if you buy or make a 3" DP for the stock turbo you'll need to get another one made for the new turbo.
Since you can get the 2.5" one cheap now, I'd just get it. It means less money wasted on your way to buying a new 300rwhp turbo/manifold/wastegate/DP combo.

 

I didn't find much of a difference with my DP. There was some, just not a lot. Don't know your other mods, but how much of a boost increase did you get?

 

For $80, why not? For that price, go ahead and get it. Then find yourself a bada$$ old school Nissan/Datsun tuner to build you the rest of your exhaust. 300rwhp used to be my goal too. But that was way overshot with this BNR Stage 2 turbo.

 

I have an '89TII fitted with a Bonez DP (2.5") & hi flow cat....A waste of money. A little more sound, and boost only 7/7.5#. Not worth it.

 

Turbonut: Do you have a 3" catback as well? I have the Bonez combo, and it was a huge difference for me compared to stock. But, if you still have stock catback, then the difference i probably not that big....
I had MazdaSpeed 3" catback with stockcats for a year, little difference but not much, but together with the Bonez turboflo = fells like 40 bhp! Not to mention spoolup....

 

I should've been a little clearer I guess. If you've already replaced the rear of the exhaust, then fitting a DP will give a very noticable increase in performance. If you still have the stock cat-back, then the restrictive rear mufflers will prevent a new DP having much effect. If you're going to replace the exhaust one section at a time, work from the back to the front to get your money's worth. Plus a cat-back will improve the exhaust note, but a DP won't. Very little because the downpipe I had made has a seperate pipe for the wastegate, so it's controlling boost very well. The turbo now boosts up much quicker and the car feels alot quicker and more responsive. With a 2.5" divided DP, a 3" into twin 2.5" cat-back and a pod filter I'm still not hitting fuel cut, but it must be really close...