I'm on the hunt to find out why it's taking soo damn long for my turbo to spool up. Have a 62mm T4 divided hotside (.82 ar) with a smaller 58mm coldside. I see positive pressure as low as 2400 rpm but it takes till 4400 rpm to see 10 psi. Recently cleared a few boost leaks I've found and it dropped from 4600 rpm to 4400 rpm. The only thing I can think of at this point is potentially the 2.5" exhaust is restricting exhaust flow severely. Here's all the details on my build:

13B-REW
3" Downpipe
3" -> 2.5" midpipe
2.5" cat-back

Opinions?

It was recommended to run 4" DP -> 3", however don't have enough room to make it fit. Do have enough room to go 3" from the mid-pipe back though. But again would the 2.5" be causing that much back pressure?

 

Is that a stock cat back?

 

don't have an answer for you but....

3 inch............... area is 7.06 sq inches

2.5 inch............... area is 4.9 sq inches

3 inch is 44% larger and works well to 600.

area is calculated on what is probably the O D rather than I D but you get the drift.

howard

 

If that's a stock (or similar chambered) cat-back, then the losses will be much greater than a couple feet of 2.5" pipe. In fact, the stock catback, especially the early ones, are the worst flowing single piece of the stock exhaust system.

Some 2.5 will affect spool, but shouldn't kill it--especially if it's relatively straight. Figure out what the your catback actually is internally then assess. A 3" catback of the straight through variety would likely help, and be the best bang for the buck, IMO.

 

Suppose pics will help out better:




Exhaust necks down to 2.5" at the cat / midpipe

 

For giggles I reached out to Comp turbo who advised that yes too restrictive an exhaust will cause a lazy spool up because it'll drop the efficiency of the turbine wheel due to the back pressure. So the question then becomes how much flow is lost (turbulence / back pressure added) when a 45º / 90º elbow is introduced into the exhaust.

 

Miata chassis? 3" will fit fairly easily. Have a 3" ArTech exhaust on my current NA and built one for my old NA. Give Abe a shout and have him hook you up.

Any restriction behind the turbo will hinder performance. While Howard, and engineers like Corky Bell, say 3" is good for 600hp, there are good and bad 3" systems. I've seen a full 3" system from a BIG name rotary shop kill spool and power at the 400 whp level. Was nice and quiet though.

 

I was actually looking at the 3" catback that flyin miata has. Only thing is before I spend more money, wanting to ensure it's the exhaust causing the issue. Not to mention I spent a good chunk of change on the existing exhaust. Few things I'm going to do is:

Pressure check the exhaust at both the downpipe O2 sensor bung and at the o2 sensor bung on the cat to see how much pressure its actually seeing.

Second thing is disconnect the mid-pipe and do a test run.

 

your exhaust is acting as a huge bottle neck.. That turbo wants to move some cfm and your exhaust is telling it to **** off.

 

Run it open downpipe would be the best thing you can do to troubleshoot the lag. I've never run anything smaller then 3inch on any build hands down.

 

If you were to run a 3" high flow cat into 2.5" exhaust, you would not be loosing much spool but rather some top end. Someone would do this for sound restriction.