Exhaust backpressure measurements

sbest

Registered User
It is sunny and hot out but instead of lounging by the pool I gotta be frigging with my car. And like the troglydite I am, gotta run in to tell you about it. I drilled a series of test ports into my stock exhaust system to test for back pressure. I know from experience that most stock systems run around 5psi +-3psi, most performance systems run 2-5psi. Cats have generally raised backpressure in most systems by a couple psi.

Well, I drilled a series of 5mm holes and tapped them M6.
I drilled a 4mm hole up through a long (2"?) 6mm bolt and cut the head and thread off to about 6mm length thread, 30mm unthreaded shank. I fitted (read, JB weld) a fitting for 4mm plastic hose and routed up to a pressure guage in the cabin on the passenger seat.

The holes were drilled:
1) between the passenger manifold and the cat
2) between the passenger cat and the "Y" cat
3) between the "Y" cat and the resonator
4) just behind the resonator (before the fuel tank)
When not plugged in to the guage the holes were plugged with a short 6mm bolt and washer and a little Hi-Temp copper RTV.

In hole #1 I noticed approximately 2.5psi per 1000 rpm at full throttle/boost(10-12psi), maxing out at 12psi at 5000rpm. This pressure would spike in excess of 15psi on full power shifts for a brief instant. Cruise pressure at 70mph was 1-2psi whether in 4th or 5th.

Port #1 max steady pressures at rpm
1000 = 2psi
2000 = 4psi
3000 = 7psi
4000 = 10psi
5000 = 12psi
5500 = 12psi

Maximum steady full throttle backpressure at ports:
1) 12+psi
2) 11psi
3) 8psi
4) 4psi

This kind of tells me the cats, especially the center "Y" one, are very restrictive. That resonator also doubles the backpressure and the tailpipe to the mufflers ain't any great shakes either. It all gotta go. A potential 10psi backpressure reduction is awesome.

The results of this testing were that I booked an appointment to get my exhaust system revamped. Unfortunately it is Aug 7th due to the guy's vacation plans. My plan is for 2¼" pipes (for better flow and scavaging) from the 2" ported manifolds, into a dual 2½" to 3" resonator, then 3" to the back and dual 2¼" (or think I need 2½"?) mufflers out the back.

Just using mild steel for economy as I may be making changes as time goes by. I have a Flowtech(?) resonator for the center but any recommendations for QUIET freeflowing mufflers?

Steve Best
 
Back
Top