Exhaust system diameter vs torque

[205freak 7 1 Cars]

Hi, I would like to know if anyone have "played" with exhaust diameter pipe and the efects on engine loosing or gaining torque along the rev range.

My current exhaust system was built for the 1.6 engine 115bhp, a 50mm pipe with middle silencer from a 406 1.8 (45mm inlet/outlet) and standard rear backbox (45mm inlet/outlet).

 

My current engine is xu10j2, with a lightened bottom end 1.9 diesel crank, skimmed/ported head, kent pt27, magnex manifold 4-1 with a compression ratio of 12.4, would the above exhaust system be restrictive through the rev range.

 

Would you think that changing the exhaust pipe to 55mm diameter, using the 306 gti middle silencer with the standard rear muffler internally modify for 55mm pipe, would loose torque mid range??

 

Looking forward to hear your thoughts.

 

[petert 630]

I’d make it at least 50mm all the way through. An Mi16 was 50mm and made 160hp. You wouldn’t have that much hp.

Edited April 8 by petert

[DamirGTI 358]

I found silencers to be the most restricting part , along with the manifold and overall pipe bends . 

 

If using Ph 1.5/2 reflection rear silencer , switch to Ph 1 absorbtion (glass pack) .

So make them both absorbtion glass pack  middle and rear , if quality made and with sensible tail pipe dia. noise level wont be much higher if any , compared to reflection silencer .

Used to run similar spec 1.9 , simply put i went much better with absorbtion silencers .

 

 

[205freak 7 1 Cars]

On 4/8/2026 at 2:50 PM, DamirGTI said:

I found silencers to be the most restricting part , along with the manifold and overall pipe bends . 

 

If using Ph 1.5/2 reflection rear silencer , switch to Ph 1 absorbtion (glass pack) .

So make them both absorbtion glass pack  middle and rear , if quality made and with sensible tail pipe dia. noise level wont be much higher if any , compared to reflection silencer .

Used to run similar spec 1.9 , simply put i went much better with absorbtion silencers .

 

 

Damir, thanks for the input. Based on the information I’ve gathered, the exhaust system can affect low-, mid-, and high-range torque. I’ve sourced an ECE-compliant unit made in Spain, featuring a straight-through middle silencer with a 50 mm perforated core. Paired with the standard rear muffler—which is also straight-through with a 47 mm internal core—it shouldn’t result in any noticeable loss of low-end torque and may even provide a slight gain at higher torque levels

[205freak 7 1 Cars]

On 4/8/2026 at 1:56 AM, petert said:

I’d make it at least 50mm all the way through. An Mi16 was 50mm and made 160hp. You wouldn’t have that much hp.

Petert, thanks for the input. Will be 50mm al the way through minus the rear silencer, which the internal diamenter is 47mm, the ball figure of bhp should be around that figure more or less.