Open/Dump Exhaust=knock

[Spl170s]

Dealing with some frustrating knock issues.

When i have the car running through the exhaust i have no knock to speak of from a dig.....a little if i romp on it during a roll though.

But, when i open the exhaust at the test pipe, i get a ton a knock all the time. From a dig it is clean at first then it goes nasty the faster i get. From a roll...forget it.

When it does show knock with the exhaust open....it always goes to 18.8 at the highest...always that exact nuumber.....

what gives?

 

[tim87tr]

Your exhaust is usually a restriction. With an open dump your will normally get an increase in boost, therefore more flow, and run lean. Increase your fuel pressure to supress knock or you may need to turn down the boost with the open exhaust. Or just add alky or some good race fuel and enjoy the extra power.

 

[Spl170s]

i turned down my boost to try and accomodate and still had the issue. I understood how the boost will go up on a open exhaust......but why to 18.8 everytime? Something just sounds wrong with that scenario?

i could up my fuel pressure some i guess...right now i am 42 line off.. octane shoudn't be my problem. that is taken care of.

 

[Ormand]

octane shoudn't be my problem. that is taken care of.

The octane is too low for the engine conditions, or you wouldn't have the knock. So it seems it is NOT "taken care of". It may be that you need less boost, less timing, more fuel, whatever, but if you have knock, the problem is not taken care of.

 

[Spl170s]

My main point being, if i know i have the fuel, and i have the octane issue fixed....could it be false knock considering i get the same 18.8 everytime it happens? My O2's are reading where they should be.

Again, has anyone experienced false knock only when they open up their exhaust?

 

[rb68rr]

How much boost are you running? What chip, what octane fuel, etc.?

 

[Spl170s]

22psi exhaust closed
Turbotweak alky chip
razor alky
94 octane

 

[Under Pressure]

car runs leaner with the exhaust open hens your knock issue....

 

[Spl170s]

so is it common for people to turn up the fuel pressure and or alky to compensate then when they dump exhaust?

 

[Roc87]

It is very common for a TR to lean out with an open exhaust. It is typical to lean out the WOT with alky (Because methanol is fuel replacement), but with an open cut out, you'll have to increase the alky delivery even more.