knock only on second turbo

[davehg]

now this seems strange to me, maybe someone can shed some light on it for me. a standard twing turbo (recent rebuild) runs perfectly fine on the first turbo up to .8 bar boost with knock readings no higher than about 20, i can rev it (NO BOOST) to the red line (knock is fine) but when the second turbo coms in around 5k RPM the knock goes mad to over 125. now whats confusing me is...

a) thie engine should of popped at these sorts of readings

b) what the hell could it be? ive changed the ingnition coils.

as soon as knock appears i take it out of gear, it can then jusmp as high as 200.....


does anyone have an idea what it could be? the map is fine

only mod is APFC and exhaust

many thanks

[Jit]

something resonating in the engine bay at higher engine speeds under load in gear?

[strange_days_uk]

yeah the knock sensor is just a microphone, so maybe something a little loose somewhere

[davehg]

could be but i do trust the knock readings, the last thing i want to do is ignore it as i could pop the engine if there real knock. ill change the sensor tomorow but im 99% sure its fine.

[strange_days_uk]

check the injector duty as you come on boost

could be the fuel system is maxxing out

did you have any porting done ?

[davehg]

at 0.8 bar on the first turbo the inj duty is only 60% and the engine etc is bog standard

[strange_days_uk]

I would monitor the injector duty as the knock increases, and see if they are related.

If not, maybe a loose earth in the sensor wiring, damned if I know.. :Santa

[davehg]

i had the plenum off today and all pips seem ok, checked the two earths on top of the rear rotor housing for the loom and there ok plus the one for the coils. the main earth to the tin plate that comes off the ancillary bracket ant the bottom of the front housing is ok as well........ its strange but true lol:Tongue1

[davehg]

anyone else got any thoughts or has had the same probem?

[Carl]

Hi Dave,
What is the injector duty like?
Also does it only happen when you back off the throttle?
If it is only when you back off then that is ok.
You do however need to be concerned if it is under load and full throttle.
Regards
Carl

[davehg]

hi carl

the duty is about 60% and it does happen on load thats why im concerned but at the same time im confused as i would expect it to happen on the first turbo as i can run that up to .8 bar up to 5k rpm

[Carl]

Then maybe a secondary injector is playing up.
Best to get it checked out I think.
Regards
Carl

[davehg]

okill change them for a set that i know are good, its just strange how i can take it to 7 k under light load and all is well, then take it to 0.8 bar up to 5k and thats fine as well but when i go onto the second turbo at 5k or above it throws a hissy fit

[davehg]

well today i replaced the secondary injectors. took it for a spin and i think i can safely say its icking up noise from else whee as a couple of times i had no knock at 6.5k and other times i did, it even hit over 200 whilst driving along out of gear lol

[Carl]

Maybe it needs to have some more fuel top end.
It might be an idea to get it to a specialist to have it checked out.
Regards
Carl