knock or no knock????? (Page 1/2)
jjd2296 SEP 17, 04:34 PM
3.4 PR swap, adding sc with 6-8lbs of boost. do I really need a knock sensor or not??
fierofool SEP 20, 08:03 PM
When I had my ECM programmed for my 3.4, it was strongly recommended that it have the knock sensor function activated. The port is there on the trunk side of the block, so it's really no big deal to use it. Due to my diminished hearing, I wasn't hearing the spark knock on my 3.4 until a passenger told me. I've since retarded the spark a bit and assume that fixed my problem. I haven't installed the 7730 yet, but the function is there when I do.
pmbrunelle SEP 20, 08:25 PM
My knock sensor hasn't picked up any knock on my 3.2 L turbo at 11 psi, non-intercooled. I only use 91 octane gasoline. My engine has a compression ratio of 7.4.

I didn't hear any knock either, and the spark plugs looked OK when I've inspected them. I know that the knock sensor nominally functions because it registers something occasionally when there's a backfire.

It's a fairly low boost level you wish to run, but an engine's propensity for knock will be setup-dependent. I'd suggest staying vigilant, and checking your spark plugs often. Besides the having the mechanical parts in good working order, you want to get the engine running with a good tune.

There's no hard answer to the knock sensor question... do you want another knock-detection method in your toolbox besides your ears, butt, and spark plugs? Many naturally aspirated engines have knock sensors.
DimeMachine SEP 20, 09:27 PM
If it is a decent knock sensor system, I would. It can sometimes hear knock before you can and validate it when you think you hear it.
Joseph Upson SEP 21, 08:42 PM

quote
Originally posted by pmbrunelle:

My knock sensor hasn't picked up any knock on my 3.2 L turbo at 11 psi, non-intercooled. I only use 91 octane gasoline. My engine has a compression ratio of 7.4....



7.4:1 is a seriously safe compression ratio. Not suggesting in anyway that you change it, but there's a lot of performance left on the table with a compression ratio lower than that of the Corvair.


quote
Originally posted by fierofool:

When I had my ECM programmed for my 3.4, it was strongly recommended that it have the knock sensor function activated. The port is there on the trunk side of the block, so it's really no big deal to use it. Due to my diminished hearing, I wasn't hearing the spark knock on my 3.4 until a passenger told me. I've since retarded the spark a bit and assume that fixed my problem. I haven't installed the 7730 yet, but the function is there when I do.



If you can install a knock sensor do it. In the mid 90s when I started experimenting with Turbocharging, I boosted a 2.8L Fiero and ran as high as 12 psi on premium fuel, retarding the timing as you have done, without a knock sensor and using rudimentary additional fuel (extra injector). I eventually broke a ring land after about 3 yrs, but that had a lot to do with the lack of tuning ability and weak cast pistons. The hypereutectic pistons are much stronger, but I destroyed some of those also after subjecting a 3.4L to excessive and repeated pronounced audible spark knock, sounded like popcorn popping in a can it was so loud. The ring lands held up, but the top edge of three pistons were chipped and a top ring broken.
pmbrunelle SEP 21, 09:26 PM

quote
Originally posted by Joseph Upson:
7.4:1 is a seriously safe compression ratio. Not suggesting in anyway that you change it, but there's a lot of performance left on the table with a compression ratio lower than that of the Corvair.



Damn right there's a lot of performance left on the table... hence why I'm not done cranking up the boost

Once I get a clutch that doesn't slip, I'll be shooting for about 18 psi.
jjd2296 SEP 21, 09:50 PM
Thanks all. Going with 6869 ecm and knock enabled.
Joseph Upson SEP 22, 08:00 AM

quote
Originally posted by pmbrunelle:


Damn right there's a lot of performance left on the table... hence why I'm not done cranking up the boost

Once I get a clutch that doesn't slip, I'll be shooting for about 18 psi.



Got it, in that case consider looking into having a clutch built locally if there's a shop around capable of doing it. I had my clutch built (kevlar) and pressure plate modified for more clamping pressure locally for the turbocharged 3900 and I never had a problem with it holding the high torque loads with the high compression, and held at 21 psi with water/meth injection. I also noted some of the advertised HO clutch options did not appear to have anything more than a performance grade lining and clamping pressure equal to, or a little higher than the original 80s Fiero pressure plate where they bothered to list it, but not higher than the later application pressure plates (which I used as a base) that retrofit back to the Fiero. Don't fall for the HO clutch advertised with an HP rating and no clamping pressure, or torque holding rating.

[This message has been edited by Joseph Upson (edited 09-22-2020).]

jjd2296 SEP 22, 10:10 AM
is inter-cooler or water/meth injection needed if you staying 8lbs or under with a supercharger??
Joseph Upson SEP 22, 12:15 PM
Superchargers are a bit more complicated when it comes to water/meth injection. I've never used one, but understand there are potential problems associated with injecting water/meth into certain types. Inter coolers are no problem, just depends on the type in use as to the efficiency relative to the boost level.

Water to air with a heat exchanger beneath the blower is usually a safe bet, but an air to air at low boost levels has to be able to maintain a net gain after plumbing restrictions are accounted for, relative to what is lost to heat when running direct air induction without one. If 8 psi is only generating about 150-170 F inlet temps and with an intercooler you are only able to get those temps down to about 120 F, but with a 1 psi loss in boost pressure, you may only be breaking even in performance results, in which case it would not be worth the trouble.

20 plus yrs ago I believe the rule was 7 psi, or less didn't need intercooling, but things have changed since then. State of the art engine management systems and high output / L factory motors may be able to make better use of small gains in inlet temp reduction.