New to boost, please help resolving knock at 7-8psi

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deathtrapmiata
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Posts: 34
Joined: Mon Mar 24, 2014 12:39 pm

New to boost, please help resolving knock at 7-8psi

Post by deathtrapmiata »

1994 mazda miata 1.8l (4cyl)...low compression built motor, forged pistons 99 headswap, header, coil on plug igniton, RX-8 425cc injectors, 190lph fuel pump

Finished installing Dual throttles to my Hot side supercharger ( a Jackson Racing M45)...no intercooler...but meth injection.
The meth injection nozzle is tapped in right before the throttle/intake...running a 3.0 GPM nozzle that sprayed based off presure (via pressure switch teed into a vacuum line)
it is a progressive spray that is set to start the mist right at 7 psi and up. the car is running 93 octane fuel, and the methyl alcohol is 49/51 ratio in the fluid for the spray.

basically the car runs just fine in the crusing cells, IE: map kpa from 20-100...and then only some of the time, (not everytime) I reach my max boost It hesitates/ knocks

Ive taken plenty of spark advance up in those cells in the spark map, ...at 2psi ~ 17-18 degrees, 4-5 psi, 15-16 degrees, and at max 7-8 psi(156kpa) ~ 14 degrees.

^^thats as low as I took it from those top cells in the spark map, Ive never seen anyone take that much timing for just 8psi, and Im starting to wonder If I took TOO MUCH spark from the get-go and thats why its knocking.

as for the VE table, it should be fueling fine. I had run VEAL to tune at first and started with having the cells higher than expected. they actually took away fuel from what I had set in there when street tuning, and so were looking at VE's of 107-118 in the high range. and I watched AFR's when the knock occured, they were in the low 13's in low boost and down in the 12's, and 11.8 at max boost.

still getting knock and I just need to now what else i need to do to stop it from happening..

1) what about spark plugs, the coil on plug ingition....possibly do i need to tweek the dwell settings or spark settings? also would tightening my spark plug gap help?
2) Meth injection? should I try to turn the pressure up, so it only starts to spray right at the end (7.5-8psi), or maybe the meth is actually choking out my engine...maybe i dont even need it!
3) is it possible that I took away too much spark, and can having too little spark advance also cause knock?

its also come to my concern that the meth-injection nozzle could be too big, and maybe it could be moved to the intake BEFORE the supercharger, it would then be placed up-stream of the intake temperature sensor and cool the blower as well, but then i dont know if the alcohol would get the chance to act as fuel in combustion by the time it reaches the cylinder head. it could help seal the roots in the S/C and create a slight increase in boost!, so ive been told

any other suggestions would be greatly appreciated.
attached is a copy of the current tune and a datalog showing some 5th gear pulls up to 150kpa
elutionsdesign
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Re: New to boost, please help resolving knock at 7-8psi

Post by elutionsdesign »

Your intake air temperatures are warmer than your coolant temperatures at high boost, over 190 F in that log. That just seems too high even with the methanol injection.
Graduate of EFI University.

I build, repair, install and tune Megasquirt systems in North Dakota and beyond!
deathtrapmiata
MS/Extra Newbie
Posts: 34
Joined: Mon Mar 24, 2014 12:39 pm

Re: New to boost, please help resolving knock at 7-8psi

Post by deathtrapmiata »

The intake temp sensor is bunged into the charge pipe about 6 inches right out of the supercharger outlet, the way the hole is welded it does set a little low.
it is a GM open element sensor.

the bad thing is that the meth injection nozzle is located down-stream of this sensor, in that same charge pipe, about 6 inches from the throttle body on the intake manifold

also it has come to my realization that the methanol-injection nozzle is way too big (has to much flow / spray)

its a Devil's own DO3 nozzle, 3gph...with my water pump set at 200psi this is spraying the equivalent flow for a 400 HP engine!

Last night I ordered a smaller (DO1 / 1gph) nozzle, which actually flows ~1.2 -1.4 @ 200 psi; we will try that because i really believe that the mixture was "choking" out the combustion...the "knocks" actually felt more like hesitations, like when you burn somthing to ECU while in throttle or the car is running

please also note that I turned the pressure switch all the way up to 8psi, to see if that was the case, Im not 100% sure on this but it did actually seem to reduce

** So If anybody needs a nozzle let me know! i have a DO3 and a DO7 that i don't need!
39ClevoUte
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Location: Australia

Re: New to boost, please help resolving knock at 7-8psi

Post by 39ClevoUte »

Hi you mentioned low compression, be interested to know value. Thanks Rob
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