Ok, engine in my lifted rig with a half a million body miles has been having detonation issues for a while now and I'm finally fed up with it. Detonation occurs any time I lug the engine below 2500 rpms in any gear but 1st gear (assuming the gearing is low enough that the engine can out pull the detonation since the wheels break loose and engine revs faster)

 

I've tried everything I know to stop detonation and I cant find a cure. I've ran snowmachine spark plugs with a 9 rating (coldest plug you can get) RMK700 plugs are identical to subaru plugs just have a colder temp rating. This helped a tiny bit in the summer when it was really hot out. I've done away with EGR completely, reinstalled a fully functional emissions system off another federal engine setup. Tried a decel valve off an old chevy 350 style carb setup to stop the raw fuel from getting sucked into the engine on decel. Turned timing down to 4 degrees (still detonates, engine doesnt like 4 degrees BTDC) Ran high octane fuel (91 is highest we can get at pump) havent tried av-gas yet cuz I'm not paying 8 dollars a gallon for 120 octane. I've used every style of octane boost known to man except Valve-Tek since I can't find a supplier up here that sells it. (Valve-tek works wonders under marine applications with these newer Fuel injected Big blocks we use in out jetboats that require a min of 92 octane.)

 

The block in the car is a 1994 EA82 SPFI engine with the heads shaved down as much as the machine shop would do (not sure how much it is, its still not a valve bender) so its got more compression than a stock carb block, but Ive seen 350's running 11:1 compression without a hint of detonation.

 

I guess the only thing that could be causing these issues is the vac advance on the carb? its not retarding the timing on acceleration? (not sure if i understand the principals of vac advance but from what I remember, timing advances under cruising rpms, and then drops under massive accel (since vac drops) correct?

 

Give me some other ideas I might be overlooking please. I know I need to ditch this carb setup and goto FI but I wont be able to do it until I get another one of my cars on the road (92 Loyale, 92 SVX which ever one comes first lol)

 

Thanks for the responses this might get appreciate ppls help.

Put the EGR back on.It will help.

 

You may need water injection w/your compression.

 

Different engines have dfferent octane requirements even though they may have the same compression ratio.How much did you say you`re running?

 

Those older 350s get away with it because they are also running high overlap cams and modified advance curves.

SPFI is 9.5:1 so thats the original ratio. not sure what it is now with shaved heads. cant be over 10.3:1 or so. Engine rips balls on Top end. it'll smoke a fuel injected wagon any day of the week.

 

If I'm going to hook up EGR again, I'm going to have to find a different style valve. I dont like the subaru one with the 1/2 inch line going to their chit decel valve. I guess I could just vent that to atmosphere too.

 

water injection would be pointless on a 90 hp engine, not to mention having to do oil changes more often due to more condensation in the engine.

Overly high compression, bad fuel, too much advance, too lean, too hot, local hot spots, oil burning... they can all cause detonation.

 

How long has it been doing it? I mean if it's just started since you planed the heads then it will be compression, if it's got gradually worse then oil burning or carbon build up.

 

How bad is it? Just a second as you first put your foot down, all the time below certain revs, only when it's hot?

its occcuring pretty much anytime below 2250. Hot/cold dont make a different, fuel quality dont make a difference. I may have to take a vacumn line off and run some water through one for a lil bit. I've heard taht will knock off some carbon off the valves/pistons/top of the cylinder.