Hey gang..

Just brought home my Hemi Superbird and did some tuning yesterday. The motor pings under load cruising down the street. At the present time, the timing is set to about 34 degrees total advance at 2500 rpm. In the event I can't get a good combo for the curb idle timing and idle speed setting, I may have to go to additive of the motor has a higher than OEM compression. Not sure what that compression is at this time. What octane additive do you all use? I'm looking for suggestions. I haven't had a need to do that in a long time. I may try the FPO Advance Limiter plate for it so I can get good initial curb timing while maintaining a reasonable total limit, say 30-32?


Cheers,
Troy

Most octane boosters are useless unless you are just barely on the cusp of knock. Read the labels, if they say they raise it a number of "points" they mean points as in 0.1. So you'd go from 93.0 to 93.6, if they raise it six points, for example.

Royal Purple's Max Boost claims up to 30 points, so that 93 octane would bump up to 96 octane. It uses MMT There are others that claim it can bump pump gas, I assume premium, to as much as 105 octane.

Basically, the good stuff isn't cheap, the cheap stuff isn't good and the good stuff tends to be "not street legal" for whatever reason. Most likely because they use MMT and the amounts needed to raise pump gas exceeds the EPA's limit and there is some disagreement as to the toxicity of MMT when handling it. It degrades readily when exposed to light, so if it's in a can it's probably got MMT in it.

Is the vacuum advance hooked up too? How much timing is that adding? Cruising down the street is a prime scenario for vacuum advance plus the fast mechanical curve adding up to a lot of timing.

The second item that few people check is if the balancer/TDC marks are accurate. It's rare that I see them match up, whether rebuilt original dampers or aftermarket.

Also what plugs are you running(heat range)?

I agree with sniper and would check vacuum advance. If hooked up it could easily be in the high 40 advance range.

Interesting; my Street Hemi had a measured 10.8:1 CR, 36 degrees with vac advance, and never pinged or detonated on even Calif 91 octane,
It did have a pretty big cam (IIRC, around 260 @ .050 & .547 lift), though, which no doubt bled off some dynamic CR.
One of the crispest-sounding engines I've ever owned, after a little stagger jetting.

I agree my hemi has no problem at 36 but still makes most power @ 34. 36 total shouldn't be a problem but if it's set to 32 total with the advance disconnected it could get close to 50 with it hooked up.

I've never been able to have vac advance connected on any of my small block Mopars. Three mild cammed Magnum 360's 9.1 compression and making 320hp. Ran great around 32 total timing and vac advanced disconnected. My E58 didn't like it either. Lots of surging could be felt and could never get the timing set to where the vac advance didn't cause some pinging.

you did not mention what octane you are running now. I have run 93 octane here in Michigan and used this additive from Mancining racing. you can get it all over the usa. it works good but at that time i was going thru gas like bubble gum. ended up just biting the bullet and ran pump swill 110 torco at the local station that had it. the turbo blue worked just fine as well.

look into this and see if this can help you out

https://race-gas.com

did you try adjusting the vacuum can [3/32 allen wrench inserted into the hose nipple] or limit the can movement ?

Honestly can't remember now it's been so long but I think so Good reminder for sure

Under conditions of high manifold vacuum, which would add advance, you are still not building
peak cylinder pressures, nowhere near what you would have at WOT.
I don't see how vacuum advance could contribute to pinging unless something besides manifold vacuum was activating it.

I’ve had a few engines that experienced pinging at part throttle. Easiest check in the world to simply disconnect and drive it. I had to dial back my Mallory made Mopar adjustable vacuum can on my current 440 to avoid it, that was in conjunction with an all in at 2300 rpm mechanical.

^^^ Yup, same experience here; on a couple engines - a SB & a BB - part throttle/slight acceleration ping, until I limited the advance or briefly disconnected the vacuum can.

A cheaper fuel fix, depending on how much the car is driven, could be Aviation 100LL.

Tons of great advice here....you can probably tune it where you need it. Maybe double check for vacuum leaks to make sure it's not going lean and cauing the ping.

But to answer your question about octane boosters (a band aid, but you might need one for a little while).... as mentioned correctly, octane boosters that say "adds 5 points" means adds 0.5 octane points (a dirty secret of the octane booster industry}. Xylene and toluene are both very effective and can be bought as paint thinner in hardware stores. Plan on adding a couple gallons to the tank (ballpark guess, it can be calculated). Toluene is 114 octane and xylene is 117 octane

Toluene is hard to find these days, Xylene can be found but at $22 a gallon it's expensive.

At 1 gallon of Xylene to 9 gallons of 91 octane gas, the level gets raised to 93.6 octane.
At 2 gallon of Xylene to 8 gallons of 91 octane gas, the level gets raised to 96.2 octane.

A can of Torco octane boost makes more sense as it will raise 10 gallons of 91 to 102 octane, using MMT, for slightly more money than one gallon of xylene.

Not sure how much xylene to gas you can run before issues occur.

VP Madditve Octanium with EF 90

Much good info and suggestions,
but since the vehicle is “new” to you
I would start simple and run compression tests of each cylinder..

With sparkplug removed you could do the damper marking accuracy check.

Ask around and see if a friend has a wide band air fuel ratio gauge.

If you know past owners telephone and have a chat.

The SDS is an interesting read. It has a touch of lead in it. Mostly just hydrocarbons with 3-4% MMT.

I agree with topside. I had a 440 six pack mildly built and messed with different thing to get it to idle and and at the same time sit you back in the seat and found that the easiest thing was to get av 100LL and mix 50% av with 50% Sunoco 91 with no ethanol and it ran great!!!!