[dragetsen]

I'm planning a 389 build for 87 octane. There's very little info on this. Anyone out there with experience on this? The plan right now is SD Perf. heads, 8.6:1 cr, Lunati 262/270 211/219 hrc,

[steve25]

Please let us have a clue, will motor be .030” over , or more?

[dragetsen]

Will be 0.060" over if the block will take it.

[steve25]

What year is the block and what is the casting number on the heads?

Do you currently have heads done by SD performance because it’s my understanding that he does not port iron heads any longer

[Goatracer1]

If it is any help to you I did a stock 1964 389 rebuild 2 years ago using 0.030 replacement Silvo-lite pistons with 0.070 inch milled off the tops. That amount was recommended by the manufacturer. The engine has an 067 cam and Tri-power. It's an auto trans with a 3.08 rear ratio. It runs great on 87 octane gasoline even in hot weather with the AC on.

[SRGN]

If you use aluminum heads, you can easily run up to 9.5:1 on 87 octane with a decent tune.

[dragetsen]

#15 '70 big car iron heads in hand. '63 389 block.

[steve25]

Those heads are 87 to 89 CCs with pressed In rocker studs .
With out running the numbers my guess without any further details is that with a .060” over bore you will have a 8.5 to 1 ratio.

Also it’s important to note that with 389 pistons and 67 and up heads that once again depending on the details you may only have enough piston to valve clearance to run .425” to .450” lift at the valve.

Also of importance is that the 1963 block will not accept a block mounted starter.

[dragetsen]

Like I said, I have a set of SD Perf. iron heads, so no longer pressed in studs. I know all about starter mounting and the valve angle change stuff. What I really want to know is how far can I safely push dcr on 87 octane. Dave softened the chambers up a bit so they burn a bit faster than stock. I figured the dcr to be 7.38:1 with the Wallace calculator. Can i push higher than this? If so, how much?

[steve25]

In my opinion now with the dcr info you have posted, no one will 100% be able to nail down the answer your looking for because what’s left hinges on your situation.

How detonation resistant the 87 octane fuel is that YOU will run because it can change slightly by brand.

What’s the average intake air temperature that YOUR motor will have?
The use of headers on your motor will put a lot more heat at full throttle up on to the inside of your hood to be sucked into your air cleaner.

What’s the altitude YOUR motor will run at ?
Air density is a big factor.

What will YOUR coolant temp be?

What will YOUR air to fuel ratio be when the motor is making peak torque at full throttle?

Having heat a resistance coating done to the tops of your pistons can noticeably provide a safety margin for running a given octane fuel while allowing more ignition timing, and the same goes for having your intake valves coated.

[b-man]

Weight of the vehicle, auto or manual trans, rear end gearing, air conditioning or not.

All of these are factors that will affect the octane tolerance of your engine.

Share more information with the members to better assist you.

[dragetsen]

The engine is for a 4000 lb. '63 GP with a wide ratio TKX and 3.23rear gear. No a/c. I live at sea level, but have to go 5000' + to leave town, so my DCR of 7.38 was figured at sea level. Cold air intake, 160' coolant temp. Only one place to buy fuel here, so I get what I get. But we all have the problem of inconsistent fuel if we are buying pump gas. As for a/f ratios, hopefully that will be dialed in at 12.8 at WOT.

[Jay S]

If the Lunati HR cam mentioned has a 112LSA with 4* of advance the maximum target compression for a fairly stock 389 should be at around 8.7:1 for 87 octane. With the cam mentioned, stock induction and exhaust in a heavy car the engine ideally would pump around 150 psi, with a good tune 160 psi max of compression at sea level for 87. Higher SCR than that and it will probably bite you at some point.

The more mods you do to the engine, bigger cam, better induction and exhaust, the higher the octane threshold is, there isn’t really a straight up answer for maximum compression for 87 octane. I have ran up to 10:1 on 87, but it wasn’t needing to climb any mountains either. Since the engine needs to climb a lot of elevation your probably going to need to have the AFR a bit on the lean side at sea leval, or it will be really rich at 5000 ft. I doubt you will regret being a little conservative on the compression.

[steve25]

Since low compression really was here to stay since the early 70s I have found a gray area in regards to a motors compression, timing and the motors power while the vacuum advance is available in relation to throttle opening .

When your dealing with a tad lower compression then you would like there’s a range where you can run a smidge more initial timing and also have the vacuum advance in play longer and at least in a part throttle range have the motor perform like it had a higher compression then what it really does.

In your case for sure I would keep your off idle fuel mixture a bit on the rich side for sea level and just deal with the slight power loss at higher altitudes.

Better to have 5 less hp then to be sending the Rod’s out the side of the block or oil pan.