ctuck0659

In a non-blower motor, at what compression ratio is 87 octane no longer safe?

jeffswav

It also depends on what timing you run. I have 8.9 comp with 36 deg total timing and I run 89 octain fuel.

dslbrnr

when you hear pinging you need more octane, or less timming. When air temp is hot you will need more octane, except when you have an intercooler.

ctuck0659

I've got EFI motors so the timing would retard if the engs knocked anyway, but there's no way I'd ever hear pinging over the thru hull exh. and wind noise. I'm asking cause my motors are around 9:1 too after I cut the heads down and am wondering if I'm losing any power (from the ECM retarding the spark) running 87 octane.

Raylar

Put a scan tool or laptop on the engine while you are running and read the timing at various rpms underway.
If timing is being pulled out at midrange and high rpms then most likely you are expierencing knock and the ecm if so equipped with knock sensor system will pull out timing under loads and you will loose power.
If the engine is not expierencing any knock from the 87 octane fuel and not pulling out timing, drive it like you stole it! and be safe , have fun!

Best Regards,
Ray @ Raylar

jeffswav

Spend the extra couple dollars and use 89. Only put the amount of fuel in the tank you will use in a couple of weeks. Over a month and you are pushing it.

ctuck0659

Raylar: I've got a rinda scanner- great suggestion- thanks-

BIG SLICK

i just burnt a set of pistons running 89 octane and 34 degrees of timing in my 515(9.8 to 1).got 60 gallons at the marina put in 2 bottles of octane boost.first time i took it above 4000 rpms since i built the engine.had it wide open about 5-10 secs.learned the hard way.will be using 93 from now on!

jeffswav

I have heard octain boost does not do much. I would think with 9.8 comp 91or better would be a must.

t500hps

I'm at 9.75/1 and 34 degrees, run 93....if I have to put any 89 in it, I keep it below 4,000.