Don Terrill’s Speed-Talk

What is the highest, dynamic CR for 87 octane gas?

Way too many variables to even make a half assed guess IMO.

Many modern engines run 12:1 or a little higher on 87 just fine.

Don't know about dynamic, but my 511 has 12.5:1 static and runs safely with 100 octane. 36* total timing. 270@50 solid FT.

frnkeore wrote: ↑Mon Jun 14, 2021 3:19 am What is the highest, dynamic CR for 87 octane gas?

Which pump gas?

No two are the same.

What intake air temp? What elevation?

The criteria here is 2V wedge and heart shaped chambers, both aluminum and cast iron. Old school, if you will, not direct injection.

A safe guild line, in a loaded engine, with a .040 - .060 squish.

80 deg 800 ft elev.

mopardave wrote: ↑Mon Jun 14, 2021 8:09 am Don't know about dynamic, but my 511 has 12.5:1 static and runs safely with 100 octane. 36* total timing. 270@50 solid FT.

That 270°cam with the right timing events makes that happen, you sure would not want to do that with 12:1 and a 230°@ .050 engine would rattle itself to death.

Well, if you believe what's printed in the car magazines of old, about 20 years ago Super Chevy put together a Vortec headed 355.
8.75:1 SCR and used Comp's XE268H cam (I believe on a 108 ICL).
Quench at .044".
87 octane fuel (from who's pump I can't say other than being California based) and netted peak TQ of 427 at 4000 and peak HP of 390 at 6000 on the magazine dyno.
DCR works out to 7.2:1.
Make of it what you will.

David Redszus wrote: ↑Mon Jun 14, 2021 11:58 am

frnkeore wrote: ↑Mon Jun 14, 2021 3:19 am What is the highest, dynamic CR for 87 octane gas?

Which pump gas?

No two are the same.

I live in Oregon and we have both 87 & 92 oct, with ethenal in the winter months. But, to any that may post, assume it's your local pump, that you pull up to, when you need gas.

I remember the old version of PipeMax spitting out an actual fuel octane rating recommendation...

I don't have the newest version so can't comment on it.
DCR, squish / quench, IAT, chamber design, lots and lots of variables.

Many smart folks on here don't even want to make a call with DCR #'s... should possibly give you pause on accepting a simple answer to a complicated question.

David doesn't even want to make a call with RON/MON ratings of pump gas included.


Adam

skinny z wrote: ↑Mon Jun 14, 2021 3:19 pm Well, if you believe what's printed in the car magazines of old, about 20 years ago Super Chevy put together a Vortec headed 355.
8.75:1 SCR and used Comp's XE268H cam (I believe on a 108 ICL).
Quench at .044".
87 octane fuel (from who's pump I can't say other than being California based) and netted peak TQ of 427 at 4000 and peak HP of 390 at 6000 on the magazine dyno.
DCR works out to 7.2:1.
Make of it what you will.

And 160F dyno water, probably...

Even the # of variables is variable in these tests.

Adam

NewbVetteGuy wrote: ↑Mon Jun 14, 2021 5:58 pm And 160F dyno water, probably...
Even the # of variables is variable in these tests.
Adam

Undoubtedly.
Like I was saying...

skinny z wrote: ↑Mon Jun 14, 2021 3:19 pm Make of it what you will.

frnkeore wrote: ↑Mon Jun 14, 2021 3:19 am What is the highest, dynamic CR for 87 octane gas?

At what temperature??? That has very much to do with, what will work.

Depends on the load on the engine as well, in my opinion. Is it a 500 cube motor pulling a 3000 lb vehicle with 4.11 gears or a 250 cube motor pulling a 4000 lb vehicle with 2.75 gears? One might detonate and one might not, with the exact same DCR and octane.

JMO,

paul

frnkeore wrote: ↑Mon Jun 14, 2021 12:36 pm The criteria here is 2V wedge and heart shaped chambers, both aluminum and cast iron. Old school, if you will, not direct injection.

A safe guild line, in a loaded engine, with a .040 - .060 squish.

80F deg 800 ft elev.

Randy and Paul. Temp and Elev are as above. Loaded is loaded, no matter the weight. Consider it a steady dyno pull at 160 deg engine temp. The 80 deg is assuming air intake, out side the engine compartment.

I know by application, there are things that work. Such as 6.8 DC but, I would like a upper end number to build a engine to.

Will 7.5 do it, in those exacting numbers, supplied above?