'58 270 Duntov: 87 or 93 Octane?

[David B.]

I know of some 270 Duntov owners who use 87 in their 270 Duntov. I've been using 93. Am I wasting my money? The manual says to use premium.

Dave

[Bruce B.]

Re: '58 270 Duntov: 87 or 93 Octane?

What is the compression ratio of your engine?

[David B.]

Re: '58 270 Duntov: 87 or 93 Octane?

Bruce, it's 9.5:1.

Dave

[Jim D.]

Re: '58 270 Duntov: 87 or 93 Octane?

I've been running regular gas in my 270 HP 1960 for over 35 years. Never had a problem.

[Mike M.]

Re: '58 270 Duntov: 87 or 93 Octane?

A few 'Valero' gasoline stations have openned in my arear (mid Ms) lately. Some (not all) have a dedicated non ethanol gas pump (90 octane). I am using that gas in some old cars that I own. The cost is less than premium ethanol. No pinging issues as yet. Was previously using premium gas that had ethanol in it.

[Duke W.]

Re: '58 270 Duntov: 87 or 93 Octane?

It should run okay on 87 PON, which is 91-92 RON. If the engine is to Flint spec the real CR is probably closer to 9:1. If it's been rebuilt with OE equivalent pistons and a typical thick head gasket it may only be 8.5:1. You can measure the head gasket thickness at the corners with a feeler gage.

Download the '57 to '61 vehicle information kits from the GM heritage site. The 270 was basically the same for all five years. For '57 the fuel recommendation was was 91-94 RON, which is regular (but I believe the centrifugal spark curve spec is wrong). In '58 they changed it to "premium". Why? That's what my presentation in San Diego will be about next month.

Do this experiment. Verify that the initial timing and centrfugal curve are to '58 spec. Let the tank get near empty and install 3 gal. of 87. Drive the car around. If it doesn't detonate with your driving style install another 3 gal. of 87 and check for detonation. Repeat a third time.

If you do detect signficant detonation (a little transient detonation on upshifting is okay) try your mid grade.

My bet is that it won't detonate on 87.

Now advance the initial timing to 10-12 degrees and rerun the test. If it detonates you can either rerun the test with mid-grade or retard the initial. Increasing the initial timing should result in more SOTP low end torque and better fuel economy.

My bet is that it will run okay with 12 degrees initial on 87 PON.

The next step, if you want to further optimize performance and fuel economy is to quicken the centrifugal curve.

Duke

[David B.]

Re: '58 270 Duntov: 87 or 93 Octane?

Duke, I just saw your amazing response from May 5. Thank you! What's the title of your presentation?

I've been driving with 87 for a week or so and am experiencing no obvious problems. By the way, I have a hearing disability (I wear hearing aids) and my 270's lifters chatter a bit. How can I discern detonation with bad ears and solid lifters?

Dave

[Duke W.]

Re: '58 270 Duntov: 87 or 93 Octane?

..."Tuning Vintage Corvette Engines for Maximum Performance and Fuel Economy".

I don't know how a specific hearing problem will affect one's ability to hear detonation. In a Corvette, it's pretty easy pick up from the background noise. It's a relatively low frequency noise that is quite noticeable to anyone with normal hearing acuity. Degradation of our high frequency hearing is typical as we age.

Take someone for a ride that has good hearing. Load it up at 1500 in top gear and and listen for "knock".

Duke

[David B.]

Re: '58 270 Duntov: 87 or 93 Octane?

Thanks, Duke. I'll take my wife's good ears for a detonation detection drive.

Best wishes for your very interesting presentation. I assume that we'll be able to read the text in a future NCRS publication.

Dave

[Duke W.]

Re: '58 270 Duntov: 87 or 93 Octane?

The presentation will be 30-something charts in Microsoft PowerPoint that will be projected onto a big screen. The charts themselves should tell a cohesive story, but there are certain things I will have to add verbally that will not be on the charts.

I think in the past couple of years, PowerPoint charts from national convention presentations have been placed on a Web site for viewing/downloading, but I don't think it was the NCRS site.

In any case, I will figure out some way to get the charts on the Web so they can be viewed and/or downloaded for those who won't be able to attend the presentation.

Of course, you'll miss all the jokes.

Duke