johnny spangle

I recently purchased a C3 with a replacement Crate 300HP engine similar/same as this one: https://www.summitracing.com/parts/n...99529/reviews/

The carb is a rebuilt original Rochester. Car is a '79

With the replacement engine I am unsure whether to use premium gas (93 octane) or stick with regular (87 octane). I can't seem to find a definitive answer on this.

Any help? Thanks.

Easy Mike

Saw a blurb on the evening news which said tests show there is no actual benefit for buying premium gas; no additional horsepower; no additional cleaning benefits. Nada. The study said buy 87 octane and get on down the road.

Alan 71

Hi js,
With an engine like this I'd think the compression ratio of the engine would be the determining factor in the octane that you choose.
I looked at the Summit site but it doesn't appear to give a required octane for this engine.
From the rest of the description I'm thinking it would use regular 87, maybe 89?
So, what's the compression ratio?
Regards,
Alan

resdoggie

Try 87. If you get some ping, try the next higher octane level. If you get to 93 and it's still pinging, retard your timing a few degrees.

Whoever wrote the article that higher octane does not provide any benefits such as performance increases, does not understand how hp engines work or the study was conducted on a Civic.

ddawson

I agree. correct octane to keep detonation at bay.

The only time benefits are seen are on cars that have variable timing or an ECU that can adjust for more timing.

I have a Honda Ridgeline and they is a 10HP benefit by using 91 octane. Not required unless your towing.

Then you have the LSx engines that reduce timing when knock is detected to work with lower grade octane.

I'm still running 91 in my 496 9.8:1 engine, but now that I have 500 miles on it I'm going to try 89 and see how it does.

Rodnok1

I've ran 87 in engines with a lot more hp than that, some timing adjustment was needed for pinging but wasn't issue unless towing a heavy trailer . Then I definitely noticed the lower octane. Short of that never found a need to spend extra cash.

Tim Ware

The TV article was referring to tests on modern, everyday cars. It went on to say that higher octane fuel might be required on high performance engines.

jim in oregon

I'd run 92 octane ETHANOL FREE gas.. with appropriate timing curve and tune up..Jim

7T1vette

As stated above, it's a function of the engine's compression ratio. If CR is less than 9.0, you can probably get away with 87 octane fuel. Between 9.0 and 10.0 mid-grade would likely be needed. Above 10 you will need premium fuel to keep from having pre-ignition problems.

I didn't see the TV show referenced, but it should have stated that the engine only needs the octane level it was designed to use. Most electronically controlled engines will run on regular fuel...even if they were designed for higher octane levels. But, they will run poorly. A non-ECM engine will not run worth poop if the fuel octane level is too low for it.

resdoggie

I remember back in 2009 I was going to purchase a new 2009 Genesis coupe with the 3.6L(?) engine rather than the 2.0L turbo. The specs for the 3.6L specifically stated that regular gas had a 300HP output and premium gas was 312HP. I ended up getting a 370Z with 332HP and glad I did.

johnny spangle

I confess I do not know the compression ratio and don't own a tester. I'll need to borrow one or ask at the garage. But it sounds like if I don't get pinging with 87 then that's as good an indicator as any. I will try that approach. Thanks.

flyeri

The specs for that crate motor say 8.0:1 compression. They also call for 89 octane fuel. This is from the summit website you listed.

johnny spangle

Really? I didn't see that. Do you have a direct link / text context so I can find it in the page? Thanks.

jim in oregon

I still advocate use of ethanol free gasoline..mostly 92 octane..here's a link so you can find it wherever you live and travel..Jim...

http://www.pure-gas.org/index.jsp?stateprov=OR

ddawson

See page 4

https://www.chevrolet.com/content/da...e-12499529.pdf

johnny spangle

It looks like they're calling for 87 octane no? Pasting below.

Fuel:
........................................ ........................................ .....................
Regular unleaded - 87 (R+M/2)

lurch59

I also do not like the ethanol/gasoline blends. There are fuel separation/hygroscopic issues that are not worth the risks. This stuff is popular due to political pressure from the "green crowd".

Stick with pure gasoline, it's easier on the fuel system.

lurch59

Well, at some point a CR above 10 will require an octane boost. I know you didn't mean a 12:1 compression ratio would run on pump gas, but wonder when an octane boost would be required. I don't have computer controlled stuff to change ignition timing, so I just use octane boost to be safe. Perhaps overkill...I think I'm right at 10.25:1, but have aluminum heads.

ddawson

Yep 87 is what they call for.

Fuel:...........Regular unleaded - 87 (R+M/2)

johnny spangle

Have the answers I need now. Thanks everyone for replying.