Fuel Octane

[nickelplated5s]

The rig has been home for 6 or so weeks after about a 1 1/2 year shop journey. Tires were aged out and new now. So... We live rural and I have 87 non ethanol here. Bought some (5 gallons) 110 non ethanol and added some. My math puts it up to low 90's octane.

Here's the Q, the shop rebuilt the MFI, amongst other things, without permission but I paid. Motor used to pop with lifted throttle coming down to 2K RPM. Now with light throttle it pops around 3 - 3.4K. 4K+ it runs fine. What do I need to look for? Air cooled shop time around here is limited. I have a fall appointment.

Motor is a 69 2.0 rebuilt to a 2.2 with mods. MFI is original.

[EC900]

The fact you have 87 ethanol-free gasoline readily available is a huge plus.
But no need to mix fuels, stick with one (87) and give it a tank test drive over the next few months. My motorcycle recommends 91 but in some rurals all they have is 87 so for a couple hundred miles I might get a little knocking on hills.

These engines were designed for 87 and anything more is unnecessary say the experts and more pricey. Yours being bored out, it may not be a racing engine (?) spec, I wonder if your timing needs a basic advance adjustment to match the higher/lower octanes.

Generally, backfire sometimes is exhaust or vac leak, any mods to the exhaust (?) could be the problem too, maybe an injector is faulty (?) wrong spark plugs(?).

Too bad the shop wouldn’t make these adjustments and tweaks when they finished those repairs and at very least take a look at the problems right of way. Anyone can check your timing if you don’t have a light.

[r lane]

So many possibilities, do all the basic non MFI tests first. What is your compression. Try an AFR setup though I haven't had much luck with those on MFI cars. I have a track MFI car and have had injector issues off and on, maybe race fuel, don't know. My 72 2.7 MFI street car began popping at steady throttle, felt left sided. I pulled all three injectors and tested them. None looked bad, maybe not perfectly simetrical, but a strong mist. Replaced the two that looked like 2nd and 3rd best and now popping, stumble gone. May reinstall them just to see if the problem returns.

[dw1]

Originally Posted by EC900

These engines were designed for 87 and anything more is unnecessary say the experts and more pricey.

I am curious about that statement.

My sources give the 1960s octane ratings for gasoline at an approximate average of 94 for regular and close to 100 for premium. Development of higher octane gasoline came to a screetching halt in 1970 with the passage of the Clean Air Act.

To quote:
"By the late 1970s/early 1980s, the last of the leaded premium grades disappeared as oil companies introduced new premium unleaded fuels that were higher in octane than the unleaded regular fuels (91-83 vs. 87) - but still lacked the moxie of leaded premium fuels, leaving drivers of older cars with engines requiring high-octane gasolines with few options to keep their classic rides intact without undue engine knock, creating a cottage industry for octane boosters and, with the recent phaseout of leaded gasoline altogether, additives to replace lead as a lubricant against valve recession.'

I do not, as of this writing, have any data on the octane ratings of gasoline in Germany during the same period.

[EC900]

“These engines were designed for 87 and anything more is unnecessary say the experts and more pricey.”

Basing that on the typical low compression ratio 8.5:1 - 9:1. for the flat of the era.

But thanks for the memories- the good old days when Mopar hot rodders in the neighborhood would run down their tank by Fri nite and mixing in leaded Purple Martin 100 for the drag strip...gosh, where did all the desolate streets go !!! There was only one or two stations that had 104 racing gas in early 70’s that were a good distance away.
But even when I first started driving mid 70’s (VW 311) it was common practice or lore to add some “premium ethyl” because of the special additives and detergents the budget brands lacked though leaded best I recall was whopping 49.9˘ gal.

The OP didn’t spec his comp ratio and is trying to even out his tune with octane that he may or may not need.

[tobluforu]

I think we might have crossed this bridge on a post of yours already. Octane, run what you want. If you had a 2-stroke bike like I have, run ethanol free gas. Since you don't, run what you have access to, no need to mix. Now, if you had a fully modded turbo car like I had, or an 800whp sc viper like I had run highest octane available. Since you don't, split the difference, run 89, etc.
There are so many variables in regard to what might me causing your car to act that way, but if you haven't ever had a MFI, get used to them running great, most of the time. Get used to their idiosyncrasies, I have. This is old tech. You have somewhat of a frankenmotor now. Did you put dif cams in, did you recalibrate the pump for the new displacement/cams, if needed. What exhaust are you using, I have gone through I think 6. How much have you messed with the pump and its settings? My advice, time to get your hands dirty. If I relied on the local Porsche mechanics to help fix my car, well god knows if it would even be running. I was also lucky as I had Grady, Warren and a few others to bounce a lot of ideas off of. There are not many who will work on these or can, because they can flat out be a hassle, very time consuming until you get it kind of right. Once you do, set and forget, or tinker.
By the way, a bunch on mfi cars have the same issue you have, a weird spot in the rpm range where it acts this way.. A little more or less rpms, and it goes away.

[proporsche]

or you can go to paint store and get yourself lacquer thinner Toluene.It is used for racing .When i lived in CA (ihave euro engine with 98 octane reg) for half a tank half gallon of toluene ..You will love it...
here
Toluene is produced in the manufacturing of gasoline, and it is also a gasoline additive that can be used to improve octane ratings for fuel used in race cars and other automobiles. The higher the octane number or rating, the greater the fuel's resistance to knocking or pinging during combustion.

[tobluforu]

Originally Posted by proporsche

or you can go to paint store and get yourself lacquer thinner Toluene.It is used for racing .When i lived in CA (ihave euro engine with 98 octane reg) for half a tank half gallon of toluene ..You will love it...
here
Toluene is produced in the manufacturing of gasoline, and it is also a gasoline additive that can be used to improve octane ratings for fuel used in race cars and other automobiles. The higher the octane number or rating, the greater the fuel's resistance to knocking or pinging during combustion.

Yep, I used to run that in a turbo car I had at times. Same with Torco before they reformulated it. Racing gas can get pricey.

[nickelplated5s]

Thanks for the replies. Mahle pistons and cylinders and other mods during the rebuild. Piston squirters, hydro tensioners, etc. Compression is fine. Stock exhaust but I may put SSI's on.

The question was octane only. My fuel supplier only has 87 non ethanol. Non eth. 110 is $22 per gallon and blending it is not an issue. Car runs the same on 87 and 91. I'll have the MFI looked at in the fall. Carbs I can tune but I'm not an MFI person.

PCA has a TT here in July. We plan on showing up for fun only. It is essentially a track car and I'll push it some though. As an aside my neighbor has a CAT scale. 1/4 tank and no people it bounced around 2160-2200 lbs.

[tobluforu]

You will find that once you get the hang of working on MFI, working on carbs will become the new nightmare. I have worked on ssi's, dual Webbers, etc. and those can be a PITA. There is no jetting on an MFI car. Once you know the pump is fine, space cams are good, linkage and pump timing good, ignition timing good, you then fiddle with high and low adjustments keeping good notes. Exhaust can make a pretty big difference as these engines like some back pressure.
Don't be afraid to disconnect the rpm transducer to test. It will make your car run rich on deceleration, but if it's in good tune, no worries. Plugs, get 7 series ngk, if you foul plugs, get iridium's, but find out why its fouling. And I'm not talking about dry fouling which a lot of these cars have, like mine. Im talking wet where it bogs, etc.
Good luck and have fun.