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Wed Apr 10, 2024 9:15 am

91 Syncro, just went over 100k, original engine rebuilt to 2.3L. As best I can tell from previous threads, 91 octane is not required, but some folks seem to prefer it.

The original owner, a German woman who purchased the van in Germany, said her mechanic told her she "ruined the engine" by using regular octane - thus the rebuild. I've had one encounter with this mechanic and would never use him.

Luckily the rebuild was subcontracted to NW Connecting Rod, who rebuild my '69 Westy years ago. The engine is tight. On the 4k mile trip to Baja and back I was getting 18-20 mpg on paved roads with 91 octane. And barely any oil consumption.

I see the new GoWesty engines require 91 octane, but for the orginal WBX or rebuild it's user's choice?

Wed Apr 10, 2024 9:23 am

No clue what the pistons GW is using look like, but the stock piston design is very resistant to knocking. The original European piston likely had the same basic design as the stock US market pistons, just a smaller dish, so should have had decent knock resistance as well.

Wed Apr 10, 2024 9:44 am

Octane needs are based in large part with compression ratio. Inaudible detonation can occur, you wont hear it, but it can cause engine damage if it continues.
piston shape can effect this, but so can the deck height, and to a lesser extent valve timing, and mixture, along with ignition timing and engine temperature
If the builder recommend high octane, Id use it it, why risk engine damage? Keep in mind that in many places the high test gas is not much better than what used to be sold as regular. 91 octane is the high test stuff in much if not all of California short of special racing fuels.

I run 91 in many of my cars because the engines have been rebulit with slightly higher compression.

You can use low grade fuels in a higher compression motor but in that case the ignition timing will need to be retarded to avoid detonation , and there will be a loss in power and fuel mileage as a result of the retarded timing. Knock sensors in modern car do this, upon sensing engine detonation (knock) the engines brain retards the timing until the
point of no more detonation.

Wed Apr 10, 2024 9:55 am

on my 4,400 mile trip, i used 91 the entire trip but had numerous full throttle periods, often for several miles, with each tankful and wanted any edge i could get to not hurting the engine. to and fro town in our flatland, i use 87.

Wed Apr 10, 2024 10:13 am

The sticker by my fuel filler use to, iirc, say 91 octane, and in small print, the formula used to calculate the octane rating. I checked how that worked out, and the formula used is/was more of a European used formula, which when converted to what we use here in the States, comes out to be between low to mid grade fuel octane rating. I think it came out to something like 85 octane rating?

The original owner of my van, used 91/supreme/hi-test/preminum all the time, in the mistaken belief that VW requires high octane in the vanagon, when in reality, regular fuel (here in the States) is all that is required.

When I had my engine rebuilt (Rocky Jennings 2.3), he HIGHLY encouraged the use of the highest octane fuel possible, while also BEGRUDGINGLY allowing the use of lower octane fuel ONLY if you're traveling FLAT GROUND at LOW ELEVATIONS.

Wed Apr 10, 2024 10:52 am

do.dah wrote: The sticker by my fuel filler use to, iirc, say 91 octane, and in small print, the formula used to calculate the octane rating. I checked how that worked out, and the formula used is/was more of a European used formula, which when converted to what we use here in the States, comes out to be between low to mid grade fuel octane rating. I think it came out to something like 85 octane rating?
USA 91 (R+M)/2 is directly equivalent to RoTW 95 RON, the normal 'regular' in Europe and most everywhere else.

Wed Apr 10, 2024 11:27 am

fxr wrote: do.dah wrote: The sticker by my fuel filler use to, iirc, say 91 octane, and in small print, the formula used to calculate the octane rating. I checked how that worked out, and the formula used is/was more of a European used formula, which when converted to what we use here in the States, comes out to be between low to mid grade fuel octane rating. I think it came out to something like 85 octane rating?
USA 91 (R+M)/2 is directly equivalent to RoTW 95 RON, the normal 'regular' in Europe and most everywhere else.

The VW recommends 91 ROM

91 RON = 87 (R+M)/2

Wed Apr 10, 2024 12:14 pm

Wildthings wrote: fxr wrote: do.dah wrote: The sticker by my fuel filler use to, iirc, say 91 octane, and in small print, the formula used to calculate the octane rating. I checked how that worked out, and the formula used is/was more of a European used formula, which when converted to what we use here in the States, comes out to be between low to mid grade fuel octane rating. I think it came out to something like 85 octane rating?
USA 91 (R+M)/2 is directly equivalent to RoTW 95 RON, the normal 'regular' in Europe and most everywhere else.

The VW recommends 91 ROM

91 RON = 87 (R+M)/2

That was many moons ago. :) 91 RON is very rare beast these days in the EU at least.

But yes, here in the USA I use 87 regular at sea level, even in the Subaru EJ22. :)

Wed Apr 10, 2024 12:46 pm

Klister wrote:
I see the new GoWesty engines require 91 octane, but for the orginal WBX or rebuild it's user's choice?

It's been a while since I had my GoWesty engine installed but, at the time, they RECOMMENDED 91 octane to avoid detonation since their engines have higher compression ratios than stock.

They explained it kinda like, "you don't really need it but its cheap insurance so please use 91 octane".

Wed Apr 10, 2024 1:05 pm

In the US and Canada, there is an additional consideration - - ethanol. The stoichiometric mixture for ethanol is 9:1 air-to-fuel by mass. For gasoline, stoichiometric mixture is 14.7 : 1 by mass. So, ethanol gives you less mpg (a slight oversimplification) and therefore slightly lower fuel mileage.

If a rebuilt WBX MV engine uses the stock compression ratio (achieved by using stock pistons and cylinder heads), then the WBX will happily run on 87 octane without knock. GoWesty's engines use raised compression ratios to achieve higher torque and better fuel mileage. They don't use knock sensors on their engines unless you go for their new EFI system. The GW engines without knock sensors have to use high octance fuel to be safe from knock. If you have a DJ WBX engine (the European version with high compression ratio), then you will have to run higher octane fuel, but this engine is seldom seen in the US or Canada.

Here in Canada, it is possible to buy pump gasoline without ethanol, but only in the higher octane grades such as 94 octane at Chevron. Our SVX-engined rig is fed only 94 octane, but apart from this engine being a high-compression engine (10.0 : 1), the main reason we do so is to avoid the problems for the entire fuel system caused by ethanol in a vehicle that is NOT a daily driver. I have recorded and noted better fuel mileage on 94 octane non-ethanol fuel than with, say, 91 octane with ethanol. Another consideration with Chevron is the higher proportion of fuel injector cleaner they include in their 94 octane fuel. So, the choice of premium grade or not can include more than just octane and knock control. A knock-control equipped engine can run its entire life on regular 87 octane gasoline, but fuel mileage will be lower and power will be reduced when the knock control system reduces spark advance and/or turbo boost.

Wed Apr 10, 2024 1:33 pm

one thing about the stock WBX motors being knock resistant.... buried under 2' of bedding, a foam lined cover, and in the rear, there's no way i am going to hear any knocking. i run extra advance for that happy peppy feeling and when traveling with full weight, full sound isolation, and full mountainous terrain, i'm buying the most knock resistant fuel to hedge my chances.

Wed Apr 10, 2024 2:30 pm

Weren't practically all European Synco WBX's built with the higher compression DJ pistons? This might be a better discussion on brick-yard.co.uk or a more active euro centric forum.

A poster over there says
"the dj uses super 98 ron. it can be run on 95 but requires the timing to be tweaked. "