Newbie Question: Anyone running a high compression motor on pump gas?

[Bax]

I'm a new owner of a '70 911S and also a newcomer to this board. In my sponge-like quest for early Porsche knowledge I was reading an article somewhere (Esses maybe?) that running a compression ratio over 10:1 on pump gas is going to cause detonation. Anyway, I did some searches on this board (at least I tried!) and conventional wisdom seems to suggest that compression ratios of greater than 9.5-10.1 will result in detonation on pump gas without twin plug heads.

So, is anyone out there running single plug heads with a compression ratio over 10:1? If so are you using some kind of octane booster, and which one? BTW, 93 octane is readily available in MA, and there's one place on my commute selling 94. The reason for the question is that my motor has been converted to a single plug 2.7 with 10.3:1 compression ratio (and 906 cams with 46 mm Webers)...

The motor was reportedly built a while ago by a fairly well known / reputable engine builder (not in Porsche circles though), Quicksilver Racengines in MD. The records of what else was done to the motor were lost in the various changes of custodianship, and Quicksilver's paper filing system doesn't lend itself to finding out exactly what other mods they did...although one of the former owners (a mechanic) is positive about the compression, cams & carbs.

Thanks in advance for any info anyone has to share.

Bax

[Adam]

Matt Holcomb's 2.7 MFI motor runs 11.3:1 compression with single plug heads on 98 Octane fuel with no (apparent) problems. He'd probably be able to elaborate.

Hope this kind of fuel is available to you guys.

[CamB]

Remembering that is 98 RON...

As best I can tell, this is the same as either 93 or 94 in the US (which is average of RON and MON).

Soooooo, what has the car been run on in the past? Did it have any problems?

Cam

[Bax]

Thanks for the replies.

"But what really perks my interest is that you've got 46mm carbs on a 2.7. Even 3.0's should run only 40mm carbs, unless they're track cars (or see quite a bit of the track). 46mm carbs are typical for 3.2 engines and larger. But not 2.7. Is your car a track-only vehicle? Probably not, being that you mentioned using it to commute. No disrespect, but there might be a reason the shop that built the motor isn't known in Porsche circles...I'm curious to know more about this motor. How does it drive?"

Unfortunately, the early history is a bit sketchy (i.e. undocumented!), but it is believed to have spent some time on the track. I neglected to mention (sorry, it was late O'clock here when I posted the question) that the carbs have been choked down a little, since it was virtually undriveable in traffic with that setup...

I can't give very detailed driving impressions since I haven't had it long, and it's been in the shop for most of that - not for engine work though! It seems well mannered on the road, pulls hard in the upper rev range, and I didn't notice any detonation. I know that the previous owner used it as a daily driver, and put about 25,000 miles on it in 3 years or so. I hope I'll have it back from the shop this week, so will be happy to comment further if you're interested.

As for the engine builder, their reputation is based on building Formula Ford F2000 engines - I think they won engine builder of the year 10 years running in US F2000 championship. I realize that doesn't qualify them as Porsche experts, but hopefully they're not complete muppets!

[jluetjen]

Hi Bax;
We should get together some time, it would be nice to see another early 911 in the area. My car has early S pistons for a 9.9:1 CR on the same gas as you and I've had no problems. I wouldn't sweat the 46mm carbs since you have them. They might be a little "over the top" for street use, but certainly not unreasonable. Keep in mind that Porsche used 46mm carbs on the 2.0 906 and R motors. Your motor us pulling 35% more air at the same rev's as the 2.0's, so I wouldn't worry about it unless the mileage or throttle response is killing you.

While Quicksilver may not be well known on this BBS, as you mentioned they are a very well known FF engine builder. With all the national championship engines that they've built, they do know how to build solid motors. They may not know all of the little speed secrets of some of the 911 experts, but I'm confident that they did a solid job on your rebuild.

[cegerer]

You need to come to Michigan to buy your fuel! Many Sunoco stations have a special little pump tucked away in a corner:



-- Curt

[Greg White]

I run 93 octane in my 84 euro Carrera that has 10.3:1 (approx.) all the time and no problem. Greg

[billwagnon]

Curt, what's the price-per-gallon on that 101 octane?!?

[Bax]

Hi All,

Thanks for the words of encouragement - I'll see how it runs on the 94 octane when I get it back from the shop. I'll keep an eye out for the 101 at Sunoco just in case!!

John (jluetjen) - I'll drop you an e-mail. I work in Concord, MA so Westford is only just down the road.

Cheers,

Bax

[cegerer]

I think it's around $3/gal. The funny part is the "Street Legal" sticker on the pump. If you read the fine print on the white sticker just above the "101" sticker, it says something like 'this fuel is for off-road vehicles only!' -- Curt

[Raymond]

curt, i notice in that photo that the 101 pump is in its rack and the other pump is 'being used.' what gives?

[cegerer]

Well, there are benefits to running 7.5:1 RS pistons - 87 octane fuel being one of them! -- Curt

[dtw]

My 3.2 Euro was 10.3:1 single plug. It ran great on 93. If I recall correctly, the DME for the 3.2 did not have any knock compensation/retard, but I bow to the FI geeks on that one.