How much compression can you run on pump gas?

[rbogh901]

Single plug, dual plug, etc, what is realistic for 91 octane using reasonable technology for a performance 2.7/2.8 or other motor? Obviously twin plugs help and so experience there will be interesting.

thx.

[Steve@Rennsport]

Hi,

There are many variables that affect octane requirements. Some of these are:

1) Bore size

2) Compression ratio

3) Single or twin-ignition

4) Camshaft profile

5) Air-fuel ratio

6) Spark plug heat range

7) Cylinder head temperature

8) Ignition timing

9) Piston crown design


A 2.7/2.8 will run OK on 91 octane fuel as long as the CR is kept below 9.5:1. One must be careful about total timing in order to prevent (expensive) problems in hot weather.

[rbogh901]

ok, ok., more specifically:

92mm bore, rsr sprint cam, A/F target @13 or so at WOT, twin plug with appropriate advance.

I did find this:

"You can run 11.3:1 on street fuel so long as your dynamic ratio is 8.6:1"
Is this an upper limit or within an acceptable range?

[Steve@Rennsport]

Originally Posted by rbogh901

ok, ok., more specifically:

92mm bore, rsr sprint cam, A/F target @13 or so at WOT, twin plug with appropriate advance.

I did find this:

"You can run 11.3:1 on street fuel so long as your dynamic ratio is 8.6:1"
Is this an upper limit or within an acceptable range?

Ahhhhhh,..thats more like it,.... Its very challenging to offer constructive assistance without sufficient information,........

A 2.8 at 11.3 with twin-ignition and RSR cams needs a minimum of 100 octane race gas in this configuration. Even then, total timing will be compromised for safe operation in warm to hot weather.

FWIW,...13:1 AFR @ WOT will result in piston expenditures so I'd rethink that one,...

[larrym]

most of the tech articles in the motoring press suggest that 9.0 CR is the limit for current pump gas in an iron engine like a chevy, and I hear that many circle track racing orgs now limit racers to pump gas and 9:1 spec engines

I believe I've also read that 8.0 to 8.5 is the limit for aluminum engines on pump gas - witness that most earlier Porsche engines were 7.5-8.5, even the 210 hp 2.7RS was only 8.5

the newer computer controlled engines get away with higher CR's cuz they have electronic timing control with knock sensors - you may never know that your engine has simply dialed back the timing to accomodate a heavy foot


yes, higher CR's existed in the olden days - but so did 100-102 octane pump gas

I built a 2.8 with 10.5 CR & webers back in the mid-70's and it was a fine engine at the track on 110 octane, but you could feel the difference and sometimes hear the pinging if you used pump gas by the mid-90's, so ya just had to back off the throttle

- when the 91 octane rule came about I started mixing race gas from 5-gallon cans - I finally gave up and built a 8.5 CR motor for the street, and used a Halmeter to dial in the carbs at about 14:1 AFR at cruise, trying to get decent fuel economy; it runs a distributor that Steve Weiner created for that engine & solex cams, all in at 4000 rpm

- and even that setup complains on a 100 degree day on no-name gas - so I have added an MSD timing control and I just dial it back if I get a tank of poor gas - and I dial it up when I switch to race gas

- yes, I could richen the jets to offset that a bit, but then I'll pay at the pump, re-jetting & re-tuning webers routinely for varying uses is not a trivial task

[Walt Fricke]

My 2.7 race motor with twin plugs and Mahle nominal 10.3/1 race pistons has run well for years on pump premium. I've had my share of problems, but broken rings or burned pistons have (knock on wood) not been among them.

Of course, the compression ratio measured with a "whistler" checker was down around 9.5. I've raced it at nearly sea level (Thunderhill, Texas World), altitude (Aspen, other Colorado tracks at 5,000+ feet), and places in between. Ignition advance modest (28 degrees max).

But a friend, whose 2.8 had run fine on pump gas, holed a piston at Las Vegas once. He thought he had gotten some "bad" gas. It wouldn't take much additional octane reduction to cause trouble if you are close to the edge.

I have always jetted based on air density. Less = smaller mains. I'd usually run a jet size or two fatter toward sea level.

For me, up in Colorado, hotter usually means less air density. So I guess I hadn't linked a need to run richer with higher temperatures. Does that work differently at sea level?

Steve - what AFR is needed for WOT on a track? 12.5?

Walt

[lvporschepilot]

If you are running a efficient quench, say .040-.045, you can run this setup on pump 91-93 octane so long as you keep total timing way down on the conservative side. I am running a very similar cam/piston setup in my Ferrari and it runs fine on pump fuel.

[larrym]

holed pistons is not necessarily a given result of running on the edge of CR vs fuel - continual mild detonation can simply erode the edges of the piston and eat its way into the top ring land - i have seen this on several engines, and you can see pics of it in most of the tech literature

there is an excellent discussion of the all these compression and combustion issues and "fixes" in Bob Barkhouse's book - "Engine Repair"; and Philip Smith's "the design and tuning of competition engines" -- if you are just now building an engine - ya might want to check Barkhouse above and The Power of Squish at Calculating Engine Compression - Formulas, Tech - Circle Track

detonation tends to occur at the hot edges due to the flame travel pattern and rising pressure - dual plugs clearly combat this since they light the fire so that it travels more toward the center - I think that BruceA discusses this porsche solution in the engine performance handbook - it seems reasonable that different 911 piston top shapes are also a factor in turbulence which affects flame travel

- if you always keep the rpm up, you won't hear this tip-in ping since the dynamic compression factor comes into play - you also won't hear it at higher altitudes since the cyl pressure is lower, and my experience with sevearal different vehicles is you can dial in more advance at high altitude and get away with it

Smith writes: "...It is usually the case that, on a particular fuel, a compression ratio on which the engine is perfectly happy, is only fractionally lower than one at which all kinds of troubles may intervene .... the noise of detonation in a high compression engine is much less easy to detect .. so that it is essential to look for other evidence that the engine is free from detonation"

- my own engine is a single-plug with only slightly domed pistons, tuned for 2500 feet altitude, so when I'm down in the valley at zero msl on those 100 degree days it is just a bit lean, inviting audible pinging on throttle tip-in at rpm below 2500 rpm - hence my msd dial-back solution - and then there are the days i drive it to Tahoe and the thing has to perform at 7000 feet

[Steve@Rennsport]

Here is something else to read: http://www.rennsportsystems.com/2a.html.

Using one's ears as the defacto "knock-sensors" in any engine, much less an air-cooled one, will seriously damage one's checking/savings account.

There are several ways to detect incipient detonation, however listening isn't one of them,....

[Borsche911]

Guys, I've just had a major oh **** moment when reading the above posts. i have just built a 2.7 with JE 10.5:1 pistons, L cams and webers. I've fitted F3 emulsion tubes and 130 mains and the recomended size air correctipon jets. I've fitted MSD 6AL ignition and all seemed well. I ran for 20 minutes at 2000rpm and it sounded great. I then f]dropped the oil and took it for its first short drive. At 3000rpm, it dont want to know. Its almost as if rev limiting is coming in but I've switched back to the original cdi Bosch box wit the same result. I removed the plugs and they were black. The engine fels as if it is choking and as the exhaust has to be fitted after the rear skirt, (its in a VW Beetle) I used old gaskets and flames can be seen through a small gap in the exhaust flange area. Its single plug running on 97 octane premium fuel from the UK. Timing is conservitavle set at TDC at full chat. Any ideas?

[Steve@Rennsport]

Lots of ideas but I need a LOT more information in order to offer anything constructive.

Might I suggest another thread so this one isn't hijacked???

[J P Stein]

These are 9.7:1 Shasta (or so the man said) pistons than I ran on Chevron pump premimum (92 octane up this way) for 7 years of autocrossing with some street driving. A bit of scotch brite to knock off the carbon. Solex cams & back yard tuned Webers. ....

[J P Stein]

Raw from the motor. Try to ignore the thumb. . ......

[kenikh]

Not bad, JP3. Seems to me they would run fine and they did. That said, I have always found measured CR lower than "spec" CR. My 9.5:1 JEs eventually measure out at 8.9:1 @ 1mm deck (.040"). 10.3:1 Mahles seem to come in closer to a real 10:1 at 1mm deck.

As far as deck/squish goes, that is a funny thing, too. .040" is the optimal for quench to "effort", meaning that the variances in a well built motor shouldn't exceed that margin. The margin is for thermal expansion, toelrance build up and pistons rocking in the bores. If you go for extra "effort" e.g. time and money and patience and pixie dust, you can go smaller and get more gain from tighter quench.

For the OCD (or deep pocketed) builder using top parts, who measures and fits everything (like grinding or lash capping valves to get height perfect), you can get down to .025" squish (or lower). That said, you'd better know what you are doing or expect a lot of expensive recycling material. I am never personally building a motor smaller than .040" nor am I paying an expert to do it, as you can easily triple or quadruple the cost of a motor in labor alone to build to the tolerances needed for a miniscule gain.

[rbogh901]

yes, diminishing returns...

Interesting to note the following with regards to CR's, (poached from the 964/993 board):


"... one prime example is the Singer 993 engines which were built by Geoffrey Ring and Collin Belton.

They use Motec fuel/ignition management w/o knock sensors and even higher cr than stock. There are different maps for different fuel quality but under any of them, hp is very healthy.

The last Ring car I was in had stockish 3.6Motec w/ cams, high comp pc and headers ran on 91, and went like a scalded cat, 300+ hp on a Dynapak"
__________________
Bill Verburg



Note there are NO knock sensors. Higher CR. Different motors but not race gas dependent so there are folks doing things right to make it work.

[Walt Fricke]

JP

Shasta made pistons for 911s? I didn't know that. I've just seen their ads for 4 bangers.

Didn't show any evidence of detonation, did they (to stay on message here).

Walt

[kenikh]

Mahle, Shasta, Wiseco, Cosworth, Wossner, JE, Arias...the names come and go.

Even stalwart JE is in transition - apparently since Wiseco has bought Carillo Industries, the rumors cropping up that some Porsche piston sizes and designs (small bore) are now NLA or limited are possibly cause for concern. It's been a few years, but the attrition seems to be showing up of late. They certainly aren't investing in upgrading the small bore piston designs to the latest FSR designs. I've pressed on JE for a year now and they continue to say they have no plans.

[Steve@Rennsport]

Originally Posted by kenikh

For the OCD (or deep pocketed) builder using top parts, who measures and fits everything (like grinding or lash capping valves to get height perfect), you can get down to .025" squish (or lower). That said, you'd better know what you are doing or expect a lot of expensive recycling material. I am never personally building a motor smaller than .040" nor am I paying an expert to do it, as you can easily triple or quadruple the cost of a motor in labor alone to build to the tolerances needed for a miniscule gain.

One big reason why Porsche specified 1mm (.040") of deck height was to make an allowance for carbon buildups normally found on higher mileage engines over the long term.

This prevents piston-to-head contact, especially when hot, after several thousand miles of carbon layers,....

Race engines, OTOH, get disassembled every 100 hours or less and cleaned up. Such maintenance regimens afford some liberties with reduced deck heights to help prevent detonation with high compression ratios.

[J P Stein]

Walt: Yes, no evidence of detonation.
He (IIRC, Richard, I no longer have the paper wrok) machines them from JE slugs.
His ads no longer list 911 pistons so
I didn't much care for the supplied rings and the cylinders became scored. This well may have been my fault but I scored on a set of these for the rebuild. Went to 110 race gas as the motor was single plug. There's nothing like a set of NOS Malhes to make a fella feel secure. .....

[Walt Fricke]

Ah, Arias. I have a set of five 90mm ones. Worked great until a wrist pin clip, which was like a standard circlip with ears with holes in them, broke off an ear. The design had the wrist pin pocketed, so to speak, so that hard steel little ear piece couldn't just drop harmlessly into the sump. It etched one side, passed through the wrist pin, etched the other, etc.

Some day those of us with broken Arias sets will have to get together to make sets of six. I'll never understand why that unstressed part of the clip (the rest stayed in place and the wrist pin was not affected) broke off.

Motor ran great for the short while it lived, but it was my first race motor so I had no way of comparing it by seat of the pants, and never got it to a dyno. The 2.7 with the Mahles like JP has made 220 RWHP @ 7,100 on a Dynojet when fresh. Another good thing about Mahles and Andial is that you can (or at least could) get a singleton a bit heavier than what you had, and carve off a bit of weight to rematch your set.

Walt