2.2 to 2.3 conversion question

obrut · 11-01-2006, 02:41 PM

doing a little research for a possible project

historic racing rules in australia allow a ST replica to run a 2.3 (i.e. 85 x 66)

can regular 84mm barrels be bored to 85mm or are new barrels required?

1/2mm of the wall doesn't sound like much...

304065 · 11-01-2006, 03:01 PM

Ryan, this is done all the time, and 85mm pistons are fitted.

Search here for posts by cnavarro, for example, for more info.

obrut · 11-01-2006, 04:00 PM

thanks john - will do

Henry Schmidt · Henry Schmidt's Garage

Boring a biral barrel to 85 mm is no problem.
Do your rules state 85 x 66 or do they say 2.3 ?
One highly successful vintage racer here in the states has been campaigning a 81 x70.4 (2300).
I am a fan of the short stroke engines but I just thought I would throw out a different option.

Correction Tadd pointed out: 81x70.4=2176 not 2300

obrut · 11-01-2006, 06:17 PM

thanks henry

they explicitly state 85 x 66 - as a 1mm overbore was the done thing "back in the day"

Henry Schmidt · Henry Schmidt's Garage

Back in the day, 70.4 X 80 was also done.

Too bad they limit options even when they were options in the day.

obrut · 11-01-2006, 06:49 PM

yep, the rules for historic racing down here are very very strict.

Henry Schmidt · Henry Schmidt's Garage

If you are looking for maximum performance in your ST project, you might consider using some of our new 84 mm Nikasil cylinders.
Less friction, better cooler and more consistent cylinder expansion rates. All can equate to more performance if optimized.

tadd · 11-02-2006, 03:39 AM

Isn't an 80x70.4 a 2.1 long stroke (actually 2123 cc)?

I would think then 81x70.4 would be 2176 cc. Is a 5% displacement difference worth it? I would assume that would be like 8-10 hp?

tadd

jluetjen · jluetjen's Garage

Quote:

Originally posted by Henry Schmidt
Back in the day, 70.4 X 80 was also done.

Too bad they limit options even when they were options in the day.

Henry;
What rules are you referring to? As far as I know, both the FIA rules and the SCCA Production rules dating back to the early '70's only allowed a 1mm overbore. Stroking wasn't allowed within a given engine specification.

Henry Schmidt · Henry Schmidt's Garage

The benefit of running a 70.4 crank in a 2.0 S generally the 69S was that you get a light size increase and also a .5 point compression increase (10.4:1) with stock pistons.
If the engine was pumped for CCs it looks like a 2.2.

As for the rules: jluetjen you're probably right about stroke. I wasn't really referring to rules, just what people actually raced. The famous racer I referred to earlier, raced with VARA, FIA and other who definitely regulated bore & stroke.
We all no that racers back in the day never deviated from the rules

Case in point:
10 years ago I had some GTP race engines from the now defunct NPTI. This team was one of IMSA's defining teams in the late 80s and early 90s.
While doing inventory on the 20 some engines, I discovered multiple engines with the same number, three 3.0 liter VG30 engines with the same serial number.
The rules at IMSA clearly stated "the same engine must finish the race as the car started with."
We also found some 3.2 liter engines and nowhere did they ever claim they were racing a 3.2.
Were they covering their butt ? you decide.

304065 · 11-02-2006, 06:09 AM

I am not sure about the dome volume, I had to backsolve into it for the 911S piston. Do not rely on these data for building your engine, consult a professional. Void where prohibited. May cause blindness.

tadd · 11-02-2006, 06:39 AM

I assume you can sill use 906 pistons for a bit more CR oomph - with twin plug of course.

I would also assume that with the S pistons due to the softer dome you could get away with the resultant 10.4:1 with single plug on premium pump gas?

tadd

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally posted by tadd
I assume you can sill use 906 pistons for a bit more CR oomph - with twin plug of course.

I would also assume that with the S pistons due to the softer dome you could get away with the resultant 10.4:1 with single plug on premium pump gas?

tadd

The deep dish dome configuration of the 2.0 head is very susceptible to detonation.

Even at 9.8:1 the dynamic pressure inside this early engine is too high for street gas. 91 octane. That is why the distributor has such a limited advance.

With twin plug you can run more advance and effectively generate increased HP safely through out the range .

tadd · 11-02-2006, 07:19 AM

Henry:
I was under the impression that the big problem with the 2.0 'deep dish' head was that at high CR the dome intrudes too much and therefore interferes with the flame front.

Since the S piston is not as extreme as the 906 flame travel is impeeded less and therefore a single plug from the factory. The factory dyno curves specify 98 ROZ which IIRC is equal to our 93 (RON+MON)/2 number. It is also my understanding that ROZ is a test more like the RON test, so given that all gas is not equal due to the way a given manufacturer blends that RON-MON average.

Anyway, aside from that rambling

, I take your point.

I guess my question is that given a gasoline that meets the factory 98 ROZ requirements is there some other fundamental design 'flaw' of the 2.0L head once dome height is taken out of the equation?

tadd

Henry Schmidt · Henry Schmidt's Garage

Hemi head design is prone to detonation even with a flat top piston.. The 2.0 is the deepest. Ergo, the 2.0 head is the Porsche head most prone to detonation.
All engines built by Porsche with 10.4 used either twin plug and/or high octane fuel.
Not being an engineer, I can't be any clearer than that. Perhaps a train driver

would like to chime in.