Ducktail vs standard lid - oil temp increase?
 

I changed out my ducktail for a standard engine lid and noticed an increase in oil temp. Probably 12-15* higher with standard lid during 3400 RPM highway cruising. No other changes to car and ambient temps same day to day. Is that just the way it is or should I be looking for an issue? Thanks

I've heard that before....it appears the duck allows for better airflow besides adding downforce.

Yes, I recall Porsche factory testing on the duck which verified addional airflow into the engine compartment (and downforce). I think that the "whale tail" offered even more airflow into the engine compartment and downforce (but additional drag over the duck). The duck seems to work quite well all around as long as speeds stay reasonable - under 150 m.p.h. maybe?

Yup. Turns a low pressure area into a higher pressure area.

Are there any grill designs that flow more air? Was really liking the flat lid look. I can live with higher temps during winter but come summer I'd be happier with that extra 12* cushion.

I doubt it unles you went with some non standard scoops/inlets. It is a function of pressure.
What kind of temps are you talking about?
If you want to lower temps try an effective oil cooler....I put a Carrera unit complete on my SC and drive in the dead of summer....100-110+ degrees and rarely see 210 F. Oil temps are never an issue now and as I have a 9.8-1 Euro SC it was a concern.

+1 for Carrera cooler, I drive mine in Texas summer now with no heat issues

Just a minor clarification: A ducktail - or any other early 911 tail for that matter - does not create downforce. But it will provide a better aerodynamic flow which decreases lift.

Thanks for posting this, news to me.

I always thought the duck-tail would just lower temperatures at high speed and high stress, not at normal cruising.

Yes to the reduction of lift, but the Duck increases drag and lowers top speed as I understand it.

That statement is incorrect. The opposite is true.

JR

You are splitting hairs! Decreasing lift or creating downforce are the same thing. The net effect of either is that there is more weight on the ground.

J Mac....You made me pull out Frere's book to look into the 'tail' issue.
The duck you removed decreases your lift by 70% and, according to the tests Porsche ran, it also increases the air intake into the engine compartment. That is your increase in temp I suspect.
The tail also starts to 'work' at everyday driving speeds as airflow is airflow.

Same thing happens with my engine, I have yet to install an auxiliary cooler as my '76 chassis never came with one.

Throw the ducktail on, temps never get higher than ~215 on 90+ degree heat driving hard. Looking forward to the auxiliary cooler since I too like to switch between the duck and the smooth rear decklid from time to time.

If you install a ducktail without the AC does this create even more air flow? I have noticed engine oil temps over 210 when driving fast.

Thanks all, lots of input.

On mine, with Carrera fender cooler, cruising 3400RPM w/ ducktail in 75* ambient = 195-8* oil temp...
Cruising 3400RPM w/flat lid in 75* ambient = 210* oil temp

That's not too hot, but I wonder how high it would go in summer 90*+ temps.

J Mac...in 75 ambient I have a hard time hitting 185 w/duck and Carrera cooler.
@ 3200/3400 cruising into Phx at 100 ambient I'm at 200-205 altho I do have a block off under the fender oil cooler forcing air to the radiator..
Your temps are still good but what afr do you run at....I've a euro and set my afr at about 13.2 idle 3.0 CO...it runs well like that so I'm in the high 11's/low 12's at wot. That richer setting also keeps things cooler....and, I keep my engine/engine oil cooler very clean.

Since I'm revisiting 'The Porsche 911 Story'...you are correct...at 152 mph the duck reduced drag by 3% according to the Porsche wind tunnel tests.

No. The net effect may be the same, but there's a difference in how you get there. The 911 ducktail is a spoiler. It improves the airflow over the back-end, so there's less of a low pressure build up at speed.

In order to create downforce, you would need to put a wing on your 911 instead of a spoiler. But that would create drag and decrease top speed.

+1 not the same. nobody here is talking about the effects on center of pressure just drag and lift?

Slush, Winders was correct. Reduction in "lift" and creation of downforce are the same thing. Perhaps you meant that a ducktail will not get you past the threshold of net downforce. A large enough spoiler would accomplish this, though a wing would be a more efficient choice to meet that goal.