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How much friction does a 911 motor have?

I'm not sure exactly how to ask this question, so a little background - I'm trying to calculate the fuel requirements for my 3.2SS MFI build. I have a 2.7RS space cam and I've chosen the cams (DC43-102) and port sizes (39/36) and inlet tracts to somewhat mimic the power/torque curve of a 2.7RS, only bigger.

The MFI system delivers whatever fuel you tell it to. No feedback, no "active" metering like a carb, where more air = more fuel. It's all up to the calibration to provide enough fuel for the power being made.

At first glance you would think it's easy, 3.2/2.7 = 1.185 so if I increase the fuel from the factory calibration by 18.5% that should be good.

However, the motor first must overcome it's own friction before putting power out to the flywheel. So in reality the 210Hp (probably more accurate to say 225 from what I've been reading) 2.7RS motor has to make, say, 300Hp, but 75Hp of it gets sucked up whirling all it's bits around, making heat and noise.

Given that frictional loss stays somewhat constant, then my 275Hp 3.2 is going to need to make 350Hp. The difference is slight (only a few percent using these numbers) but what if the real frictional loss is much more? If for instance it takes 400Hp worth of fuel to make 225 at the crank, and 450Hp worth of fuel to make 275, the increase in required fuel would only be 12.5 percent.

Does that even make sense to anyone?
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Old 01-24-2019, 08:12 AM
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Old 01-24-2019, 08:17 AM
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I don't have a number for you but we do know that in 911's it's common to see a measurable increase in friction when fitting heavy valve springs for racing.
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Old 01-24-2019, 09:09 AM
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I've got dyno numbers from a bone stock mid-80's Chevy 350 with a 2 barrel carb.


Back in College we had that on a hydraulic engine dyno. you could apply varying load to motor using the the measurable resulting torque.


Memory recalls it put out only like 165-185 hp,

but the cool fact/data was at idle.


it takes 12-15 horsepower to spin over the engine at idle, taking into account Engine Oil resistance and rotating mass in motion, thermal heat and so on..............


not a direct answer to your post, but accurately measured data that I can share with all.......
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Old 01-24-2019, 09:23 AM
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Someone may have put a 911 on a Spintron, and so would have relevant data. Might have to warm the engine to operating temperatures before spinning? I can't say as I have seen this.

With rear wheel dyno numbers, there are estimates that the transmission losses overall are in the 10-15% range.
Old 01-24-2019, 10:53 AM
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Seems like you would have to have some very sophisticated equipment and know-how to test and achieve an accurate answer.
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Old 01-24-2019, 01:05 PM
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Jonny, fuel needs are based on airflow. If volumetric efficiency of the two engines are equal, and it sounds like you are doing everything you can to make it so, the fuel needs will be 3.2/2.7. The engine doesn't care where the energy goes. What it wants is a near-stoichiometric ratio for efficiency and avoidance of detonation. Change the air in a/f and the fuel should change by the same ratio.
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Old 01-24-2019, 01:41 PM
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What heads, compression ratio, and what stacks. I am running a 2.7, 9.5 to 1.0 with 2.4 S stacks and S cams and a 2.7 space cam. Strictly a street car, have never dynode it, but the car flies. Have a track car, 3.0 9.5 to 1.0, street rod cams, twin plug 3.2 heads. Taper bored out a set of 2.2 stacks to accommodate the additional displacement, but stayed with the 38mm throttles. Have just made a second set of throttles, 40mm and am heading for the dyno this Sat. As was stated, the pump doesn't know nor sense air flow speed or volume, and presently, this engine is too rich, so hoping the larger throttles will improve the mixture, and more power. Bob
Old 01-24-2019, 02:38 PM
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One more note, when making comparisons using imperical reasoning, very few things will be linier. That is what makes engine development so difficult and frustrating. We must like it though.
Old 01-24-2019, 02:42 PM
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Quote:
Originally Posted by Mixed76 View Post
Jonny, fuel needs are based on airflow. If volumetric efficiency of the two engines are equal, and it sounds like you are doing everything you can to make it so, the fuel needs will be 3.2/2.7. The engine doesn't care where the energy goes. What it wants is a near-stoichiometric ratio for efficiency and avoidance of detonation. Change the air in a/f and the fuel should change by the same ratio.
Dan

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Hmmmm........ you know, I may be overthinking it! Wouldn't be the first time.

3.2/2.7 it is.

Thanks for the voice of reason!
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Old 01-24-2019, 02:53 PM
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What are your pieces, intake, CR, exhaust, single or twin plug. 3.2 heads are 41-42 mm at the intake port, so don't know what you have. factory space cams were very engine specific. a 2.7 cam may serve a 3.2 in as there are mixture adjustments, just don't know how well.
Old 01-25-2019, 05:11 AM
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Quote:
Originally Posted by Rusty Heap View Post
it takes 12-15 horsepower to spin over the engine at idle, taking into account Engine Oil resistance and rotating mass in motion, thermal heat and so on..............
Very interesting ... one thing to consider for the 911 is the cooling fan, which of course draws hp. We can infer how much hp it takes to drive that by looking at the difference in SAE hp figures vs. DIN hp from early 911 motors, back when Porsche commonly rated its cars using both standards. The basic difference is that SAE hp is measured without ancillaries, which for a 911 is basically just the cooling fan / alternator.

If I remember correctly the 2.7 RS was rated at 210 DIN and 230 SAE hp, so the fan/alternator draws 20 hp at around 6500 RPM.

Scott
Old 01-25-2019, 05:23 AM
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Quote:
Originally Posted by r lane View Post
What are your pieces, intake, CR, exhaust, single or twin plug. 3.2 heads are 41-42 mm at the intake port, so don't know what you have. factory space cams were very engine specific. a 2.7 cam may serve a 3.2 in as there are mixture adjustments, just don't know how well.
I'm building the motor to have a similar torque/hp curve as the 2.7 in the hopes that the space cam will be a good match. Based on what I can tell from others it should be and I'll dial in the mixtures using the AFR. There are more details in the most recent pages of my build thread. Basically I'm upsizing everything about the airflow path by the same 18.5% (or thereabouts).

For instance, my throttle body exits will match the ports at 39mm, the RS had 36mm, so that's a 12% increase cross sectional area - the best I could do within the constraints of the 2.4 MFI throttle bodies.

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Old 01-25-2019, 06:03 AM
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Quote:
Originally Posted by Mixed76 View Post
Jonny, fuel needs are based on airflow. If volumetric efficiency of the two engines are equal, and it sounds like you are doing everything you can to make it so, the fuel needs will be 3.2/2.7. The engine doesn't care where the energy goes. What it wants is a near-stoichiometric ratio for efficiency and avoidance of detonation. Change the air in a/f and the fuel should change by the same ratio.
Dan

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^^^^This is going to cover the bulk of it, with some nuanced tuning to cover the differences in VE. If the 2.7 didn't come "on cam" until 4500, and the bigger engine starts to get efficient around 3500, there will be some recovery tune to make the system work
Old 01-25-2019, 06:59 AM
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Originally Posted by smokintr6 View Post
^^^^This is going to cover the bulk of it, with some nuanced tuning to cover the differences in VE. If the 2.7 didn't come "on cam" until 4500, and the bigger engine starts to get efficient around 3500, there will be some recovery tune to make the system work
I went with a fairly "cammy" setup (DC43-102) for this reason. Wish me luck!

Of course I always like to supplement luck with knowledge and tools and information.... so looking forward to it.
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Old 01-25-2019, 08:20 AM
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