I curious if anyone knows why Aston has such a large displacement with only ~450 BHP from it?

 

Is the AM super-charged? If not, then that would be compared to the XK's 5.0L with 385 bhp.

Bruce

 

Because it doesn't have forced induction. The Jaguar NA 5.0 makes only 380 hp.

Anyway, the 5.9 in the DBS does make 510 hp naturally aspirated.

 

Bruce beat me to it.

 

Ahh, yes, the SC makes a difference there.

But, it seems that a liter more displacement, 4 more pistons that the engine could be tuned to put out a lot more power.

 

Was the DBS engine modified mechanically?

 

Also depends on the desired character of the car.

Think about the 5.0L V10 in the previous M5, it's naturally aspirated & puts out 500BHP.

The holy grail with N/A engines is that figure 100BHP per litre.

BUT, it's a monster redlining at 8250 rpm, it's loud, harsh, not exactly in the mould of Aston or Jaguar.

Tune the Aston V12 to make 600HP N/A and it wouldn't be an Aston anymore...

Here you go Let me google that for you

 

Cambo, thanks for your comments. As to the arrogance of the Google reference, my question on the board was to engage board members in a discussion, not to show any inadequacies in using the internet to search for information.

I have a degree in Computer Engineering and been in IT for over 25 years. I also have a patent on a 70 Million dollar software system in use by one of the largest airlines in the world.

I know how to use Google - but thanks for your help.

 

Displacement is just one factor affecting the amount of air/fuel mix the engine can ingest, and doesn't too accurately indicate the power it can make. A key factor is the engine's volumetric efficiency, which is a measure of the engine's ability to ingest air, and that VE varies across the rpm band. Camshaft specs, cam and valve timing, intake manifold design, exhaust restrictions, etc, etc all impact VE and the amount of air that will flow through that displacement. Some engines are designed and tuned to make gobs of power at low and mid rpm, and sometimes at the cost of higher rpm where the peak hp you've quoted is made. Variable valve timing is one technique that allows an engine to make more power across a wider rpm band, but other factors can make power trade-offs.

After those mechanical considerations you have the air/fuel and ignition tuning programmed into the ECU. Power will vary rather widely from tuning, as will the accuracy of advertised power figures!

Bruce

 

Well I see someone has no sense of humor.

 

To broaden the discussion slightly...hp isn't always a particularly useful measure of power since it only applies to a very narrow rpm band, and generally at high rpm where most street use seldom ventures except under hard accelearation. A more useful measure that recognizes power throughout the entire powerband is Area Under the Torque Curve (AUTC), and when combined with hp gives a much more accurate picture.

If you were to examine a dyno plot you would see both a hp and torque curve. Torque is what you feel as a push into the seat back, and the thing that thrust you forward as the smoke pours off your rear tires!

The amount of area, or space, that you see under the torque curve is a visual representation of the total amount of power that is available across the powerband. You can quantify that by noting the actual torque at various rpm and add them together for a total that could then be compared the same way with the dyno plot from other cars. If your engine will most often be in the 2000 rpm range while cruising, and accellerating from there up to say 6500 rpm, then it would make sense to add torque at 500 rpm increments from 2000-6500 rpm. 500 rpm increments is close enough to account for fluctuations in the torque plot, but not so close that it would be too onerous to calculate.

This analysis will clearly demonstrate the dramatic and meaningful difference between a 2L turbocharged engine making 510 hp and a 5L supercharged or normally aspirated engine making 510 hp. The 2L would only make modest torque up to 3500-4500 rpm compared to strong torque at 2000-2500 rpm. Huge difference in normal driving where a wide powerband is the holy grail, but not so important for drag racing where only high rpm power is useful.

If someone has dynos of the 5L XKR and 5.9L DBS to post we could do an analysis.

Bruce

 

Bruce - excellent twist. I don't have you racing guys knowledge of this stuff. Very interesting though and would also like to see such a comparison.

Perhaps I will try and Google it.

 

Here's a dyno from a 2011 XJL 510 hp S/C. The thread can be found here, and dyno in post #53...https://www.jaguarforums.com/forum/x...upgrade-64589/


Please note that different dynomometers will yield different results from the same vehicle, so different vehicles measured on different machines will do the same. Varying test conditions including ambient air temp and facility altitude above sea level will also affect results. Two vehicles run on the same day on the same machine would yield very comparable plots. The real value of analyzing plots is to see the shape and nature of the torque curve even if the actual numbers may not be completely comparable.

This dyno shows the advertised 510 hp engine produced ~420 horsepower at the wheels as measured on this particular Dynojet dynomometer. The 90 hp difference between hp measured on an engine stand and measured on the dyno represents the consumption of power in driveline losses, and affects all vehicles to one extent or another.

Bruce

 

Thanks for posting the dyno curves. Does anyone have similiar info for the 4.2 supercharged version? If this is an old question, can someone point me in the right direction?
Thanks, Glenn

 

I don't understand the 90 hp driveline losses.

90 hp equals 66 KiloWatt. A coal fire of 20 inch diameter puts out around 9 KW. This means I need a BIG fire to put out 66 KW. If so much heat was generated in a car it would burn, not drive.

Next. Dyno losses are mostly calculated as a percentage. So let's assume I have an engine that puts out 200 hp. According to the 20% losses rule 160 hp would be left. Now I modify the engine to put out 300 hp in the same car. According to the 20% rule now the loss is suddenly 60 hp? But it's the same engine, same car with the same driveline.

So an XK with 298 hp loses 59 hp and an XKR with 510 hp loses 102 hp? almost twice as much? Not very likely.

 

sorry, double post.

 

Years ago I had a TVR400 on order and while I was waiting for it they lent me a TVR350 which was basically the same car with the engine but in a lesser stage of tune (V8 3.5L). In normal use the 350 felt faster as the power was delivered at lower revs which made it a more relaxing car to drive.

 

You need to be careful with drivetrain losses, first of all we can't measure it on the street, and each dyno type will cause different losses (i.e. 1 large drum per wheel dyno or a 2 small drum per wheel dyno). On dynos with 1 large drum per wheel 17% (or maybe 15%) would be more appropriate, but it is just an estimate which seems about right for the R cars that I have seen dynos from.

More force means more friction, so there is definitely a difference, especially also with our torque converters that don’t lockup. I think my car would start to burn if I let her running at full power without proper cooling ;-)

Anyway, 420rwhp (on a dyno with 1 large drum per wheel where a 17% drivetrain loss could be used) would mean 506 about engine hp.

Not advocating that this is all exact, which is why I personally only look at dyno type and rwhp to get some idea, and only use it as a ballpark figure.

 

Here's an article that dyno tests a 2012 LF-A and 2010 GT-R on the same dyno. The 552 hp LF-A spins the rollers at 430 whp, and the 480 hp GT-R at 382 whp. Open the dyno link and note how the shape of the torque curves are so different, with the GT-R starting off with a big turbo-boosted push peaking at 3800 rpm and then falling off, while the LF-A's none boosted engine makes a more consistant amount of torque. As the GT-R runs out of breath it 's power dips below the LF-A's from 6900 rpm and up. The LF-A's big high speed advantage is its much higher redline that allows more effective use of each gear, and its lighter vehicle weight.

The XKR/XJ S/C dyno graph shows a torque curve "shape" that is very similar to the LF-A's, but stronger due to the benefits of super-charging. The LF-A then dominates with an extra 2000 rpm...and the Jag would become a tiny spec in its rear view mirror. Flat torque curves are the most desirable and the Jag 5.0L S/C engine does it perfectly. When a friend with a GT-R drove my XKR he just couldn't get over how much power it had at any speed and any rpm.

Bruce

 

Here's a thread with a dyno of an Aston Martin 4.7S, before and after exhaust mods. You'll see a desirably smooth and flat torque curve again, with the ultimate goal shape often described as "table top" flat.

http://www.6speedonline.com/forums/a...-4-7-dyno.html Post 11.

Bruce