Compression ratio of NA vs Supercharged engine...
 

I was looking at the specs for the 2010 5L and found this interesting:

5.0L V8 N/A 5.0L V8 S/C

ENGINE & TRANSMISSION
Engine Capacity (liters) 5.0 5.0
Engine Capacity (cc) 5000 5000
Cylinders 8 8
Valves per Cylinder 4 4
Compression Ratio 11.5:1 9.5:1 <<<<<<<<<<<<<
Bore Stroke/inch 3.64/3.66 3.64/3.66
Bore Stroke/mm 92.5/93.0 92.5/93.0
Trans Final Drive Ratio 3:31:1 3:31:1

So the compression ratio is *lower* in the S/C engine, which I expected to be higher. I understand that there are many components of power, but for you guys that have raced/have a lot of experience with building/tuning engines why do you think they designed the S/C engine with a lower compression ratio?

The supercharger is cramming more air into the cylinder, and I don't have any numbers on the difference between volume in the N/A engine as opposed to the S/C engine, but still I would expect the compression ratio to be higher than 9.5:1...

It's typical for any force-induction motor to have lower compression than a normally aspirated version. Afterall, when boost exists the additional air forced into the cylinder raises the combustion chambers pressure more than when ingesting air at atmospheric pressure with the NA. Not a very technical answer, but the goal is to avoid detonation and minimize engine wear.

Bruce

Bruce is correct, forced induction motors tend to come with lower compression ratios to accommodate the additional boost pressures and to reduce fuel octane requirements. A normally aspirated engine that requires 91 octane fuel, if boosted to say 8 psi, would likely require fuel that is several octanes higher to prevent high combustion chamber temperatures "knocking", which could damage the engine.

My twin turbo charged engine came with a 10 psi factory boost for 91 octane gas. Now, I am running closer to 20 psi boost but, if I tried to do that on 91 octane my engine would be history in a very short order. I'm using 103 octane racing fuel for that level of boost to prevent knocking.

I'm still curious about:

1) How much boost the supercharger provides on the Jag SC engines?

2) What kind of boost control exist on the SC engines? This last question comes out of the discussions on the effects of installing a smaller aftermarket pulley that supposed to raise boost 1.5 psi. I wonder how that additional boost does not get regulated back down to the standard boost by the boost control?

Albert

I am curious about this as well. If the additional boost is still within the range of the engine ECU software and mechanics to support it then I'd like to install the upgraded pulley on my XKR.

Also, I am curious if the additional boost will cause a degradation in fuel economy. I would guess "yes", and if that's the case how much would be the next question...

I don't know either. I wonder if S/C's use a drive clutch to regulate boost vs a wastegate. I've never looked into it, although a search under Eaton might turn up something. You should look into that in case one day you consider swapping the RX7 TT and XK for a beast that runs on pump gas and won't bust a spleen, I don't know..maybe an XKR, or an XJ S/C when you're old enough to pull off that look :)

Bruce

Ha...ha...ha... I don't know if I ever get old enough for that look. I'm still a kid enough to enjoy a wild beast under my butt at times but, just not for regular riding. It's like riding a wild bull; a thrill for that short time but a real PAIN for extended times.

The last time I had dropped my car at the dealer for software update I asked for a brand new SC XJL for a free loaner and, to my surprise, they agreed to hand me the keys to a window stickered SC right from the lot. Not even their regular loaner. Well, I had a blast driving the SC XJL for a full day and loved it. I will very much consider it for my next purchase, even though my rational part of the brain (as tiny as it may be) tells me that for the type of driving I use this car for, I would be better off with the normally aspirated 5.0. But, than, if I turn that rational part of the brain loose it may convince me that a Prius would even be better... ah...blasphemy....!!! forget that thought...!!!!

Albert

Unfortunately, everything in life is a compromise in varying degrees...lol...

And I'll spend the rest of mine trying to prove you wrong :icon_lol:

Manual for the 07-09 shows a pressure release bypass valve mounted to the throttle body. Also says it is diaphragm controlled, which is very "old school" in the turbo world. Could be a good chance of installing a manual boost controller on that diaphragm control valve an nurse a few extra PSI out of the system that way?
Element of danger here, as turbos have a inherent boost limit, and can only develop pressure up to a point. Chargers don't have that limit, so massive over-boost to self destruction is possible.

Just thinkin out-loud...
Vince

I can't see any "pressure relief" feature actually allowing the release of airflow from the supercharger and intake as that air has been metered through the MAF sensors downstream from the airfilters, with that air volume used to calculate fuel to be injected. Bleeding off air after the MAF sensor would cause the air/fuel ratio to end up being rich...and that would never have been designed that way. But if a pressure monitor activated a something that reduced the supercharges outputed airflow that would make sense. I'll look into it when I've got more time.

Bruce

Here what I was reading... Maybe I have the wrong area...

Referring to item6

Bypass Valve
The bypass valve attaches to an opening in the induction elbow and controls a bypass flow from the intercoolers back to the inlet side of the supercharger. At closed or partially open throttle settings (i.e. idle and most cruise conditions), the bypass valve is fully open to provide maximum bypass and optimum fuel economy. As the throttle opens, the bypass valve progressively closes to reduce the bypass flow and increase the pressure of air supplied to the engine for optimum power output.

Bypass Valve Actuator
The bypass valve actuator is a diaphragm-operated actuator attached to the valve spindle of the bypass valve. A vacuum pipe connects the actuator to the induction elbow.

Vince

There is no boost control, only a bypass valve that allowes (metered) air to bypass the supercharger under high vacuum (so very low load) conditions.

I asked for help in the General Help section...here's the thread:

https://www.jaguarforums.com/forum/g...engines-72136/

Brutal comments are very informative. Thermo is convinced it will void the warranty, but this is not shared by Brutal.

Okay, now that makes perfect sense...thank you! So that means under brisk/full acceleration and load there is no by-pass at all, and therefore no increase in boost pressure. I recall ECU Tuning claiming that their tune increased boost, and I can now see that they are probably adjusting this by altering the by-pass function at low or part load, and not actually increasing the amount of full boost at wide-open throttle that I think you and others questioned without reply.

I found Brutal's comments interesting as he seems to suggest that a pulley ugrade does not require an ECU tune to adjust as the ECU is able to compensate for that extra boost already. If that's true I would feel a lot better about going that route to get a little more power because they have failed to convince me that they are able to do a very good job of re-mapping the ECU based on the one tune they did on a 5 L XJ S/C.

Unfortunately there would not likely be much of a power increase from a 1.5# pulley upgrade alone, but at least with a little more information we might be able to consider it to be safe, and we wouldn't lose it if the dealer did an ECU update. Do you think 1.5# would equate to maybe 15-20 lb-ft of torque? And do you know what the stock boost pressure is on the 5L?

Bruce

Do

From Mina's website:

"...This kit is designed to deliver an extra 17-25 HP (at the wheels) by getting your Jag supercharger to produce 1.5 Lbs of additional boost."

It does not mention torque. But an increase of 17 to 25 HP at the wheels, assuming a 15% loss through the drivetrain would mean an increase of 19.6 to 28.8 before loss...

Their price is $320, and (just for debate sake) let's say install is $75...if we use the higher hp number, and a price of $340 for Mina's pulley (cost plus shipping) then the cost per hp is $14.41...

Superchargers are limited by the pulley ratio (crank pulley/supercharger pulley). They won't produce more boost than what is set by the pulleys. To change boost, you need to change pulleys. Whereas on turbos, you can change boost on the fly by fiddling with the boost controller.

Forgive my ignorance, I've never dealt with super charging before, only turbo charging.

So, you are saying that max boost is limited by the pulley ratio. That also implies that max boost is dependent on the RPM that the super charger and the engine are turning at.

So, my question to clear things up: Is the pulley ratio chosen to provide max boost at max engine RPMs? In that case, if I understand correctly, the supercharger would only provide partial boost, say 2# at 2000 RPM, increasing that level of boost all the way up to the engine red line?

I've remember reading the advantage of super charger vs. turbo charger that the supercharger provides almost instant low RPM boost while the turbos need to spin up which tends to occur at higher RPMs. So, the supercharger only provides a low level of boost at low engine RPMs?

Albert

I too am starting to understand parts of the equation... Supers will build boost indefinitely strictly depending upon the rotational speed you provide them. The only limits would be if you starve them of air due to intake restrictions.
Seems like a safety valve should be incorporated in there somewhere, but I guess if they set the blower upper deck pressure to trip a fuel cut off, they have accomplished the same function?

Vince

I'm no expert either, just know some basics.
Yes, max boost is limited by the pulley ratio and/or gearing. Also they can't spin to infinite rpm but are limited depending on supercharger. 15K or so for the roots type Eatons and about 20k for twin screws.

Centrifugal supercharger work as you described with linear boost rising with rpm. So low boost at low rpm with the boost rising with engine rpm.

Positive displacement superchargers such as the roots (Eaton) and twin screws/lysholm provide more boost at low rpm than centrifugal superchargers. They pump a fixed volume of air per rotation.

Here's a formula for figuring out boost:

Theoretical Boost = (((Supercharger size in liters x Pully Ratio) / (Engine L / 2)) x 14.7) -14.7

Then parasitic losses take away a couple psi.