Regular or Premium Gas: Is Engine Still High Compression After Supercharger Disconnect

gasoline
#1

The dealer told me that the supercharger on my '99 Buick Regal was starting to wear out and it would take the entire engine with it unless I disconnected it. So, I sadly said goodbye to the supercharger. Now I’m wondering if I still need to use premium gas. Do I still have a high compression engine?



PS - Notes of condolence for my supercharger are welcome.

#2

Well, no. In fact, you have a lower-than-average compression engine now, you can use regular safely.

#3

You can find these superchargers all of the time on eBay, Craigslist, etc. for a reasonable price and I don’t quite understand the logic behind a worn out supercharger taking out the rest of the engine unless they’re referring to a castastrophic supercharger explosion.

I bought a good used unit a couple of years ago off of eBay with all of the plumbing, intake, etc. for 75 bucks.
If you’re even mildly mechanically inclined you can buy a rebuild kit for these superchargers and DIY. A small handful of bearings and seals, a few gaskets, some oil, and that’s about it.

#4

How is the power level with no supercharger? For the disconnect was it required to only remove the belt drive.

What symptons brought you to the Dealer? what part of the supercharger was wearing? Did the method of “taking the engine out” get explained? Did you do all oil changes and on time? type of oil?.

Did you get a quote on new supercharger?how much. Are you considering a repair? I would think performance is poor.

#5

The only logic I can surmise from the dealer’s comment is that the lower compression without the supercharger would reduce blowby thereby preventing high crankcase pressures and their effects, those being oil leaking from your engine’s every seal and gasket.

I’ll disagree with my respected friends here. If in fact your engine’s old and tired and that’s the goal, simply stretching a few more miles out of it, and the car is working fine for your needs without the boost, he may be doing you a favor.

There is an old axiom: it’s cheaper to keep a new car running right but just keep an old beater running.

You should not need premium any more. Run with regular and if you do hear pinging you can always reconsider.

#6

In normal driving the supercharger is not doing much anyway. These things have a bypass designed into them and the induction system works the same as a normally aspirated engine unless the accelerator pedal is stomped.

#7

To answer your question, without the puffer boosting manifold pressure, you can use regular gas safely.

#8

I would really like to know, does the supercharger really increase the compression ratio?? I doubt it very much.

#9

Increase compression ratio? No, certainly not. A super charger increases combustion chamber PRESSURE, though.

Think about it: without the supercharger, assume that a given cylinder will take in , let’s say 50 cubic inches of air/fuel. It’ll then compress it to a volume of about 6 cubic inches, for a compression ratio of 50 / 6, or a little over 8 to 1.

But with the supercharger running, the same cylinder is pressurized before the piston begins it’s compression. The blower may force feed the cylinder 75 cubic inches of air/fuel. That 75 cubic inches compresses to the same 6 cubic inches, for an effective compression ratio now of 75:6 or about 12.5 to 1.

The cylinder has been filled with more air than it normally would suck in, it has been charged more than normal, hence the name SUPER charger.

#10

Actually, when you take in to account the dynamic compression ratio (and you would because the super charger only operates while the engine is running), it does in fact have a lower compression ratio with no super charger than it does with a super charger.

#11

The same principle is in play when you drive in high altitude regions of the country. In places like Utah, New Mexico, and Colorado, “regular” gas is 86 and even 85 octane instead of 87 octane, because in the thin air at mile high altitudes, engine don’t need as high an octane rating as they need at sea level.

A supercharger just artificially takes the engine to a lower altitude so it can make more power. In fact, the earliest use of superchargers was to permit aircraft engines to make sea level horsepower at high altitudes. There’s no way the engines of a B-17 bomber could fly at 30,000 ft if the engines didn’t have superchargers and the crew didn’t have oxygen masks.

#12

I would really like to know, does the supercharger really
increase the compression ratio??

Compression ratio is a ratio of the volume of the combustion chamber; from its largest capacity to its smallest capacity.

A supercharger cannot change it.

#13

Correct, the supercharger does not change the compression ratio, but it still make an engine need a higher octane rating.

Have you ever noticed that engines stop pinging when you lift your foot off the throttle? Lifting the foot off the throttle also doesn’t change the engine’s compression ratio but the high vacuum in the intake manifold causes the engine to need less octane rating.

A perfect vacuum compressed to 1/100 of its original volume is still a perfect vacuum.

#14

Don’t positive displacement superchargers actually perform their best at low-mid engine speeds? And since all superchargers are driven off the engine, they are working at any time the engine is running?

#15

As suggested, use regular unless you notice pinging.

My deepest regrets for your supercharger.
:frowning:

#16

Correct, the supercharger does not change the STATIC compression ratio
but it does have a significant effect on the DYNAMIC cr.

#17

A supercharger does not change either static or dynamic compression ratio.

Static compression ratio is based on volume ratios between BDC and TDC.
Dynamic compression ratio uses the position of the piston at intake valve closing rather than BDC.

Dynamic compression ratio is always lower than the static compression ratio.
Dynamic compression ratio does not change at any time during the operation of the engine.

Dynamic compression ratio should not be confused with cylinder pressure, which is always changing.

#18

Wow, I’ve been using that term incorrectly for years. Thanks for setting me straight!

#19

The reason why the superchargers do not provide boost during normal driving conditions is because there is a vacuum pod controlling a bypass flap. At a certain point the spring loaded diaphragm in the vacuum pod will override the vacuum, the flap will close, and boost will begin.

I could not find a good pic of a Buick unit but here’s one of a Ford. Same thing.
Note the vacuum pod at the top of the item on the left. A rod connects that pod to the flap inside the ductwork and it all works, as convoluted as it may seem.

http://cgi.ebay.com/ebaymotors/Thunderbird-SC-M90-Supercharger-3-8L-Very-good-1992_W0QQitemZ370155388226QQihZ024QQcategoryZ38634QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

#20

What math are you using to determine all of that?