Without causing anyone else to go searching through the forums, can anyone tell me the compression on a 2005 STR motor?

Hard to find many specs on this motor online, i was also curious to find out what model eaton blower is on the car. Maybe I'm looking in the wrong places...? Chevy+Ford specs are A LOT easier to find lol.

THANKS!

 

I believe it's 9:1. Non-R is 11:1

 

Can't recall but it's in the brochures and they're on the site plus all over the net.

Eaton M112

 

I must be searching wrong if its all over the net. Thanks for the compression specs and blower model guys!

The M112 is the same blower used in the Cobra's isnt it?

I was asking about the compression because i had heard the R was different from the non R. I was going to try running 87octane in the car if the compression was under 9.5 to 1... Looks like it will be just fine as long as I'm not getting on it. I'm tired of paying 4.35 for 93 octane if its un-necesary. I got rid of my diesel truck recently too because diesel was up to 4.42 a gallon and even with my programmer exhaust etc i was only getting 12-13 mpgs.

 

The R is lower than the NA but I suppose the SC ups it when it's engaged?

I've posted multiple (many!) brochures surely some made it to the FAQs. I hope.

Maybe also in JTIS?

OK - looks like NA is 11.0:1 and SC is 9.1:1

 

Using static compression ratio to determine octane requirements is an overly simplistic and misleading hotrodders trick that became outmoded in the 80s. Adding a super or turbo charger to the equation makes it foolish.

The pros and cons, plus myth, legend and misunderstandings of running lower than recommended octane gas have been beaten to death here many times, almost as much as synthetic oil or K&N filters.

Without sounding like an a-hole, if you can't afford the gas, sell the car.

 

I can afford the fuel just fine...but why waste money? Like i said been running a sub 12mpg average GM diesel p/u for three years as a daily driver my man. you do sound like an a-hole but i didnt take it that way no worries lol. And before the Vette for the weekend car i had a built big block tubbed fox body mustang that probably got 4mpg and i usually had to run a mix of 110 and 93 because i couldnt keep my fingers off that n2o button lol.

I disagree with you however, i also dont think its an outdated hot rodder's trick. However apparently im an outdated hot rodder compared to most on this site.

My guess, this is strictly based on my guess, is the computer has knock/ping sensors. Sooooo if running 87 and the car starts to ping, timing would be knocked back a few degrees automatically to save the car from detonation? Most modern domestics have this feature...

Also, an aluminum headed car can usually run 87-89 octane completely safe up to 10 to 1 compression...at least thats what ive found in my travels. When under boost, i agree it can be a bad scenario, but not when any driving i do while on 87-89octane would be sub 3000rpm's. (keep in mind pumps are minimum rated in NYS so octane has the potential to be higher, and i dont buy fuel from mobil who has the reputation of pumping out lower octane than advertised nor will i buy from a hodge mart) IDK what the curve is as far as boost levels go on the car, without a gauge its impossible to tell as all motors will vary by ambient air etc... but i doubt sub 3000rpms 87 octane would cause any issues.


Oiled air filters ( for example: K&N are pointless in my opinion) and i run synthetic oil, however I have read millions of pros/cons to it. I choose to spend the extra $ on the synthetic...cant hurt i suppose.

 

10-4 Mikey. Zee0hsex, as jagv8 states; the blown STR has a static compression ratio (CR) of 9.1:1. This will start the motor and run her smoothly. Thereafter, the blower begins to pound 13 lbs. of boosted fuel and air mix, effectively raising the CR to 12:1 or more. If less than 91 octane is used, the ECU will pull back timing (thus power) when it senses pre-ignition (knock). Unburnt low octane fuel can accumulate and foul the valves, pistons, and wash down the cylinder walls causing premature wear.

 

Yes, the cars do have knock sensors and the ability to retard timing. The supercharger builds boost in proportion to engine rpm meaning that the static compression number of 9.1:1 is meaningless as soon as you turn the key.

It's very probable that timing would be retarded under even gentle driving conditions cutting into performance accordingly. The point is that using 87 octane defeats the purpose of having an R.

If you find that gas is too expensive now, it's not going to be cheaper in the future- are you going to run the car on 87 forever?

 

Er... it's entirely possible the car will NOT cut back timing with 87 octane because the software may be written on the basis that 87 is never used.

That's "may" - wanna risk it? Not me. Yet our fuel is a little over double your price.

 

jagv8- it's known that the knock sensors and accompanying software to retard timing does exist on all S types. I can't think of any modern car that doesn't have this.

I do agree with you that worrying over a few dollars per tank seems 'unusual' given the type of vehicle, not to mention that the OP's fuel price is far cheaper than what you or I pay!

 

I doubt the software is written on the basis 87 is never used, because i dont think thats how it functions... If the car has knock/ping sensors like some of the guys have stated the car will adjust timing accordingly, you could always get a bad tank of 93, then what would it do, even though technicaly "87 wasnt being used" in that case.

No i didnt plan on running it on 87 forever, then again I probably wont have the thing for much more than 2 years. I get bored fast....So whats forever?

And when i was stating this idea, it was more for when going on a trip or when my week would require mostly highway etc... jump on the highway and cruise at 1800rpms, with 87 octane, i see no damage potential there.

Also my R is a daily driver, so the comp cutting into performance accordingly with my octane level/driving habbits when on lower octane DEF doesnt defeat the purpouse of having it in my eyes. I didnt buy it strictly on its performance capabilities. Its appearance, comfort, ammenities, buying price and mileage, etc was why i bought it. The performance is just a plus for me, I currently and have had something far more geared for performance to drive other than my daily drivers. So if and when I want to run my R hard ill put good fuel in it. Only downfall is if some idiot pulls up next to me on the highway that wants to go when im not prepared, but then again, who the hell cares, i know what it can do, and ive burned and been burnt by PLENTY of vehicles on the road. Im over that.

Also i apologize for bringing up a topic that has been beaten to death, Im failry new on the site...but i appreciate the comments whether you agree or disagree. This is without a doubt an age old argument that will never come to a head.

Jagv8 where the hell are you located that fuel is $8+ per gallon? thats insane!!! I thought NY sucked. Yea its only a few dollars per tank, but that adds up when you drive a lot. And those few dollars can add up to the savings it will take to have a repair performed that has a high potential on this type of car that may be beyond my wrench level.

 

Of course the car will NORMALLY look at the knock sensors - only at times of its choosing, though. My point is that the software may not do so in a way that can cope with 87 octane.

BTW, it almost certainly cannot look at them at all in some circumstances, such as open loop or any time the PCM is rather busy.

Any debate about bad gas is pointless as that could trash the engine for all the PCM cares. Probably throw codes but remember not until 2 consecutive drive cycles, generally.

 

I guess you guys are just a bit more worried about it than I?

I agree with your arguments, they are all 100% valid...but i think the motor is a bit more resilliant (sorry couldnt figure out how to spell that correctly and too lazy to look).

I dont think that the static compression numbers are thrown out the window when put under boost either, Im not a forced induction guy by any means. I know the basics and thats about it, but i dont think the compression changes when boosted, i was under the impression that the combustion chambers were just under a different type of circumstance with the same amount of compression. The air is compressed in the supercharger right? hence the term boost. So why would the static compression of the combustion chambers change? All your doing is using pre compressed air and fuel in the combustion chamber rather than the natural atmospheric pressure of the air and the same fuel.

However i know it has the same effect as n2o or a car with stupid high advanced timing when burning, hence detonation. So if i was to REALLY push the argument i could use a few stage colder plugs. But theres not point to that when 93 octane is available. I suppose you guys have me beat as far as wear potential compared to actual savings, but then again maybe not?

 

Ouch. Your engine theory and physics are way off (again not trying to be an a-hole)

A turbo or supercharger gives a net increase the dynamic pressure in the cylinder- that's their job. The 'detonation' couldn't care less whether the pressure comes from the piston all by itself or from some external means. Same end result- far more air compressed to a far higher degree which allows more fuel to be injected give a huge boost in power.

As to N2O- that's a whole different ball game. Changing plug heat ranges doesn't eliminate compression-related detonation, it eliminates hot spark plug related preignition. Apples and oranges.

 

Actually a very informative post, thanks! So i do know the n2o factors without getting into them here.

Like i said im a newbie with forced induction, thats why i placed to many ?'s in that last post.

 

Also when i was using the term detonation, i was referring to pre-ignition as you put it. Ive always learned white plugs means too hot which equals detonation.

But I dont think my engine theory and physics on how it works is off, i think i just am ill knowledged when it comes to forced induction as i explained. Thats why i thought the air was being compressed in the turbo/blower which made it a more dense air being put into the combustion chamber, which is why more fuel was allowed to be burned. I understand now youre not forcing more dense/compressed air, but more air all together.

 

Er... it IS forcing denser (compressed) air in and that IS more air. Being already compressed, when it's compressed in the cylinder it's now even more compressed. So, higher compression. In round terms, multiply the two together to get the overall figure.

Why is this so hard............. it's not!

Oh, I don't mind if you run 87, just saying that it's not a good idea (may be a very bad one) and no way would I do it.

 

I didnt say it was hard.

Thanks for the replies

 

Zee-

Detonation and pre-ignition are NOT the same thing and the terms are not interchangeable!

Here's a brief explanation of the differences:

Detonation

Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

Pre-ignition

Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.

Engine Basics: Detonation and Pre-Ignition: Streetrod Stuff