77amc

Has anyone looked into lowering their compression ratio to utilize less expensive gas?
Would there be a thicker head gasket that can lower the ratio by .5 or more?

[EDIT = political comment removed] I thought of using thicker head gaskets (after I port my heads) to lower it for an octane rating of 'regular' or 89. (I guess i could de-shroud the chamber some to get it lower..)
I'm really not looking to build a race engine and would be just happy as a lark to have 280-300hp out of a 5.7 or 230-260hp for the 265.
I've read many threads on how the owner had tuned his LT1 car to get better mileage but would it help any to lower the comp any?

Just a thought.

E

Injuneer

You could just retard the ignition timing. If you use low octane fuel, the knock sensor/PCM will do that for you, and prevent engine damage.

77amc

OK, so all I would need to do is just pump in regular gas and the O2/ecu will accommodate? (by retarding the ignition timing)
Well, that's reassuring..

Thanks again sir.
E

Injuneer

Has nothing to do with the O2 sensors. Your engine has a knock sensor. Your engine is tuned to run on 91 octane fuel. If you use lower octane, you will likely experience detonation (spark knock). The knock sensor picks up the knock vibrations, and the PCM (powertrain control module) retards the timing. Run low octane fuel long enough, and the PCM will set a "low octane fuel" flag and the timing will be retarded until the engine is switched back to the correct octane fuel.

The specifics are somewhat dependent on the year of the engine and PCM. You haven't provided that info, so you are getting a "generic" response. Best thing to do is set up a "Signature" with basic info about your car - year, model, engine, trans and any major mods. You'll get more specific info that way.

Finally, the octane requirement of the engine is highly dependent on the altitude, referenced to sea level, where the car is being operated. The higher the elevation, the lower the fuel octane requirement. Again, you haven't filled in the "Location" in your Profile, so we can't get specific about what octane your engine requires to avoid knock.

KW Baraka

In any case.....increasing your gasket thickness will have some effect on lowering compression ratio.....but it will also increase your quench, so you'll lose lots of horsepower.

My suggestion would be to retune the PCM......pull a bit of timing THAT way. Don't depend on the PCM to compensate.

And at least that way, you don't have to open up the engine in any way.

KW

Injuneer

Just a warning to all..... political topics and discussions are strictly forbidden on this site.

Don't go there.

slick1hunting

I would be curious if anyone has tested to see if they get better fuel mileage with the higher octane as I assume you would have a small increase. The other concern I would have is the higher grade fuels have more detergants and if any issues there would be worth the added expence of higher octane. Id also be leary if the knock sensor didnt work and you didnt hear detonation. I also think there is the discussion of using premium with no ethenol and the use of ethenol mixed gas. I would bet that non ethenol premium will yield the highest mpg and the least issues with repairs. (Ethenol fuel is tough on my 2 strokes if not burned quick and letting it sit for any length of time). I know with e-85 it can lose octane and water down so to speak in time(shelf life) as well as the issues of condensation. Just some of my thoughts. I prefer to pay a little extra to avoid other potential problems. I know there can be pretty good gains in the tune itself. I would be more apt to focus more there. If im not mistaken leaning it out also adds heat and heat takes engine life. Alot of things to consider.

Injuneer

If the knock sensor doesn't work, it sets a code, and the PCM defaults to a calculated ignition timing retard that pulls up to 15*, depending on engine operating conditions.

The stock PCM is hard coded to maintain 14.7:1 A/F ratio in closed loop/part-load driving. That value is selected because it yields the lowest combined total of HC, CO and NOx emissions, not because it provides the best fuel economy. There is no harm to "engine life" if you were to run a bit leaner than that. You would get better fuel economy, but the inspection sniffer would pick up increased NOx content. Leaning it out in closed loop is not easy.

bw_hunter

The use of higher octane fuel can produce more rear wheel horsepower but shouldn't yield significantly better fuel economy. All gasolines have the same energy content per pound and the octane rating relates to the burn rate. Higher octane fuels burn more slowly allowing more timing advance, making the engine more efficient. Lower octane fuel burn more rapidly and will detonate with a lot of timing advance...the knock senors 'hear' this knocking and the PCM pulls timing to protect the engine. (Alcohol containing fuels have higher octane ratings but lower energy content per pound leading to poorer fuel economy)

What I don't see is how this could equate to better fuel economy at part throttle conditions. If it does I can't imagine it would be significant enough to notice.

The difference in fuel prices here amount to about 25 cents a gallon between mid-grade and premium. That works out to about $4.25 a tank full or about 1.4 cents a mile. Is it worth retuning the engine or making mechanical changes for this meager savings?

RamAir95TA

IMO if you're that concerned about fuel mileage that it causes you to come up with the idea about changing the entire dynamic of the LT1 engine then maybe you shouldn't be driving a 17 year old RWD V8 sports car.

Just a thought.

77amc

WOW.. I didn't think that was such a sensitive 'political comment'..

OK, added to the 'profile' but I've got several PROJECTS (more like 9) and this one is not a GM. Just the engine. The rest can range from adapting a Jetski to the bottom of my 12' jonboat to making a Kubota 3cyl diesel motor into a generator to power my house in the near future.
It's a '98 BMW 328i convertible that I'm adapting a LT1 motor into.

The LT1's that I have:
1st-from a Camaro (not sure the year, bought it as a basket case or KIT form) Supposedly a low mileage one, 60K
2nd-from a 94 Fleetwood (with harness and ECU and trans)
3rd- from a 94 Caprice with as complete a harness that i could get out. And ECU.

I thought of starting out with the Caprice motor and go from there to swap the Fleetwood engine later on after I get the bugs wrung out.
I really wouldn't want to start out with the 5.7 and break $$tuff.

On Bimmerforums.com there are several that have done the LSx swaps and have VERY nice results. BUT, i don't have that kind of money just laying around and there are some that have accomplished the LT1 swap also.

I would eventually like to get a 740iL and drop a LS motor into.
(I've already got a 5.3 but it needs a rebuild and that's expensive)

SO, Getting back...
From what I'm reading is that the ECU needs to have the 'right' knock sensors from the vehicle that came with that particular ECU.. Right?
Are there any identifying marks or numbers on them?
I don't think that I kept them separate.

E

77amc

That made me laugh and reminisce.. Not changing the dynamic. Just asking questions about lowering the compression.

Those are mostly all I've had long ago. I built a 72 Monte and with a 331 gear and a 200r4 I was getting 19. And that was with a Bracket master 284 cam..
Gas was cheaper then.
But with these Honda's that I've been messing with for a couple of decades, don't have the torque but have been surprising me and keeping a smile on my face (and with getting over 30mpg)
(I did go for a ride in a 380hp civic that was a blast, and couldn't pull my head from the headrest when @50 the front tires started smoking)

And reading of tuning of pretty radical cams getting in the 20's with a 5.7.
That's why I'm starting with the 4.3 with a Camaro cam in a 3100lb car should be closer to high 20's if not better. Considering it wasn't bad in a BIG Caprice.
Since the BMW's gear is 390, the engine should be quite spirited.
As things go, I can look for different ratios if need be.

I had a '67 ralley sport convertible Camaro (and sold it to pay off a loan. stupid,stupid,stupid) and have wanted to get another convertible since.
That one i had put a 283 in, with 2bbl, a powerglide and 273 gear. It got 31mpg driving 65mph with the top down. Had NO get up and go but it cruised nice.

E

Injuneer

It looked like a candidate for deletion when you first posted it. It became one when someone decided to challenge it and started throwing in smilies.

It's a judgement call.... please accept it.

The knock sensor has to match the PCM. 93-95 PCM's (OBD-I) from the F-Bodies require the 4,500 ohm knock sensor. 96-97 PCM's (OBD-II) from the F-Bodies require the 100,000 ohm knock sensor. I have no idea what the mix is for the B-Body cars. If you mismatch them, you will get a code for the knock sensor circuit voltage.

From Shoebox's parts listing:


1993-1995 knock sensor AC-Delco # 213-96 GM 10456126

1996-1997 knock sensor AC-Delco # 213-325 GM 10456287

And a little info about your project would have been helpful up front, since the L99 is not an LT1.... just a diminutive cousin, with it's own, unique combustion chamber sizes and gasket thicknesses.

rngilliland

I tested my completely stock 94 Z28 auto using 93 and 87 octane fuel. There wasn't any difference between the two, except when accelerating using half throttle or more. The 87 octane had a good bit of timing retard, but the 93 octane didn't have any. Under all other conditions there wasn't any timing retard or knock count. That was five or six years ago and I don't have the files anymore to give more exact information. The two tests were done a couple of weeks apart, but in about the same weather and around 300 to 500 ft altitude.

LowRyter

I had ran a leg of the Hot Rod Power Tour and needed gas but the station only had 87. Car ran fine. I wish I had tried it 160k miles ago when it was new. Would've saved some money. Could feel no difference in 100+ heat.