84redta

iTrader: (34)

In the midst of doing all the research on this, and just looking up some EPA ratings on different LT1s, and noticed Fbodies and Corvettes recommend premium while the Caprice and other iterations using the LT1 use regular. Is that part of the 260 to 275HP difference? People who have done LT1 swaps on your cars, what do you use? Regular or Premium, and if you use premium, what grade? 91 or 93?

For reference, i'm using these two different years:
1988 Firebird : http://www.fueleconomy.gov/feg/bymake/Pontiac1988.shtml
1993 Firebird : http://www.fueleconomy.gov/feg/bymake/Pontiac1993.shtml

1988 even says that some of the TPI motors should be using premium although i've seen the owners manuals recommend 87 regular octane so i'm taking/using this source with a grain of salt. Just want to hear what people have to say and are doing with thier LT1 powered cars. Also please include where your LT1 came from, if it was Caprice, Cadillac, Buick, Corvette or Fbody, thanks.

Jon

Klortho

F-body LT1 can run on mid grade, but premium is recommended and 87 octane is bad.

I run premium in mine f-body, 93 octane, I also run premium in my LS1, 93 octane.

It could be the difference between the iron head and aluminum head and the CR difference.

84redta

iTrader: (34)

Do you happen to know if the owner's manual would call for 91 octane? I don't want to cause internal damage, but the more money i save on a minimal point of view, the more money the car can get down the road so thats why i'm so curious.

And while it does make sense why the caprice calls for regular and the fbody and corvette call for premium, aluminum heads are the biggest if not only difference between the two platforms. Anyone else with insight into this and personal behaviors?

Klortho

Well, when I talked to my guy who knows these motors inside out, and told him I was running 89 he immediatly chastised me for doing that saying I needed to be running premium if I wanted to get the best gas milage and performace out of the motor.

Remember, these motors have 10.5:1 compression and the more pinging the knock sensor picks up, the more the PCM is going to retard the timing to keep it from knocking. The aluminum heads transmit the pinging more than cast iron does which makes it even more critical to run the right gas in the car.

All we have in Tn is 93 octane premium, I was getting 26 mpg in the GTA with the LT1 (6-speed, 3.70 gears) and I get about 26 in the '98 with the LS1 (6-speed, 3.42 gears) but I tend to drive the '98 harder than the LT1 as well (it's actually more fun)

84redta

iTrader: (34)

well thats great to hear on the mileage because i plan on using a t56 relatively soon after the LT1 install (maybe a year after) and installing a torsen LSD and 3.42s out of a '99 firehawk with low miles in my 4th gen rear. The carrier still looks nice and new. I was reading people getting like 25 mpg average but then that website brought my hopes down real quick, but hearing that someone else out there with the same body style (3rd gen) with the same drivetrain is looking at better gas mileage than my LO3/T5. So the extra cost of fuel will balance out in the end i guess for the added MPG i'll be looking at.

EFIhead

I know this is an old thread, but I was wanting to run mid grade in an f-body LT1. I know that the factory combustion chambers are 54cc. Summit Racing has dished pistons that add 10cc, 12cc, and 23cc to the combustion chamber figure. What do you guys think? I can't imagine needing all that much dish since these motors can kinda run on midgrade stock. I'm also not worried about power loss from the lower CR because I intend to do LS1 style coil-per-cylinder and LS1 PCM mod using EFIconnection's 24x kit. Thoughts?

Pocket

iTrader: (24)

Recalculate your CR and make a decision

How will LSx PCM control affect fuel grade?

EFIhead

It's not just the PCM I want to use but also the coil per cylinder set up the LSx's do. I believe the power lost from a lower CR won't be as bad as it would if I retained the LT1's ignition system. You wouldn't happen to know how to calculate CR would you?

Pocket

iTrader: (24)

True, but all thats external to the combustion chamber means diddly when picking a fuel type. Even if you had some NASA PCM giving 2 coils and 6 valves per cyl your CR still determines minimum octane

http://www.csgnetwork.com/compcalc.html
http://www.rbracing-rsr.com/compstaticcalc.html

EFIhead

I know that. I also know that the lower the CR, the less power the motor has the potential to make. What I meant to say was that the coil per cylinder set up would run the engine more efficiently. So even though I lowered the CR and run the motor on 89octane, the power lost wont be as significant because the coil per cylinder can give a hotter spark and burn the air/fuel mixture more completely, making the most of the lower CR. And thank you for the calculator links. Any ideas on what CR I should shoot for? I know stock is 10:4:1. I think the rule of thumb for Gen 1 was that 9:5:1 was the limit for 91-93 octane.

torque_is_good

Lt1's have reverse flow cooling which permits higher compression.

A question though. Why the discussion about performance and then insisting on saving a whopping $2 per week by running cheap gas?

EFIhead

Well, I didn't want to stoop as low as 87 octane, than I'd be looking at what, 250hp? But you have a point, the savings isn't that much from running premuim. On the other side of the coin, a 250hp LT1 is better than a 190hp 305.

scamaro355

its all in the tune and static compression ratio.. im running 10.2:1 on 87 with not issues at all.....

EFIhead

Thats pretty cool LT1 right? Is this without running a ton of spark retard?

scamaro355

no. just a regular sbc not LT1 but its the tuneing that makes the difference... im running 35* total timing its at full at 2800rpm under full throttle... all my mods are listed in the sig....

torque_is_good

octane is the ability of the fuel to resist "knock"

there are 2 ways to resist knock

A: increase your octane level

b: retard your timing which reduces performance

there is nothing magic about it, no special tunes etc can deny, or compensate for the benefits of increased octane. If that were the case, then normally aspirated cars at the drag strip would run 87 octane.

so again, it comes down to 89 costs 10 cents more per gallon than el cheapo

93 costs 20 cents more per gallon than el cheapo

so, at 20 gallons per week you save $4 per week using el cheapo vs premium

If you want economy, then retard the timing and run 87

if you want to discuss performance, then there is no place for 87 octane in the conversation

scamaro355

it also has to do with the design of the combustion chamber.. and weather the heads are aluminum or cast also make a big difference... on how compression effects the engine.. i get a kick out hearing guys try to brag.. oh i have to run race fuel.. HAHAHA really?.. wanna race... then they get all pissed when they lose... good stuff! but im sorry what would an educated automotive technician know about this stuff... my bad...

torque_is_good

All good points that you make but we're talking about an LT1 that has high compression already and reverse flow heads.

To discuss heads, metals etc is again a great topic but it in no way shape or form can change the benefits of increased octane, or have a bearing on the LT1.

It's great that there are other educated and knowledgable people in the forum to help folks with these questions.

Can an engine be "de-tuned" to run on lower octane? Absolutely, but why?

I don't understand, and never will, the desire to save $2 per week on fuel cost but ask questions about performance.

EFIhead

Well, I'm going to have to give scamaro355 props for what he does with 87 octane. If the cost of premium is an extra $.20 at the pump, and hypothetically I were to get 15 gallons every two weeks, the extra cost of Premium at the end of the year is $73. However, I haven't found the extra cost of Premium to be a linear value. If memory serves right it was an extra $.30-$.40 in my area when prices were nearing $5.00 a gallona few years ago. As much as I don't want to say it, we most likely will run into this situation again. I guess I'm just trying to cushion my self a little from the unstable future of gas prices. And we all know the old saying, different strokes for different folks.