ohh nooo - ive been running 92 octane in my tbi530... could that decrease hp/tq??????

[rai]

like the title says - im running a L05 350 in my 65 Chevelle. its mainly stock but it just has long tube headers and a x pipe. looking into doing the power charger mod, tb ridge removal, and a injector spacer next weekend.

what kind of gas should i be running? im not too worried about fuel prices, focusing on performance mainly

[87] [89] [91]

thanks guys!

first post btw

[rai]

heres a picture of the mut lol

a8.sphotos.ak.fbcdn.net/hphotos-ak-snc6/hs050.snc6/168073_103273089749382_100002000741823_23660_29691 70_n.jpg

[dcZ71]

87. your wasting money on anything higher imo.

ethanol free preferably if you can find it.

[rai]

any decrease in performance when running 92?

[RichLockyer]

Octane has nothing to do with power.
Higher octane may allow you to advance the timing a few degrees, but with modern computer controls, it's not going to make a huge difference.

However, running 92 in an engine that doesn't require it CAN cause an increase in carbon buildup. Probably not a bad idea to run a course of Seafoam to help clean it out, but short story, if your engine does not knock on 87, then 87 is the correct fuel.

[2500ak]

No, higher octane is just more knock resistant. Unless you've got a high preformance engine that'll knock without it (high compression, nitrous, forced induction, etc...) you won't see any benefit.

Now if the ignition system is changed to add more timing advance, sure you'll see some benefit from it. Not a lot, but some.

[RichLockyer]

ethanol free preferably if you can find it.

LOL! That's a good one.

He's in California.
There are presently three sources of ethanol-free gas in California.
Two are marinas near Paso Robles, and one is a marina near San Rafael in the San Francisco Bay.
There are seven places to get ethanol-free in Arizona. One is a Texaco station near Phoenix.
There is no ethanol-free gas in Nevada.
There's a bunch of places in Oregon, but the only one south of Roseburg is a marina in Brookings.

[davo727]

I found that my stock tbi trucks (not the 1993 it runs premium do to higher conmpression) run good on 89 octane. I tried 87 and they run so crappy that they get worse milage so that its actually cheaper to run 89 than the crappy 87 octane.

[kemble]

For best performance and cleanest burn, it is wise to use the lowest octane of fuel your engine will run on with no detonation. My last car made 370 hp at the back tires with 10:1 compression and it drank 87 octane.

[someotherguy]

I found that my stock tbi trucks (not the 1993 it runs premium do to higher conmpression) run good on 89 octane. I tried 87 and they run so crappy that they get worse milage so that its actually cheaper to run 89 than the crappy 87 octane.

So fix what's wrong with your truck instead of band-aiding it with midgrade fuel it doesn't need - when it's operating correctly.

Stock TBI 350's are designed for use with 87. Period.

Richard

[Fast305]

So fix what's wrong with your truck instead of band-aiding it with midgrade fuel it doesn't need - when it's operating correctly.

Stock TBI 350's are designed for use with 87. Period.

Richard

I disagree they were tuned to operate well on 87 octane, by limiting the timing. The factory timing specs are nowhere near Minimum Best Torque Timing numbers. With a 9.3:1 compression ratio, less than 195* @ .050 intake duration, cast-iron heads, and a 200*F operating temperature, the engine should have been tuned to run on 93 octane. The stock TBI engine needs 6-8* more of timing at high RPM and 93 octane fuel to reach its peak HP potential. The timing in the higher load/lower RPM range needs as much as 12-14* more advance. The air/fuel ratios at wide open throttle are typically about 1.5 points too rich at peak torque and 1 point too rich at high rpm.

[someotherguy]

Also, are we 100% sure (I know you've done tons of homework, and real work, on these engines - so I'm not arguing, just debating) - that the TBI truck 350 is 9.3:1? I thought I'd read numerous times that they were in the high 8's or very low 9's. I can't dig up the specific info at the moment, dunno where I saw it, but if you have some supporting info to post that would be cool.

Richard

[someotherguy]

Let's agree to disagree. The key word in my very short post was: STOCK. In other words, I'm considering stock being an untouched engine with factory equipment, at factory settings.

Richard

[Rotbody]

87. The thing is in the low 9's compression wise. The engine doesnt have alot of compression, so doesnt need it. If it did the engine would ping on ya telling you it needs better gas anyways.

[rai]

thanks for all the help guys!!! my next fill up will be 87 for sure :)))

[rai]

.

[rai]

here was the pic above(finally reached the 5 post link limit)

[kemble]

sweet ride.

[Fast305]

87. The thing is in the low 9's compression wise. The engine doesnt have alot of compression, so doesnt need it. If it did the engine would ping on ya telling you it needs better gas anyways.

9.3:1 unless it is an 8,600+ GVW and that is 8.75:1. The 9.3:1 engines have 12cc semi dished pistons with 4 valve reliefs. The 8.75:1 engines have an 18cc dished piston with larger eyebrow shapped dishes and a deeper dish overall. These are real world numbers that I have measured and calculated. Calculate it for yourself. The stock piston is .025" in the hole nominal, 4.00" bore, 3.48" stroke, 65cc chamber, .028" thick compressed head gasket with a 4.100 bore, with either a 12cc or 18cc dished piston.

As for detonation, none of the TBI engines or even CCC Q-jet engines with Knock sensors should have audible knock in them. If they do something is wrong in a major way. The knock sensor is sensitive enough to pickup detonation at an un-audible level and retard the timing. The only way to know if the engine is knocking on 87 octane is to datalog. I datalogged a bone stock engine at bone stock settings and under load in 3rd gear it would often show 4* of knock retard. On some of the PCM part numbers and programming this would put the truck in Low Octane spark retard mode. 93+ all have low octane modes.

[someotherguy]

9.3:1 unless it is an 8,600+ GVW and that is 8.75:1. The 9.3:1 engines have 12cc semi dished pistons with 4 valve reliefs. The 8.75:1 engines have an 18cc dished piston with larger eyebrow shapped dishes and a deeper dish overall. These are real world numbers that I have measured and calculated. Calculate it for yourself. The stock piston is .025" in the hole nominal, 4.00" bore, 3.48" stroke, 65cc chamber, .028" thick compressed head gasket with a 4.100 bore, with either a 12cc or 18cc dished piston.

Thanks for the #'s and I do believe you, I don't need to calculate it. I know you've put in enough work that if you say it's true, it's true.

As for detonation, none of the TBI engines or even CCC Q-jet engines with Knock sensors should have audible knock in them. If they do something is wrong in a major way. The knock sensor is sensitive enough to pickup detonation at an un-audible level and retard the timing. The only way to know if the engine is knocking on 87 octane is to datalog. I datalogged a bone stock engine at bone stock settings and under load in 3rd gear it would often show 4* of knock retard. On some of the PCM part numbers and programming this would put the truck in Low Octane spark retard mode. 93+ all have low octane modes.

Interesting to note about the knock retard you're seeing, and I do agree the knock sensors are very sensitive. I've got a '96 7.4 right now that seems to be triggering the knock sensors (it has 2) with tons of knock counts, and giving lots of knock retard, because it has serious valvetrain wear/noise and the exhaust is loose all the way up to the collectors due to broken mounts (we should hopefully have a chance to fix the mounts today.)

My daily is a '94 C2500LD with 5.7; right now I have it set at factory spec of 0 BTDC base timing, and the engine is bone stock. I can graph OBD-I knock activity and knock retard with the OTC Nemisys; should hook it up and see what I see.

Richard