350 tbi timing

[Overland Suburban]

So many variables! At 3mpg increase you save about $6 per 1000 miles if you fill up at the shell.
What are you after? Performance or efficiency?

Also, make sure your knock sensor is working. Dangerous knock is not necessarily loud enough to be heard.

I'm OCD! I can't stand a sluggish engine when I know good and well that with a little tuning it will become throttle responsive and a light touch of the throttle moves you down a road. Not to mention the extra 75 miles per tank of fuel that gets you a lot further when road tripping and trying to make time.

At .20cents a gallon more for super unleaded it cost $5 more to fill a 25 gallon tank. At 3 mpg better you get 75 miles more on a tank. For arguments sake let's say we went from 12 mpg to 15. That means every tank of fuel we saved 5 gallons. 5 gallons times $4 a gallon is $20 per tank or approximately $50 per 1000 miles. To put it in perspective thats only $15,000 dollars savings for a 300,000 mile rig!

 

[F-64]

I think my math is off...nationwide difference between regular and premium right now according to AAA is $ 0.61. Regular average is $2.440. Premium average is $3.050.
Regular----300,000 miles at 12 mpg is 25,000 gallons. 25,000 x $2.44= $61,000.
Premium---300,000 miles at 15 mpg is 20,000 gallons. 20,000 x $3.05=$61,000.
So you are not really saving any money as long as the price split is $.61
Engine longevity may be another issue.

AAA gas info
https://gasprices.aaa.com/?state=US#state-metro

Regards,
mike

 

[Frankenchevy]

Our 87 is a little over $3/gallon at Costco and Arco. All taxes.

 

[RecklessWOT]

Yeah, for real. 15 years ago mid grade was .10 more per gallon and premium was .20 more. I haven't seen that in years no matter what part of the country I'm in. Even at the cheap off brand station in town, right now regular 87 is $2.109 and 93 is $2.799 /gal. I know this because I have to run 93 in my daily, and since a lot of stations these days have digital signs that only show regular price I have learned that the station with the cheapest sign does not always have the cheapest high octane fuels, the price gap is huge at some stations. Regardless, I haven't seen it simply .20 /gal in a long long time.

Spending that much extra is not justified for a few MPG. My wife's wagon gets a mile or two per gallon more if I run 91+ instead of 87 and makes noticeably more power (owner's manual even recommends 91 though does not require it), but I can't justify spending an extra $8-10 on every fill up for that slight gain. Even if it was a full 2mpg more, that saves me about a gallon and a half per tank which is less than $5 no matter what grade of fuel I'm using, so still ends up costing more. It doesn't knock without it, just has slightly reduced performance. She's not racing the thing, and even though I am essentially racing every time I get behind the wheel of any vehicle, it hardly seems worth it when we're not talking real race cars...

 

[Overland Suburban]

I think my math is off...nationwide difference between regular and premium right now according to AAA is $ 0.61. Regular average is $2.440. Premium average is $3.050.
Regular----300,000 miles at 12 mpg is 25,000 gallons. 25,000 x $2.44= $61,000.
Premium---300,000 miles at 15 mpg is 20,000 gallons. 20,000 x $3.05=$61,000.
So you are not really saving any money as long as the price split is $.61
Engine longevity may be another issue.

AAA gas info
https://gasprices.aaa.com/?state=US#state-metro

Regards,
mike

Mike, thanks for posting it appears my math is off. These figures have me leaning more towards just doing a 12 valve cummins swap instead of trying to milk this 350 tbi

Tracy

 

[bucket]

Mike, thanks for posting it appears my math is off. These figures have me leaning more towards just doing a 12 valve cummins swap instead of trying to milk this 350 tbi

Tracy

That's a whole nuther ball of wax. Those swaps tend to be expensive and the price of diesel is quite often higher than gasoline. Math hurts me but I know it takes a lot of miles driven to break even on the cost.

If you simply want a 12v under the hood, there's certainly nothing wrong with that. But imho, it's not really worth it if the main goal is to save money.

 

[JoeR Jr]

The computer can only advance the timing so far and the stock setting is absurdly cautious.

If the engine is in fairly good health and runs properly, a TBI 305/350 likes anything in the 8-12° btdc range and will usually do it on 87 octane without knocking. Hot weather and high loads may make 10° the max without changing the fuel.

I agree with this. I have always set these trucks at 8 degrees with no trouble. The couple of extra degrees you might get out of it by running 92 octane would not be worth it, unless for some reason you needed to squeeze every singe HP hour of it. So set it at 8 degrees and run 87 octane. You'll be happy with the extra power and mileage.
Joe

 

[F-64]

hmmmmm.
That's Cummins is a heavy one at 1100lbs. Your 5.7 is maybe around 475lbs. Your suspension and brake system will need upgrading with all that extra weight on the nose. Torque will be outstanding but fuel costs will still be an issue with the Cummins.

Getting better mpg out of these flying bricks is not easy. Not exactly the best aero. I would probably just make sure the tires are inflated properly. I'd also reduce weight if possible.

I'd be tempted, if it were me, to grab a 4.3 V6 from a wrecked 2014 or newer Silverado 17city 22hwy 285hp 305tq.
Your 5.7 hits around 11city 15hwy 210hp 300tq.
But I'm weird...I'd probably also turbocharge the V6 at a later date..

 

[Urbanburban22]

Hey all, I was reading through this post before I went and checked my Base timing on the ole 91 Tbi burban. I disconnected the advance wire (tan and black) and got her up to temp. The PO had timing set at (approx.) 14Degrees Atdc! This made the old 350 make some sweet back firing noises on a deceleration (true dual with long tubes, 3" full 304 stainless exhaust), but she was a slow Blue turd. So I set timing initially, at 13 degrees Btdc and HOLY SMOKES! Doughnuts and burnouts! I was very impressed with it! I had rebuilt the transmission (700r4 "stage 2" with all the basic upgrades and a 1,800 stall HD converter, corvette servos and a shift kit) and did some small ignition up grades (6AL MSD OLD SCHOOL model, and MSD blaster coil, MSD custom plug wires), as well as rebuilding the Tbi with new injectors. Fuel pressure set to 13Psi. I was getting some bad pinging at 1/4 throttle loaded down in second so I backed her off to 10degrees Btdc and seems to have cleared up (running 87 octane). In all thanks for the knowledge, I can finally keep up with modern traffic and have a lot of fun doing it! Fuel economy is definitely a lot better, idles smoother (IAC issue though), a lot less heat from the long tubes coming through the fire wall too. Can't believe I drove it like that for over a year before thinking it could be a timing issue.

 

[KnuckleBuster]

Advance 10-14 degrees?! On a factory stock motor? Wow, that's news to me. I've had these trucks come in pinging with just a few degrees variation.

The computer does not just run timing up until the knock sensor triggers and then back off. Timing is set according to a specific algorithm and has a max. The knock sensor will reduce from the point of knock, but even then it can only subtract so much timing.

One consideration: The computer will check the knock circuit by momentarily advancing the timing to the point of detonation. If it receives a knock signal during this test it will know the system is functional. If you run high octane fuel it may prevent the detonation, which the computer may recognize as a system fault, set a code, and turn on the Check Engine lamp.

 

[bucket]

The computer can only advance the timing so far and the stock setting is absurdly cautious.

If the engine is in fairly good health and runs properly, a TBI 305/350 likes anything in the 8-12° btdc range and will usually do it on 87 octane without knocking. Hot weather and high loads may make 10° the max without changing the fuel.

Advance 10-14 degrees?! On a factory stock motor? Wow, that's news to me. I've had these trucks come in pinging with just a few degrees variation.

The computer does not just run timing up until the knock sensor triggers and then back off. Timing is set according to a specific algorithm and has a max. The knock sensor will reduce from the point of knock, but even then it can only subtract so much timing.

One consideration: The computer will check the knock circuit by momentarily advancing the timing to the point of detonation. If it receives a knock signal during this test it will know the system is functional. If you run high octane fuel it may prevent the detonation, which the computer may recognize as a system fault, set a code, and turn on the Check Engine lamp.

I still stand by my previous post. I've run many personally and had no pinging, with the added benefit of more power and better mpg.

 

[gmbellew]

I still stand by my previous post. I've run many personally and had no pinging, with the added benefit of more power and better mpg.

10-14 BTDC is more than I've ever run in a 350tbi, but I'm a newbie. the Knock sensor will detect predetonation before it is audible and pull timing. it only pulls so much, though. so you could add too much, and it will pull it down to a point that wont audibly ping. whether that is too much or not is anybody guess. also. sometimes the tune is set up to not detect ping by the sensor near WOT.

 

[bucket]

10-14 BTDC is more than I've ever run in a 350tbi, but I'm a newbie. the Knock sensor will detect predetonation before it is audible and pull timing. it only pulls so much, though. so you could add too much, and it will pull it down to a point that wont audibly ping. whether that is too much or not is anybody guess. also. sometimes the tune is set up to not detect ping by the sensor near WOT.

Right, in my post I suggested 8-12 degrees. The amount you can get away with is determined by engine health and how the vehicle is used. Typically, a TBI Camaro can get away with more timing than a 3/4 ton Suburban pulling a load in the summertime. But I've run 8-10 on heavier work trucks many times.

 

[JACK34]

I just sold my 1994 1500. It drove me nuts those TBI trucks are junk. I got a 73 K10 with a fresh 350 4 speed I could not be happier. I run 91 octane fuel in everything I own. If you are worried about gas mileage get a Prius.

 

[Rusty Nail]

I also think gmt 400 trucks suck.

Hey guys and gals I know you can't believe everything you read on the internet.

Thanks

LoL
Says the guy -from the internet- as he poses his question...to the internet.

Just pokin fun.

I ran 10 degrees in a 75 Blazer (and 305 heads built #416s _1.94/1.60 valves) on 91 octane with a Quadrajet and my 88 Blazer with tbi and bullstuff #191 heads and sees 9.5 degrees but it's a tad too high on 87.

There is something to be said for the allowable advance differences between the two. I don't assert to know what it is but agree it's there.

35 year Old TBI systems really like new MAP sensors though and I 100% agree gmt400s are junk. I used to own one but it was a "gift " it was red ffs.
I didn't buy it to be clear. I'll never own another one that's for damn sure.