Correct timing, carburetion for GM Performance Parts engine #12499529

[rich weyand]

I'm putting this in a new thread so title searches will turn it up if people are looking for it. I'm confident enough now about the setup to post it for others to use, without having to go through all the tuning work I did.

A lot of people on the web knock this engine. It's low-compression (8:1), with the 76 cc heads, and what looks like it could be the cam off the 1967 Corvette 350 cid/350 hp, 11:1 compression ratio engine. However, now that I have it set up right, I like it. Lots of torque (as much as the TH350 is rated for), including down low, nice horsepower, it'll run on cheapo gas, it starts no matter what, and it sounds nice. The people who knock it probably either 1) haven't had one, 2) never got it set up properly, or 3) were willing and able to spend much more money.

These settings are for the following configuration. They may apply or not if your configuration is different

GM engine #12499529
Edelbrock 1406 Performer carburetor
Edelbrock 2601 Air-Gap intake manifold (requires thermostatic air cleaner and heat riser; otherwise use Edelbrock 2101)
Hooker headers
Dual exhaust all the way back, no crossover

TIMING
static: 16 degrees BTDC, total: 36 degrees BTDC (GM says 32 degrees total timing; they're wrong)
vacuum advance connected to manifold (not port) vacuum; this is the driver's side port on the front of the Edelbrock 1406 (GM says don't hook up the vaccum advance; they're wrong. In fact, not hooking it up is a really, really stupid spec from GM. You will get poor off-idle and low-throttle performance and horrible gas mileage.)

CARBURETION
secondary metering jets: Edelbrock 1426, 0.095" (stock jet)
primary metering jets: Edelbrock 1425, 0.092"
metering rods: Edelbrock 1440, 0.060" cruise, 0.057" power
step-up springs: 8#

These carb settings will result in 14.5 A/F ratio in cruise, 13.5 A/F ratio in power, and 12.5 A/F ratio on the secondaries.

So if you don't know anything about engine tuning, don't have the time to mess with it, or don't have the A/F Ratio meter and other equipment, and you want a $2000 engine replacement that will perform nicely and sound good, you should be able to get this engine, drop it in, and set it up as above, and be good to go without any other messing around.

Rich

[GreaseDog]

Just out of curiosity rich, what gear ratio are you running, and what is your tire size? What kind of economy are you getting out of this setup? You're still running the th350 right?

[rich weyand]

Hey there, GreaseDog:

The RPO sheet says 3.43:1 for axle ratios, though I haven't measured it. It probably still is 3.43, since the people who rebuilt it went crazy on the bodywork and paint, but the mechanicals were left pretty much intact and original. I'm also running 31" tires, so the effective ratio is about 3.21:1 if it was on stock 29" tires. I am still running the original TH350 and NP203. The NP203 is modded.

I should have mileage numbers within a week or so and will add them to the thread when I have them. This will be around town and driving my hilly, winding lane, so they will be low-end numbers. Highway numbers will have to wait until I get a chance to get out of town with it.

Rich

[GreaseDog]

Right on, just a bit curious for myself, but mostly curious for the sake of the thread.

[SUBURBIAN]

Thanks for listening to me and fast305 in the other thread and ditching the gm specs. You'll absolutely see some mpg gain and increased performance from the current tune. Post comparison results when you have them. :thumbsup:

[rich weyand]

SUBURBIAN:

Thanks for all the help, to you and many others on this forum, over the last couple years. I try to give as good as I get, with advice to others when I know it, and write-ups of my own work when I'm successful, but I would be in a world of hurt without everybody's help!

Rich

[rich weyand]

OK, so I got a mileage chop today. 8.5 mpg. Better, but not good. So I swapped the step-up springs out today from 8psi to 5psi, which will keep me on the cruise circuit more often, particularly in all the stop and go traffic around town. So far, driveability seems unimpaired by this change. I originally went to the 8psi springs when other things were messed up, to clean up off-idle performance, but with the timing and carburetion now dialed in, that is unnecessary. I could see another .5 mpg from this change.

So I am looking at 9 mpg, maybe. I'll do another couple tanks and see what I get and report back. That 8.5 number was all in-town and winding country lane, with lots of construction and hills, and trips averaging maybe 5 miles. So, worst case mileage is what I'm saying.

[rich weyand]

OK, so I finally went out of town with the truck and got a mileage chop. I topped off both tanks on the way out of town, then topped them both off at the same pump at the same pump setting on arriving back.

545 miles, 59.8 gallons. Call it 9.1 mpg.

500 miles of this was interstate driving, with about half of that at 75 mph. The other half was mostly at 70 mph, with some at 65 mph. About 50 miles of around town driving in Chicago burbs on arterial streets, so 45 mph or so.

This is with transfer case unlocked and hubs free.

[GreaseDog]

Not bad considering no overdrive, going to keep tinkering with it i assume. If you're like me, you can never leave well enough alone. Lol

[rich weyand]

Well, I set it up with the NGK A/F ratio meter, and it was dead on: 14.5 cruise circuit, 13.5 power circuit, and 12.5 on the secondaries. I will probably check it again this summer now that the engine is run in, but I don't expect it to change much.

Next tinker has to do with new Tuff Country EZ-Ride +2F/+3R and new exhaust system, then I'm about done.

[vic_V8]

9 MPG....wow, I was hitting 15 mpg with 35" tires and a 700R4.

My 1977 F250 with a 460, C6, and 4.10s gets about 8 MPG.

[rich weyand]

A 700R4 would make a big difference. Near as I can figure, with my tires and axle ratio, 75 mph is about 2600 rpm, continuously. A 0.6:1 overdrive would get me down to 1560 rpm, which would make a big difference on mileage! 75 mph in third gear with a TH350 is like driving around at 50 mph in second gear, except it's even worse because you are pushing that big square cab through the air at 110 feet per second.

[rich weyand]

A little checking shows the OD on the 700R4 is 0.67:1, so that would get the RPMs at 75 down to 1750.

[vic_V8]

the 700R4 has a 3.06 first gear (th350 has 2.52) and a 0.70 OD, I really like this trans for an all around truck. I could see you getting a nice 15 MPG or more with a 700r4 and a TBI unit. And it would be fun reliable truck.

[rich weyand]

But I have a fun, reliable truck. And I can adjust it all myself.

As for fuel consumption, I just don't give a damn. When it runs dry, I put gas in it.

[vic_V8]

But I have a fun, reliable truck. And I can adjust it all myself.

As for fuel consumption, I just don't give a damn. When it runs dry, I put gas in it.

but I thought this tread was about performance and economy?

I have to say though, this is the reason I'm part way through putting the Cummins in my truck, WAY more power and double the MPG. The Crate motor replacement has gone the way of the Dodo bird. For the same money today, people can swap in a LS series motor, make 100+ more hp and better mileage than the crate motor of equal value. I love old push rod motors, but dollar for dollar, they can't keep up with the new push rod motors.

EDIT: Sorry, I got this totally off subject.

[rich weyand]

but I thought this tread was about performance and economy?

Uh, nope. It was about how to properly set up this particular crate engine if you don't want to have to go get an A/F meter and put a bung in the exhaust pipe and spend three months dicking with it. I tried to put in all the keywords a person would need to hit it easily on a google search, even if they were not on this forum, and were dropping this engine into, say, an old Bonneville or something.

The mileage discussion was in response to GreaseDog. I don't care about the mileage, much. I mean, 2 mpg would be a little low, right? What I most wanted was a truck with NFC -- No F***ing Computer. I do that for a living, and just am not interested. So, no TBI, no computerized transmission, no ECM. More horsepower is good, and I went from 175 hp and 275 fp to 300 hp and 340 fp by going to this engine, with not a bunch of bucks. I gave up about 1-2 mpg to do that, and that's OK.

What I would REALLY like, and might even put up with a computer for, is a 350 fp CVT that went from about 5:1 to 0.5:1. If it had a TH350 footprint so it would be a drop-in replacement, that would be even better. I'm toying with some stuff right now, to see if I can build one for myself. THAT would be fun!

[GreaseDog]

A 700R4 would make a big difference. Near as I can figure, with my tires and axle ratio, 75 mph is about 2600 rpm, continuously. A 0.6:1 overdrive would get me down to 1560 rpm, which would make a big difference on mileage! 75 mph in third gear with a TH350 is like driving around at 50 mph in second gear, except it's even worse because you are pushing that big square cab through the air at 110 feet per second. 1600 is a little low for a 700r4 to survive, however, a 700s overdrive is also.7:1, so that should help. I like to keep mine around 2000. The only 700 or 60e I've ever fried were the multiple used units in my 86 with 2.73s, and my 93 c1500 with 260k on the clock.

[GreaseDog]

A little checking shows the OD on the 700R4 is 0.67:1, so that would get the RPMs at 75 down to 1750.

That it's a 2004r, which it's not reapply adapted to a 4x4 application due to it's non removable tail shaft housing.

[vic_V8]

CVT= Constant variable transmission?

I had a polaris with CVT so I'm guessing that what you meant? Or constant variable timing?

[GreaseDog]

What I would REALLY like, and might even put up with a computer for, is a 350 fp CVT that went from about 5:1 to 0.5:1. If it had a TH350 footprint so it would be a drop-in replacement, that would be even better. I'm toying with some stuff right now, to see if I can build one for myself. THAT would be fun! wow, that would be Badass.

[rich weyand]

What I would REALLY like, and might even put up with a computer for, is a 350 fp CVT that went from about 5:1 to 0.5:1. If it had a TH350 footprint so it would be a drop-in replacement, that would be even better. I'm toying with some stuff right now, to see if I can build one for myself. THAT would be fun!
wow, that would be Badass.

It certainly would. A drop-in TH350 replacement that was a continuously variable transmission from 5:1 to .5:1 would yield a 5:1 first gear hole shot, then hold RPM constant at the max horsepower point as long as you held your foot in it. And gas mileage would be phenomenal at any speed with an overdrive that could go all the way to .5:1.

Driving a CVT is an interesting experience, because when you punch it, the RPM immediately goes up to the max horsepower point as the tranny drops the ratio, then the engine holds at constant RPM while the car accelerates. If you haven't ever driven one, pop into the local Ford dealer and test-drive a hybrid Fusion, then imagine that setup on something with real horsepower.

Not so far away, either, I don't think.

[rich weyand]

Check this out:

http://www.truckpartsandservice.com/allison-to-produce-gearless-transmissions/

[Fast305]

It certainly would. A drop-in TH350 replacement that was a continuously variable transmission from 5:1 to .5:1 would yield a 5:1 first gear hole shot, then hold RPM constant at the max horsepower point as long as you held your foot in it. And gas mileage would be phenomenal at any speed with an overdrive that could go all the way to .5:1.

Driving a CVT is an interesting experience, because when you punch it, the RPM immediately goes up to the max horsepower point as the tranny drops the ratio, then the engine holds at constant RPM while the car accelerates. If you haven't ever driven one, pop into the local Ford dealer and test-drive a hybrid Fusion, then imagine that setup on something with real horsepower.

Not so far away, either, I don't think.

I never have been that impressed with them and they fail at an alarming rate. I believe honda tried building them and they were JUNK. The Nissan ones are JUNK. We replace 3-5 of them a week and have 4 of the 2013 Pathfinders on the back lot at work waiting on CVTs to come it. Even in a Maxima with 290 HP in front of the CVT I find the performance lacking. My brother had a Jeep Patriot with the 2.4 CVT and in CVT mode it was slow, slow, slow, but if you auto sticked it with 6 defined ratios it was quicker. An engine actually does more work running between peak torque and 10% over HP peak than it does sitting at HP peak.