73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Performance => Topic started by: Capt. Killingfield on February 16, 2013, 11:39:47 PM
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Alright guys, first post. I really didn't want to start a new thread but I've searched the internet and this forum, and still can't find any threads that answer my specific questions.
I'll let you know from the start that I'm kind of a newbie at this: my experience so far only encompasses bolt-on mods.
The Vehicle:
1985 Chevy C10 1/2ton 2-wheel drive shortbed
low-mileage GM Performance sb 350 long-block crate engine I bought new a few years ago (before I was hip to the junkyard Vortec craze >:()
BOTTOM END IS COMPLETELY STOCK
Eddy Performer Intake
Eddy Performer 600cfm Carb
Eddy TES Headers with an exhaust leak in the center of the pass. side collector
MSD 6AL - with HEI cap/rotor upgrade
2.5" exhaust
Flowmaster Delta Flow muffler
Eddy Stoicometer
hard-lined vacuum gauge
I've recently become emissions free (got an address in another county ;D) and have carefully pulled off all of the emissions equipment, except for my fuel canister. I've also plate-blocked the EGR boss.
Timed vac to dizzy
PVC to PVC
Full vac is tee'd to carb bowl vent and canister <<<==== IS THIS WRONG??
TH350 through 3.08 gears
P235/75R15 all the way around @ 35psi
The Issue:
After yanking the ugly unneccesaries, eliminating possibilities of vacuum leaks and TVS mis-routes, properly tuning my carb, new plugs, new air filter, and adjusting the nut behind the steering wheel, I am only getting an avg 9mpg with 12mpg all-freeway.
The only thing left that I'm not sure of is timing. My gearhead friends (and 'net research) tell me my initial timing should be around 10-12*btc. My underhood sticker says 4*btc. My truck seems to run best at 8*btc. I use an inductive timing light. I've tried to set my initial by ear, but the engine always diesels. I've tried to set it by vacuum, but there's not much difference in psi unless I'm way off.
At 10*btc -RPMs raised and engine seemed less bogged down, but diesels and is harder to start. MPG also seemed to drop.
At 4*btc - the truck really seemed to hum when i set it during the day, but later that night I couldn't get it above 40mph. It seemed to misfire or something.
At 8*btc - it starts fine, shuts down fine, even runs fine, just the mileage sucks.
I don't want my engine fighting itself.
The Question(s):
How do I find the perfect timing for my specific combo, and how will I know it?
Is 9-12mpg the best I can hope for with factory low compression heads/pistons?
What mods need to be made to a vehicle to necesitate a change in timing?
Could I be a tooth off?
Thanks for any and all info!
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A dyno session will help you tune the timing. If you have an extra $400 bucks you can get an MSD distributor that is configurable with a computer. You can have lots of control over the timing curve.
For fuel mileage I'd consider getting a wideband O2 sensor and dialing in that carb. What you will probably find is that the carb tuning is hard to get perfect everywhere and fuel injection is easier to get fine tuned.
Get a transmission with a lockup feature.
Drive it like Miss Daisy was in the bed sitting on an unsecured rocking chair.
:))
Dan
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My situation is almost identical to yours. 84 short bed 2wd, Goodwrench 350, 700R4, 2.79 gears, 255/60/15 tires. The motor has no smog and the EGR blocked off, quadrajet, shorty headers dual exhaust and a regular HEI.
I have my timing at 12 initial with full manifold vacuum adding another 10 degrees (its adjustable). The truck starts great hot or cold, no detonation, shuts off fine, no dieseling. Its my daughters truck and I know she drives it conservatively, but it gets about 8-9 mpg in the city, which is terrible. I thought someone may have been stealing gas out of it at her school, so I put a locking gas cap on it, but alas that was not the case.
I have decided to build a vortec motor, with a carburetor and a small torque cam. I believe the problem is the Goodwrench motor just isn't designed for efficiency or performance with the crappy heads. Yes, you can buy some vortec heads and bolt them on the goodwrench, but I found a used low mileage vortec motor for 350.00. I like the heads and I like the roller cam and it will drop in place of the Goodwrench, with a few parts. A flexplate for one piece rear main seal, electric fuel pump (its possible the block may be machined for mechanical, if so I will use that), timing cover and carb intake for vortex heads. I will have the block hot tanked etc.... and then assemble myself. I am doing a budget build with used parts where I can use them and new where it is necessary.
I can also sell the complete goodwrench motor and recoup some of the expense. I have been reading and reading through the internet and talking to my motor building friends and I think I will pick up quite a bit of mileage this way. I'm hoping for 13-15 in the city.
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I am no expert by any means but... If I can pull 15 mpg from my 454 which is untouched with dual air pumps you should be able to do at least that with your set up. Sounds like timing issue and even though you have a low cfm carb, a stock quadrajet is easier to deal with IMO.
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Thanks for the advice guys.
....For fuel mileage I'd consider getting a wideband O2 sensor and dialing in that carb. What you will probably find is that the carb tuning is hard to get perfect everywhere and fuel injection is easier to get fine tuned...
I have an Edelbrock Air/Fuel Ratio Monitor (wideband) PN:6593 that has been a lifesaver when it came to finding the proper rod/jet combo and setting my idle mixture. It's one of the first things I bought for the rebuild. I've been recommended the MSD dizzy before. I should look into this. I've been having a difficult time trying to decide on either a 5.3L Vortec swap, or get FAST EFI and just stroke to 383. Decisions. Either way I'd prefer to let a computer do the fine tuning for me.
I am no expert by any means but... If I can pull 15 mpg from my 454 which is untouched with dual air pumps you should be able to do at least that with your set up.
Exactly.
Yeah, 8-9mpg almost sounds pointless. A taxi may be cheaper in the long run. I'm in the same boat...er...truck.
It's amazing to me how much of an improvement was made when I actually read the carb adjustment manual, used that tiny paper ruler, and re-bent those stamped linkages. It's like a totally different engine.
"OUT OF THE BOX PERFORMANCE" can be slightly misleading.
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Alright guys, first post. ...I'm kind of a newbie at this: my experience so far only encompasses bolt-on mods.
The Vehicle:
1985 C10 shortbed
low-mileage GM Performance 350 long-block crate engine
Eddy Performer Intake
Eddy Performer 600cfm Carb
Eddy TES Headers
MSD 6AL
2.5" exhaust
I carefully pulled off all of the emissions equipment, except for my fuel canister. I've also plate-blocked the EGR boss.
Timed vac to dizzy
PVC to PVC
Full vac is tee'd to carb bowl vent and canister <<<==== IS THIS WRONG??
TH350 through 3.08 gears
P235/75R15 all the way around @ 35psi
The Issue:
After properly tuning my carb, new plugs, new air filter, I am only getting an avg 9mpg (12mpg freeway).
My truck seems to run best at 8*btc, just the mileage sucks.
The Question(s):
How do I find the perfect timing for my specific combo?
Is 9-12mpg the best I can hope for with factory low compression heads/pistons?
What mods necessitate a change in timing?
Could I be a tooth off?
To begin: Welcome to the site from California!
Consider the following...
- Correct the hose routing to your carburetor and canister "vents." You should NEVER have vacuum plumbed to the float bowl of the carb! This alone could cause severe mileage and drivability issues. The carb bowl should be 'vented' to the charcoal canister. The canister, depending on application, may also have a 1/4" PCV connection that tees into the PCV valve/vacuum hose, plus a ported vacuum connection to the carburetor, in addition to the carb bowl vent connection. Check the vacuum routing diagram on the radiator support.
- The PCV valve should be properly plumbed between manifold vacuum and either valve cover, via a 3/8" PCV vacuum hose. The opposite valve cover should be vented (optimally to the air cleaner housing) through a fresh air filter. The PCV system is a passive device that uses no power to function and dramatically benefits engine health and longevity. It also has a metered effect on the air/fuel mixture ingested by the engine.
- Using manifold vacuum versus ported vacuum to operate the distributor vacuum advance is arguable. There are as many opinions about this as people posting. Neither method is perfect for every application. In the process of dialing in your vehicle, you may switch back-and-forth several times before settling.
- "Perfect timing" is best determined using a chassis dyno. For your build, that's probably an unnecessary expense. Start by making sure the mechanical and vacuum advance mechanisms are functioning properly and not binding or worn out. Set your timing for a maximum of 31° to 34° total advance (initial timing + mechanical advance + vacuum advance, combined), all in by 3,000 to 3,200 engine RPM. This will be a good starting point, from which you can deviate, plus or minus, for best drivability and mileage. When setting your initial timing, be sure your vacuum advance hose is disconnected and plugged. All said and done, 8° BTC may be the optimum timing for your app.
Unless you're having a problem with the distributor you currently have, replacing the distributor may be more of a "feel good" (aesthetic) upgrade than an actual benefit.
- Once you get the carburetor vacuum plumbing sorted out, recheck the carburetor jetting/fuel calibration.
- The worst mileage you'll ever get is with the engine running without moving. The next worst mileage is during acceleration. Minimize your idle time and accelerate modestly for a potentially surprising improvement. Or, you could do what a friend of mine does... never use the brakes! :D
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"Set your timing for a maximum of 31° to 34° total advance (initial timing + mechanical advance + vacuum advance, combined)"
That seems low. Did you mean to say a maximum of 31 to 34 NOT including vacuum advance? I wouldn't try to set the maximum timing advance with the vacuum advance hooked up.
Also, the timing marks on your balancer are not guaranteed to be correct if the outer ring has slipped at all relative to the crank. Probably not likely an issue, but it could cause huge confusion if it has happened.
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To begin: Welcome to the site from California!
Consider the following...
- Correct the hose routing to your carburetor and canister "vents." You should NEVER have vacuum plumbed to the float bowl of the carb! This alone could cause severe mileage and drivability issues. The carb bowl should be 'vented' to the charcoal canister. The canister, depending on application, may also have a 1/4" PCV connection that tees into the PCV valve/vacuum hose, plus a ported vacuum connection to the carburetor, in addition to the carb bowl vent connection. Check the vacuum routing diagram on the radiator support.
- The PCV valve should be properly plumbed between manifold vacuum and either valve cover, via a 3/8" PCV vacuum hose. The opposite valve cover should be vented (optimally to the air cleaner housing) through a fresh air filter. The PCV system is a passive device that uses no power to function and dramatically benefits engine health and longevity. It also has a metered effect on the air/fuel mixture ingested by the engine.
- Using manifold vacuum versus ported vacuum to operate the distributor vacuum advance is arguable. There are as many opinions about this as people posting. Neither method is perfect for every application. In the process of dialing in your vehicle, you may switch back-and-forth several times before settling.
- "Perfect timing" is best determined using a chassis dyno. For your build, that's probably an unnecessary expense. Start by making sure the mechanical and vacuum advance mechanisms are functioning properly and not binding or worn out. Set your timing for a maximum of 31° to 34° total advance (initial timing + mechanical advance + vacuum advance, combined), all in by 3,000 to 3,200 engine RPM. This will be a good starting point, from which you can deviate, plus or minus, for best drivability and mileage. When setting your initial timing, be sure your vacuum advance hose is disconnected and plugged. All said and done, 8° BTC may be the optimum timing for your app.
Unless you're having a problem with the distributor you currently have, replacing the distributor may be more of a "feel good" (aesthetic) upgrade than an actual benefit.
- Once you get the carburetor vacuum plumbing sorted out, recheck the carburetor jetting/fuel calibration.
- The worst mileage you'll ever get is with the engine running without moving. The next worst mileage is during acceleration. Minimize your idle time and accelerate modestly for a potentially surprising improvement. Or, you could do what a friend of mine does... never use the brakes! :D
Wow, thank you very much for taking the time to write that up, bd. And thank you for the Welcome. By "PCV to PCV" I meant I was utilizing the large center "PCV" vacuum port on the front of the carb. It is hooked straight to the PCV on my DR side valve cover through a 3/8" hose. There is a filter on the opposite valve cover.
I must tackle this fuel "vent" routing. I must've read my routing diagram wrong, for I thought the fuel bowl vent was tee'd into the PCV line somwhere, which was full manifold vacuum. It didn't seem right to me, either. I've noticed that some carbs seem to delete that vent altogether. Could I just cap it? Maybe delete the canister altogether? Sounds rather counterproductive to conserving fuel....
Either way, thanks for the tip. That sounds like a big mistake on my part.
Off topic, I took a ride to a town called Buckeye earlier today to help a neighbor find some parts for his '68 C10. He got original M/T valve covers and a Weiand Intake for $60. It's funny the things people have rotting in their backyards for decades. At least it's dry in AZ!
For your efforts, guys, I offer you eye candy, haha:
(http://i50.tinypic.com/28aqqs5.jpg)
(http://i47.tinypic.com/jqjdw9.jpg)
(http://i45.tinypic.com/rr8sa0.jpg)
a Mach 1 Twister?!
(http://i50.tinypic.com/sctcom.jpg)
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I must tackle this fuel "vent" routing. I must've read my routing diagram wrong, for I thought the fuel bowl vent was tee'd into the PCV line somewhere, which was full manifold vacuum. It didn't seem right to me, either. I've noticed that some carbs seem to delete that vent altogether. Could I just cap it? Maybe delete the canister altogether? Sounds rather counterproductive to conserving fuel....
Some carbs don't have an 'external vent' connection, because they are "internally vented" into the air horn area. The conventional fuel bowl must be vented (externally or internally) to prevent undue pressure from building in the float bowl, which would affect fuel level and tip in. Conventional carbs rely on atmospheric pressure as their base reference for fuel delivery.
The canister serve a beneficial purpose. The fuel bowl and fuel tank(s) 'vent' into the canister. Captured fuel vapors are subsequently burned by the engine rather than evaporating to atmosphere where they are lost. I would leave the canister in place... just plumb it correctly.
Sounds like you had fun hunting parts!
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My original qjet had a port for the canister, the replacement was off of an early 80s chevy and does not have the port. How much does this effect everything?
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If the fuel bowl wasn't fitted with an external vent, it's internally vented and will be fine. But, the 3/8" carb vent port on the canister should be capped.
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MSD 6AL - with HEI cap/rotor upgrade
Personally I feel as though this is an area in which you would benefit from adding the MSD distributor. You'd be amazed. That being said, play with your timing, advance it til it pings and then retard it. Record your timing settings for reference and tweak it from there. See where it performs the way you like it.
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Such a wealth of knowledge here! If only I could get you guys in my garage for an hour.....
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advance it til it pings and then retard it.
hey vile, could you describe a little more of what the ping sounds like, and uh which way you turn the distributor for advance/ retard, just trying to figure out my timing too lol
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Pinging can sound like a can with a handful of popcorn seeds being rattled. You turn the dis. clockwise to advance. It's helpful to have the vacuum advance diss connected and in its place a vacuum gauge. Aim for highest vacuum.
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Actually you turn the distributor counterclockwise to advance it.
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so (since my timing tab is the wrong one) i would turn the distributor counterclockwise until i head pinging, and then turn it clockwise until i get the highest vaccum reading? something like that?
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Actually you turn the distributor counterclockwise to advance it.
Riiigghhhtt... haha thank you!
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so (since my timing tab is the wrong one) i would turn the distributor counterclockwise until i head pinging, and then turn it clockwise until i get the highest vaccum reading? something like that?
What I did was adjust to highest vacuum, go for a drive (up hills, full throttle starts, etcetera) and listened for pinging and back it off accordingly. I found that my truck ran really great advanced(lots more power and response) but pinged under load. So maybe when I am rich I'll run mid or premium gas to rid of the ping. Lol make sure you use your normal gas too. Different qualities of gas can effect results.
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thanks 79!
one more question, if i were looking at the front of the engine, about where would the plugs(where the wires plug into the coil on the cap) on the dist be at if i were in the ball park of the right timing? mine right now is about 3oclock ish
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The old school dial in works something like this. You would incrementally advance the timing and go for a ride. If at any time under load it starts to ping you've gone too far. Conversely it should start easily without straining the starter. Once you find where you have the best performance record your readings and fine tune, adjust idle speed etc. It's OK to play around with it.
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Whats wrong with your timing pointer?
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Whats wrong with your timing pointer?
OJT ! (http://forum.73-87chevytrucks.com/smforum/index.php/topic,25996.msg216652.html#msg216652) :)
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Maybe I missed something in the posts, but why don't you get the correct timing pointer and set the timing properly?
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it turned out that i was putting the timing light plug on the #2 cylinder and not the #1 lol :o :P
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Ok, so now that you know which is the number one cylinder, hook up your timing light and see where you are. Try 10-12 degrees advanced (vacuum disconnected and plugged) that should be safe.
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i set it at 8* for now, thats what my Chilton's manual said lol. in a couple weeks ill set it to 6* and see where it goes and then ill start going back the other way up to 10* and 12* and see where the best performance is! (gas mileage lol)
-Nic
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i set it at 8* for now, thats what my Chilton's manual said lol. in a couple weeks ill set it to 6* and see where it goes and then ill start going back the other way up to 10* and 12* and see where the best performance is! (gas mileage lol)
-Nic
I wouldn't bother with 6*. The more retarded the number the likelihood that you'll get worse gas mileage, the whole reason for the 10* is emissions not MPG. Your motor will be more efficient more advance and therefore better MPG.
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hmm alright thanks 79! i will have to change my timing up to 10* then when i get back this weekend! thanks for the advice! 8)
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hmm alright thanks 79! i will have to change my timing up to 10* then when i get back this weekend! thanks for the advice! 8)
I've read that what you want is be fully advanced @32* by x rpms. I've seen people saying it's best to be "all in" by 3000 rpm. That includes your initial plus vacuum. I am sure ill be corrected if I am wrong.
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Also the more retarded your timing the hotter the motor will run.
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hmm alright thanks 79! i will have to change my timing up to 10* then when i get back this weekend! thanks for the advice! 8)
I've read that what you want is be fully advanced @32* by x rpms. I've seen people saying it's best to be "all in" by 3000 rpm. That includes your initial plus vacuum. I am sure ill be corrected if I am wrong.
Correct...
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When referring to 32 degrees total, that is mechanical timing only, it does not include vacuum advance.
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Yes all mechanical not including vac... This is best achieved by using the 375/41 dizzy center plate and weights from a 80s suburban. I was so skeptical b4 I put them in... Now the truck is a total diff beast. 10 degrees initial, 32 all in by 3k plus 8 degrees vac using the Accel vac can.
Put it this way. B4 I couldn't break the tires loose for nothing.even power braking Now they beak loose from a roll.
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so if i went to my local junkyard and pulled the distributor and replaced that center peice (not sure what that is) and the weights (not sure either) i could achieve alot more power?
-Nic
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Yes... The parts im taking about are what you'd find in a dizzy recurve kit. , but they have the correct weight abs curvature. (http://img.tapatalk.com/d/13/05/03/vaba9e9u.jpg) the boomerang looking parts are the weights... They will have 41 stamped on them. The center plate is next to them, it will have 375 stamped on it. That is what you want to look for at the yard. This will take all guess work out. Set to 10 or 12 degrees and go... B side the springs to bring it all in by an approx 3k that is it.
All u have to do is take the Cato and rotor off and look at them for those #s
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thanks 454! 8)
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No prob let us know when you get them. We can go through the process...
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will do! also will i need to get the springs out of it too? and does it matter if its out of a sbc or bbc?
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Dave the ignition man says you could use the springs that come with, but Idk about used springs. i got three moroso advance kit and used one light and one medium spring. Once you've gotten set on the mechanical and all is well. Then its time to bump up the power a little more with the vac advance... I choose the accell vac can, because it operates down to 5 inches of vac. This totally its an unrated step. This bumped up the low end tq and part throttle response. with that said every engine is diff. My engine build is 60K into, comp cams 260h, flat tops, bummer tbi heads... a said 9:5 comp. Headers and summit race mufflers backed by a th350. Rolling on 31in all terrains
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Oh and long as they have those #s stamped on them get them
:D
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Oh and back to your first post. Eddy carb is not a good performer. The q jet is. 25 buck will get you one. Get a 76 qjet. Rebuild it our have it rebuilt... Then you'll see what i'm talking about. I have the Eddy 1406 and its is nothing even when dialed in compared to a stock qjet. Mpgs you may gain 2 or 3... Keep out them secondary:-)