Ok, You have a set of Vortec heads on a 350 c.i. engine. What brand camshaft do you have and what are the specs? What brand distributor is being used? Did you assemble the engine or someone else?

 

So it's a comp cams thumpr with intake 227 and exhaust 241 at .050. I also have the msd heI distributor. I haven't changed any weights or springs.

 

So it's a comp cams thumpr with intake 227 and exhaust 241 at .050.

That's the reason for poor gas mileage. It's a cam designed for a "look at me" idle with little or no concern for low RPM manners, driveability, and gas mileage. Simple fact: Excessive duration and overlap means intake charge gets diluted by exhaust gasses, and unburned fuel goes out the tailpipes at probably anything less than 3500 RPM.

I honestly don't see how timing or quench height, etc, etc, can help at all. IMO, your best bet is to install a cam with at least 10 degrees less intake duration and 20 degrees less exhaust duration, and with a 112-114 LSA to counter the 10:1 compression.

 

I built the engine. It was my first build and I had no guidance other than the internet and it run alright only problems I'm having are my mpg and I have bad throttle response which I'm sure has to alot to do with mpg and I can't quite figure out the best timing on the engine but I've got it to where I can daily it without any real worry

 

It is hard to get the kind of mileage EFI delivers with a carb, but yes this is bad.


A 2000 S10 to a carbed 350 is quite a project a lot of stuff gets involved when taking these computer intensive vehicles through such a conversion.


Let's start with the distributor since you didn't make any mention of what's in there. Hopefully you used an HEI with mechanical can vacuum advance.


Knowing the cam by part number would be most helpful. The cam timing and drive position relative to the crankshaft are important pieces of information to have down in precise numerical terms as this drives the ignition curve and has a strong relationship to compression ratio and total driving gear ratios. If these things don't line up the gas mileage and overall performance will be poorer than expected.


Transmission data is needed a major component to modern fuel mileage form larger V8s is the use of an overdrive transmission whether an automatic or manual. The same can be said about knowing the final drive ratio, whether the differential(s) are limited slip and of course tire size. Too low a ratio spins the engine high which burns fuel at a high rate at the other end too high a ratio loads the engine too much which burns a lot of fuel.


So in the end to be of any help a lot of detail information about the build is needed. One thing you can take as a for sure is it takes quite a bit of work to tune these beasts once they are built.


Bogie

 

Talk to Eric32,he has a similar combo and gets 20 or so mpg

 

To start with set the initial timing at 14*-16*. Then check the total timing with a timing light with built advance. You might have to rev the engine to 4000 rpm and higher if you have never changed springs. Once you have the total subtract the initial timing this is your mechanical advance the distributor puts in. BTW what the model number of your distributor?

 

So here is all the info i know off the top of my head about how everything is set up.
Engine
- comp cams 12-600-4 installed with no advance or retard
- stock vortec heads
- performer rpm intake dual plan
- holley 600 or 650 with 65 jets
- 10:1 comp

Tranny
- TH350
- as far as i know stock
- about 2100 stall tc

Rear end
- 3.73 gears
- 31" tires
- open diff

 

Fairly hot cam
3 speed automatic
high(er) stall converter
Stiff gears
big tires


Ouch, none of this is conducive to good gas mileage.


The 350 is very edgy when it comes to bettering 9 or 10 MPG. This really could use a 700R4 with overdrive and a deep low with a lockup converter.


The deep low of the 700R4 helps get you launched from stops without having to bury your foot to get that cam to come up with some torque. The overdrive high and converter lock up get the cruising RPMs down under the gas hog line which seems to be about 2000-2300 RPM on these engines. A built 200R4 provides closer ratios because it's low is not so low as that of the 700R4. This is good for racing in a lighter vehicle say under 3000 pounds but costs more fuel in city stop and go since the throttle has to be used deeper to get launched rfom a stop. There are good arguments on both sides of the 200R4 vs 700R4 the way you actually use the machine needs to be the controlling choice. In either a lock up converter in cruise is almost like a 5th gear especially with a high stall converter. Lock up is not a racing clutch, it will not stand up to those loads so in these manually TV controlled transmissions it's a good idea to have a vacuum switch and/or a normally off hydraulic switch in the 4th gear apply passage. The vacuum switch drops the TCC out when the throttle is opened suddenly, the 4th gear (actually an old Ford/Chevy brake switch) drops the TCC lock if the trans shifts out of 4th and doesn't allow converter lock anywhere or anytime 4th is not signaled "on".


The Vortec head can make a lot of power on fairly conservative total advance like about 35 degrees all in about 2500-2800 RPM. Vacuum advance is very helpful in cruise with a light throttle as it keeps the advance up to give the mixture time to burn when the RPMs are too low for full mechanical. It is on the street to keep these things separated as they are doing two different and important things. The vacuum advance is sensitive to throttle position and engine loading while the centrifugal (mechanical) is sensitive to RPM. There is a no mans land between these things that takes a computer to negotiate so a compromise always has to be struck between how fast the vacuum goes away as the throttle is opened and how fast and soon the mechanical comes in with an RPM gain. That's actually the easy part the vacuum is the PIA because you can get into a loading situation where more throttle opening is called for which reduces manifold vacuum thus vacuum advance but at the same time the throttle opening does not equate to an RPM increase. Engines operating in this hole get really hungry. The flip side of this is too much vacuum and mechanical at the same time you get detonation.


The rest becomes tune of finding how lean you can run the motor with how much advance, this will keep you back and forth between carburetor and distributor for a while. A trick with the Holley is to use the two step power valve from motor homes. This lets you lean down main metering more than can be done with the typical one step Holley power valve to the point where it's almost as good as an Edelbrock AFB (Performer) or AVS (Thunder) on gas mileage.


OK I'm closing up and going home.


Bogie

 

my distributor is a MSD street fire 8362 i believe

 

Bogie, here is the camshaft he has going by the duration he had listed. https://www.summitracing.com/parts/cca-cl12-600-4/overview/

 

yeah that looks to be the cam im using. i have the cam card and the numbers look to match

 

Please read this article before you get started on tuning the engine. http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor

 

SS10 Jordan, Does your carb look like either one of these Holleys? https://www.summitracing.com/parts/hly-0-1850sa/overview/ or this one https://www.summitracing.com/parts/hly-0-4777c

 

the carb is a basic cheap reman that i got on ebay with single inlet and one metering block

 

i will add that i bought it from holley on ebay, at least that was what ebay said was the seller but it came with the electric choke installed up-side-down

 

Holley 0-80457SA 600CFM 4bbl Factory Refurbished ALUMINUM Carb Electric Choke
is what i bought for a carb. i just went and found it in my ebay purchase history

 

Getting back to your engine did you measure from the top of the decks (block) to the crown on top of the piston at TDC. Are the piston flat tops? When taking this measurement it's best to lay a straight edge across the cylinder bore inline with the piston pin. Using a set of feeler gauge slide the blade under the straight edge until it is snug. Don't feel bad if you didn't most people new to engine building do not do it. Do you remember the part number to the head gaskets you used? Looking at your distributor, MSD says it is tuned to 22* at 4000 rpm. This might need to be changed when tuning your advance curve. You said you have an electric fuel pump is it a Holley?

 

i didnt measure my pistons when i built it. there have been alot of things ive learned and should've done different since i started to drive this truck and get to know people with more experience than myself. i plan to use everything ive learned to build a 4-bolt main i have sitting around. but back to the mpg problem. is it possible that this is more of a combo problem than a tuning problem? i do plan to change foe 33" tires and a 3.08 rear gear and a 3500 rpm stall convertor. i know the gears would help me but will the tires and the tc hurt me?

 

my current tc is the cheapy summit one that is rated at 1700-2100 stall.. if that helps

 

oh and the fuel pump is just another cheapy that i bought just to get the truck running. https://www.summitracing.com/parts/atx-e8012s/overview/

 

Thank you. I think it has to do with timing alot too. I cruise the truck to and from work at like 45 to 50 and the max mph and it does rev above 2100... Unless I'm trying to have fun. I don't fully understand how you were saying to go about testing

 

Timing is something that I can't really wrap my head around with all the reading I've done trying to figure it out

 

Try this for timing.
Where should I set the timing on my performance engine?

 

This sounds like a 1/4 tonne 4X4? With a 3 speed auto you are geared correctly with a 2400 rpm= 60 mph. If you run a 4 speed auto?

If you went to a 4.3:1 gear and a 700r4 with lock up you will have a great launch and 2200 rpm in O/D is just over 65 mph.

Shifting at 2600 rpm(spritely) would net shift points at 20 mph 37 mph then 56 mph. Those numbers sound good to me. Tuning the timing and carb will take a little work and time.
3.08 gears are not a good choice for 31 tall tires with any transmission

 

the goal for my s10 is to make it into a prerunner. 2wd with some large m/ts. im not too worried about 1/4 mile times just want to be able to daily it with better mpg until i can afford to buy something else to daily.

well thanks for the help ill see what i can do and if i cant figure it out i will be back

 

I have two different 350 builds and one I just recently had build and its got some new things on it so I can't tell you what the mileage is but before it on both 350 builds I got 19 mpg highway and could crack 20 on my milder build if I worked a little more at primary jetting but it runs pretty good as is and I don't have a o2 monitor but is in my plans. Both of my s10's just have regular 15 inch tires and I do believe are around 31 inches high and are just old every day tires and nothing fancy. Pretty much all factory except the rear posi unit's are the Detroit trutrac ones along with the v8 transplant and upfront cooling.

86 build with basic hei distributor with timing in by 3000 rpm or a few rpm over.
350 9.5 to 1 cr or little higher weiand dual plane intake high rise intake 650 holley double pumper
200cc dart iron eagle heads ported and gasket matched milled 69cc combustion chamber
flat top forged pistons on a decked block
Lunati voodoo hydraulic roller cam 282/290 231/239 @50 535/550 lift
Turbo 350 3.73 rear posi unit and cruises at 3100 rpm at 65 to 70 mph
3000 plus stall non lock up

96 build with basic hei distributor with timing in by 3000 rpm or a few rpm over.
350 9 to 1 cr weiand dual plane intake high rise with 650 holley double pumper
Lunati voodoo 270/278 219/227 @50 515/530 hydraulic roller cam

180cc dart shp aluminum heads with 72cc combustion chamber
flat top hyperutectic pistons on a decked block
Turbo 350 3.42 rear posi unit and cruises at 2800 rpm at 65 to 70 mph
2000 plus stall non lock up