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Discussion Starter · #1 ·
I'm no stranger to building playtoy engines and putting together a performance package pretty well.
But this time around I'm goin the other direction for as much torque down low as possible with good fuel economy.

The project in question this go round is a 1980 c10 with the dreaded integral 250 (all smog equipment removed and plugged) th350 with 2:73 gears and 31" tires. I wanted an efficient cruiser and ended up with a very weak 12mpg pig.
I'm thinking with the idea of throwing together a few things I have sitting around and see if I can come up with something better.

Starting on 350 with D-dish pistons, 305 H.O. heads with 58cc chambers, 165cc ports, 1.84 Int. 1.50exh. valves. An Edelbrock performer intake and Holley 600 vac. secondary or quadrajet.
Theory is that most torque down low would have better vacuum and be in the secondaries less providing better fuel mileage.
How would the gm929, summit 1101 or an equivalent comp cam perform in this combo.

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The 929 cam was the standard Chevy cam for many years, it ran in 350’s from 180 to 300 horsepower depending on the choice of heads 64 or 76cc chambers and carburation being 2 or 4 barrel. So it is a good all around cam.

2.73 rear gears and 31 inch tires are working against using whatever torque any engine in this size range will make. Essentially the engine is always lugging at street speeds, this is a condition where too much throttle is needed to get the power up to what is needed. Without a tire size change I be looking at more final drive gearing to get more leverage on the tires which although you may spin more RPM you will do it with less throttle.

Bogie
 

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I would put in a 4 speed auto and increase rear gear set to get 18-1900 rpm in O/D @ 60 mph. Probably use a 305 under the 305 heads. Complete blue print on engine. Cam specs, around 194-198º duration intake side. decide cr before build to match fuel you will use. Intake would likely be a street master or performer rpm not air gap as you dont want the slot the air gap has for more top end. A 750 cfm edelbrock or thermoquad or Qjet. 1 1/2" long tube headers into a full 2 1/2" true duals.
 

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id build a 454 with peanut heads, and a 262 or 268 cam. you'd get decent mpgs, and nearly 500 foot pounds of torque. if you have to stick with a sbc, add a 383 kit, and similar cam.
 

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Discussion Starter · #6 ·
What gear ratio would you recommend to keep the th350 and still cruise 65-70. I may have another rear with 3.42. I went with the same combo in my dad's truck with a GM crate sp350-th350 and 2.73 gear. His averages between 18-22 mpg.

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Generally speaking, when I do this for customers, We try to keep the cruising RPMs with a traditional carb/basic throttlebody inj in the 2100-2300rpm with a mild camshaft. This gets you off the idle circuit and tends to be more efficient in most applications. When you run a rowdy engine with a high idle; the cruise RPM needs to increase as well. Now, multiport, sequential EFI will allow you to drive comfortable at ridiculously low RPM. However, as you've seen it takes more throttle input and a lot of gear changes when you arent on the highway.
Calculate GEAR, RPM, MPH, TIRE DIAMETER (wallaceracing.com)
if you run a 3.31/3.36 W/ your 31" tires at 65mph, thats 2331rpm. A 3.23 at 65 is a tiche over 2200. I'd go get an axle with one of those (3.23/3.31/3.36). That will help your around town driving (remember the hardest thing to do (in daily driving) is to move a 2 ton object from a dead stop - it only takes about 70hp to keep it going 65mph)
Your 2.73 with a 2.52 1st gear and a 31" tire, is giving you an effective takeoff of 5.77. You should be a lot closer to 8 or 9. As Bogie said, this is killing your mpg.
Wallace Racing - Starting Line Ratio Calculator make sure you set the existing tire height to 26 inches and your 'new' tire to 31 inches, and the 1st gear for a TH350 is 2.52

I think a customized converter will definitely pay dividends as well. Freakshow is the most reasonably priced for the performance.

I'd go with a 3.36 rear, a converter from freakshow (about $300 last i knew), and your 31s. Thats about as good as it'll get on paper as a compromise. Takeoff is still a little shy, but the customized converter will definitely help there; cruising is 22-2400rpm which is very efficient and easy to tune. Think about that quadrajet or thermoquad longterm. I'd also try to find a shop with a distributor machine to maximize your ignition curve.
A lock up TH350 (TH350c) is an interesting idea in your application.
If you go to a 700r, I'd like to see a 3.55 rear axle.
 

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Nothing except maybe a hyper-tuned EFI will get you better MPG on that 350 than a properly tuned Qjet. For many years the TBI truck guys would swap from EFI to Qjet and pick up 2 mpg.

For a cam, I've built several torquey truck engines with Melling's 22124 grind. It's a 203/213 duration with a 110 LSA. It's actually the Mercruiser Marine grind for their 235hp motors. lot's of torque just off idle. Stock stall would probably be lovely. Choose gears for about 2000 rpm at 70 with this cam. A lockup TH350c would be brilliant.

Mild cam, properly geared, well-tuned Qjet you should see 16 easy on the highway depending on your aero. Big difference between a lowered truck with highway tread versus a lifted truck with fat mud tires.

Also, when you're dealing with smog equipment, I would urge you to give some though to what you remove. I'm not hugging trees, I'm honestly saying this for your MPG goals. EGR costs exactly zero hp and can help tremendously in the MPG category. EGR only operates at mid-throttle cruise, not idle or WOT.

If you're in the "smog equipment is for babies" category, feel free to remove it all, but it actually doesn't cost any HP and can help MPG.
 

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What gear ratio would you recommend to keep the th350 and still cruise 65-70. I may have another rear with 3.42. I went with the same combo in my dad's truck with a GM crate sp350-th350 and 2.73 gear. His averages between 18-22 mpg.

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3.42s would be very, very close to Autogears suggestion. He really knows his stuff, and has been setting up his customers with sweet combinations for many years.
That rear end swap will be the best thing for you right now. You will notice a big difference, and probably wont want to mess with anything else after that. Plus the big bonus is, you already have it!!!
 

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Discussion Starter · #12 ·
Curtis73 the original smog equipment was butchered up by previous owner and pump was seized. Which worked in my favor the pump had the belt bound up and owner thought the engine was "locked up." Sold to whole truck to a buddy of mine for $200

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Discussion Starter · #14 ·
Imsport blew my mind as well. I was expecting to see 14-15 when we put it in. He swore up and down it was too much motor but now he chugs along 60mph with a grin.

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I think putting more gear in the back will help.
Back when Mustang GT came with 220hp and 2.73 gears, putting more gear in them would pick up MPG for most drivers not driving exclusively on the interstate---it just takes less throttle to get them going around town.
 

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Curtis73 the original smog equipment was butchered up by previous owner and pump was seized. Which worked in my favor the pump had the belt bound up and owner thought the engine was "locked up." Sold to whole truck to a buddy of mine for $200

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That reminds me about when I bought a 62 Caddy DeVille for $1500 because it wouldn't start. I showed up, noticed a crooked distributor cap, paid him $1500... then waited for him to go inside before I put the distributor cap back on, started the car, and drove it on the trailer. He was not happy.

Good score. And yeah... smog pumps suck.
 

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Small world. I just sold an 82 C10 that I had for 7 years. It came to me with the 250 six, 3-speed manual, and 2.73 axle. Calling it a dog was an understatement. What it really needed was an optional 3.42 or 3.73.

Ended up with a 355 and TH350 based on a 97 roller cam block. I kinda liked the relatively low revs on the highway so stuck with the 2.73. Low RPM torque came from 9.4:1 compression, Vortec heads, and a take-out RamJet roller cam @ 196/206. It also had full length headers, 2-1/4" exhaust, and Dynomax Super Turbos. While it wasn't a rocket, it would spin the tires (OK, one tire) like crazy in first gear, and it certainly wasn't a slug. Tires were 29".

If you're looking for max low RPM torque and don't have a roller cam block, I'd suggest a flat tappet cam in the range of 194-204 intake duration, However, a cam with more aggressive lobes, like a Comp XE or Lunati Voodoo should work with a little more duration. I suspect Mike Jones at Jones Cam Designs would probably recommend his 204/208 grind, depending on head flow data, if available.

Edit: Just remembered your 31" tires. If that's what you must use, then that Summit K1101 cam is sounding like a good fit!
 

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3.42 and 2.76 are a truck ratios; 3.31, 3.36, 2.73 are car. Both are 12 bolt but big difference in pinion shaft diameter so no crossing lines. Carriers interchange. Personally, I like the plate type over the locking diff. Some people don't like Qjets but set up properly, they will make the best mpg and power.
 
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