Want to build a good horsepower, good fuel economy engine for a 1978 c20 with stock 350. I don't know much and need help.

[Quinn_Hanifan]

it runs pretty well and as far as i can tell its in good condition

Do you know your motor is in good shape to start with? Tearing the engine completely apart to replace pistons is MUCH more complicated & expensive than just pulling heads.

 

[Quinn_Hanifan]

I hate to say it, but it sounds like he needs a LS motor. I think those parameters are the reason the small block chevy GM developed into the LS.
I am a SBC fan, but power, mileage, and "cheap" aren't really achievable.

it sounds silly but I'd rather stick with carburation than go with fuel injection grated yes i know it's more efficient and powerful (I think). I'm really just trying to get the best out of the carburation I suppose. thanks for the recommendation though

 

[Hunter79764]

You're asking a lot of good questions, and of course you are going to get a lot of answers. I'll throw some of my questions and answers in as well...

What kind of shape is your truck in now? Does it basically run good, or are you dealing with lots of drivability issues? What do you use the truck for, and what do you want to use it for?

As mentioned, you have a 3/4 ton truck without overdrive, so MPG is not as easy to achieve, but at the same time, going from 10-11 up to 14 will be a significant savings on your gas bill. There's ways to get it closer to 20, but its likely going to be out of your league (like it is out of most of ours here). Modern trucks don't get to 20 without some major technology, so there's that. And putting myself in your shoes, there's something to be said about taking a portion of your "MPG" budget and buying a cheap honda you can drive around town, which also gives you a ride when a weekend project on the C20 doesn't get finished before school on Monday. Of course, insurance costs are a factor too. Just something to kick around.

As for more power, there are a number of ways to go, and a lot of it depends on what you have and what the condition is. If you're sitting on a good truck that isn't burning oil etc, then you should leave the bottom end of the engine (including the pistons) alone to save money, but it does limit your compression ratio options. If you are leaking oil and have minimal oil pressure and it's generally worn out, then you are probably needing new pistons anyway and increasing compression is as close to free horsepower as anything at that point. A matched heads/cam/intake system can give you a lot, but again, you have to have some realistic goals defined up front to pick those items together.

The LS swap is something to consider, although it really does open a big can of worms. Even a 4.8 liter (the smallest LS engine) has an extra 100 hp and 50 ft lbs over the stock 350, but will have a significant cost and complexity to install it. Of course, if you are doing all of the proposed engine work yourself, that has a steep learning curve too. But a 4.8l with a modern overdrive transmission is probably going to give you the best MPG potential, all other things held constant. If you know and understand carburetors, great, but shops that can tune a quadrajet are getting fewer. And putting a carb on an LS makes it more complicated, more expensive, and generally does not have as good of driveability as the fuel injection.

To answer another question, the rear gear ratio can be found out from your options code or sometimes on the axle itself. A higher number like 4.11 is called "lower" gears, and is better for take off of "lower speeds". "Higher" gears (which are lower numbers) like 3.23 is better for "higher speeds" like highway cruising. (Note, that is not why they are called that, but it's a good way of remembering it). You can swap rear gears (or whole axle assembly) to get better highway driving, but it will be slower off the line. Or you can gear it to be a stoplight warrior, but highway manners will suffer. In reality it is always a compromise, and there's no single right answer.

"RV Cam" was a type of cam made for motorhomes or trailer pulling. It turns out that getting 10,000 lbs of RV up to speed in a reasonable amount of time uses pretty similar equipment to getting 4,000 lbs of truck up to speed a lot quicker while keeping the ability to tow when needed, so it has been a common choice for many folks over the years.


The secret to doing a car project on a budget is to make sure you only buy parts once, and you buy the right ones. We can help you with that, but it really needs a clear set of goals and plans. Otherwise, you might give a nice tune up on an engine that will only get torn apart, halfway rebuilt, then scrapped in favor of something else that doesn't use any of the parts you just bought. And you'd be amazed how many times all of us have done that exact kind of thing...

 

[Quinn_Hanifan]

I had a similar goal for my 1975 K25, so I can provide my input.

- original smog era 350s and GM replacement crate engines are seriously hobbled by low compression and poor head flow. Compression ratio with typical 76cc heads and dished pistons is about 8-8.2.
- switching to an iron Vortec head or aftermarket aluminum head with 64cc chambers can add about 1.0 point, and thinner head gaskets will add another .1-.2. Switching to flat top pistons also helps a lot, but requires a lot more engine disassembly.
- Vortec heads require a different intake, so look at the price difference between Vortec and older intakes (it’s not really that much). Some aftermarket heads have much better chambers, but retain the old intake bolt pattern. I used Blueprint 8002K heads for that reason.
- Once you budget for better heads, you then look at camshafts. Original cams were very low duration (maybe 192/195 @ .050), and switching to an RV cam with a duration up to about 214 @ .050 was very common. Once you go much higher duration than that you will still gain power, but start losing gas mileage. I used a cam duration of 213/217 @ .050 because I wanted to favor power a little over gas mileage.

From there, look at headers and lower gear ratio to improve performance. Gear ratio is always a trade-off if you use a TH350 or other non-overdrive transmission. Around 3.50 is usually a good compromise, but larger or smaller tire diameter has to be factored in.

okay, thanks alot. that answers my question of where to start, which would be the heads if im understanding you correctly then the cam and then gear ratio. how should i incorporate the intake manifold carburetor and others (as in aftermaket) ? ive heard it should all be a package kinda.

 

[85K304SPD]

The place to start would be a good compression test. You need to determine if the engine needs a rebuild before you add a bunch of stuff to the outside. Assess what you have and determine what you want it to be in the end, as well as what your budget is and do the math and go from there.

 

[Quinn_Hanifan]

You're asking a lot of good questions, and of course you are going to get a lot of answers. I'll throw some of my questions and answers in as well...

What kind of shape is your truck in now? Does it basically run good, or are you dealing with lots of drivability issues? What do you use the truck for, and what do you want to use it for?

As mentioned, you have a 3/4 ton truck without overdrive, so MPG is not as easy to achieve, but at the same time, going from 10-11 up to 14 will be a significant savings on your gas bill. There's ways to get it closer to 20, but its likely going to be out of your league (like it is out of most of ours here). Modern trucks don't get to 20 without some major technology, so there's that. And putting myself in your shoes, there's something to be said about taking a portion of your "MPG" budget and buying a cheap honda you can drive around town, which also gives you a ride when a weekend project on the C20 doesn't get finished before school on Monday. Of course, insurance costs are a factor too. Just something to kick around.

As for more power, there are a number of ways to go, and a lot of it depends on what you have and what the condition is. If you're sitting on a good truck that isn't burning oil etc, then you should leave the bottom end of the engine (including the pistons) alone to save money, but it does limit your compression ratio options. If you are leaking oil and have minimal oil pressure and it's generally worn out, then you are probably needing new pistons anyway and increasing compression is as close to free horsepower as anything at that point. A matched heads/cam/intake system can give you a lot, but again, you have to have some realistic goals defined up front to pick those items together.

The LS swap is something to consider, although it really does open a big can of worms. Even a 4.8 liter (the smallest LS engine) has an extra 100 hp and 50 ft lbs over the stock 350, but will have a significant cost and complexity to install it. Of course, if you are doing all of the proposed engine work yourself, that has a steep learning curve too. But a 4.8l with a modern overdrive transmission is probably going to give you the best MPG potential, all other things held constant. If you know and understand carburetors, great, but shops that can tune a quadrajet are getting fewer. And putting a carb on an LS makes it more complicated, more expensive, and generally does not have as good of driveability as the fuel injection.

To answer another question, the rear gear ratio can be found out from your options code or sometimes on the axle itself. A higher number like 4.11 is called "lower" gears, and is better for take off of "lower speeds". "Higher" gears (which are lower numbers) like 3.23 is better for "higher speeds" like highway cruising. (Note, that is not why they are called that, but it's a good way of remembering it). You can swap rear gears (or whole axle assembly) to get better highway driving, but it will be slower off the line. Or you can gear it to be a stoplight warrior, but highway manners will suffer. In reality it is always a compromise, and there's no single right answer.

"RV Cam" was a type of cam made for motorhomes or trailer pulling. It turns out that getting 10,000 lbs of RV up to speed in a reasonable amount of time uses pretty similar equipment to getting 4,000 lbs of truck up to speed a lot quicker while keeping the ability to tow when needed, so it has been a common choice for many folks over the years.


The secret to doing a car project on a budget is to make sure you only buy parts once, and you buy the right ones. We can help you with that, but it really needs a clear set of goals and plans. Otherwise, you might give a nice tune up on an engine that will only get torn apart, halfway rebuilt, then scrapped in favor of something else that doesn't use any of the parts you just bought. And you'd be amazed how many times all of us have done that exact kind of thing...

This answers a lot of my questions, great explanation. i think what i want is 300 325 horse. what classifies as low medium and high torque? what are their capabilities and how does that affect fuel economy? I'm OK with sacrificing some fuel for power if that means i can get more work done. but in all honesty, i think what i SHOULD do is go for better mileage only because the work i have to do with my truck is kinda far and it sucks to get half the money i would make just to get there. I have a TH400 transmission which ive heard is good but it doesn't have overdrive i don't think. i don't know my gear ratio and in not sure how to find that. i think it burns a little oil but im not sure, pretty smoky out the exhaust when i start it up but goes away after it warms up. My truck is in okay shape i think, i replaced the radiator and alternator due to failure, new air cleaner, thermostat, and pretty much all the fluids other than power steering and brakes. I need new brake pads at least but the bolts for the wheels are seized on so i haven't been able to get them off. the body is rusty but the frame is really good. ive done other smaller stuff like lights and some electrical stuff.

 

[Quinn_Hanifan]

The place to start would be a good compression test. You need to determine if the engine needs a rebuild before you add a bunch of stuff to the outside. Assess what you have and determine what you want it to be in the end, as well as what your budget is and do the math and go from there.

How do i go about doing a compression test?

 

[Ricko1966]

I got a great idea,drive the truck a while as is,evaluate it,sort out the known problems. Next we can evaluate,does it burn oil,does it use water,does it run hot,does it have low compression. Is the gear too steep. ETC. Those things will help in the decisions.

 

[RustyPile]

This begs to be discussed, but no one has posted a comment along those lines.. The following is only my opinion but should be considered...

One word: WEIGHT.. Personally, I would never consider a 3/4 ton when designing/building a performance vehicle. They're just too heavy. A 1/2 pickup is considerably lighter and will outperform (acceleration, mpg, and comfort) a similarly equipped, and geared, 3/4 ton, simply because it is lighter in weight.. And the routine maintenance items -- brakes, tires, suspension and steering components are more expensive to replace on a 3/4 ton.

Quinn, this is totally your decision, but is it possible for you to retain the 3/4 ton for daily driving. Maintain it properly for dependability.. Locate and purchase a suitable 1/2 ton short bed and build it to your goals.. Trust me.. If you stick to a single vehicle, there will be times in its construction where it will be disabled while you're working on it.. There will always be situations where the particular project can't be completed on days off or in a weekend. You'll find yourself afoot, looking for a ride until that particular project is completed - and there will be plenty of these situations....

 

[ChuckN]

I think your information is great!! I think I'm starting to understand the compression ratio stuff better. where on my engine can I find the casting number? its all gummy and it's kinda hard to clean since it's in the engine bay(?) so an exact location would be great to try and find the number. Thanks!

The cylinder head number is cast in the area by the valve springs- you’ll need to remove the valve covers to see them, but they may be hidden under some sludge depending on how clean your engine is.

As far as the block goes, you can find that in the driver side rear of the block cast into the top of the bell housing more or less. For a more accurate idea of what engine and when it was made, you can look on the front of the engine, passenger side in front of the cylinder head on a flat “pad”. Hope that helps!

 

[Hunter79764]

To check your rear gear, here's a decent write up.

HOW TO: Determine the Rear Axle Ratio you have - Modern Driveline

Knowing your rear end’s gear ratio is crucial when you are considering a transmission upgrade

www.moderndriveline.com

You can also look on your code sheet if you still have it, it should be a sticker with "Service Parts Identifier" or something, with a bunch of 3 number/letter codes in your glove box, I think. What you are looking for is "G80" to tell you if it has a locking rear end (or "Posi/Positrac/Gov-Loc"), and one of the codes at the link below. Of course, that will only tell you for sure what it had in 1979, if a previous owner changed it out the above method will be much more reliable. If you have that sticker, feel free to post it here and we can help you decode it to see what your truck came with.

GM RPO Codes - Axle Ratio and Differential Identification

The GM RPO codes label in your glovebox or on drivers door identifies axle ratios and more. Decipher the codes and get the right parts!

www.differentials.com

To confirm, your TH400 is a 3 speed automatic, no overdrive. They are very solid transmissions, but were never intended for MPG. You mentioned working with your truck, what kind of work? Hauling a heavy trailer, or loading up the bed full of brush and taking it to the dump?

300-325 is a reasonable goal in general, and you're right, torque can and will change things. For example, I have a large motorhome with an 8.1L/496 big block. It makes about 325 hp. A 4.8L also makes about 300-ish in a swap (with decent exhaust and tune etc.). But I would NEVER want the 4.8 pulling my 17,000lb motorhome pulling a 6,000lb truck on the highway, because it has about 325 ft lbs, where the 8.1 has around 500 ft lbs. But if I were to swap the 8.1 into a Miata, I'd likely never get any real use of the torque, as I'd constantly be moving up to the next gear or just shredding the tires and running out of RPM, since the 8.1 redlines at something like 4500 rpm but the 4.8 can run to 7000 with minor tweaks.
Again, keep in mind what you have now and how well it does what you want. On a different car, I went from a 4.3 V6 to a stock 5.3. The horsepower went from 160 to about 300, and the torque went from 250 to 350. The difference in how it accelerated was night and day. That V6 and your 350 probably aren't too far off on power production. Either a mild rebuild on your stock engine or a 5.3 swap will get you in that 300-325 range, with about 350 ft lbs. Without knowing exactly what you're looking to do, that is a reasonable place to look.

 

[Quinn_Hanifan]

I got a great idea,drive the truck a while as is,evaluate it,sort out the known problems. Next we can evaluate,does it burn oil,does it use water,does it run hot,does it have low compression. Is the gear too steep. ETC. Those things will help in the decisions.

I've just picked up a compression tester so I will be doing that soon. I'm not sure if it burns oil, though it's thick white smoke out the exhaust whenever I start after sitting a bit, but once it's running it's fine. it doesn't use water I don't think but I replaced the thermostat housing and it was leaking pretty bad but I'm working on sealing it up now. I've never had it run hot, its always been on the cooler side never really getting above half way on the gauge. how would i tell if the gear was too steep?

 

[Quinn_Hanifan]

This begs to be discussed, but no one has posted a comment along those lines.. The following is only my opinion but should be considered...

One word: WEIGHT.. Personally, I would never consider a 3/4 ton when designing/building a performance vehicle. They're just too heavy. A 1/2 pickup is considerably lighter and will outperform (acceleration, mpg, and comfort) a similarly equipped, and geared, 3/4 ton, simply because it is lighter in weight.. And the routine maintenance items -- brakes, tires, suspension and steering components are more expensive to replace on a 3/4 ton.

Quinn, this is totally your decision, but is it possible for you to retain the 3/4 ton for daily driving. Maintain it properly for dependability.. Locate and purchase a suitable 1/2 ton short bed and build it to your goals.. Trust me.. If you stick to a single vehicle, there will be times in its construction where it will be disabled while you're working on it.. There will always be situations where the particular project can't be completed on days off or in a weekend. You'll find yourself afoot, looking for a ride until that particular project is completed - and there will be plenty of these situations....

Understood, i love the idea of what you're saying and a 1/2 ton will most likely be in the relatively near future. i plan on keeping the 3/4 ton as long as i can, i probably should start looking at brakes, a tune-up, and maybe some new tires for dependability. Im just curious if i can get around most of the old emmision tech to at least make it a better daily driver than it is. if that doesn't matter much though please let me know!

 

[cmichels83]

Before you get too carried away buying parts to get more "power", you need to determine where you want your power. You expressed the desire to maintain good mpg, which leads me to believe that this is going to be a daily driver and not a race vehicle. Most things people are going to tell you to do to get more power are actually going to shift your power curve up in the rpm range outside of the typical daily driving rpm. When you chase these high rpm hp gains, it comes at the sacrifice of pep on the bottom end and you lose mpg really fast. If you want to learn, I suggest watch Uncle Tony's Garage. He explains things very well. Here is his daily driver Playlist.

https://youtube.com/playlist?list=PLA6afW-Gw7oxMYSAQTxKZcODloc-6FtfU&si=xdNnbvpRVO9TFFbT

 

[wandererd2000]

Put vortec heads from speedway, a .450/451 roller cam edelbrock vortec intake and avs 650 carb and have been pleased with the results ,link bar lifters about 1500in the top end