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.

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

Quinn_Hanifan

Member
Joined
Feb 29, 2024
Posts
49
Reaction score
14
Location
Toledo Ohio
First Name
Quinn
Truck Year
1978
Truck Model
C20
Engine Size
350
i need a relatively budget build but Was thinking about putting either Vortec or 2.02 headers on it and a new cam, intake manifold, carburetor, exhaust, etc. I would also love to know the other necessary things for said parts. I've been trying to do research but can't find anything for what I need or really understand. I would appreciate it greatly for some help!

-Quinn
 

GTX63

Full Access Member
Joined
Nov 27, 2022
Posts
728
Reaction score
2,966
Location
Tennessee
First Name
Ty
Truck Year
1985
Truck Model
K10
Engine Size
350
Welcome Quinn. Vortec heads are a nice addition to a gen 1 small block chevy, just make sure you have used heads checked as they are prone to cracking. I have owned some very strong small blocks with Qjets, stock exhaust manifolds and modest cams and intakes. Probably just start firing away with your questions and we'll help out as best we can.
 

Quinn_Hanifan

Member
Joined
Feb 29, 2024
Posts
49
Reaction score
14
Location
Toledo Ohio
First Name
Quinn
Truck Year
1978
Truck Model
C20
Engine Size
350
Awesome!! thanks for responding so quickly! With the Vortec Heads would I need to make any modifications to the block or head or would it bolt on just fine? I don't have big shop kinda tools so ease of installation or modification would be the best. also curious about whether or not I would have to or should change the stock intake manifold, i would like to but if it's not necessary at the moment that would be optimal. for context, this is my first truck, and I'm 16 so the moneys tight and it's an everyday driver at the moment.
 

RustyPile

Full Access Member
Joined
Oct 7, 2017
Posts
901
Reaction score
1,117
Location
Elkhart, TX
First Name
Nick
Truck Year
1983 GMC
Truck Model
1500
Engine Size
350 SBC
For starters, these old square bodies aren't very aerodynamic, therefore fuel economy isn't one of their strong suits.. Second item on your agenda -- good mpg and power don't fit very well in the same sentence.. You'll find yourself giving up some of one to gain the other.. The carburetor, rear gear ratio, intake manifold and camshaft should be considered as a "package".. Determine what RPM range your engine will be spending most of its time and select the package to fit that.
 

Quinn_Hanifan

Member
Joined
Feb 29, 2024
Posts
49
Reaction score
14
Location
Toledo Ohio
First Name
Quinn
Truck Year
1978
Truck Model
C20
Engine Size
350
Okay, I dont have an rpm gauge because the truck came stock with just the big fuel gauge, duly noted on the power or mpg trade off, i feel like I want more power because i do know for sure that the late 70s has the emissions tech which makes them low horsepower and kinda sucky. is there a way I can find the RPMs without a gauge and more accurate than listening to the engine? Also dumb question but what exactly does the rear gear ratio do? and how to change it when I need to. Thanks!
 

SquareRoot

Full Access Member
Joined
Mar 18, 2017
Posts
3,694
Reaction score
6,674
Location
Arizona
First Name
Mike
Truck Year
85
Truck Model
K20
Engine Size
350
Awesome!! thanks for responding so quickly! With the Vortec Heads would I need to make any modifications to the block or head or would it bolt on just fine? I don't have big shop kinda tools so ease of installation or modification would be the best. also curious about whether or not I would have to or should change the stock intake manifold, i would like to but if it's not necessary at the moment that would be optimal. for context, this is my first truck, and I'm 16 so the moneys tight and it's an everyday driver at the moment.
Vortec heads require a vortec intake manifold that isn't cheap. I have vortecs but I wouldn't do it again. There are other heads that perform better at the same or slightly lower cost.
 

Quinn_Hanifan

Member
Joined
Feb 29, 2024
Posts
49
Reaction score
14
Location
Toledo Ohio
First Name
Quinn
Truck Year
1978
Truck Model
C20
Engine Size
350
Okay, I feel like I've heard that but wasn't sure. what about the 202 heads? I've heard different things about them but what peaked my interest is that they would just fit right on the manifold I have now. its okay if not I'm curious if that would be cheaper than Vortec head and manifold though. also open to other ideas that would work, I'm. still just trying to learn about all this and figure it out. also thank you for your service sir!
 

ByrdDog

Junior Member
Joined
Dec 4, 2023
Posts
18
Reaction score
28
Location
Amarillo TX
First Name
Robbie
Truck Year
1979
Truck Model
Chevy
Engine Size
6.2
RPM range will vary a little depending on what rear gear ratio your axle has, but being a C20, I'd suspect 3.55 or 3.73. Regular highway driving with either of these gears would probably put you at 2200-2500 RPM's. Most daily drivers live in this range, so if that's the case for you, that's what I'd build for. Tire size can also affect cruising RPM, so if you plan on running taller tires, this will lower cruising RPM. I would agree that the Vortec heads should not be specially sought-out; if you find a good deal on a set, by all means use them, but I wouldn't pay a premium...
 

fast 99

Full Access Member
Joined
Dec 31, 2021
Posts
1,615
Reaction score
2,181
Location
Spokane, Washington
First Name
Brian
Truck Year
81,85
Truck Model
K20
Engine Size
350
If you know the gear ratio and tire diameter RPM can be calculated from that. Unless changed gear ration should be able to be decoded from VIN. Lower cruise RPM higher millage. Higher RPM more power, less millage it's a tradeoff.

What millage are you trying to achieve? By today's standards SB aren't light, particularly efficient or aerodynamic.

Vortec heads are a completely different casting. Inner 2 intake bolts [for starters] are at a different angle. By the time a used set of heads is bought, machine work performed cost will be fairly close to a new aluminum set that will not require the costly intake.
 

Quinn_Hanifan

Member
Joined
Feb 29, 2024
Posts
49
Reaction score
14
Location
Toledo Ohio
First Name
Quinn
Truck Year
1978
Truck Model
C20
Engine Size
350
Understood, any parts recommendations for that rpm range? Byrddog
 
Last edited:

Quinn_Hanifan

Member
Joined
Feb 29, 2024
Posts
49
Reaction score
14
Location
Toledo Ohio
First Name
Quinn
Truck Year
1978
Truck Model
C20
Engine Size
350
If you know the gear ratio and tire diameter RPM can be calculated from that. Unless changed gear ration should be able to be decoded from VIN. Lower cruise RPM higher millage. Higher RPM more power, less millage it's a tradeoff.

What millage are you trying to achieve? By today's standards SB aren't light, particularly efficient or aerodynamic.

Vortec heads are a completely different casting. Inner 2 intake bolts [for starters] are at a different angle. By the time a used set of heads is bought, machine work performed cost will be fairly close to a new aluminum set that will not require the costly intake.
Mileage probably somewhere around the 20 mpg range? she gets about 10-12 right now. if not ill just suck it up and go for more power but if I can achieve that I may do that. still thinking on it though
 

RustyPile

Full Access Member
Joined
Oct 7, 2017
Posts
901
Reaction score
1,117
Location
Elkhart, TX
First Name
Nick
Truck Year
1983 GMC
Truck Model
1500
Engine Size
350 SBC
Okay, I dont have an rpm gauge because the truck came stock with just the big fuel gauge, duly noted on the power or mpg trade off, i feel like I want more power because i do know for sure that the late 70s has the emissions tech which makes them low horsepower and kinda sucky. is there a way I can find the RPMs without a gauge and more accurate than listening to the engine? Also dumb question but what exactly does the rear gear ratio do? and how to change it when I need to. Thanks!
Let's get the terminology correct so we understand each other.. It's not "rpm gauge", it's tachometer.. Now to answer your RPM question.. I've been building engines since the late '50s when I was about your age.. I raced cars and built race engines for some of those years.. I can listen to an engine and make an educated guess as to the RPMs but no way is that a reliable method..

Your engine building project, in the end will cost you somewhere north of $3,000.. And that's only the actual cost of the engine.. Savings can be had by the use of used/reconditioned parts, and /or existing parts. You'll need many tools, both special and "contemporary". One of those tools is definitely a tachometer.. You can get a small "hand-held" tachometer for test and monitoring, and/or you can get a nice big one to mount near the driver's position inside your truck for constant monitoring..

I'm saying the following to enlighten you and is not in any way to belittle or insult you, absolutely not.. I was once 16 years old and, much like you, I had a very low level of "automobile knowledge".. The members and myself are here to help you in any way we can.. I suggest you spend a lot of time searching the internet for articles that explain the many faucets of automotive repair and service.. A good starting point is a Factory Service Manual. You'll find answers as to how something works. There are sites that offer free downloading of these manuals..

The rear end ratio is important in that it changes the driveshaft spinning rate (RPM) to a lower rate on the rear axles, a sort of torque divider or multiplier. Typically, with the transmission in high gear the driveshaft spins at engine speed (RPM).. The higher the rear ratio (lower numbers), the axles speed is nearer to the driveshaft speed.. Conversely, the lower the ratio (higher numbers), the slower the axles spin in relation to the driveshaft.
 

Quinn_Hanifan

Member
Joined
Feb 29, 2024
Posts
49
Reaction score
14
Location
Toledo Ohio
First Name
Quinn
Truck Year
1978
Truck Model
C20
Engine Size
350
All understood, I really appreciate the help and can't wait to learn more. I have the original manual and other books so I will be sure to do more reading, and the internet is something I am for sure looking around. it's just hard to find specific answers to questions that I have. I'll be sure to look for a Tachometer so I can get an accurate reading and work from there. thanks again!
 

mcarlo86

Full Access Member
Joined
May 17, 2019
Posts
533
Reaction score
931
Location
ND
First Name
Brian
Truck Year
1990
Truck Model
Suburban
Engine Size
350
Torque is your friend in a pickup. A good RV cam, properly tuned Qjet carb, and a good tune-up (cap, rotor, plugs, wires) would be things to look into. More than likely if your engine is stock, it probably has dished pistons. Flat top pistons and/or heads with smaller combustion chambers can make some difference as well. Compression is your friend for torque (up to a point - need to be realistic so you can run pump gas yet though). If the engine is tuned up and runs well, then maybe shift your focus to either changing gear ratio (3.42's if you have 4.11's now) in the rear end or get an overdrive transmission. Realistically though, I don't think 20 mpg is going to be achievable. A better target might be 15 mpg for a 3/4 ton squarebody. I remember being your age and working on my '68 C10. If you can find time, get an extra job to earn more money and save, save, save. It will be worth it in the long run. (I still have my '68 C10). Good luck!
 

Quinn_Hanifan

Member
Joined
Feb 29, 2024
Posts
49
Reaction score
14
Location
Toledo Ohio
First Name
Quinn
Truck Year
1978
Truck Model
C20
Engine Size
350
okay, i will for sure start to look into the tune-up, that's something I've wanted to do but i don't exactly know where to start. Torque sounds like a good idea, im not sure i fully understand the smaller combustion chamber for that. Thanks!

Also What exactly do you mean by RV cam? i dont think I've heard that before
 
Last edited:

Latest posts

Forum statistics

Threads
42,029
Posts
907,657
Members
33,521
Latest member
Laqota
Top