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.

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Ricko1966

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Vortec heads,carbureated are not cold weather friendly. When you keep referring to 2.02 heads, that's valve size,what's more important is casting numbers. Some heads with smaller valves outflow other heads with bigger valves. Port velocity is important too,too little and you lose low end.A lot depends on your budget,and goals. M carlo has given you some solid advice,but budget and what you are willing to trade off on to reach your goals is going to change any kind of advice. On a budget? Got dished pistons? Get a cam,headers,083,113,or 601 heads. The small chambers will help with the compression along with not being bad heads. P.s. a stock 350 with a cam,headers,and a 3.73 gears feels like a rocket to most people.
 
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ChuckN

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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!
In case someone hasn’t already covered this- smaller combustion chambers increase your compression ratio. More or less, the compression ratio is the ratio of the area inside the cylinder itself compared to the combustion chamber. That’s where you get a number, for example, of 8.5 to 1. That can change slightly depending on head gasket thickness, if your pistons have a “dish” to them and/or valve reliefs. But let’s keep it simple for now- you get the concept.

Increasing the compression ratio generally increases the power due to the physics. But, with too much compression comes detonation/spark knock, etc- the air/fuel mixture burning too fast and/or at the wrong time. That will hurt an engine.

SBC heads from the factory for the most part came with 76cc or 64cc chambers, though double check the casting number to verify. That info should be easy to find on the interwebs. 76cc was to lower the ratio for gas after they dropped octane ratings and did away with leaded fuel when I was little kid. 64cc chambers were also largely in the 60s during the muscle car era when you could run 11:1 compression on pump gas that was available at the time. Vortec heads also have a smaller chamber, but I believe they were more forgiving against detonation due to the chamber design. Outliers like the 283 and the 305 had smaller chambers, but they’re less common and generally perform poorly on something like a 350 because the intake and exhaust ports are smaller.

On the highest octane pump gas, 10:1 or a bit lower (generally speaking) is a good idea. Most engines in the 70s/80s ran about 8.5:1 compression.

Ok I’m rambling, I’ll shut up now. Why do I talk so much?
 

Quinn_Hanifan

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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!
 

Quinn_Hanifan

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Vortec heads,carbureated are not cold weather friendly. When you keep referring to 2.02 heads, that's valve size,what's more important is casting numbers. Some heads with smaller valves outflow other heads with bigger valves. Port velocity is important too,too little and you lose low end.A lot depends on your budget,and goals. M carlo has given you some solid advice,but budget and what you are willing to trade off on to reach your goals is going to change any kind of advice. On a budget? Got dished pistons? Get a cam,headers,083,113,or 601 heads. The small chambers will help with the compression along with not being bad heads. P.s. a stock 350 with a cam,headers,and a 3.73 gears feels like a rocket to most people.
would it be better to get dished pistons if I don't have them already? I haven't torn into my engine due to convenience and time so I don't exactly know whats in there yet.
 

Quinn_Hanifan

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Also when we are talking about cams, what is there to know about compatibility? what exactly does it rely on and how will it affect the performance of the engine?

sorry for all the questions, I learn better when I actually can have a conversation with someone rather than reading an article that only answers some of my many many questions.

Oh and am I correct to think that a higher gear ratio means more torque and power? and a lower ratio would mean less power and torque but more efficient.
 

Ricko1966

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would it be better to get dished pistons if I don't have them already? I haven't torn into my engine due to convenience and time so I don't exactly know whats in there yet.
No if you have flat tops that would be preferred,but I'm assuming you have dished pistons, that would be a good build,it could also be a good build on flat tops.It would depend on some more as of yet unknowns,like how far down the pistons are parked in the cylinder,but one step at a timeThe engine code you are looking for is stamped on the block on a pad behind and below the alternator pictured in this pic.
 

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fast 99

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RV cam is generally designed for lower RPM applications, torque and heavier GVW. They used to make cams for more MPG, unknown if they are still available.
 

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Quinn_Hanifan

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75gmck25

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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.
 

tadslc

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I applaud your effort and want to give you some advice. Although I am a total SBC novice I know this has been true for the vehicles I've worked on in the past.

Find a reputable vendor and talk to them about what you want to achieve within your budget. They should be able to put together a package for you that will include everything you need to achieve this goal. It's all too easy to mismatch parts buying here and there as a rookie.

Who this vendor is, I have no clue. Others on here can offer that advice.

As far as 20 mpg, I don't think it's going to happen. I just spent a lot of money on my truck trying to achieve a significant mpg increase from my normal 10 mpg. New Blue Print engine, Holley Sniper, 700R4 transmission, etc. Runs absolutely fantastic and is so much more user friendly on the highway but...

Bottom line is I'm achieving a best of 12 mpg so far. More tuning still to go but if I can hit 15 mpg I'll be thrilled.


Good Luck
Tad
 

ByrdDog

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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.
 

85K304SPD

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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.
 

Ricko1966

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The LS big advantage,isn't the engine, the difference is engine management. Excellent,Fuel and Timing control. Compare a 99 burb vortec 5.7with an 01 burb with a 5.3 there is not some night and day difference.or a 5.7 impala ss and a 5.7 LS GTO again not some earth shattering difference. And if you cripple either buy installing a carbureator and regular old distributor ignition,they both go back to about what you have with a well built,properly tuned old school 350.
 
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