C3 mod help
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1st Gear
Joined: Dec 2025
Posts: 1
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From: Germantown Ohio
C3 mod help
So I recently got into the corvette scene after inheriting my Grandpa's 1978 C3 and he had put some mild mods on it but I am now wanting to put in a higher performance cam. Specifically I'm looking at the comp cams "Big ***** Thumpr" and I'm hesitant because I know about the problems that a bigger cam like that can cause I just have no clue what/how to measure everything out to do it right. If anyone has any tips or advice for me going into this it would be greatly appreciated. Any guidance helps. (for clarification it is a 350 small block as well)
Race Director
Joined: Nov 2006
Posts: 13,068
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From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Tell us a little more about your engine. 350 SB is a little vague. The cam you suggest is just about making noise. Not power.
Race Director
Joined: Nov 2006
Posts: 13,068
Likes: 7,678
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Let's just guess that you have a mostly stock L48. Stock pistons and heads. Your compression ratio is somewhere around 8.2 - 8.5-1.
this kind of compression isn't going to support a big healthy cam. Just won't work.
Imagine that your car has the factory automatic transmission and torque converter. Just won't work.
And your rear gear ratio? Imagine it's 308. Just won't work.
Now of course I'm just guessing.
So think through all those things. Then and only then choose a cam. And perhaps not the one with a cool sounding name.
this kind of compression isn't going to support a big healthy cam. Just won't work.
Imagine that your car has the factory automatic transmission and torque converter. Just won't work.
And your rear gear ratio? Imagine it's 308. Just won't work.
Now of course I'm just guessing.
So think through all those things. Then and only then choose a cam. And perhaps not the one with a cool sounding name.
Melting Slicks
Joined: Apr 2021
Posts: 2,549
Likes: 2,009
From: Minnesota
Welcome to the forum!
Like what was mentioned above, the camshaft you’re talking about has a lot of overlap. Which creates lots of drivability and tuning issues. It also greatly reduces the amount of vacuum the engine produces, which if you don’t know already, a lot of things on a C3 need it to function correctly.
Here is a great thread about modifying an L48, the base engine that came in 1978 Corvettes. The optional L82 350 had a bit more power from the factory, but can greatly benefit from better cylinder heads and a free flowing exhaust too.
The camshaft I have in my ‘76 is similar to the one mentioned in that thread. With the car in gear and idling at about 600 RPMs it definitely has that “cammed” sound, and drives great!
Like what was mentioned above, the camshaft you’re talking about has a lot of overlap. Which creates lots of drivability and tuning issues. It also greatly reduces the amount of vacuum the engine produces, which if you don’t know already, a lot of things on a C3 need it to function correctly.
Here is a great thread about modifying an L48, the base engine that came in 1978 Corvettes. The optional L82 350 had a bit more power from the factory, but can greatly benefit from better cylinder heads and a free flowing exhaust too.
The camshaft I have in my ‘76 is similar to the one mentioned in that thread. With the car in gear and idling at about 600 RPMs it definitely has that “cammed” sound, and drives great!
Drifting
Joined: Jun 2025
Posts: 1,577
Likes: 555
So I recently got into the corvette scene after inheriting my Grandpa's 1978 C3 and he had put some mild mods on it but I am now wanting to put in a higher performance cam. Specifically I'm looking at the comp cams "Big ***** Thumpr" and I'm hesitant because I know about the problems that a bigger cam like that can cause I just have no clue what/how to measure everything out to do it right. If anyone has any tips or advice for me going into this it would be greatly appreciated. Any guidance helps. (for clarification it is a 350 small block as well)
Is Grandpa's motor original to the car ? No guesses answer this. Verify. You may have to go thru drill of verifying casting numbers and casting dates and stamp pad in order to achieve any real level of confidence.
After you Verify above, is 1978 engine a base rpo L48 or rpo L82.? If OE 1978 C3, it's either one or the other.
No matter what's between the fenders, pass on any noisemaker cam akin to that thumpr. In general, having a part number helps to ensure appropriate advice.
What mods has Grandpa made to engine ?
