Need help with cam for 350 motor - The 1947 - Present Chevrolet & GMC Truck Message Board Network

crazyjim · 01-13-2016, 08:52 PM

I have a rebuilt 350 thinking of putting a thumper cam in it something to make it sound badass with a little more power what cam should I put in it it will have a turbo 350 behind it.

jokerz71 · 01-13-2016, 09:32 PM

There are a few things you need to know before using that cam.You need to know your actual compression ratio on the motor.If this a basic 350 with smog heads, it is probably close to 8.5:1.However, if it was rebuilt with shorter "rebuilders pistons" & then had a thick .041" rebuilders head gasket, you will be closer to 7.5:1.Either way, not enough to support a Thumper, or, any other cam much bigger than a small RV type cam.To accurately figure your compression, you need to know which heads you have.What type pistons & their volume in cc's.You will need to measure how far they set in the bore at TDC.
On another note, if your main concern is sound over power, then go ahead with a Thumper cam.There are much better cams that run better & more efficiently than the Thumper.These cams need a minimum of 9.5:1 to perform very well at all & even then, they are a PITA to tune.You'll also be making very frequent stops at the gas pump.That lope you seek from the cam, is actually the sound of an inefficient motor.Those cam's power range doesn't begin to kick in until 3000 rpm, so, you will also be looking at a high stall convertor along with stiffer gears to get the cam up into it's operating range.
If you don't have the required compression, stall & gears, this motor won't be able to get outta the way of it's own shadow.

**MikeB** · 01-13-2016, 09:33 PM

I'd say "a little more power" is the last thing you'd get with a Thumper cam in an otherwise stock engine. But you will get a bad (and erratic) idle, driveability problems, and terrible gas mileage.

Check with CompCams to see how much compression, stall, and gearing you'll need to be able to live with it.

**MikeB** · 01-16-2016, 05:16 PM

Quote:

Originally Posted by MikeB

Check with CompCams to see how much compression, stall, and gearing you'll need to be able to live with it.

BTW, my guess would be 10.0:1 compression, 2,500 RPM, and a 3.7x axle, depending on your transmission ratios and tire height.

So you have to ask yourself: Is my truck engine and drive train a good fit for a Thumpr cam? And can I live with crappy power below 3000 RPM or so?

rich weyand · 01-13-2016, 11:39 PM

Agreed on all of the above. A lumpy idle means the engine is tuned for high rpm horsepower, which makes sense for a race vehicle, not a street vehicle.

A good street vehicle is going to have torque out the ass from very low rpm, which means it will idle very smooth. It LIKES running at low rpm.

Long discussion and some recommendations here: http://67-72chevytrucks.com/vboard/s...4&postcount=11

01-13-2016, 08:52 PM	#1
crazyjim Registered User Join Date: Sep 2014 Location: The Dalles OR Posts: 123	Need help with cam for 350 motor I have a rebuilt 350 thinking of putting a thumper cam in it something to make it sound badass with a little more power what cam should I put in it it will have a turbo 350 behind it.

01-13-2016, 09:32 PM	#2
jokerz71 Registered User Join Date: Mar 2013 Location: Stanton,TN. Posts: 57	Re: Need help with cam for 350 motor There are a few things you need to know before using that cam.You need to know your actual compression ratio on the motor.If this a basic 350 with smog heads, it is probably close to 8.5:1.However, if it was rebuilt with shorter "rebuilders pistons" & then had a thick .041" rebuilders head gasket, you will be closer to 7.5:1.Either way, not enough to support a Thumper, or, any other cam much bigger than a small RV type cam.To accurately figure your compression, you need to know which heads you have.What type pistons & their volume in cc's.You will need to measure how far they set in the bore at TDC. On another note, if your main concern is sound over power, then go ahead with a Thumper cam.There are much better cams that run better & more efficiently than the Thumper.These cams need a minimum of 9.5:1 to perform very well at all & even then, they are a PITA to tune.You'll also be making very frequent stops at the gas pump.That lope you seek from the cam, is actually the sound of an inefficient motor.Those cam's power range doesn't begin to kick in until 3000 rpm, so, you will also be looking at a high stall convertor along with stiffer gears to get the cam up into it's operating range. If you don't have the required compression, stall & gears, this motor won't be able to get outta the way of it's own shadow.

01-13-2016, 09:33 PM	#3
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,597	Re: Need help with cam for 350 motor I'd say "a little more power" is the last thing you'd get with a Thumper cam in an otherwise stock engine. But you will get a bad (and erratic) idle, driveability problems, and terrible gas mileage. Check with CompCams to see how much compression, stall, and gearing you'll need to be able to live with it. __________________ Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then! Last edited by MikeB; 01-16-2016 at 01:21 PM.

01-13-2016, 11:39 PM	#5
rich weyand Registered User Join Date: Apr 2014 Location: Bloomington Indiana Posts: 1,041	Re: Need help with cam for 350 motor Agreed on all of the above. A lumpy idle means the engine is tuned for high rpm horsepower, which makes sense for a race vehicle, not a street vehicle. A good street vehicle is going to have torque out the ass from very low rpm, which means it will idle very smooth. It LIKES running at low rpm. Long discussion and some recommendations here: http://67-72chevytrucks.com/vboard/s...4&postcount=11 __________________ Rich Weyand 1978 K10 RCSB DD.



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