327 Thumpr- Tune or Trash? - Team Camaro Tech
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post #1 of 15 (permalink) Old Jan 1st, 17, 05:05 PM Thread Starter
Kevin
 
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327 Thumpr- Tune or Trash?

I have a 68 327, 4-speed, with a 12 bolt 3.08 rearend that is basically stock. The heads are 1.94/1.5 "double hump, the pistons are flat top, and the cylinders have been bored .030". Distributor, carb, intake, and exhaust manifolds are also stock.
The cam that was recommended to me was a Mutha Thumpr Cam. (.489/.476 lift, .235/.249 [email protected]")
Since I've installed it its been a nightmare to tune.
I am trying to stay as close to stock as possible (externally) so I kept the Quadrajet and rebuilt/modified it by doing the following: increased the down idle tubes, replaced the secondary butterfly bushings, put the lightest APT spring in it an at this point trimmed it.
The cam makes very little vacuum (7-9") and the APT was opening up all the way causing it to run extremely rich. Since I pulled the carb back down and trimmed the spring it is not running as rich, but still bad enough that I cant stay in the garage while its running (and the idle adjustment screws are out 1/2-3/4 of a turn).
I am having a really hard time tuning the engine. I have a spring kit and an adjustable vaccum advance , but have not installed them.
Currently I have 30 degrees initial timing and have taken a break from running the engine because I cant get it tuned, and the front end is being rebuilt so I cant pull it out of the garage yet.
I don't have power accessories or an automatic transmission so vacuum (for accessories) isn't a big deal to me right now.

Is there a way to tune these cams without the exhaust being so rich? If so, how do I do it?
I really want a nice sounding cam that has a lot of torque in the lower RPM range (<3800).

Would I be better to go ahead and pull the cam out and go with something else? If so which cam?
Any opinion or advice would be greatly appreciated.

Kevin
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post #2 of 15 (permalink) Old Jan 1st, 17, 05:38 PM
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Re: 327 Thumpr- Tune or Trash?

I believe the Thumpers were made to make the choppy idle sound everyone desires.
But, for your engine that is way too much cam. If you think it's bad now, just be glad you don't have an automatic with those gears and that cam.
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post #3 of 15 (permalink) Old Jan 1st, 17, 06:52 PM
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Re: 327 Thumpr- Tune or Trash?

I remember the day, (not proud of it) took a stock 283 back in the days of primitive drag racers, stuffed a 650 double pumper and a single plane intake on it to make it run. Put out so much black smoke at idle the neighbors complained about having to washer their cars daily while I was changing spark plugs daily. Barely idled, but sounded mean like a hot cam. Lost all of the torque for launch when I buried the throttle.

Also later stuffed the big ticket .500 lift cam in a stock 350, it wouldn't pull the hat off of an old man sitting in a rocking chair with no breeze. Didn't start pulling until 2500 or more. But man did it sound cool when you could keep it running. Until I figured out the push rods were bending because I didn't have the long slot rockers. A couple of broke valve springs, broken rocker arms, etc. Also, couldn't get into the usable range of the cam as I needed new heavy valve springs, along with the correct breathing of the intake, carb, exhaust, transmission ratios, and rear gear ratio.

I always had to learn the hard way.

You need to determine what you want first, an economical cruiser for use at less than 5000 rpm or an engine that starts pulling at 2500 and doesn't quit until 7000 or so. With a flat tappet cam, don't think you will find one that has torque and will go 6500 rpm +. Sorry, but the Hemi Lope from John Force's funny car sounds cool, but at a great price. A roller cam might help, but you still need a balanced system. With 3:08 gears, you are going to need a truck cam to get the torque.

After determining what you want most, start building the engine around the breathing system - compression ratio, heads, cam, intake, carburetor, and exhaust. One without the matching others is almost worthless.

Back in the stone age, before the internet, we didn't have the wisdom of the manufacturer's sites and forums to guide us in the right direction. The good cam manufacturers will tell you what compression ratio, intake and carburetor, and exhaust system is needed along with recommended transmission and differential ratios.

30 degrees initial timing, assuming with the vacuum advance plugged, is a recipe for disaster on todays fuels. I set the total advance at 34 to 36 degrees with the mechanical advance all in. Then work back to your initial advance reading.

Good luck, it is a long hard road to learn the hard way.
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post #4 of 15 (permalink) Old Jan 1st, 17, 07:43 PM
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Re: 327 Thumpr- Tune or Trash?

Quote:
Originally Posted by 69rsconv3spd View Post

Would I be better to go ahead and pull the cam out and go with something else? If so which cam?
Any opinion or advice would be greatly appreciated.

Kevin
Yes, pull it.

A good street cam for a 327 is a 268H Comp Cams.
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post #5 of 15 (permalink) Old Jan 1st, 17, 07:57 PM
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Re: 327 Thumpr- Tune or Trash?

Bad choice, too much cam as said above. Curious who recommended it...

Don

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post #6 of 15 (permalink) Old Jan 2nd, 17, 08:03 AM
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Re: 327 Thumpr- Tune or Trash?

You have way too much cam for your current combo. I totally agree with Anne. Pull it and replace it with a 268H Comp Cams.
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post #7 of 15 (permalink) Old Jan 2nd, 17, 09:50 AM Thread Starter
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Re: 327 Thumpr- Tune or Trash?

Two Guys I know are running the Big Mutha Thumper cam. One in a 350, the other in a 413 and they both love it. My coworker (413) recommended it, unfortunately it was until after the rebuild that he informed me they are hard to tune and hes running a lot of after market parts.
Thank You for the replies
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post #8 of 15 (permalink) Old Jan 2nd, 17, 10:22 AM Thread Starter
Kevin
 
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Re: 327 Thumpr- Tune or Trash?

Ron,
What I'd like is a fun car to drive red light to red light, something will push me back in the seat like the car did when I was a kid.
My dad collected cars while I was growing up and when he passed 15 years ago he left us 64 cars (some in poor shape). This was his 68 that he never finished because he built my 69, my brothers 67 and several others first. The only thing he would do is crank it up every month or two, drive it to the gas station and leave several tire marks between our house and the gas station. Then it would sit in the garage until the next time dad wanted to "play".
When he passed he left 12 quarts of Quaker State ND in the garage and told me no matter what only use that oil in his '68. When I finally got the engine running after 10 years and pulled the valve covers I found out why.
What started as "just getting it running" has turned into a complete rebuild.
I didn't harden the seats when I rebuilt the heads and intend to run straight gas with a lead additive.
This car is intended to stay "my dads car", so fun to drive, but not taken on long trips.
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post #9 of 15 (permalink) Old Jan 2nd, 17, 03:13 PM
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Re: 327 Thumpr- Tune or Trash?

It sounds like you want in town street performance. The 3.08:1 gear is great for the highway and gas mileage, but won't put you back in the seat very hard. I assume you have a M20 Muncie which is a wide ratio. If so, that helps with a tall gear (i.e., 3.36 to 3.08). If you get down to the 3.73 and 3.54 gears, you will get much better off of the line performance with a sacrifice in milage. May want to change to a M21 Muncie. We put 3.73s in the wife's 67 Camaro with a pretty much stock 350 / 300 hp and a M22 Muncie. She ran 25" tall slicks. It would run in the high 13 second bracket in the 1/4 mile. (She Granny shifts) Gearing is a large part of acceleration performance. Not a wheel puller, but she also drove it to the grocery store. A five speed is always a good way to get performance, especially on the street.

A good hydraulic cam like the Comp 268H will give you good performance in a 10:1 compression engine. I always liked the GM 300 hp cam for a street car, and might go up to the 325 hp hydraulics. Just me. Put some good valve springs on the heads. You don't need the super hard springs unless you want to spin over 5500 rpm. Chevy long slot rockers are good insurance and they are cheap. Roller Rockers are a PIA and not really built for longevity. I do like using Poly-locks and good pushrods.

Once again, the intake system (I use dual plane for the street) and carb should be matched to the cam and heads. A good tuner for a QuadraJet (I am not one of those guys) can make it run better than most other cars on the small block Chevy. About 600 to 650 cfm is all you will need for a slightly modified small block (say up to 350 hp) and will carry you to 6000 rpm (which I do not recommend turning it that high with stock rotating assembly). You can't beat electronic fuel injection if you have the $$$$s.

1.94 intakes and 1.60 exhausts will do the job, you don't need 2.02s except for bragging rights. A good 3 angle valve job and matching the intake and exhaust ports to the manifolds will help. Unless you have to or plan on driving the car 150,000 miles, I would not have hardened seats put in the heads. Some heads will take the seats, others can be ruined if the machinist breaches the water jacket on some thin castings. Then you have boat anchors (I'll give you a set if you want them). I have over 100,000 on the set in our 68 without hardened seats.

Believe it or not, a good electronic ignition and MSD 6A box will help wake up any car. I love mine. The distributor must be in good shape (bushings) and mechanical weights matched to the torque curve of the cam and compromised if needed to avoid detonation of cheap gas. The vacuum advance is for cruising and will help milage. As I stated, I would tune the engine to 34 to 36 degrees total mechanical advance, then worry about initial advance. Once again, I am still learning at 68 so am not an expert.

I ran a full tilt 302 in a full bodied (3200#) 71 Vega with a Nash 5 speed and 4.11 gears. It would pull the wheels easily, but wasn't much of a street cruiser at 6 mpg. Ran low 12s and high 11s at the track. Broke almost everything once or more. But, it would put you back in the seat. Wasn't much fun on long cruises.

If you are looking for more of a race car than a mild street car, go with more cubes (big block) and get out your billfold.

Good luck with your toy. I still have 3.
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post #10 of 15 (permalink) Old Jan 2nd, 17, 04:11 PM
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Re: 327 Thumpr- Tune or Trash?

Lotta cam for small cubes and compression.

Something in the 22X/22X on a 110-112 LSA 106-108 ICL with a mid .4XX lift would be soooo much better. (up to a 280H cam MAX)

Not saying you can't make it work, but gonna be VERY temperamental.

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post #11 of 15 (permalink) Old Jan 2nd, 17, 05:43 PM
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Re: 327 Thumpr- Tune or Trash?

With stock intake, exhaust,ignition, small valve heads don't expect much. There's a reason people change all that. It's to increase the amount of fuel and air in and out of the motor.
My first 327 was a stock short block '65 Vette motor. I put it together with extreme attention to detail and kept it tuned sharp. I'm guessing 9 or 10 to one flat tops. Large valve double humps, headers, dual plane intake, big Holley, 460 lift cam, Mallory dual point distributor. It also had stock 3.08 gears and a four speed and it screamed. It was pretty decent. Everything worked together real well.
Like the man said, you gotta figure out what you want first. Stock or figure out a well fitting combination.
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post #12 of 15 (permalink) Old Jan 3rd, 17, 12:07 AM
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Re: 327 Thumpr- Tune or Trash?

I replaced a Comp Cams XE268 camshaft in my 327 with a Sealed Power CS-179R and what a difference it made. The CS-179R is a copy of the L79 350 hp 327 cam from the 60's. Sure it's old technology but my 327 loves this grind. It has more lift and duration than a stock cam and pulls great through 5500 RPM. I occasionally take it to 6500. The 114 lobe separation angle provides a good idle with great manifold vacuum. It's a blast to wind it out occasionally, but still has good street manners in heavy traffic.
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post #13 of 15 (permalink) Old Jan 3rd, 17, 06:01 AM
R66
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Re: 327 Thumpr- Tune or Trash?

Zora and the GM engineers spent a lot of time and money on high HP cams for the 327 back in the day. Not that aftermarket cams aren't good, but I have had a lot better luck with GM designs. Your 1.50 exhaust valves and stock exhaust manifolds are going to limit your top end RPM, but should be fine for street use.

Back in the day, some aftermarket cams had to be degreed to get the best performance out of them. They were not all perfectly timed when ground. I assume this has been improved over the last 1/2 century, but wouldn't hurt to check.

Your 30 degrees initial advance is way too much initial timing in my opinion. You will never get it to idle. Set the initial at 4 to 6 degrees and then run the engine to set the total mechanical advance to 34 degrees (36 degrees max.). A good Chevrolet manual on the 68 327 will give you the guidance for tuning the engine.

The "MOUSE" (SB Chevy) is a great little power plant and will live forever with the right care.
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post #14 of 15 (permalink) Old Jan 3rd, 17, 07:39 AM
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Re: 327 Thumpr- Tune or Trash?

If you want stop light to stop light fun then the problem isn't so much the cam as the rest of the engine and drivetrain.
I looked up the cam. It wants to run between 2000 and 6100 rpms, and has a rough idle. At a bare minimum, you need 3.55 gears. Your 3.08 gears are killing that cam.
The Comp Cams website says you need 9:1 compression. I would have thought that cam would have wanted at least 10:1 compression. I would also think you would need 2.02 heads, not 1.94's. Vortec heads built for high lift would work well too. That cam has a large overlap, so you will probably need a vacuum tank if you want consistent power brakes. You also need headers and true dual exhaust. Your stock exhaust manifolds are strangling your engine. Likewise, the intake isn't helping things. I'm not a fan of them, but a single plane intake or a dual plane with the divider trimmed down would probably help that cam find it's sweet spot.

For what you want, I'd suggest a new 268 cam and lifters, and go to a 3.42 or 3.55 gear. That should work nicely without changing any other part of your drivetrain.
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post #15 of 15 (permalink) Old Jan 3rd, 17, 08:00 AM
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Re: 327 Thumpr- Tune or Trash?

Quote:
Originally Posted by R66 View Post
Zora and the GM engineers spent a lot of time and money on high HP cams for the 327 back in the day. Not that aftermarket cams aren't good, but I have had a lot better luck with GM designs. Your 1.50 exhaust valves and stock exhaust manifolds are going to limit your top end RPM, but should be fine for street use.

Back in the day, some aftermarket cams had to be degreed to get the best performance out of them. They were not all perfectly timed when ground. I assume this has been improved over the last 1/2 century, but wouldn't hurt to check.

Your 30 degrees initial advance is way too much initial timing in my opinion. You will never get it to idle. Set the initial at 4 to 6 degrees and then run the engine to set the total mechanical advance to 34 degrees (36 degrees max.). A good Chevrolet manual on the 68 327 will give you the guidance for tuning the engine.

The "MOUSE" (SB Chevy) is a great little power plant and will live forever with the right care.

It's fairly common now days with a new rowdy cam to idle best with 15 to 25 initial timing with a total maybe around 36. mild cam around 8~10-12 initial. Easier to get the idle RPM where you want it without opening the throttle blades too much and exposing the transfer slot or idling off the main circuit,then you have watering eyes, fouled plugs,way too rich at idle...
Lots more dyno information to tune with now days than 40 years ago.

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