[RONNYinWAXHAW]

Thank goodness for these posts. I searched "SBC Thumper Cam" and read all the responses in those posts. There was a lot of good info there. I too am considering a 383 in a one-piece rear seal block. I was going to use flat top hypereutectic pistons with the coated skirt and stock rods in a daily driven pick-up (in my project journal). I was looking into a set of heads with 180cc intake runners and 2.02/1.60 valves with 64cc chambers. This block is set up for a roller cam (bosses for the spider in the valley, cam retainer drilled and tapped) so I was looking at the Extreme Energy hydraulic roller that specs: 270/276 .495/.502 with the powerband being 1600-5400 rpm. I plan roller rockers, RPM Air-Gap intake and the carb is undetermined. A 2000 lock-up convertor in a 700R4 and 3.42 or 3.73 rear gears. Any suggestions?

[wwilliams181]

I honestly think you will be fine with this cam. The problem most are talking about is that the Rpm range is narrowed and the loss of cylinder pressure due to the added overlap. That cam already has 5 deg. built in. I would install it straight up which would be an intake centerline of 102. If you ar a little high on compression.. this cam will work to your advantage when using pump gas. The upside to the sound is the throttle response. The head choice you are looking at will be fine. If you havent bought pistons yet.. i would tell you that this is one place you should spend your money.. i buy almost the lightest thing i can get. You will really gain here.

[Greg T]

I have the info....first hand. I have 388 with Etec 200 heads and ran on a chassis dyno. Round 1 is a big m/thumper and round 2 was an Edelbrock 2201 . Round one on the chassis dyno ran 347 ponies and 379 twist.

Round 2 came in at 380 ponies and 397 twist. All else was the same, except for a little change in tuning to match each came. The 2201, rather old school cam, gained 33 ponies and nearly 20 lb/ft, and at about 800 rpm lower than the Thu per. Also, much easier to tune and excellent street manners. Also, it idles nicely at 850 rpm. This is no dyno sim. These are real numbers.

[techinspector1]

Originally Posted by wwilliams181

and the loss of cylinder pressure due to the added overlap.

Let me help you out here in case there are any newbies reading this and misunderstand what you have written.

There is little or no cylinder pressure at overlap. The piston is coming up to TDC on the exhaust stroke and has evacuated the cylinder of exhaust gases from the previous power stroke. The intake valve is just beginning to open. So, we have two valves open and cannot possibly have any cylinder pressure.

Cylinder pressure is determined by the static compression ratio of the motor combined with the intake valve closing point after bottom dead center.

[cdminter59]

Don, I checked on Comp Cams website where they have CamQuest a camshaft selection software with dyno results. I used United Engine & Machine (Keith Black) COMPRESSION RATIO CALCULATOR Static and Dynamic Compression Ratio
(Considers Cam Timing and Rod Ratio). I used the 75cc chamber from the AFR 195 heads and SRP flattop pistons with a piston head volume of +5. (#1) 299THR7 PN 12-602-8, 2500-6200, .569/.554 lift, 243/257 duration @ .050, LSA 107, ICL 102, CR 9.895, DCR 8.341, 359hp@5500 rpms, 369 lbs torque@4500 rpms. (#2) 12-443-8 XR294HR, 2800-6100, .540/.562 lift, 242/248 duration @ .050, LSA 110, ICL 106, CR 9.895, DCR 8.118, 390hp@5500 rpms, 413 lbs torque @ 4000 rpms. (#3) 12-433-8 XR288HR, 2500-6000, .520/.540 lift, 236/242 duration@ .050, LSA 110 ICL 106, CR 9.895, DCR 8.287, 394hp@5500 rpms, 426 lbs torque @ 4000 rpms. Send the thumpr back and swap for 12-433-8 XR288HR.

[cobalt327]

Originally Posted by wwilliams181

I honestly think you will be fine with this cam. The problem most are talking about is that the Rpm range is narrowed and the loss of cylinder pressure due to the added overlap.

While it's 100% correct that overlap alone doesn't cause a loss of cylinder pressure, a large amount of overlap implies that there will be more duration, all else being equal. An increase in duration in turn implies there will be a later-than-normal intake closing (IC) point. A late IC point will cause a loss of cylinder pressure, reflected by a lower dynamic compression ratio. So in a round about way, many times excessive overlap could be said to contribute to a loss of cylinder pressure- just not the way that some think it happens.

Overlap can also cause reversion at low rpm along w/a loss of vacuum. This contributes greatly to the 'lope' of a performance cam- which is really nothing more than the engine running like crap at low rpm.

As it is, cam companies have been stretching the envelope for a long time. It's only because of advances including CAD/CAM and in metallurgy that we can get away w/cams as intense as the Comp XE series.

Personally I'd like it if a cam could be ground w/o any lope and retain 20 in/Hg vacuum while retaining all the lift, duration and overlap needed for high rpm efficiency. But because of the limits on materials, valve spring pressure, lobe intensity, etc. this can't happen. Not talking about variable lift/duration/timing valvetrains here- I want a magical cam w/square lobes w/o needing any bells and whistles!

[techinspector1]

Originally Posted by cobalt327

I want a magical cam w/square lobes w/o needing any bells and whistles!

Us fellows used to sit around and bench-race this concept.