Comp 276hr in 383

[POPO1984]

Comp cams recommended me the 276hr for my 383 build. I see on there site a dyno sheet which shows this cam as making 373 hp and 427 tq in a 356 ci. motor. does anyone have a guestimate at what it should make in a 383. I will use the same set up exactly except i am gonna use vortec heads and a about the same cr. I asked the tech about the hp and tq rating but he said they dont give out those numbers. I plan on using just a little more stall than stock


heres the link http://www.compcams.com/Technical/Dy...6HR-10_001.asp

[au03z71]

I have a 383 with the XR276HR and Vortec heads. I'd like to know this too, but I think it's going to be around 425 hp/450 ft-lbs.

[POPO1984]

thats what I am thinking too

[Marv D]

Not exactly apples and apples, but.....
Scott Forte was a employee of a well known carburetor, cam and head manufacturer and we talked at length about the "420 HP TOTAL PACKAGE" They offered. I also asked him if they had ever put the package on a 383 and what improvements they saw. He was quick to explain that the 383 they built did NOT see any higher "PEAK HP" numbers than the 355 dyno mule that was built to test the package and make the 'advertised' numbers. They DID see improvements of as much as 30-35 pound ft of torque in the 3000 - 4500 range. With HP being nothing but a function of torque and rpm you can say the 383 did make more 'overall power'. But heads, cam and all had not offered any great benefit to 'PEAK HP' between the 355 and 383. With most smallblocks making a peak in the 6000-6500 range, and being pretty much all done by 7000, that's not surprising with a milder cam.

Myself I suspect that would be the very much case here with the 276HR in a 383 vs a like built 350. A hydraulic roller is a HEAVY lifter and RPM's above 5500 - 6000 are typically nothing but beating up on the valve train,, and torque falls so quickly that peak HP suffers (again, torque and HP are exactly the same at 5250rpm, so were just talking a function of TORQUE with a cam that is not going to be real happy above 6000). So especially with a hydraulic roller motor,, forget about any advertised HP numbers. (There usually bogus BS anyways) Put it in a real simulator like Engine Analyzer (calling Super73!!!! LOL) and you can get some pretty decent ideas of how much torque increase you will see. and just keep in mind,, that increase will be in a very DRIVABLE range.

[POPO1984]

Thanks for the info MarvD. so basically what you are saying is that there wont be much hp gain and that it will be just gains in the torque.

[Marv D]

That's a two edged sword. HP is just torque / 5250, times rpm. Torque is all a dyno can measure. HP is just a function of that torque and the rpm.

So.... a motor that makes 600 ft pounds of torque at 4000rpm is making a whopping 371 HP (not impressive is it)


Same 600 ft pounds at 5250 is 600HP (at 5250 HP and torque obviously are exactly the same) at 7000rpm 600 ft pounds is 800HP. See what I'm getting at. Just because a hydraulic roller motor is all done early in the rpm game,, it is STILL making buttloads of power.

A high reving small block making 420HP at 6500rpm is only making 340 ft pounds of torque.

Think of it this way. TORQUE accelerates the car. HP will make MPH in a heavy car.

As an example I rode in a buddys Powerstroke, chipped POS with propane injection, 1ton dually 4wd crew cab,, this animal had to weigh 7500 pounds and we had 4 guys in the truck. When he stabbed the throttle I about crapped my pants. He would DEFINATELY outrun my 3600 pound 12 second pickup until he ran out of gear / motor. Torque is KING on the street. Torque is the twisting effect on the drive shaft,, more of it AT ANY RPM is going to accelerate harder than slowly building RPM to get the same job done.

The 383 making more torque below 5250rpm is a more powerful motor. BUT because the 355 is making similar torque in a higher rpm range, the two could EASILY have the same PEAK HP. Even if the 383 had a buttloads more torque in a lower rpm range. Stroke will do that for you. More stroke = more torque. More stroke also means more piston speed, more friction loss,,,, more reasons to keep the RPM's down and accelerate away from the high winding little 355 that has to gain more rpm to make the same 'power'.

So Scott saying there was no PEAK HP improvement was VERY believable. But if the dyno were a race car,, the motor that had more torque (in a usable rpm range) would be at the finish line first.

Is that about clear as mud. I don't 'splain' things too well sometimes.

[Marv D]

I should also add,,,, most smallblocks, reguardless of 302, 355, 383 or 400 based motors will generally make their torque peak in the 4000 to 5000rpm range. Typically only higher end smallblocks making 650+HP will have torque peaks above 5250.

[POPO1984]

Clear as Mud. I think I got it now. thank you for the help.

[KQQL IT]

Ive got a 288hr in a 406 sbc . Its a riot to drive, ran a 12'' verter 2500ish stall. Big fun...

[KQQL IT]

^^^ also 9:7 to 1 dart 200's rpm air gap and a 850dp holley.