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SBC 355 Vortec puts out low HP on Dyno?

 
Bmd781
I love my Chevy Chevette!
Posts: 3
Joined: 11/07
Posted: 11/18/07 03:53 PM

Guys,
I hit the chassis dyno last week for the first time.  The motor is in a 1991 Camaro.  I built an engine very similar to some builds on here that gave 425 HP at the flywheel.  I got a whopping total of 238 HP/250 Ft.Lbs at the rear wheels.  Around 100 HP off of what I had thought I would have at the rear wheels.  Here is my component list, everything was new except block & crank:

SBC 350 bored .030 over
Compression Ratio is 9.53:1
Vortec Heads - Valve guides milled for lift clearance, Added Stiffer Springs.
Roller Rockers
Edelbrock RPM Air Gap Intake
GMPP Hydraulic Roller Cam - (More aggressive than GMPP Hot Cam) GM P/N: 12370847
Edelbrock Performer 750CFM Carb
SLP 1 3/4" Headers
HEI Distributor - Accel Coil

The dyno had AFR readouts, it was around 14.8 pretty much the whole run.  The car sounded excellent.  I'm stumped on this one, does my combo just suck or is something wrong.  I've read people getting 400+ with similar combos.  Desktop Dyno has it in the 450 HP range (I know this may be inaccurate in the first place)  Anyone have any suggestions.  I need to understand what is goin on.

Thanks,
Brad  

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bigcam406
My first time was in a Chevy
Posts: 206
Joined: 03/07
Posted: 11/18/07 11:08 PM

first thing i'd do is a leak down test,then a cranking compression test.i looked up the specs on the cam from the gm website,just gives the duration at .050",no lift or lobe seperation angle.sounds like u did your homework in building a good combination,but im thinking it might be something internal causing a loss in cylinder pressure.good luck Grin  

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whiteghost
I love my Chevy Chevette!
Posts: 3
Joined: 11/07
Posted: 11/19/07 11:43 AM

If your AFR was 14.8 across the whole WOT pull, you are waaaay too lean to make good power.  You'll want to shoot for an AFR target in the low 12s for WOT to make good power.  Have you adjusted the jetting at all on the carb?  What's your fuel system made of? Whats your initial and final timing set at? The answers to these questions will help us figure out whats going on.  

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Bmd781
I love my Chevy Chevette!
Posts: 3
Joined: 11/07
Posted: 11/19/07 04:52 PM

I have not adjusted the jetting on the carb yet.  This was the first time I've seen any data on the engine.  I've heard 14 or so was what AFR should be.  I thought that for any natural aspirated engine burning regular gasoline there is one single optimum AFR.  If this isn't true, how do I know what to shoot for?  Is low 12's a common known fact?  My fuel system is a mechanical fuel pump from GMPP.  It is a high performance high flow pump.  Also, I set my timing by feel.  I increased it slightly until I got a spark knock and then backed it off a hair.  I did the same thing swapping springs in the distributor.  I can't remember what my initial timing was but it was set with a light.  

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Bmd781
I love my Chevy Chevette!
Posts: 3
Joined: 11/07
Posted: 11/19/07 05:04 PM

The cam specs are:

Lift @ .05:  Intake - .539
            Exhaust - .558
Duration:    Intake - 234
            Exhaust - 242
Lobe Center Angle:  112
Intake Centerline:  107  

I called up the performance shop that dyno'd the car and talked to the guy.  He said I might be able to squeeze an extra 20hp or so with tuning.  I still kind of stumped, I thought this combo would do a lot better than that.  Am I smokin crack or is there power to be unleashed because of something being off.  

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bigcam406
My first time was in a Chevy
Posts: 206
Joined: 03/07
Posted: 11/19/07 09:00 PM

as far as the AFR is concerned,it should be in the high 12's to low 13:1 ratio at wide open throttle,set the timing with a light,not by ear,initial advance should be in the 10-12 degree range and limit the distributor advance to 24 degrees,totalling 34-36 degrees,all in by 2500,as with vortecs,you dont need alot of total timing as they have very efficient combustion chambers.once you get the timing curve in where the engine likes it,then play with the jetting on the carb.if everything else fails,do the compression cranking test and a leakdown test.hope this helps,good luck with it. Grin  

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stempak
V-6 Camaros rule!
Posts: 47
Joined: 11/08
Posted: 11/23/08 03:01 AM

Ya something sounds wrong, compression isn't very high and your header size/diameter's could be losing you power too I forget the size these 350's like might have been the torque steps, and porting of your heads or brodix's or something better than vortecs but something sounds way off maybe a lunati or comp 2  

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chaseturbo
V-6 Camaros rule!
Posts: 86
Joined: 09/08
Posted: 11/23/08 04:46 AM

What kind of chassis dyno did you run it on. I have had some very low numbers on a mustang dyno, and almost 100 more on a dynojet.  

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JW454
I have an SS396 tatoo
Posts: 345
Joined: 07/08
Posted: 11/23/08 06:16 AM

The comp ratio for a cam in the 235-245 range needs at least 10.5:1 to make optimum power. You can get away with your 9.5:1, but decent power wont be had till at least upper mid range when cylinder pressure doesn't have enough time to be lost. Also sounds like your tuning is far from optimum. How are the plugs reading after a wide open pull? Also substantial power could be lost through a less than perfect transmission. Standard or auto? A cam with your dur.@.050 will make it's best power around 5500+ rpm. Is your trans and gearing allowing the engine to run in this rpm. range? Too late, but best way to find your power distribution is to dyno engine alone and work out the bugs, then chassis dyno to find your losses in the drivetrain.  

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10second.racer
I love my Chevy Chevette!
Posts: 13
Joined: 11/08
Posted: 11/23/08 02:43 PM

IT SOUNDS LIKE YOU WANT YOUR CAR TO GO FASTER
WHEN YOU TAKE INTO CONSIDERATION ALL OF THESE FACTORS THERE IS IS ALOT MORE INVOLVED THAN JUST HORSEPOWER. EXAMPLE A 200 HP ENGINE WITH THE RIGHT GEARS COULD MAKE 400 REAR WHEEL HORSEPOWER SO YOUR DRIVE TRAIN IS WHAT EFFECTING YOUR CARS PERFORMANCE YOU CAN SPEND THOUSANDS ON A MOTOR AND STILL HAVE A CAR THAT IT SLOW.
ITS THE GEARS THAT WILL GET YOU THERE FAST!!

                                     RESPECTFULLY,
                                  THE 10 SECOND RACER  

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10second.racer
I love my Chevy Chevette!
Posts: 13
Joined: 11/08
Posted: 11/23/08 02:53 PM

ALSO, I BET THE CAR RUNS GREAT ON THE HIGHWAY? IF NOT, YOUR ENGINE'S THE CULPRIT!  

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SMOKESHOW
I mow my lawn and find Chevys
Posts: 1001
Joined: 09/08
Posted: 11/24/08 12:28 PM

It looks like you have a lot of cam, header, and, carb for your combo's compression.  I'd check into a 600cfm ish carb some 1 5/8 headers, definitely time the engine with a light, and maybe do a compression test. 14.8 sounds a bit lean as well, are you sure your carb is functioning properly?  How does the car "feel"?  

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SMOKESHOW
I mow my lawn and find Chevys
Posts: 1001
Joined: 09/08
Posted: 11/24/08 12:29 PM

OOPS here I am posting on year old thread again.  Who keeps waking the dead???  

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GibTG
I mow my lawn and find Chevys
Posts: 1985
Joined: 10/03
Posted: 11/24/08 03:18 PM

You're surely excused Mr. Belosi...

I'm not going to reprimand every newcomer for bring back ancient threads.  
~Gibs

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SMOKESHOW
I mow my lawn and find Chevys
Posts: 1001
Joined: 09/08
Posted: 11/25/08 07:31 AM

Thanks Grin  

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