fwayne

I took my car to Performance Technical Center (Elk Grove Village, IL) today for a local Corvette "Dyno Day". They're nice guys and, even though I was the only one with a C3, they ran it two runs on their Dynojet.

The motor in my car is a new (~1,500 miles) GM Fast Burn 385 with the optional Hot Cam. The transmission is a fresh TH-700R4 from Bow Tie Overdrives. When the new motor and trans went in, the differential was replaced with a new 3.70:1 gear. The induction is a Holley Street Avenger 670 cfm carburetor. The exhaust starts with Hooker Super Comp headers and goes through 2.5" dual pipes to Flowmaster 50s.

The results: 212.9 hp @ 5,600 rpm, 233 ft-lbs torque @ 3,450 rpm.

That's the dissapointing part. I expected closer to 300 hp SAE net. Below is an averaged graph of the two runs, which plotted very close to one another.



The weird part is the "bump" from 4,800 to 5,300 rpm. Why does the torque drop off, then jump back up?

The air-fuel ratio stayed between 12.5:1 to 13.2:1 throughout the entire sample range (3,400 - 5,900 rpm) for both runs.

Has anyone else dynoed their FB385 with an automatic? What were your results?

Anyone with any ideas or suggestions? Should I just put a big muffler on my Civic and sell the 'Vette? :crazy:

45ACP

What are the specs on the cam?

What intake are you using?

fwayne

The cam duration at 0.050": 218 degrees intake, 228 degrees exhaust. Lift at valve is 0.525" for both. Lobe angle is 112 degrees.

The intake manifold: GM 12366573, aluminum, dual plane.

MotorHead

Take it to a tuner, can't suggest anything, would be guessing but a pro will have it at 300 HP in no time, something is seriously out of whack there.

Well I will give it a try what exhaust setup do you have ?

1) Check to see if you vacuum advance is opening the secondaries.

2) No vacuum leaks around carb, manifold, lines etc.

3)Is your distributor setup for 34-36 degrees advance over 3000RPM ?

There are more but that's a start

Smokehouse69

Things don't look right, other guys here on the Forum have dynoed straight ZZ-4s and gotten quite a bit more rwhp than your car showed. The only things I can think that would limit your perfomance would be wrong timing and advace curve or your carb isn't opening completely. Both things can really limit your performance and horsepower.

fwayne

The exhaust is nothing fancy; just Hooker Super Comp headers, a 3" to 2.5" reducer, 2.5" pipes equivalent to a big block exhaust on a 1974 vette, Flowmater 50-series mufflers. Loud and stinky. That's it.

I've checked for leaks in the hoses a couple of months ago, but didn't find any. My distributor has Lars' curve kit in it. It has 20 degrees mechanical advance at 3,000 rpm and peaks to 22 soon after. Basic advance is set to 14 degrees. I've tried it set at 10 degrees (32 total, the recommended max. for Fast Burn heads) but I don't notice much difference.

The carburetor secondary opening is one of the possible problems that came to mind, but I don't know how to check it. I doubt if any of my friends will volunteer to hang on to the valve covers and stare down the barrels while I punch the throttle on the highway. Maybe. :D

79Shark

Did you also install the 1.6 rockers with the hot cam, I beleive that is how they rated the cam, also the zz430 had a 750 cfm carburetor, which is basically the same motor that you have.

fwayne

Yup, 1.6:1 roller rockers.

45ACP

Well, just for the hell of it I did a conservative run of your combination in DD2000.

It comes up with just under 420hp, which I believe is flywheel hp. Assuming 20% loss for the trip out back gets you to 336rwhp.

Man, it looks like you are down over 100hp from where you should be!

GrandSportC3

That seems very low....

fwayne

GM claims 425 flywheel hp for this combo. I would have been satisfied with about 300, a drivetrain drain of 30%.

PRNDL

But how does it feel? Have you taken it to the dragstrip? If you can run low 13s or better the dyno is bs. I am not exactly sure how those things work but for example if your torque converter isnt locking up I dont think you will get an accurate read. MJ

PROSOUTH

fwayne, I 'm not on top of this but there is a 385 Fastburn sitting in my garage floor waiting until I finish the el Camino to be installed along with my TKO 5 speed and 308 gear.

One thing I noticed right off is that on page 5 of sheet nine FB385 E$ngine Specifications it states "A 750 CFM Holley Carburator ....is recomended for the 385FB engine."

Now combine the problem of 750 - 670 = 80cfm negative and then adding the fact that the LT1 Hot Cam and the 1:6 Roller Rockers you probably installed all of this raising the possible horsepower to 430 Hp with proper fuel availability that you are running out of air and fuel needed to properly supply the engine.

I honestly think you need an 850 Holley to start with. JIM :yesnod:

fwayne

It feels like it could be faster. :D The other cars tested today scored between around 300hp to over 600hp. I haven't taken it to the dragstrip yet, mainly because I'm running on old tires. For what it's worth, the torque converter lock up does engage.

GrandSportC3

I agree that a 670 cfm Holley will not deliver enough fuel... I also would never use a vacuum secondaries carb on such an engine...
Another reason not to get enough fuel would be the fuel pump.. I put a $50 Carter 120 GPH fuel pump on the new engine and it works great!!!
I have a Holley 80528 carb (750 HP Double Pumper) and it's enough to feed my 465+ HP...so it should be enough for this engine...

:cheers:

Olivier

fwayne

I agree that I'd probably lose some power as I approach 6,000 rpm because the carburetor is a 670, but that doesn't explain why I have only 170hp at 4,000 rpm. If the secondaries are opening late, it could explain the low figures AND the hop at 4,800 rpm.

fwayne

I think that if fuel pressure was a problem, I'd see a rise in the air-fuel ratio. As it is, the mixture is good through six grand.

PROSOUTH

Well, I see that you didn't reply to my post and I guess you have your reasons. But I see no other reason for your combination not to work.

My 2 cents whether you see it or not says that your Carburetor is the problem. I don't care if you believe me or not and just because you think it only runs out of breath at the far end, it ain't so. It's like saying that because I stick one finger in my nose and don't try to run a race I will get plenty of air.

A carb has a venturi that is adjusted to size and the air fuel ratio is balanced as such. I'm not on top of this, but I have identified your problem whether you believe me or not and I may be wrong. But if you read the book that came with the motor it says exactly what I stated, A 750 Holley is required for the std 385FB engine. And then you up it to 430 HP requirements and still think you have the right carb? You should get great gas milage at least. JIM :confused:


[Modified by PROSOUTH, 12:44 AM 9/29/2002]

fwayne

I did respond to your post and I do appreciate your help.

However, with one finger in my nose, I will get plenty of air as long as I don't try to run a race. Your colorful analogy is exactly on point.

An engine can only draw as much air as the product of its displacement and speed. Unless that volume does not approach the flow rating of the carburetor, the flow rating of the carburetor cannot be the power-limiting factor. A big block that makes 500hp at 6,000 rpm can produce just as much power at 3,000 rpm with a 500 cfm carburetor as with a 1,000 cfm carburetor.

My air-fuel ratio is fine. My car's problem is that it doesn't produce enough power even at speeds below those which can be attributed to under-carburation.

BSeery

My stock ZZ4 with a 700R4 behind it pulled 220 HP on the dyno.

That is running a Q-Jet with an "emissions tune", stock exhaust manifolds, and stock 2 - 1 - 2 exhaust.

I agree that a bigger carb is needed, a 750 minimum. I would stay with a QJet or Edelbrock as they are more of a demand sensing carb. A mechanical secondary carb is more apt to drown it or cause other tunning problems.
Also, with the smaller primaries in the QJet you get better lower RPM punch and better driveability.

I am thinking of going fuel injected once I get my exhaust changed to something better.