Compression Check & Questions - Team Camaro Tech
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post #1 of 6 (permalink) Old Mar 11th, 00, 06:57 PM Thread Starter
Join Date: Feb 2000
Location: Cincinnati, Ohio
Posts: 9

Today i checked the compression on my 69 350.
Results: 1 195, 2 195, 3 180, 4 180 , 5 185, 6 190 , 7 200, 8 190
The compression ratio is supposed to be 12 to 1.
The cam card shows exaust lift is 617 at valve.
The car still not running right. Backfiring through carb when stomping the pedal. Just trying everything people are telling me. Is the cylinder pressure good?
Help me with this cam card - what is my duration? Card says:
Lift: intake @ cam 3994 at valve 599
exhaust @ cam 4114 at valve 617
Valve timing: intake open 50 closes 86
exhaust open 90 closes 54
Tappet lift of .020
050 Cam lift timing:intake open 35 closes 65
exhaust open 75 closes 33
Min Rpm 4500
max Rpm 7500
valve float 8500
What does all of this mean?
The motor has 205 heads with alot of speed shop receipts. Solid Crank. Performer intake.
Holly 750, Muncie 220, 12bolt 411
This is everything i know. Is the HP around 500? Any ideal or suggestions as to why its backfiring when stomping the pedal?
Im stripping the paint right now because im feed up with the mechanical problem right now.
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post #2 of 6 (permalink) Old Mar 12th, 00, 05:10 AM
Join Date: Nov 1999
Posts: 285

I hope this is a race car and not a street car. 12:1's and .600+ cam in a small block on the street equals NO torque, big bucks for racing gas and no fun. Sounds like she's running lean and/or timing problem.
70 RS/Z-28 is offline  
post #3 of 6 (permalink) Old Mar 12th, 00, 06:09 AM
Senior Tech
Join Date: Sep 1999
Location: Idaho
Posts: 219

When the backfire happens, is it just when you initially rev the engine, or does it continue to backfire when the rpm's are up there around 3000 or more? If it's just on initial rev, I would think timing's a little to far advanced or the power valve in the carb is gone. As a matter of fact, I'd almost put money on the power valve unless you have a power valve blowout preventer. The power valve is designed to dump fuel into the carb on initial rev up. But in earlier design holley carbs, if you backfired through the carb, the power valve was toast.

Now then, on the cam, at .50 your intake is 275 degrees and exhaust is 288 degrees duration.(my best guess with int. opening BTDC and closing ABDC, and the exh. opening BBDC and closing ATDC) This is definately not a street cam profile. You'll be lucky to idle around 1200rpm. If you have power brakes, you'll need a secondary vacuum source. One thing you didn't mention on your cam profile was the LSA (lobe seperation angle). I would assume that it would be around 102 degrees or so. I would bet the HP would come close to 500 especially if the heads have had any work done to them.

How do you plan on driving this car? This is definately a strip engine as your HP band is 4500 - 7500. My suggestion would be to check your overall timing (distributor+crank) and if that is good (11-12 at the crank and about 24-26 at the distributor) then I'd look at the power valve.

Good Luck

67 Camaro LS6 454/TH400/12bolt 3.73
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post #4 of 6 (permalink) Old Mar 12th, 00, 04:43 PM Thread Starter
Join Date: Feb 2000
Location: Cincinnati, Ohio
Posts: 9

The carb is new out of the box. I hope the power valve isn't bad. It revs great. starts great, drives good when slowly accelerating, but if you give it alot of pedal at once is sputters throught the carb. This weekend with my 2 teenage boys help we got about 60% of the paint stripped. I have an appointment this saturday to have someone look at it. I would love to figure it out before then because he is not cheap. I bought the car to use as a street rod. I have 4 cars so i won't be driving it every day. I would like to take it to the strip and have some fun also.
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post #5 of 6 (permalink) Old Mar 13th, 00, 05:42 AM
Join Date: Nov 1999
Posts: 285

A stumble when the go pedal is mashed is often attributable to a lean condition. ie. stomping on the pedal allows a lot of air into the engine and the accelerator pump squirts in extra gas to cover "the hole". I'd check that first. Suggest you get a basic carb tuning book as well. These usually aren't that difficult to figure out. Regarding your power valve, often one backfire through the carb is sometimes all it takes to blow it out. Good luck!
70 RS/Z-28 is offline  
post #6 of 6 (permalink) Old Mar 13th, 00, 06:05 AM
Senior Tech
Join Date: Jan 2000
Location: Chickasha, OK
Posts: 105

Is it a flat tappet cam? If it is you might want to check the exhaust lift. When an exhaust lobe goes down, it doesn't let the combustion pressure escape, and when on the throttle, this pressure will be released into the intake tract, sometimes causing a backfire. Anyway, with that much cam, and probably a HUGE spring, it wouldn't hurt to check. It's happened to me. SBC are horrible about knocking lobes off cams. Just my 2 cents. Take care.

69 SSRS Frame-off Resto
81 Z-28 377ci Drag Car
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