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Old 05-23-2013, 07:58 PM
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chevy 355 low compression question?

OK guys I am new here and looking for help. I have a Chevy 355 that is in my dirt track car. We were having problems with it blowing blue flames out exhaust and popping on deceleration. We thought it was header leak and lean mixture. We tried everything even timing. Well long story short we found the problem in the distributor the mechanical advance lockout was allowing the distributor to retard to about 17 degrees btdc when it should of been at 34. We ran two nights this way. I have now checked compression and there all at 100 except for 5 and 7 they are at 95. I didn't check the compression when I first got the engine I kick myself in the but now. But to me these numbers sound low I was thinking I should be up in the 150s+? the motor is built to be a 9:1 compression motor. Here are some of the specs Gm 3.480 crank had been sent out and counter weights cut and nitride coated gm 350 block prepped 0 deck piston height is + 002 out 4.040 bore . Scat 2 -1cr5700 rod pushrods lingth 7.950 piston -7cc
Felpro 1142 .041 thick 4.100 bore head gasket clevite ms-909p-10 main bearings rods torque 45 lbs .0047-.005 stretch mains 70 ft lbmain .0055 thrust
Rings total seal cr3690-45.
Rod bearings clevite cb-663 p-10 .23 Side clearance bullet cam Chs 278/286-064f at .50 in 248 exh 256
1.5 rocker ratio exh 532 lift in 353 cam is in at 102 heads are done with stainless swirl polish valves true 76 cc lifters are bullet with ls9925spl with oil hole

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Old 05-23-2013, 08:24 PM
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whats the question? are you asking if the engine is damaged? Do a leak down test.Or are you thinking the cam timing is bleeding off CR?
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Old 05-23-2013, 08:36 PM
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Quote:
Originally Posted by Duckhnter83 View Post
OK guys I am new here and looking for help. I have a Chevy 355 that is in my dirt track car. We were having problems with it blowing blue flames out exhaust and popping on deceleration. We thought it was header leak and lean mixture. We tried everything even timing. Well long story short we found the problem in the distributor the mechanical advance lockout was allowing the distributor to retard to about 17 degrees btdc when it should of been at 34. We ran two nights this way. I have now checked compression and there all at 100 except for 5 and 7 they are at 95. I didn't check the compression when I first got the engine I kick myself in the but now. But to me these numbers sound low I was thinking I should be up in the 150s+? the motor is built to be a 9:1 compression motor. Here are some of the specs Gm 3.480 crank had been sent out and counter weights cut and nitride coated gm 350 block prepped 0 deck piston height is + 002 out 4.040 bore . Scat 2 -1cr5700 rod pushrods lingth 7.950 piston -7cc
Felpro 1142 .041 thick 4.100 bore head gasket clevite ms-909p-10 main bearings rods torque 45 lbs .0047-.005 stretch mains 70 ft lbmain .0055 thrust
Rings total seal cr3690-45.
Rod bearings clevite cb-663 p-10 .23 Side clearance bullet cam Chs 278/286-064f at .50 in 248 exh 256
1.5 rocker ratio exh 532 lift in 353 cam is in at 102 heads are done with stainless swirl polish valves true 76 cc lifters are bullet with ls9925spl with oil hole
You don't have enough static compression for the camshaft in your engine. That will cause low cylinder pressure. Either get your SCR up to around 11:1 or your camshaft duration down to around 208-216 deg @.050"

Read this

http://www.crankshaftcoalition.com/w..._compatibility



Duke
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Old 05-23-2013, 08:50 PM
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Quote:
Originally Posted by Duckhnter83 View Post
I have now checked compression and there all at 100 except for 5 and 7 they are at 95. I didn't check the compression when I first got the engine I kick myself in the but now. But to me these numbers sound low I was thinking I should be up in the 150s+? the motor is built to be a 9:1 compression motor.

Bullet cam Chs 278/286-064f at .50 in 248 exh 256
1.5 rocker ratio exh 532 lift in 353 cam is in at 102
I'm uncertain as to how you circle track guys build a motor, but what I hear you saying is that you have taken a camshaft that should have been screwed into a 11.00:1 to 12.50:1 static compression ratio motor according to the 248/256 @0.050" tappet lift figures you listed, but you have, instead, screwed it into a motor that has only 9.0:1 static compression ratio. Is that correct? And you're wondering why the cranking compression is low?

If you install a camshaft that is compatible with the static compression ratio, you can expect the cranking compression to be in the 150 range. If you install a camshaft that is incompatible, as you have done, then you can expect 100 cranking compression.

I figured your Dynamic Compression Ratio at 7.4:1 and I'd be surprised if you can get out of your own way on the track.

Duke was posting while I was typing, but both of us think alike.
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Old 05-23-2013, 09:53 PM
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Yes my question is does the compression sound too low?

So what you guys are saying is that I should probably be seeing low compression numbers because of the cam being to big for the motor? I was afraid I had hurt the motor running it while the timing was retarding and the engine was popping and shooting flames out the exhaust maybe rings or something because the motor does have a fair amount of blow by but it has always had some. I am no engine builder these are the specs I was given on the engine and the engine was supposedly built by a reputable race shop. And the motor when it was running right It was one of the fastest cars at the track.
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Old 05-23-2013, 10:12 PM
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how long since it was built?
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Old 05-23-2013, 10:23 PM
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It has maybe 10 races on it. At an average of 30 laps a race on a 1/2 mile track.
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Old 05-23-2013, 10:42 PM
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If the performance has dropped then the engine is hurt.read my first post
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Old 05-23-2013, 10:46 PM
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With my setup could the cam timing bleed off compression test?

I need a good quality leak down tester the one I have is one of those low pressure ones I think it only uses 15 psi. Although with that I do notice the air coming from the valve cover breathers.
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Old 05-23-2013, 10:51 PM
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Quote:
Originally Posted by Duckhnter83 View Post
Yes my question is does the compression sound too low?
If the specs on the engine that you quoted in your original post are correct, you have a fundamental problem, a mismatch with your SCR and your camshaft selection.
Read my first post, did you check the link?



Duke
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Old 05-23-2013, 10:52 PM
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most race car crews use a leak down tester both on new engines and after every race.If competition is not that toug then just freshen the engine when the driver whines about not enough power.
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Old 05-24-2013, 08:28 AM
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What RPM are you turning? You can likely drop 10-20 degrees off that can and pick up speed depending on the rpm you're turning.

Buzzing an engine high with crappy heads does require more duration but you're likely way over that.
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Old 05-24-2013, 08:30 AM
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A common cheap cam for a circle track engine like yours is the prc 108 cam, still a little big but getting better. The 108 cam is good for a max rpm of 6500 or so, lower redline would mean lower duration.
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Old 05-24-2013, 08:40 AM
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I'm running 6500 rpm. When the car was running with the retarded timing the car seemed to blubber or really hesitate in the high rpms. I would assume because it wasn't burning properly in the cylinders and the combustion was happening as the piston was on the down stroke.
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Old 05-24-2013, 08:47 AM
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Running the timing way retarded will cause the engine to run HOT. have you done a leak down test yet?Not looking for you to make more power before you establish the engines health
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