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Old 09-17-2012, 01:51 PM
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big cam idle blues

OK, just tired of breathing pig rich fumes at idle (or staying on the pedal while staging), both suck. Timing locked at 38, SBC 355, 283/296 @ .050, 108 lobe center installed at 105 centerline, what's the secret? I see other SBC cars with pretty big cams and they somehow manage it. I've tried different things, drilling butterflies (works with smaller cams), using a pcv port on the carb for even more idle air, IAB's, IFR's, etc..

Before I get slammed for the big cam I just want to say that I like the sound of 9,000 rpm's, but it will never idle right with a small cubed SBC.

Please let me know what's the trick or give me some examples of long duration cams that still idle on their own. (automatic cars of course)

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Old 09-17-2012, 02:04 PM
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With a cam that big the engine will have a higher idle speed.
Especially in a small CID engine. 1200+rpm.
The staging rpm will be higher too.
You need a full race 8" converter 5500++ stall.

Just for grins information what happens if when you advance the timing further, at idle.
It may need up to 50deg timing at idle to burn the fuel fast at idle.
Does this clean up the exhaust odour?

full details of the engine, carb model IAB IFR etc. idle manifold vacuum... power valve rating. Did you bench test your PV?
carb throttle position at idle.
If the engine has little no vacuum at idle (less than 2" vac) you may need to eliminate the power valve and jet up and retune the idle circuit.

How much bypass air are you adding? PCV or drilled throttles.
Remove the carb and flip it over. How much transfer slot is showing at idle? Pri ? sec?
does more idle timing allow better throttle position at idle?
Does blocking or partially blocking the idle air bleeds with your 4 fingers change the idle speed as the engine is running?

Full race cams can be tricky.

Last edited by F-BIRD'88; 09-17-2012 at 02:13 PM.
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Old 09-17-2012, 02:38 PM
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With a cam that big the engine will have a higher idle speed.
Especially in a small CID engine. 1500+rpm.
The staging rpm will be higher too.
You need a full race 8" converter 5500++ stall.
if you are not leaving the line at 6000+ rpm with this cam
its not going to go.

Just for grins information what happens if when you advance the timing further, at idle.
It may need up to 50deg timing at idle to burn the fuel fast at idle.
Does this clean up the exhaust odour?

full details of the engine, carb model IAB IFR etc. idle manifold vacuum... power valve rating. Did you bench test your PV?
carb throttle position at idle.
If the engine has little no vacuum at idle (less than 2" vac) you may need to eliminate the power valve and jet up and retune the idle circuit.

How much bypass air are you adding? PCV or drilled throttles.
Remove the carb and flip it over. How much transfer slot is showing at idle? Pri ? sec?
does more idle timing allow better throttle position at idle?
Does blocking or partially blocking the idle air bleeds with your 4 fingers change the idle speed as the engine is running?
Look down the carb while idling (with a mirror) are the boosters dripping/flowing fuel at idle?
Idle RPM??.

Full race cams can be tricky.

Installing a exhaust system for testing will help you get it dialed in. Will allow the use of a AFR gauge.
Will need approx 30" of tail pipe after the heated type o2 sensor to read at idle.
All vacuum leaks, carb internal cross leaks and ignition faults must be eliminated, first .
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Old 09-17-2012, 02:48 PM
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It'll never idle well. you can get it to idle better though with a properly matched exhaust- it'll help cut down on reversion some but it'll always be there at idle with that huge cam. reversion is what's killing you.

a well developed mpfi system can also help.
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Old 09-17-2012, 04:00 PM
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what heads and valves?injection might be the only way.auto or stick?what size carb are you using?
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Old 09-17-2012, 04:07 PM
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This motor will idle great at 1,400 rpms on the stand but once you put the converter and vacuum pump on it the idle will go away. It will idle ok at 50+ timing and this is one thing I haven't tried yet, I suppose I could keep one of the retard stages from my 4 stage retard box (1 stage retard during the run and none at idle) but that leaves it at the mercy of the box. No power valves, currently has a 950 HP on it with a 4150 to 4500 adapter sitting on a dart 4500 intake, when I used the pcv port I put a multi turn flow valve on the hose end but it would still only help slightly with valve fully open. Headers are stepped 2" to 2 1/8" to 2 1/4" with 3 1/2" collecter. Also I'm running a Hughes 4000 stall with a 150 shot of happy stuff to get off the line. It seems that no matter which adjustment you make to get a decent idle (IFR,IAB,4-corner), it still is rich one way or the other. Right now, the transfer slots are good and you have to keep it running with constant pedal action. no room for exhaust (tube chassis Vega)

Thanks for the replies and keep it comming, I'm not busy and you need the post counts,
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Old 09-17-2012, 04:37 PM
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here is the latest adition to the 355, a set of Crower stainless shaft rockers with needle bearing tips, you'd show them off too if you paid over $1,400 for a set of rocker arms.
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Old 09-17-2012, 04:42 PM
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with those big tube headers,big cam and big intake,I bet the signal is quite weak. 1,400 might be where it idles.Can you switch/block the vacuum pump from **** pit?
did you build those headers?
maybe use an idle stop solenoid that engages when car is in gear
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Old 09-17-2012, 04:49 PM
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well the converter is way too tight for this cam.
Me and other people that use full race 280++duration@.050 cams like this launch the car at 6500-7000rpm and
stage at 2500+rpm.

if you want to run nitrous, the cam is all wrong anyways.
A full race serious nitrous cam will by its very nature idle a lot nicer.
You will go faster too.

You have been posting on this since 2009. Are you sure you don't have a simple fault like ignition
or a bent valve or manifold vacuum leak?

try the increased idle timing. Tell me what it likes for timing at idle.
Its takes a lot of idle timing to over come the effect of the exhaust dilution at idle.
Cam overlap. A nitrous cam has a lot less overlap.
Now you can make use of that 150hp shot all the way down the track.
if you want to use a 4000 stall get the cam duration less than 267@.050"

if you can afford $1400 rockers you can afford to cam the motor correctly and or get the right racing torque converter with the right stall speed, for the job.
And some dyno testing. Why are you idling it on a stand?

This motor needs a tunnel ram unless you have something against going fast.

Last edited by F-BIRD'88; 09-17-2012 at 05:03 PM.
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Old 09-17-2012, 04:50 PM
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what is the inside of an airplane called? the part where the pilot sits?lol,sorry for setting off the language blocker
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Old 09-17-2012, 04:56 PM
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$1400 rocker arms. Your problem is you have too much money.
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Old 09-17-2012, 05:14 PM
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I reckon 1400rpm idle is GOOD with a cam that long..
I ran a sig erson cam back in the 70's with 274/286 @50 in a 383ci idle at 1500 -lovely with 5000stall. Would want nothin less.
4000 is toooo small ..... Listen to EVERYTHING that F'BIRD-88 says!

Duke
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Old 09-17-2012, 05:15 PM
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correct on all counts, guilty as charged, this a different 355 but still using the same cam. I put it in, then I complain about it, it's a vicious cycle, but man does it scream! It's definitely not a nitrous cam and to make things worse, the tire is a 33-10.5w with a 4:10 gear and Jimmy Arabie glide. I got this roller (tube chassis, strut front, 20 point cage, aluminum center chunk, glide etc.) very cheap so I threw this motor in it. This cam has never been able to idle without choking you, even with a loose converter. It's a fresh build and pulls fine, just won't idle. I know that before putting in the multi stage retard, I bumped it up some and when I checked it it was at 55 deg and idled fine. Obviously I backed it down to 39 deg. and that's where she sits now.
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Old 09-17-2012, 05:26 PM
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One trick I use on full race motors with a very big cams is to idle, stage and launch the car with a lot more timing than what it needs for max horsepower @WOT.
Find the sweet spot to bring the timing retard back after the launch. 1 foot out 5 feet out, 1mili sec , 5 mili sec right at nitrous engagement or.... etc etc etc.
The engine and car will show you what it likes. Just remember the retard box effects the distributor cap rotor phasing. Something else you can look at.

The stinky exhaust at idle is from slow late combustion at idle that is still burning when the exhaust valve is opening.
The extra spark timing at idle corrects the fuel burn timing at idle. now it is burning in the chamber instead of in the exhaust pipe.
If it needs 55deg to idle, and stage clean give it 55deg.
The engine will tell you what it likes
The multi retard box is not just for high speed-high gear retard or nitrous retard.
Getting the idle timing correct is where you start. Then you can work on idle air bypass amount, and idle air fuel mix etc etc.

Some times the polarity of the mag pick up wire on the distributor has to be reversed for some spark boxes to work right.
Proper ignition and engine ground is critical.

In my opinion the engine should be idling at 1500+++rpm.
a slow idle speed is not your friend on a full race motor.
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Old 09-17-2012, 05:29 PM
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What is the ET/,,, MPH 60ft and car weight with driver.

not near enough gear or converter just off hand.
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