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396chevelle 06-05-2012 08:27 PM

Holley 850 DP Question
 
In a nut shell my 496 was tuned on the dyno before it was put in the car. Now that it's in the car it has ran great, but the only issue I'm having is that when idling the exaust will burn your freaking eyes out. Any recommendations? I did want to see what size power valves were in it after I checked my vacuum. I have 6" of vacuum in drive at idle and the carb has a #4 power valve in the primary metering block and a plug in the secondary metering block would you guys recommend droping to a #3 or is the #4 close enough? Primary jets 78-78 and secondary jets 86-86. Carb is set to highest vac seems to run great, but the exaust at idle will make you cry!

cobalt327 06-06-2012 01:54 AM

Quote:

Originally Posted by 396chevelle
In a nut shell my 496 was tuned on the dyno before it was put in the car. Now that it's in the car it has ran great, but the only issue I'm having is that when idling the exaust will burn your freaking eyes out. Any recommendations? I did want to see what size power valves were in it after I checked my vacuum. I have 6" of vacuum in drive at idle and the carb has a #4 power valve in the primary metering block and a plug in the secondary metering block would you guys recommend droping to a #3 or is the #4 close enough? Primary jets 78-78 and secondary jets 86-86. Carb is set to highest vac seems to run great, but the exaust at idle will make you cry!

The eye watering exhaust is unburned hydrocarbons, caused by the EGR effect of the cam timing. This is common w/a cam that has a lot of overlap/tight LSA/late closing intake.

If you can, add initial timing. Just be sure to balance the total timing by removing timing from the mechanical advance so the total timing doesn't go too high.

In some cases, dialing in some vacuum advance could help. But having the vacuum at idle that low will make that impractical, though. I'd advise you to use vacuum advance, but in your case use ported vacuum for the vacuum advance. Limit the vacuum advance to 10 degrees.

396chevelle 06-06-2012 07:55 AM

Quote:

Originally Posted by cobalt327
The eye watering exhaust is unburned hydrocarbons, caused by the EGR effect of the cam timing. This is common w/a cam that has a lot of overlap/tight LSA/late closing intake.

If you can, add initial timing. Just be sure to balance the total timing by removing timing from the mechanical advance so the total timing doesn't go too high.

In some cases, dialing in some vacuum advance could help. But having the vacuum at idle that low will make that impractical, though. I'd advise you to use vacuum advance, but in your case use ported vacuum for the vacuum advance. Limit the vacuum advance to 10 degrees.

OK, Thanks for the input! I'll give it a shot.

ap72 06-06-2012 09:23 AM

Its very possible that your wot performance was tuned on the dyno with the jets rather than the pvcr's. A LOT of people do that and it is the WRONG way to tune WOT. You may need to step down a handful of jet sizes and open up your pvcr to get your cruise tuned right.

For idle you can adjust the idle feed and bleeds to get an acceptable idle and off idle transition.

Tuning a carb for peak power has very little to do with tuning a carb for peak performance. Peak power is easy, peak performance is much much harder.


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