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Best A/F ratio and timing for low rpm cruse with big cam and CI engines

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Best A/F ratio and timing for low rpm cruse with big cam and CI engines

 
Old 04-07-2016, 02:42 PM
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69427
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Originally Posted by zwede View Post
But he has EFI so he doesn't have the limitations a mech/vacuum distributor has. I've had very good luck with lots of advance at very low load. At WOT I don't max out until 3000+ rpm, yet I run over 40 dgrs at 1500 rpm low load.



As cuisinartvette mentioned earlier, just give the engine what it wants.
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Old 04-07-2016, 03:56 PM
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Originally Posted by diehrd View Post
So your saying it idles fine at 850 , but if you try to slowly move the car it gets choppy or wants to stall ?
yes

Originally Posted by zwede View Post
But he has EFI so he doesn't have the limitations a mech/vacuum distributor has. I've had very good luck with lots of advance at very low load. At WOT I don't max out until 3000+ rpm, yet I run over 40 dgrs at 1500 rpm low load.
thx
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Old 04-07-2016, 04:03 PM
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Here's my combo:

372ci small block
13.5:1 compression
268/[email protected] 108 ls solid roller
AED 950 Holley
1,400rpm idle
7.5" vacuum
26 degrees initial timing with custom 10 degree advance stop bushing
14.4-14.7:1 idle afr
12.0-12.5:1 cruise afr might go to 13:1 under very light load)
11.5:1 WOT afr

That is with stock AED air bleed calibration and drives pretty nice for an 8,000rpm race engine. I'd like it to be a bit leaner under load.

Last edited by Neil B; 04-07-2016 at 04:06 PM.
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Old 04-07-2016, 04:42 PM
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Originally Posted by zwede View Post
But he has EFI so he doesn't have the limitations a mech/vacuum distributor has. I've had very good luck with lots of advance at very low load. At WOT I don't max out until 3000+ rpm, yet I run over 40 dgrs at 1500 rpm low load.
Higher compression atomizes fuel better. atomized fuel burns faster. Combine that with modern fastburn heads like in Paul's case AFR 227 cc with small mid 60 cc chambers and being Heart shaped are not a good comparison to BBC large chambers.


If anything highly atomized fuel droplets like out of a 8 port injection system would require even less timing that it's carburated counterpart
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Old 04-07-2016, 05:09 PM
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Originally Posted by gkull View Post
Higher compression atomizes fuel better. atomized fuel burns faster. Combine that with modern fastburn heads like in Paul's case AFR 227 cc with small mid 60 cc chambers and being Heart shaped are not a good comparison to BBC large chambers.


If anything highly atomized fuel droplets like out of a 8 port injection system would require even less timing that it's carburated counterpart
OEMs do the same thing. I checked the timing map on my Gen II LT-1 (port injected). Even with the tiny OEM cam (206 dgr duration) GM gave it over 40 dgrs advance at 1500 rpm at low load. Mech/vacuum distributors are so off ideal advance it's a wonder they run at all.
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Old 04-08-2016, 09:25 AM
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Originally Posted by zwede View Post
OEMs do the same thing. I checked the timing map on my Gen II LT-1 (port injected). Even with the tiny OEM cam (206 dgr duration) GM gave it over 40 dgrs advance at 1500 rpm at low load. Mech/vacuum distributors are so off ideal advance it's a wonder they run at all.
You can't even compare the two. The timing on a modern computer ignition car is fluctuating 100 times per second always trying to give max advance until it senses knock.

I had a laptop hooked up and the timing numbers are just a blur. So your 40 degrees might have been for only one cylinder and then ping and it dropped right back down. I have never seen a constant advance number on distributor less ignitions.

As for me I have rebuilt enough HP motors and looked at the bearings to know that stupidly fast advance curves just hammer the barrings.

I'm also not a fan of aftermarket FI systems. My friends find these super dynotune places and they cost about $600 for 4 hours of tune time to get their aftermarket systems running the best possible. They don't have 100's of millions of dollars to perfect them like the car manufacturers have.

Last edited by gkull; 04-08-2016 at 06:20 PM.
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Old 04-08-2016, 09:53 AM
  #27  
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Originally Posted by gkull View Post
You can't even compare the two. The timing on a modern computer ignition car is fluctuating 100 times per second always trying to give max advance until it senses preignition.
I don't really feel like arguing, but I've never seen an OEM system intentionally running timing into ping. They have a timing table set for the octane the engine is speced for. If it senses pinging it does a pretty big retard, then slowly advances back to zero retard unless it senses more pinging.

Also, pre-ignition is different from pinging and will destroy an engine in seconds.

OP wanted to know how to optimize low speed drivability. Lots of timing advance is the answer. Even if he goes too far, some pinging at low load isn't going to hurt anything.
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Old 04-08-2016, 01:37 PM
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Originally Posted by zwede View Post
I don't really feel like arguing, but I've never seen an OEM system intentionally running timing into ping. They have a timing table set for the octane the engine is speced for. If it senses pinging it does a pretty big retard, then slowly advances back to zero retard unless it senses more pinging.

Also, pre-ignition is different from pinging and will destroy an engine in seconds.

OP wanted to know how to optimize low speed drivability. Lots of timing advance is the answer. Even if he goes too far, some pinging at low load isn't going to hurt anything.
You're batting a thousand in this thread. Keep up the good work.
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