Best A/F ratio and timing for low rpm cruse with big cam and CI engines
04-05-2016, 01:40 PM
#1
Race Director
Thread Starter
Best A/F ratio and timing for low rpm cruse with big cam and CI engines
On a large CI engine, with big cam..... what A/F and timing do you feel is best to get a low RPM idle/cruse ability...
yes I have a big cam.. 611/592 244/248 109 Sep
and some CI also 427
I would like to idle in first gear as low of an RPM as possable and still move...
NOT JERK... I also have a lightweight drivetrain, with a lightweight flywheel and a lightweight clutch.
So... do you run a big cam and big CI?
if so... what have you found to be the perfect setup as far as A/F and timing???
I do run a EFI, so most all parameters can be changed/modified via computer.
yes I have a big cam.. 611/592 244/248 109 Sep
and some CI also 427
I would like to idle in first gear as low of an RPM as possable and still move...
NOT JERK... I also have a lightweight drivetrain, with a lightweight flywheel and a lightweight clutch.
So... do you run a big cam and big CI?
if so... what have you found to be the perfect setup as far as A/F and timing???
I do run a EFI, so most all parameters can be changed/modified via computer.
Last edited by pauldana; 04-05-2016 at 01:40 PM.
04-05-2016, 02:19 PM
#2
Race Director
Lots of timing and you need to fatten it up some at low rpm. Start at 13.5:1 under 2,000 rpm. As for timing, you can run 40 dgrs at 1,300 rpm at low load, no problem.
04-05-2016, 03:53 PM
#3
Race Director
I talked to the inventor of the LM-1 and he said the Gen 1 small and big blocks like it fat at idle and off idle. Mine is around 13.5-14 AFR at idle and off idle it stays less than than 13.5:1 all the time. My timing is 34degs at 2000RPM don't know exactly what it is at idle but around 15-28 degs. I do have a distributor that I cam put different stops in and I put an 18deg mechanical advance in. So whatever it is at idle I add 18degs mechanical to achieve 34 degs total. This is without a vacuum advance. Hope that helps, the most important thing to remember is the gen 1 engines like it fat
04-05-2016, 04:02 PM
#4
Race Director
Thread Starter
I talked to the inventor of the LM-1 and he said the Gen 1 small and big blocks like it fat at idle and off idle. Mine is around 13.5-14 AFR at idle and off idle it stays less than than 13.5:1 all the time. My timing is 34degs at 2000RPM don't know exactly what it is at idle but around 15-28 degs. I do have a distributor that I cam put different stops in and I put an 18deg mechanical advance in. So whatever it is at idle I add 18degs mechanical to achieve 34 degs total. This is without a vacuum advance. Hope that helps, the most important thing to remember is the gen 1 engines like it fat
Report back tomorrow...
Last edited by pauldana; 04-05-2016 at 04:02 PM.
04-05-2016, 04:18 PM
#6
Melting Slicks
Member Since: May 2002
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St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19
496 BB with Lunati solid roller 695/710 250/250 112 Sep (don't remember the exact specs but this is close)
I try and stay in the 14.0 to 14.7 area if I get above 15.1 you can feel the power loss, slight hesitation when driving.
I've tried multiple timing curves and locking it out at 36* has been the best all around cruising and racing performance.
I'm running a 200r4 trans but the OD don't engage until 55, took a lot of tunning to get it to perform the way I wanted it to but its paid off.
A good ignition is critical to the combo,
Neal
I try and stay in the 14.0 to 14.7 area if I get above 15.1 you can feel the power loss, slight hesitation when driving.
I've tried multiple timing curves and locking it out at 36* has been the best all around cruising and racing performance.
I'm running a 200r4 trans but the OD don't engage until 55, took a lot of tunning to get it to perform the way I wanted it to but its paid off.
A good ignition is critical to the combo,
Neal
Last edited by chevymans 77; 04-05-2016 at 04:20 PM.
04-05-2016, 04:57 PM
#7
Safety Car
To lean is no good ever. I run 28 idle 900 rpm and 13.5 a/f . Auto trans.
04-05-2016, 07:22 PM
#8
Team Owner
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St. Jude Donor '05
#s are just a guide, give it what it wants and where its happiest
04-06-2016, 08:14 AM
#10
Melting Slicks
I'm running a carb with annular boosters on my 406, and with my 254@.050 , 108LC solid cam and auto trans (3000 stall), had trouble idling in gear. Solved the problem with 36 degrees locked timing, and IFR jet changes....to fatten it up. Idles in gear at 1000 rpms.
I think the biggest change was timing, and the jetting was more of a fine tune. I do not use an AFR meter,..just look at the plugs once in a while. They are light tan, just as you'd expect from a well tuned engine.
I think the biggest change was timing, and the jetting was more of a fine tune. I do not use an AFR meter,..just look at the plugs once in a while. They are light tan, just as you'd expect from a well tuned engine.
Last edited by The Money Pit; 04-06-2016 at 08:15 AM.
04-07-2016, 06:54 AM
#11
Aluminum Flywheels Not Intended For Street Engines
Street engines require heavy flywheels because of the lower rpm range they operate in. That's why the lighter aluminum flywheels are only recommended for high rpm track racing events. To be able to idle your street engine smoothly you'll need to use a 35# flywheel (or torque converter).
Last edited by awesome71vette; 04-07-2016 at 07:41 AM.
04-07-2016, 09:20 AM
#12
Race Director
Thread Starter
Street engines require heavy flywheels because of the lower rpm range they operate in. That's why the lighter aluminum flywheels are only recommended for high rpm track racing events. To be able to idle your street engine smoothly you'll need to use a 35# flywheel (or torque converter).
The car is used for road race.... Thus the ensemble works great for what it is intended to do.... I am just doing a little fine tuning as possible for low (as possible with a big cam lightweight rotating assembly) rpm in gear idle....
TBTR???????
Last edited by pauldana; 04-07-2016 at 10:54 AM.
04-07-2016, 10:30 AM
#13
Team Owner
Street engines require heavy flywheels because of the lower rpm range they operate in. That's why the lighter aluminum flywheels are only recommended for high rpm track racing events. To be able to idle your street engine smoothly you'll need to use a 35# flywheel (or torque converter).
I put the modern HP street flywheel to be right about 22 lbs. I thought that my 427 sbc ran really nice with it. I since changed to a 11.5 pound and it idles fine, but it requires more slipping of the clutch.
Paul if those cam numbers are actually correct you have a mild cam compared to the rest of us on the forum with 427 type small blocks with solid roller by 6-10 degrees and your lift is low
04-07-2016, 10:40 AM
#14
Race Director
Thread Starter
,,,, a bit inadequate..
.lol.... just trying to get her to do a low as possible low rpm in gear idle,,, I think the lowest i can achieve without the pulsing is around 1500rpm... been trying to get it lower, but she does not like it:-)
best RPM for her is 2000-70000
04-07-2016, 10:45 AM
#15
Tech Contributor
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Street engines require heavy flywheels because of the lower rpm range they operate in. That's why the lighter aluminum flywheels are only recommended for high rpm track racing events. To be able to idle your street engine smoothly you'll need to use a 35# flywheel (or torque converter).
04-07-2016, 11:38 AM
#16
Team Owner
Lets say with your cam requires a 1000 rpm idle. So set the timing to something like 18 and get the a/f ratio down to around 13 ish.
04-07-2016, 11:39 AM
#17
Safety Car
So your saying it idles fine at 850 , but if you try to slowly move the car it gets choppy or wants to stall ?
04-07-2016, 11:52 AM
#18
Race Director
Holley 900cfm with mechanical secondaries
Street engines require heavy flywheels because of the lower rpm range they operate in. That's why the lighter aluminum flywheels are only recommended for high rpm track racing events. To be able to idle your street engine smoothly you'll need to use a 35# flywheel (or torque converter).
I didn't make the connection until you posted this.
04-07-2016, 01:44 PM
#20
Race Director
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.