Set idle timing to highest RPM??

Tom Mobley · Aug 15, 2009

oops, forgot to mention: There's a psychological or mental part to this too. If an engine is responsive and peppy a guy doesn't have to be lead footing it all the time to get around town. After awhile you just drive with a lighter foot, really helps the city mileage. There's not a lot else that will, especially with a stick car.

What tranny do you have?

onebad82z · Aug 15, 2009

Tom Mobley said:
oops, forgot to mention: There's a psychological or mental part to this too. If an engine is responsive and peppy a guy doesn't have to be lead footing it all the time to get around town. After awhile you just drive with a lighter foot, really helps the city mileage. There's not a lot else that will, especially with a stick car.

What tranny do you have?

T56 6 Speed :yes:

vrooom3440 · Aug 16, 2009

So some of my experience to add here...

I generally do not recommend more than 18* of initial. The reason is this is also your cranking ignition timing and at the slow speeds of cranking this is about the limit before cylinder pressures rise too much BTDC. That is hard on starters and engine parts and just is not the way it is supposed to be. Yes this differs a bit from Scott's advice a bit, but is actually right in the middle of what you were proposing and what Scott proposed.

I would bet your idle vacuum went up a fair amount? My BBC 402 with a HR296 236/242 @ 050 cam idled with 7" of vacuum initially. When I got the 18* initial with 12* of vacuum all in at idle tune setup it jumped up to 11" of vacuum. As you observed a much happier motor. I have also found for me the 600 CFM carb is much more street friendly than a 750 CFM on this engine. Yeah it is under-carbed a bit. But much easier to drive.

I get around 8 MPG around town and 11 MPG down the highway with the Muncie 4 speed and 3.31 gears. Changing to a TKO-500 with the .6x overdrive bumped the highway up to 13 MPG. I used to cruise in the 60-65 MPH range around 2500 RPM, now do 70 MPH at 2000 RPM. Much quieter down the highway :thumbsup:

I have a vacuum gauge and wide band O2 permanently in the car. I find that carbs are really tough to tune for all around operation. As vacuum drops they lean out and vice versa. On the road vacuum varies all over so AFR varies. The best you can hope for is a workable compromise. I leaned out the cruise on the Edelbrock carb until I would notice just a bit of hesitation sometimes on slight grades, then I backed off one notch.

Ultimately I plan on going to EFI for better control.

onebad82z · Aug 16, 2009

vrooom3440 said:
I generally do not recommend more than 18* of initial. The reason is this is also your cranking ignition timing and at the slow speeds of cranking this is about the limit before cylinder pressures rise too much BTDC. That is hard on starters and engine parts and just is not the way it is supposed to be. Yes this differs a bit from Scott's advice a bit, but is actually right in the middle of what you were proposing and what Scott proposed.

Understood, thank you for the explanation!

I would bet your idle vacuum went up a fair amount? My BBC 402 with a HR296 236/242 @ 050 cam idled with 7" of vacuum initially. When I got the 18* initial with 12* of vacuum all in at idle tune setup it jumped up to 11" of vacuum. As you observed a much happier motor. I have also found for me the 600 CFM carb is much more street friendly than a 750 CFM on this engine. Yeah it is under-carbed a bit. But much easier to drive.

Idle vacuum went from 9" to 11" at same RPM. And yes, it makes for a much happier motor.

I get around 8 MPG around town and 11 MPG down the highway with the Muncie 4 speed and 3.31 gears. Changing to a TKO-500 with the .6x overdrive bumped the highway up to 13 MPG. I used to cruise in the 60-65 MPH range around 2500 RPM, now do 70 MPH at 2000 RPM. Much quieter down the highway :thumbsup:

I am shooting for 13-15 highway and I think for a street/strip 355 I am not doing too bad. I know it could be better, but just seat of the pants tuning I can't complain. I cruise at 2500 in 5th on the highway (right around 75MPH).

I have a vacuum gauge and wide band O2 permanently in the car. I find that carbs are really tough to tune for all around operation. As vacuum drops they lean out and vice versa. On the road vacuum varies all over so AFR varies. The best you can hope for is a workable compromise. I leaned out the cruise on the Edelbrock carb until I would notice just a bit of hesitation sometimes on slight grades, then I backed off one notch.

Ultimately I plan on going to EFI for better control.

Yeah I know carbs can't really be tuned for ALL conditions. Just looking to make mine a lil more driver friendly. If I were to DD the car again I would easily be looking into some form of EFI.

69-CHVL · Aug 16, 2009

wildman926 said:
Hello John,

Yes, you can set it that way, turning the dizzy until no more rpm's are achieved. And Yes, this is what YOUR engine will like as for initial.

You can achieve this setting by using the set screw method that Vince G./69-Chvl came up with. I had this EXACT same setting on my SBC HEI dizzy, using the screw to obtain 26* initial and 36* total. I then used PORTED vacuum to add the extra timing during cruise or NO LOAD. I had a grand total of 52* timing (initial + total + vacuum advance). You have to look at it that way.

If you use manifold vacuum for extra timing at idle, that is ok too, but it will be gone as soon as you take off from a stand still (engine under load), and will come back at cruise (no load). So at idle, you may have your initial at 26*, and say 16* vacuum timing, which would give you a total of 42* at idle (initial + manifold vacuum), but drop down to your initial of 26* once the car takes off.

Here is Vince's pic for reference.

Hey Walt - you have to revise wording in that pic. The screw to the right is the lock-out screw, pinning the mechansim. Should be labled "lock-out screw". Screw on the left, which is abandoned in this case, should be labled "advance limiting stop screw".

creeper72 · Aug 17, 2009

Anybody tried Vince's set up on an MSD Pro ?

BowtieAaron · Aug 17, 2009

i run 20 inital, 18 mechanical in at 3k for 38*, and vac advance hooked up, not sure what the total is then. msd probillet ready to run, which is i think the 8361.
i had timing at 18* base, and 20* stop in, and picked up 2rwhp by bumping timing to 20* inital and leaving the mechanical go. vac advance was hooked up too. but i then went to the combo i have above.
i want to throw mor mechanical in and get the llower stops, and bring my total down, i think that will help wth my slight bog.

aaron

BowtieAaron · Aug 17, 2009

i want to add.. my car stopped surging, and pulls better with anything more than 16* intial. it likes 20 the best. i can lug it in 4th with 3.55's at about 1200rpms with almost no surge (231/239 @ 50 in a 350). i am also getting 17" of vacuum at 900rpms.
john... what cam do you have again?

aaron

onebad82z · Aug 17, 2009

BowtieAaron said:
john... what cam do you have again?

aaron

CStraub spec'd Bullet solid...

236/244 @ .050"
.533/.540 Lift
110 LS/106 ICL

This is with 1.6 intake rockers, and 1.52 exhaust.

onebad82z · Aug 25, 2009

Just thought I would update!

Only change since last fuel mileage check was adding the additional vacuum timing and leaning the idle mixture screws 1/8 turn.

I just drove it in to work today, 50 miles one way and already has some in town cruising on the tank... 13.69863 MPG!!! Wow, now that shocked me!:thumbsup:

vrooom3440 · Aug 25, 2009

You want shocked? I thought it was remarkable when I got 14.8 MPG running up the freeway at 70-75 MPH the other day with my BBC. Then I pulled down a 15.6 MPG on the way home at the end of the weekend

I think something must be broken... I never expected to see upper teens without EFI.

My WBO2 is non-functional currently, so I can only guess at cruise AFR. But based on prior experience and tuning, with a cruise vacuum of about 10" I am probably just about right on 15 or so. I have leaned out the cruise until it started to stumble just before the rods popped in the Edelbrock 1406, and then backed off one notch.

I still only get around 8 MPG around town though

onebad82z · Aug 25, 2009

vrooom3440 said:
You want shocked? I thought it was remarkable when I got 14.8 MPG running up the freeway at 70-75 MPH the other day with my BBC. Then I pulled down a 15.6 MPG on the way home at the end of the weekend

I think something must be broken... I never expected to see upper teens without EFI.

My WBO2 is non-functional currently, so I can only guess at cruise AFR. But based on prior experience and tuning, with a cruise vacuum of about 10" I am probably just about right on 15 or so. I have leaned out the cruise until it started to stumble just before the rods popped in the Edelbrock 1406, and then backed off one notch.

I still only get around 8 MPG around town though

Great. I was on a high thinking I had a sb doing fairly well. Now I gotta beat those numbers since yours is a BB!!

wildman926 · Aug 25, 2009

onebad82z said:
Just thought I would update!

Only change since last fuel mileage check was adding the additional vacuum timing and leaning the idle mixture screws 1/8 turn.

I just drove it in to work today, 50 miles one way and already has some in town cruising on the tank... 13.69863 MPG!!! Wow, now that shocked me!:thumbsup:

Next time you take that trip, try it with the vacuum advance unplugged and compare the mileage, and then you will see what I was talking about. :thumbsup:

onebad82z · Aug 26, 2009

wildman926 said:
Next time you take that trip, try it with the vacuum advance unplugged and compare the mileage, and then you will see what I was talking about. :thumbsup:

No thanks. Use to run around like that and always had horrible mileage! Reason why I was trying to dial it all in with this thread! New there was gains to be had and I got 'em! Looks like the old camaro is doing commuting duty (100 mile round trip) again tomorrow since it will be nice out!:yes:

Now the question is how much mileage am I giving up by running around on ET Streets at 24psi hot vs a regular radial????

I know radials are better just don't have the cash this summer for them, reason why I run the ET/S tires.

SWHEATON · Aug 26, 2009

When i checked the mileage with my mild 396(now 402)/approx 365-370hp /331 gear/m20 trans/26" tires & it got 13.7 mpg .

Thats with approx 80% intown with some stop go @ lights & 40-50mph crusing along with approx 20% open road-interstate cruising at 65mph/approx 2800 rpms.

I would think i could get at least 15+ mpg on open road only 60-65-mph crusing /WOW!!!/LOL!!!!!

Scott

trmnatr · Aug 26, 2009

69-CHVL said:
Hey Walt - you have to revise wording in that pic. The screw to the right is the lock-out screw, pinning the mechansim. Should be labled "lock-out screw". Screw on the left, which is abandoned in this case, should be labled "advance limiting stop screw".

My HEI is locked out

I have a steel plate with 4 holes that slides on each post then you install the e-clip, Done locked out

If you would like a template i will send you one while i have the distributer out

onebad82z · Sep 23, 2009

Ok guys I know this is old BUT I have a new question that applies. Reading another members new post on vacuum advance sparked a thought.

(http://www.chevelles.com/forums/showthread.php?t=290732)

My mechanical is "all in" by 2800-2900. BUT, I typically cruise anywhere between 2200-2750 in 4th on back roads or out on the highway in 5th.

So even though I have 36* total timing and 16* additional from vacuum giving me a theoretical 52* at cruise I really don't since I am cruising IN my mechanical curve! I would have to cruise at 3k and up to have an actual 52*!

Should I have my mechanical curve "all in" UNDER my typical cruise RPM?

Did want to add.. Tom M did mention this in post #15!

vrooom3440 · Sep 23, 2009

There are really a bunch of variables at work here and in reality the mechanical and vacuum advance mechanisms are no more than an approximation of engine optimization. This is where the computer mapping for ignition in most EFI systems really shines. Plus the fact you can tweak it while driving down the road and find out what optimum really is... pure magic.

But enough of me waxing on about EFI

Real point is you may have to expirement to find out if your engine cruises better in your gearing/weight/power curve combination with a bit more advance. You can bump things up on initial/total for an expirement without hurting things so long as you keep your foot gentle.

The other trick you can explore here is using extra vacuum advance to compensate for the partial activation of the mechanical. I suspect that is what GM did in many cases as suggested by the 20* of advance common to so many vacuum advance cans.

onebad82z · Sep 23, 2009

Well I am using all of the vacuum advance available with my current canister so that option may not be one right now. And from what I can remember in searching through all of the cans specs, none with more than 16* available come in low enough to work with my idle vacuum. So that makes this easier with one less option.

So I can always mix/match springs and using my timing light to see when they start and are "all in" (and keeping a good log) I can test drive to see what the car does (n't) like. Thanks! Guess I knew that part but needed confirmation. OCD...

I understand what you man about EFI. I got into that realm a very (very) small amount years back but what fun is that. The keyboard doesn't burn your finger tips and wrists like a nice hot intake or header does?!

onebad82z · Sep 25, 2010

Ok I now this thread is old as hell but I wanted to update with some timing changes I made on the advice of Steve and Tom. They both mention how cars with OD rarely cruise with the timing all in. And I had great results with the progress I made with this thread last year. I have been tuning my carb to the best of my abilities this summer and have been having great results as well. Well I am at the point aside from WOT tuning at the track the car runs great and gets excellent MPG. But the weather has recently cooled off around me and with it brought back some mild low cruise surging. Earlier this summer I had tuned this clean with IFR and IAB changes, no problem. But with the temps dropping she would surge here and there. First idea was to decrease the IAB and clean her up. Then going over some notes I had in the car I remembered the idea Tom and Steve brought up with a quicker curve to get more cruise timing since I run an OD trans.

So I decided to go that route and see if adding some more timing with a quicker curve could clean up this lean surge. I currently had the MSD C curve in the dizzy. Never mapped it out ( I know I know) but for ****s and giggles I swapped out the heavy silver spring for the blue spring and kept the other light silver that has been in there. So I went to the most aggressive curve I could run with my 8361. The quickest curve with 2 light silver springs makes for an unstable idle on my car, BTDT (F curve). So I jumped right to the E curve. MSD literature shows this curve to start around 1300 and be all in by 2000. I find it to be more like this on my combo.

1000 idle she is rock solid at 28*

1250 is 35*

1500 is 44*

1750 is 44*

2000 is 44*

2250 is 50*

2500 is 52*

Anything above 2500 it stays right at 52* so I think she is all in at that point (2500)

This is with the vacuum can connected as it adds 16*, verified.

Again I wish I had mapped out the C curve on my combo but I was excited to try this curve. No guts no glory right. Anyway just took a nice 25 mile ride as it is beautiful and 85* today. Combined back road, in town, highway including a long uphill mountain pull that is about 4.5 miles long that I hit right at the end of the cruise so the motor was nice and hot. Not a single hint of detonation! I expected some especially on the hill as I kept rolling out of it and then slowly roll into it keeping it out of the PV to try and lean it out as hard as I could. She seems to love this curve. As a side benefit yet again this quicker curve really improved the midrange, it pulls alot harder with alot less pedal. A couple WOT blasts to 3rd still no signs of pinging. As a steady state cruise anywhere between 2000 and 2500 she ran smoother than ever. Another side benefit was that I was just about to start playing with cams and shooters on the secondaries as I steady roll in 1st at 2500 she had a pretty nasty bog as I mat it. Well with this new curve the bog is so slight I bet a simple jump to a 35 shooter and she rips when mashed. I love this stuff!!

Currently getting 18 MPG on the highway. I would love to see 20 with this little bit of added timing. Last year just adding 6* at cruise increased me from 9MPG to 15 so I think I just may see 20!

Set idle timing to highest RPM??

Tom Mobley

onebad82z

vrooom3440

onebad82z

69-CHVL

creeper72

BowtieAaron

BowtieAaron

onebad82z

onebad82z

vrooom3440

onebad82z

wildman926

onebad82z

SWHEATON

trmnatr

onebad82z

vrooom3440

onebad82z

onebad82z

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