locked out timing/any downsides on street cars?

[GREG K]

When you advance the timing and have a nice fine, lean well saturated airstream out of the t-slot you start to complete the burn earlier on the power stroke. What this does is it allows more of the heat ENERGY to go into the crank instead of out the exhaust.
It’s another form of energy management.
When you get the t-slot delivering a very fine lean well saturated mix exiting the carb and you advance the timing you end up with lower HC and CO amounts going out the exhaust, but higher O2 amounts. It couldn’t get higher O2 before because it was full of HC and CO, now its not. This O2 mixes with the fuel that comes from the t-slots and the rpm goes up.
When you have lean mixtures you get a slower burn so you have to advance the spark so that the burn can finish earlier. This way you have less fuel in the exhaust which would normally be stopping the cylinder from sucking back in atmosphere. When the fuel is burnt in the cylinder then the cylinder can suck in atmosphere because there is no burning in the pipe, blocking the cleaner atmosphere.
Cylinder psi goes up due to the extra ENERGY (heat) in the cylinder now, not out the pipe..Comprenda?
Its just another example of how you can manage energy flow.
That’s what tuning engines is all about. Energy management.

That’s why you have to tune carbys on engines live to get engines real good, because you have to fiddle with everything as it is all interrelated. Each engine sucks and blows and bangs and vibrates different to the next one, so you gotta sort it out.
You have to set your timing so that it allows enough vaporisation and gasification of the fuel load to eliminate detonation.

When you move your timing around you are firing the plug in a different environment. When you advance the timing you are firing it in a less vaporised environment. When you retard the timing you are firing the plug in an environment that has had more compression time, more vaporisation time, hence the fuel load has increased amounts of energy in it before the plug fires. The state of molecule conversion in the cylinder is different for firing when you move the timing around. Engines will have differing conversion rates so they will like different timing settings, you have to find out what yours likes, but you have to adjust the fuel mix with the timing, not one without the other.
You need to have the timing advancing as the rpm and load increases due to the changing environment in which the plug fires. The time in which the energy input is delivered into the fuel load is reduced as the rpm’s go up and any change in the gassed afr from what your combo likes at firing time will require earlier firing as the burn speed will be reduced.It takes time for the molecule conversions to take place.
When you boil your kettle for a coffee, it takes x amount of time to boil x amount of water. When you reduce the amount of time that the switch is on for without changing the amount of water, you wont get it to boil, will you? So now you can reduce the amount of water in the jug (read fuel load) or start the energy input earlier,to get the water to boil.
It doesn’t matter whether you have a big convertor and low gearing or not. You still have to tune the engine at all the load points. It’s a good practice to get into doing it this way. The cylinder tune needs to be built upon from the foundation of smooth combustion at the lower load points, which includes getting the timing right for these, then when you go to WOT the cylinders go to full power much quicker.
If you don’t get the molecules converting correctly at the lower load points from bang to bang, this can affect power right up through the rpm range and the cylinder does not go to full power fast when you mash the throttle.

[Adger Smith]

In response to what Rabbit ask:
I think you are on to something there. My quick, down dirty and simple response is: It all depends, because all engines are different (esp, their combustion chambers) and all load conditions on them are different. Is it fair to just give blanket answer for ALL Engine or All applications? I think anyone who has done testing and racing with extreme high output engines know they want different fuel curves and different timing curves at both peak Tq and Peak HP.
Then when you get them off the dyno and into the car you can tweek those areas to get max "on track performance"
To answer the Why on Mech advance with the loose converter on the street: It all depend on if the converter stall is above peak Tq ,The point peak Cylinder pressures are usually achieved, or below P.TQ. On street, reaching P.TQ, heavy load and a lean mixture at the same time are disasterous to engines and esp. internal engine parts.
I agree with the assesment that there is quite a lot of good information in this thread. Great techncal discussion!

[GREG K]

[b]If the engine can get up to 4000 RPM without changing the throttle angle, and just leaning and advancing, it means the initial point was way too rich or way too retarded and needed a very open throttle just to idle or just wasn't pulling fuel in well.[/b]

It doesnt mean that anything is wrong,im just describing what happens when you get the dizzy and turn it.i doesnt mean that anything was rich or too retarded,its just what the engine does.
rabbit mentioned that when he did that the vac went up 5",so i just explained why it did this,thats all.
Its not a way that you would run your idle necesarliy,its just what happens,thats all.

[MadBill]

When an engine is idling there is a lot of vacuum that sucks in whatever there is to suck in.
If you lean the idle fuel out and advance the timing,then the exhaust is not full of fuel anymore so it can suck in O2,where as before it couldnt with rich retarded settings.
The only test you need to do is grab the dizzy,turn it and lean the mixture screws.now your engine runs smooth and rpms go right up.keep going if you want,2,000--3,000---4,000 rpm sometimes.
You havent opened the carb,its shut,so you know that you are sucking in O2.
The tire tube test wont show you.
Tuning engines shows you.

It doesnt matter how long the pipe is.

simple,isnt it.

Ummm...I always though the 'normal' idle target (absent emissions requirements and tip-in knock) was to adjust for the 'highest idle speed' mixture (which I've usually found to be between ~12.0-12.5:1 AFR for a big cam down to ~14:1 for a stocker) and use enough spark advance (initial plus perhaps some vacuum) to produce the highest idle speed consistent with smooth running. (unstable RPM and excessive load sensitivity develop at some point; retarding enough from peak idle RPM to drop ~2 "Hg. is usually about right) The suggestion to: "...lean out the idle fuel and advance the timing..." merely implies that it was retarded and too rich to start with.

Let me just summarize by saying that one of us is badly misinformed, but at least now the ST members have some alternative realities to ponder and that they might want to consult a combustion text if they are in doubt as to which is right..

[dieselgeek]

If you can take an idling engine and make it rev to 4000RPM just by advancing the timing - then your idle tune was so badly setup in the first place that you probably shouldn't be tuning engines. Seriously. "Mean Best Timing" is what the EFI/EMS guys call it, some choose to idle at MBT and others just a few degrees below it. But the reason the engine increases RPM with increasing timing is NOT because it's picking up O2 from the exhaust. Even an amateur like me, knows this.

[Matt Gruber]

i do try my best to minimize fuel consumption at idle by turning the dist. to find max rpm.
then i close the throttle to get the rpm down and do my best to get the needed advance from the vac adv & base timing. (i don't think the rpm climbs anymore, very little maybe) Of course i re-set the idle mixture along the way.
i'm using a 4.5#, 4cyl radiator from a VW in my 61 vette, so this seems to be working(less wasted fuel equals less heat). mpg is 19 around town w/4.11 and no OD, (if traffic isn't too heavy).
Oops! another TUNING SECRET slips out.

[6sally6]

i do try my best to minimize fuel consumption at idle by turning the dist. to find max rpm.
then i close the throttle to get the rpm down and do my best to get the needed advance from the vac adv & base timing. (i don't think the rpm climbs anymore, very little maybe) Of course i re-set the idle mixture along the way.
i'm using a 4.5#, 4cyl radiator from a VW in my 61 vette, so this seems to be working(less wasted fuel equals less heat). mpg is 19 around town w/4.11 and no OD, (if traffic isn't too heavy).
Oops! another TUNING SECRET slips out.

VW radiator...hhmmmmmmmmm ???#-o J/K !
6sally6

[Matt Gruber]

i do try my best to minimize fuel consumption at idle by turning the dist. to find max rpm.
then i close the throttle to get the rpm down and do my best to get the needed advance from the vac adv & base timing. (i don't think the rpm climbs anymore, very little maybe) Of course i re-set the idle mixture along the way.
i'm using a 4.5#, 4cyl radiator from a VW in my 61 vette, so this seems to be working(less wasted fuel equals less heat). mpg is 19 around town w/4.11 and no OD, (if traffic isn't too heavy).
Oops! another TUNING SECRET slips out.

VW radiator...hhmmmmmmmmm ???#-o J/K !
6sally6

1st i rode around with NO FAN (1982 stock replacement copper rad) .
Everyone said it would overheat, but it didn't.
Then i tried the JEGS RACING RADIATOR. at 1st i felt duped, because it was just an ordinary Modine VW rad. BUT, jegs swore it could cool a V8 with their $170 fans. I didn't buy their fans, just 5 cheap fans. one 12" and 4, 5". Total fan weight <5#. Draws 7.9 amps TOTAL. Now the rad is on sale for $99 #555-51170. I'm still testing it out, but, so far it works fine. Would not use it with A/C or in the desert SW. But here in fla it is ok so far. Ask me again in august.

[twinturbo496]

a stock convertor, 2.xx rear gear and WOT at 2000rpm might not like locked timing, but what about 4k stall and 4.33 gears under the same conditions?

My coupe is 3800#, 4.86 rear gear, 3K+ stall, & 496 cid, 8.5:1 cr, 254/258 duration @ 50. No problems starting or street driving with 87 octane, including in summer downtown traffic. Crank trigger & 6AL with 30 degrees advance.

[MadBill]

Of course with that big cam and low CR, the cranking pressure is likely only ~ 140 psi...

[twinturbo496]

Of course with that big cam and low CR, the cranking pressure is likely only ~ 140 psi...

Yep.

BDS 14-71 is on top, I have never heard it ping yet, including with the converter locked WOT from 3000 rpm. AFB's tuned with LM-1 wide band. WOT ~ 11.5:1, cruise ~ 14:1

Pretty weak power output for such a big motor, but it runs fine on the cheapest gas in town.

[Troy Patterson]

When you advance the timing and have a nice fine, lean well saturated airstream out of the t-slot you start to complete the burn earlier on the power stroke. What this does is it allows more of the heat ENERGY to go into the crank instead of out the exhaust.

Greg K - you have an obsession with the "t-slot" thing. Seems you have an agenda. You trying to build a case of obviousness and part of the "Art" with misinformation? Just wondering? You doing this for Holley? Is this a case of unfair business practices?

Troy Patterson TMPCarbs.net TMP Carbs

[GREG K]

Awe shucks...your on to it..
Yep,im the Aussie secret agent for them..

[wjnielsen]

When an engine is idling there is a lot of vacuum that sucks in whatever there is to suck in.
If you lean the idle fuel out and advance the timing,then the exhaust is not full of fuel anymore so it can suck in O2,where as before it couldnt with rich retarded settings.
The only test you need to do is grab the dizzy,turn it and lean the mixture screws.now your engine runs smooth and rpms go right up.keep going if you want,2,000--3,000---4,000 rpm sometimes.
You havent opened the carb,its shut,so you know that you are sucking in O2.
The tire tube test wont show you.
Tuning engines shows you.

It doesnt matter how long the pipe is.

simple,isnt it.

Uh, I'm not one of these guys who's real big on theory, but I did make my living as a mechanic, and was a driveabliity/emssions specialist for about three years of that time.

I've found that the engines I've had on a gas analyzer tended to actually be slightly leaner than stoichiometric at idle. Leaning 'em out didn't bring up rpm; it lost it. I agree w/ the previous poster; I've never seen gains leaning an idle unless it was over-rich to start with.

BTW, I have one of these stupid street cars ( big mopar, solid roller cam 262 @ .050 ground on 108, tunnel ram, 4.10's, stall convertor). My distributor has weak springs installed, so that it's all in (35 deg) at its 1200 RPM idle. Might as well be locked for all intents as far as driving the car is concerned.

The other day, I noticed the idle speed came up, and the engine didn't run as smooth; found dist. clamp rattled loose and timing jumped to 45 deg. Way it ran, I thought I would find a vacuum leak.

Anyway, the engine liked the timing at idle, but it didn't like it under any kind of load whatsoever. Didn't hear it ping (like I'd be able to over that noisy valvetrain!) but it didn't
run as smooth as usual.

As far as having a locked advance, I don't see the big deal for a car like mine. Maybe I'm a degree or three short of 'ideal' at some points. But WTF? How much increase am I going to really see by going from my current 35 degrees everywhere to 38 deg. at one speed and maybe 34 at another?

I've seen a number of guys post up that they got within a horse or two with a few degrees less advance on a number of different engines and dynos. Call me crazy, but I suspect that the last 3-4 degrees of advance coming up to 'optimum' show diminishing gains; especially getting very little increase in that last degree or two. Yeah, I've seen guys claim to make major gains by changing the timing 1/2 degree, but then, I've seen a lot of claims on the internet...

-Bill

[Troy Patterson]

wjnielsen - every engine is going to be different. To be at the top of the "tuning" game and a consistent front runner in competition one can't make assumptions.

Troy Patterson TMPCarbs.net TMP Carbs