Effects of ignition timing - Chevy 350 4BBL?
sbourg
(heavy-duty service), no cat converter or ECM, basic HEI ignition.
Checking the timing yesterday, I found it at 8 deg BTC, where spec is 6
deg. The engine basically runs fine with no audible knocking, and
though it often 'smells' hot, the temp gauge stays low, barely reaching
the bottom of the 'normal' range when climbing hills.
This is a small powerplant for this use, and a rather low-revving
engine. Power seems adequate near its peak, but on upshifts it
definitely struggles under acceleration in the new gear.
My question is: if I retard the 2 deg to spec, will this improve or
reduce or have no effect on the low-rpm performance, and what effect
also as the rpms go up? What effect too on engine internal temps?
Thanks.
Steve
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ralph
>
> >My question is: if I retard the 2 deg to spec, will this improve or
> >reduce or have no effect on the low-rpm performance, and what effect
> >also as the rpms go up? What effect too on engine internal temps?
> >Thanks.
>
> differece in temps.
> Almost all engines with a timing chain and over 40,000 miles on them need to be
> 1 -2 deg advanced over stock due to chain stretch/slop.
> If you want more low end power advance it 2 more, drive it and advance it 2more
> again and drive. You will get more power after the shifts. You want to go
> about as high as you can without getting a ping under slight to mod
> accelleration.
> The vacuum advance also can effect all of this.
> An 83 in stock form will have full vacuum to the dist at idle. (SORRY DESIGN!)
> This give flat spots. I have had 83 305s and 350s. I re-route so the
> dist is a ported vacuum so it has no vacum at idle and vacuum when the throttle
> opens.
> Then reset timing to the advanced side until there is a slight ping and then
> back it off to no ping is evident on slight uphill slow accelleration.
> Many have their own methods, but this have been the best way I have found to
> get the most, smoothest power out of them.
Ooh good, somebody who knows stuff. So why do they use manifold vacuum
for advance instead of ported vacuum> The replacement Weber carb I put
on had provision for either, but I don't know enough theory to figure
out the pros and cons.
--
Fast as lightning, slick as glass.
Out from Hell on a three day pass.
ralph
>
> Our '83 class-C Chevy motorhome has a (CA emissions) 350 4BBL HD
> (heavy-duty service), no cat converter or ECM, basic HEI ignition.
>
> Checking the timing yesterday, I found it at 8 deg BTC, where spec is 6
> deg. The engine basically runs fine with no audible knocking, and
> though it often 'smells' hot, the temp gauge stays low, barely reaching
> the bottom of the 'normal' range when climbing hills.
>
> This is a small powerplant for this use, and a rather low-revving
> engine. Power seems adequate near its peak, but on upshifts it
> definitely struggles under acceleration in the new gear.
>
> reduce or have no effect on the low-rpm performance, and what effect
> also as the rpms go up? What effect too on engine internal temps?
> Thanks.
>
>
> * Sent from RemarQ http://www.remarq.com The Internet's Discussion Network *
> The fastest and easiest way to search and participate in Usenet - Free!
point knock sets in unless you start using higher octane. Keeping an eye
on the plugs is a good idea until you are sure whether they are standing
up under the hotter combustion or not. If they are looking melted, go
colder. You might have to adjust the idle speed and mixture; you might
have to retard the timing again to pass smog test. Don't worry, though,
it won't turn into a gross polluter who should be guilty. Overheating
will only be a problem in as much as you use the extra power, and if the
colling system can't deal with it; if you are climbing the Rockies with
the engine floored and the cooling system doesn't have the reserve
capacity, that kind of thing. Doesn't sound like you will have the
problem, though.
Tuning by ear to just below knock is really the best way to do things;
setting the timing by the numbers is a generic procedure that doesn't
optimize for any individual engine.
Uncle Dodo
>reduce or have no effect on the low-rpm performance, and what effect
>also as the rpms go up? What effect too on engine internal temps?
>Thanks.
If you retard it 2 deg you will have less power and will probably not notice a
kingsnake
absence of pinging, is overheated spark plugs and/or high NOx
emissions.
If your NOx emissions are within limits, and the spark plugs look
good, then over-advancing the timing will save you fuel and increase
performance slightly.
"Timing by ear" is my favorite method. I advance it until it knocs
when test driven, the retard it a little bit at a time until the knock
just disappears, or maybe a little more retared. After a couple
hundred miles, I'll pull a plug or two (the four center ones on V-6's
and V-8's), and examine them for signs of overheating.
I had an old Ford (1968) F-100 pickup that ran best at 12 deg BTDC.
When I took it in to get it smogged, they had to set it back to 2 deg
BTDC. I drove it home, and put it back the way I had it.
On Mon, 08 Nov 1999 11:24:44 -0800, sbourg <bo...@hrl.com> wrote:
>Our '83 class-C Chevy motorhome has a (CA emissions) 350 4BBL HD
>(heavy-duty service), no cat converter or ECM, basic HEI ignition.
>
>Checking the timing yesterday, I found it at 8 deg BTC, where spec is 6
>deg. The engine basically runs fine with no audible knocking, and
>though it often 'smells' hot, the temp gauge stays low, barely reaching
>the bottom of the 'normal' range when climbing hills.
>
>This is a small powerplant for this use, and a rather low-revving
>engine. Power seems adequate near its peak, but on upshifts it
>definitely struggles under acceleration in the new gear.
>
>My question is: if I retard the 2 deg to spec, will this improve or
>reduce or have no effect on the low-rpm performance, and what effect
>also as the rpms go up? What effect too on engine internal temps?
>Thanks.
>
cla...@yahoo.com
You should always go with the factory recommended settings, but some
people like to advance a little more and I'll tell you why. Advance
setting (BTC) will increase torque at low RPM and reduce high RPM torque
(the effects aren't exactly drastic in either case, but noticable).
Running advance timing will make the car hotter but 2 degrees probably
wont even show on the temp gague, so I wouldn't worry about it. The
cheapest way to fix the low acceleration on upshift is to replace the
rear differential gear to something a bit numericly higher and remove
all emission controls (probably illegal). Installing dual exhaust
couldn't hurt either. The engine is small...but short of installing a
larger engine, the lack of torque on a vehicle that large will shorten
the life of the engine (makes it work harder). So take care to change
oil often.
In article <000b8d9b...@usw-ex0101-007.remarq.com>,
sbourg <bo...@hrl.com> wrote:
> Our '83 class-C Chevy motorhome has a (CA emissions) 350 4BBL HD
> (heavy-duty service), no cat converter or ECM, basic HEI ignition.
>
> Checking the timing yesterday, I found it at 8 deg BTC, where spec is
6
> deg. The engine basically runs fine with no audible knocking, and
> though it often 'smells' hot, the temp gauge stays low, barely
reaching
> the bottom of the 'normal' range when climbing hills.
>
> This is a small powerplant for this use, and a rather low-revving
> engine. Power seems adequate near its peak, but on upshifts it
> definitely struggles under acceleration in the new gear.
>
> My question is: if I retard the 2 deg to spec, will this improve or
> reduce or have no effect on the low-rpm performance, and what effect
> also as the rpms go up? What effect too on engine internal temps?
> Thanks.
>
> Steve
>
Sent via Deja.com http://www.deja.com/
Before you buy.
Uncle Dodo
>for advance instead of ported vacuum> The replacement Weber carb I put
>on had provision for either, but I don't know enough theory to figure
>out the pros and cons.
>Fast as lightning, slick as glass.
>Out from Hell on a three day pass.
>
>
>
>
I believe GM did the manifold vacuum thing on the 80-84s(approx) for gas
mileage reasons. My only problrm with that is the milage is great when you are
at idle to super slow accelleration. Anytime you give it over 1/4 throtle or
more you loose advance until about 1800-2400when the centrifical weights are
supposed to kick in the advance. Most of my driving is done NOT at idle.
I have checked mileage on mine both ways and it gets about 1.5-2 mpg better
with ported vacuum to dist and timing set higher than specs to right below the
ping threshold. It also give a very noticible power increase and eliminates
flat spots.
sbourg
uncl...@aol.com (Uncle Dodo) wrote:
> >My question is: if I retard the 2 deg to spec, will this improve
> or
> >reduce or have no effect on the low-rpm performance, and what
> effect
> >also as the rpms go up? What effect too on engine internal temps?
> >Thanks.
Thanks to all who responded - a lot to think about.
I think my first project in light of this will be to replace the 160
deg thermostat that was installed 6 yrs ago when we had the heads
replaced with a hotter one (180 or 190?). Warmup takes a long time, the
heater output is poor, and normal running temp barely reaches the
bottom of the scale. Mileage is worse than it should be - and I need to
help it pass emissions. I was unable to remove the thermo housing when
I tried this once before - it is located where it is a stretch from
either the front or inside, and I don't want to crack it. I guess I'll
just try harder. Anyway, I think I should do this before I play any
more with the timing, since I need to do it anyway.
Clagman, the rear end is 4.10 already, so any higher and this
low-revver won't get me fast enough to drive on the freeway. It has
dual exhausts now - big mammas from Walker - and is a bit louder than
I'm comfortabe with during 'quiet hours' in a campground. An honest
muffler man told me he could install nothing better powerwise that
would not also be even louder.
Dave Allen
>>My question is: if I retard the 2 deg to spec, will this improve or
>>reduce or have no effect on the low-rpm performance, and what effect
>>also as the rpms go up? What effect too on engine internal temps?
>>Thanks.
>
>If you retard it 2 deg you will have less power and will probably not notice a
>differece in temps.
>Almost all engines with a timing chain and over 40,000 miles on them need to be
>1 -2 deg advanced over stock due to chain stretch/slop.
>If you want more low end power advance it 2 more, drive it and advance it 2more
>again and drive. You will get more power after the shifts. You want to go
>about as high as you can without getting a ping under slight to mod
>accelleration.
>The vacuum advance also can effect all of this.
>An 83 in stock form will have full vacuum to the dist at idle. (SORRY DESIGN!)
>This give flat spots. I have had 83 305s and 350s. I re-route so the
>dist is a ported vacuum so it has no vacum at idle and vacuum when the throttle
>opens.
>Then reset timing to the advanced side until there is a slight ping and then
>back it off to no ping is evident on slight uphill slow accelleration.
>Many have their own methods, but this have been the best way I have found to
>get the most, smoothest power out of them.
I've got an 82 Buick Regal (sunroof, 150K, really in nice shape) which
my wife drives, originally a diesel & re-engined with an olds 350 &
Quadraject, HEI when I bought it from the engine rebuilder (former
Olds dealer mechanic and drag race motor builder -- put a 455 in my 81
olds diesel SW). Emissions are not a concern.
I have been having a lot of problems with tuning. The vac advance was
not functioning, so I replaced it with what was supposed to be an 83
vac advance unit (but I noticed it was missing a small hole in the
front for the vac advance case which the original unit -- marked "AS6"
-- had; but throttle anything over idle runs the advance to full. How
can I get a more gradual timing advance -- are there "adjustable" vac
advances?
I rebuilt the carb - a great project which turned out fine, but I
simply reconneced the vac advance to the same carb port from whcih it
came.
I didn't realize that others routinely do what I had to learn by
necessity -- tune by ear -- going to ping and backing off. Spec 1976
350 called for 8 deg TDC, but it would just cough, sputter & die when
shifted into gear. I finally went up to about 15 TDC (which
immediatley pegs at 17 when trottle is cracked). But even then the
idle is very high, and the trans (recently rebuilt) slams into gear. I
was convinced the problem was in the distributor, but now I am not
certain.
Could timing chain be a problem?? This motor has about 100K on it. I
want to slit my wrists every time I think about timing chain. In a
moment of extreme stupidity, I sold for $50 a 69 Cutlass S (white, red
buckets, console) which I had purchased new for $3K and maintained
perfectly for 12 years (when I made the very intelligent decision to
buy an 81 Olds diesel SW.)-- I mistakenly thougt the 350's valves were
burned. And the fellow who bought it had the ill-grace to inform me it
ran like a top when he replaced the chain. I can't bear to think about
it -- the horror of it all.
sbourg
I may be hazy on timing issues, but I do know vacuum systems! If the
vac-advance unit has a larger diaphram than the old one, then it will
pull harder against the spring in the distributor for the same vacuum
'pressure' in the line. This would explain a faster 'action'.
Uncle Dodo
line hole to adjust. Performance shop can order.
15 initial to 17 sounds funny.
The vacuum advance should increase it a lot more than 2 deg.
If idle is too high you probably have vacuum at idle. Pull the hose off
advance when at idle. The engine speed should remain constant and there
should be no vacuum at the hose.
If so the throttle plates are too far open.
Adjust idle screw on carb or idle solenoid.
Dave Allen
>They do make performance vacuum advance units with an allen head in the small
>line hole to adjust. Performance shop can order.
I think I may look into this.
>
>15 initial to 17 sounds funny.
>The vacuum advance should increase it a lot more than 2 deg.
>If idle is too high you probably have vacuum at idle. Pull the hose off
>advance when at idle. The engine speed should remain constant and there
>should be no vacuum at the hose
I've done this and it is as you describe. No advance action.
>If so the throttle plates are too far open.
>Adjust idle screw on carb or idle solenoid.
>
I've noodled with these from the get go --
set throttle plates per Jim Roe's book at time of carb overhaul --
same on idle speed screws -- these are really perplexing -- they have
an effect, but very difficult to control. Well over a hundred
different tweaks with short test drives in between to get near where
it should be -- about 2.5 out on one, 3.5 on the other. The RPM drop
lean method just didn't seem to work for me.
My problem: Even Rowe's book does not specify which carb ports control
the vac advance -- undoubtedly due to model/year variations. I have a
78 carb on a 76 engine. But I still am not confident I have the
correct port hooked to the advance. I know it should be one of the
ports that respond to throttle, but am sure which.
Thanks for the info on the adjustable vacuum pressure advance unit. I
might be able to get a greater range of adjustment on this. I was
wondering if there is any chance I have weak counter-weight springs,
or possibly some slop or other problem with the distributor unit. I
replaced the pick-up coil some time back when the leads failed with
age and excessive flexing from the vac advance. Module is sealed and
should be ok, right?
Anyway, thanks again for your thoughts on this -- it has given me a
few leads and ideas of my own to track down. I am virtually certain I
am not dealing with a vacuum leak. I hope I don't have a timing chain
problem.
Regards,
Dave Allen
Falls Church, VA