Recurving timing with distributor kit for performance and mileage

[gary19734]

Recurve the distributor and advance timing. This not only lowers exhaust temp by as much as 600° but picks up another two miles per gallon and a lots more torque.  cost 125.00 wires 80.00 distributor kit well worth the money  Gary

[Oz]

Reduces exhaust temp by 600° and improves torque and gets 2mpg more.

What were you getting for mpg before?  What is it now?  How did you measure the exhaust temp to determine the drop of 600°?  What was the temp before?  Exactly how much is "lots more torque"? 

Sounds more like an ad claim than actual fact, but I'm sure that I and thousands more RV owners would love to hear how you came up with these results so we can get the same results as you are!

[cncsparky]

Mark, here is a webpage I bookmarked partly describing the timing adjustments needed for our RV's, by a user named 'JIM83Itasca' .  I followed a bunch of his posts on multiple sites explaining his modifications to his RV's.  No doubt in my mind that timing plays a significant role in power and mileage of our beasts.  In fact, I sent my distributor to be recurved based on Jim's recommendations, but won't have my rig running till next spring.  Do a google search for Jim's username and read what he has to say.

http://forums.motorhomemagazine.com/Index.cfm/fuseaction/thread/tid/25062446/gotomsg/25077917.cfm

[Oz]

Will be looking forward to the results if there are no other mileage/performance mods made in addition to the recurving of the timing as it would be impossible to tell how much benefit was derived for each.

[bluebird]

Be very careful doing this, as you can destroy an engine very easily. You have to have enough octane in the fuel to compensate for increasing the ignition timing, or you can really hurt performance and mileage. I would have to see in person a vehicle that had the exhaust temp reduced by 600 degrees, by recurving the dist and increasing the timing. 

[gary19734]

These rigs where set up with an air pump and retarted timing witch burn alot of unused fuel in the exhaust manifolds burning out exhaust donuts cracking manifolds etc. temperatures running up to 1200 degrees recurving and advancing timing brings these temps down to around 600 at full pull been doing this for years.I do this to all of our older tow vehicles tow from delaware to the salt flats yearly and to our land speed track (ECTA) in ohio several times a year.Advance timing untill slight detination then adjust vaccum advance if anything ive got up to 80000 more miles out of a rig by having the fuel burn properly along with better torque, throttle responce,fuel milage.  This info can be found in any performance magazine out there check it out you will be happy you did.  Recurve kits can be purchased for 80 dollars at pep boys.   FYI Checked temp will infrared thermometer at full pull. Have same wires on motor home 80 thousand miles later havent even began to harden. air pump needs to be removed also.

[Oz]

So, this applies to specific motors with air pumps and not every motor?  The original stament sounds like it would be the thing to do for all motors. 

[DaveVA78Chieftain]

This discussion applies to Chevy 454 Carburated versions (89 and earlier).  Chevy changed over to computerized TBI in 1990 in order to gain more control over the combustion process and meet stricter emission requirements.

Air pump removal will depend on state where the rig is registered.

Dave

[Oz]

Got it, thanks for defining the topic better.  If it's applicable to Dodge and Ford, it sounds like the info may be beneficial for everyone.

[DaveVA78Chieftain]

The principles can be applied to any make however the "values" you set would be a tad different.   All mfg's detuned the engines to meet emissions regulations.  Ran the gambit from distributor curve changes, carburator adjustments, head modifications, cam changes, Vacuum advance port changes, to lower compression ratio's.   Much of this is discussed in the book "Rx for RV Performance & Mileage" (in CW site store) however they unfortunately never published the tuning specs for the different engines that the "kits" were developed for back in the day. 
Note: Make sure you know your states emission control requirements for your year rig (can be different by year also) before attempting any modifications.  You may get a rude awaking at inspection time if you don't.

The 454 setup on that link:

Problem is the vacuum advance must be cut WAY BACK from the stock 25* to 10* and so your ignition setup will be "10* base, 22* mechanical and 10* vacuum advance for a grand total of 42* while you lite foot it down the road, Remember stock is 4* base and 16* mechanical and generally 25* for a stock vacuum advance for a grand total of 45* while lite footing it down the slab.....

means you have to modify the initial timing setting, the mechanical advance curve, and the vacuum advance curve such that you NEVER produce more that 45 degrees maximum advance at any engine RPM.  With "ported vacuum advance" (72 and later), no vacuum, or at least very minimal vacuum, is applied to the vacuum advance so the timing at idle is basically equal to the initial timing.  For "ported vacuum advance" engine vacuum is not seen by the distributor until after the pedal is depressed.   For "intake vacuum advance" (pre-emissions era), the distributor sees full idle vacuum (17-25Hg).  So, at idle, you have full vacuum advance PLUS initial advance.  However, as you depress the pedal, engine vacuum drops thereby reducing the amount of vacuum advance.  Vacuum advance is a function of engine loading.  At idle it's at it's highest (17-25Hg).  Floor the pedal and it drops to 0Hg (applies to both ported and Intake versions).  As the engine approaches the cruise RPM, vacuum increases resulting in more advance.  The amount of vacuum depends on how hard the engine is working: lower going up a hill; highest on flat terrain; higher still coasting down hill.   In the end, how much vacuum advance is applied is directly related to engine load.

Mechanical advance however is a function of engine RPM.  Higher the RPM the more advance until the MAX allowed SET POINT is reached.  Where a race engine could have a much higher set point, a "over the road" vehicle needs a variable adjustment zone above this in order to allow timing to be tuned (vacuum advance) for maximum MPG.  Going past the maximum value (i.e 45 degrees for the GM example) results in pre-ignition and detonation.

Power/torque is basically governed by mechanical advance.  Mechanical advance does impact fuel efficiency by placing the spark far enough ahead of TDC so that nearly all the fuel mixture is burned on the power stroke.

Vacuum advance tweeks that setting even higher based on engine loading.    However, it should not push it high enough that pre-ignition occurs.

For Dodge engines, my post "http://www.classicwinnebagos.com/forum/index.php/topic,3424.msg12457.html#msg12457" provides the "stock" settings you can start from.  However, recurving the MOPAR distributor is not as easy as a GM distributor.  For MOPAR, the mechanical weights are buried under the "reluctor plate" (figures 16 thru 19 of http://www.classicwinnebagos.com/forum/index.php/topic,3424.msg12455.html#msg12455) where they are up top under the rotor on a GM distributor.   Can be done, just a bit more difficult.

For Chevy TBI engines you have to change computer mapping which is a whole different bag of worms and lots of $ invested in electronic toys to reprogram the ECU.

Dave

[M & J]

On TBI engines you can "cheat" a little. The computer counts on base timing set at 0 degrees and the map responds accordingly to the engine load from there. We used to put 2 - 4 degrees base timing in and the ECM ran the map from there. Need to make sure the knock detector was working or even 4 degrees would make it ping with poor gas.

[tmsnyder]

Here's a great read about timing:  http://www.camaros.org/pdf/timing101.pdf

I highly recommend reading it to understand what happened to these SMOG engines

It specifically talks about what GM did to timing in order to increase exhaust temperatures at idle to allow re-burning with AIR smog pumps by using initially late timing (only 4 degrees before TDC), and ported vacuum advance .  Poor performance and warped/cracked exhaust manifolds were the result.  My state doesn't require original emissions equipment on 25+ year old rv's, so it's not a concern for me, I'd rather have it running well and burning less fuel in the process.

Based on this I'm taking my 454 engine back to a pre-1971 setup regarding timing anyway.   Unfortunately I can't do much about the compression, and I'm not too interested in swapping out the cam.  But I have removed all the AIR pump hardware (including the exhaust port restrictors (air injectors) and have purchased a new mechanical advance kit and adjustable vacuum advance.   It's going back on manifold vacuum and it's going to be set at 12 degrees initial timing, 23 degrees mechanical for a total of 35 degrees at WOT.  The new adjustable vacuum can is going on as-is out of the box, it's set up to start adding advance at around 6 "Hg and max's out at 14 degrees engine timing at 12" Hg.   

This is more in agreement with how GM was setting timing before the government stepped in to 'help'. 

I won't have anything to compare the mpg after these changes, except the previous owner's claim of 8mpg.   I will report the 'after' mpg however, once up and running. 

Just some FYI and one person's plan in case someone is working on their air pump equipped RV

This discussion applies to Chevy 454 Carburated versions (89 and earlier).  Chevy changed over to computerized TBI in 1990 in order to gain more control over the combustion process and meet stricter emission requirements.

Air pump removal will depend on state where the rig is registered.

Dave

[Rickf1985]

This has been discussed before. In stock form the vacuum advance on an A.I. R. equipped vehicle is set up as manifold vacuum. AIR does nothing at idle, it is only injected above idle. The cylinder temps are dictated mostly by the EGR valve and when that is open it drops cylinder temps. The AIR system is designed to burn any leftover hydrocarbons as they leave the cylinder by injecting oxygen into the system. The whole emissions system plumbing diagram is on a decal on the side of the air cleaner. The key part of the system is the TVS mounted on the thermostat housing. And 4 degrees BTDC is perfectly normal for a big block truck engine, this is NOT in a Camaro remember. Do not go by everything you see on a Camaro website since they are talking about totally different versions of the same engine plus the vehicle weights are about 12,000 lbs. different!

[tmsnyder]

Hi Rick,

It  was written by a GM engineer.  Yes, RV's and passenger cars are different applications, but that would change the cam, head design, intake design etc to tailor the performance for more torque and hp at lower rpms.   Timing, not so much.  Something on the order of 34 degrees BTDC (initial plus mechanical) is what's needed regardless of the engine.  And the smog engines do deliver this, but it's what they did to the timing between idle and 3/4 throttle (where we run most of the time) which was messed up by the smog engines.


The ported vs manifold vacuum difference is covered in the article, here's a cut and paste on that section.  The highlights are added by me b/c these are what gave me an 'Ah-haaaah!' moment when I read them.  Trouble with cracked exhaust manifolds, retarded initial timing, inefficient performance (reduced power output and poor mpg)... yup, those are all a result of the gov't getting involved in the name of air quality. It was great for people in cities that couldn't breath at the time, but 25 years later one or two vintage RVs driving around the country with pre-1972 emissions isn't going to hurt anyone.

Here's a portion of the article, see if it resonates with you:

“PORTED” VACUUM: Now to the widely misunderstood manifold vs. ”ported” vacuum aberration. After 30-plus years of controlling vacuum advance systems with full manifold vacuum, that “free” indicator of engine load and fuel mixture, along came early emission control requirements (seven years before catalytic converter technology was introduced), and all manner of crude band-aid systems were introduced to try to reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these crude, but effective systems was GM’s Air Injection Reactor (A.I.R.) system, which pumped fresh air into the exhaust ports to “afterburn” pollutants in the exhaust manifolds. The key to making this system work at maximum efficiency was retarded spark at idle; with retarded idle spark timing, the “burn” begins late, and is not complete when the exhaust valve opens, which does two things that were important for emissions. The incomplete burn reduced combustion chamber temperatures, which reduced the formation of oxides of nitrogen (NOX), and the significant increase in exhaust gas temperature ensured rapid “light-off” and combustion of the hydrocarbons in the exhaust gas stream when the fresh, oxygen-carrying air was introduced from the air pump. As a result, these engines ran poorly, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to “run hot” at idle; cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it. “Ported Vacuum” was easy to implement â€" they just moved the distributor vacuum port orifice in the carburetor from below the throttle plate (where it was exposed to full manifold vacuum) to above the throttle plate, where it was exposed to manifold vacuum only after the throttle plate opened. This meant that the vacuum advance was inoperative at idle (retarding idle spark timing from its optimum value), and these applications also had very low initial timing settings; they were usually set at four degrees before TDC or less, and some even had initial timing settings as much as two degrees after TDC. The vacuum advance still worked at highway cruise, but not at idle, which caused all manner of problems. “Ported Vacuum” was strictly an early pre-converter crude emissions strategy and nothing more. Don’t believe anyone who tells you that ported vacuum is a good thing for performance and drivability â€" it’s not. Anyone with a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don’t understand what vacuum advance is, how it works, and what it’s for. There are lots of long-time experienced mechanics who don’t understand the principles and operation of vacuum advance either, so they’re not alone.


4 degrees initial timing or less, or ported vacuum to the distributor, AIR pumps....those are indicators that the engine was set up to run poorly in the name of SMOG, it was not set up to run efficiently using, up to 1971, proper timing.  By 1971, they knew exactly how to set timing for best all around performance.  What changed with SMOG were the emissions requirements, they had to sacrifice performance for emissions.

I'm taking my 454 back to the timing with which it will actually run decent   $15 for the mechanical advance kit (I used the medium springs), $25 for an adjustable vacuum advance (I installed as is), 15 minutes to install.  Just running it in the driveway, it sounds real good.  Can't wait to try a road test.

[Rickf1985]

So what are you setting your initial base timing at? How much total vacuum advance will you have and how much does it drop at 5" of vacuum? That is about half throttle. What will the total mechanical advance be at 2500 rpm which is about where they mostly run. You have me interested, now feed me the facts.

[legomybago]

So what are you setting your initial base timing at? How much total vacuum advance will you have and how much does it drop at 5" of vacuum? That is about half throttle. What will the total mechanical advance be at 2500 rpm which is about where they mostly run. You have me interested, now feed me the facts.

I want to know this too, and mileage numbers.

[cook elandan]

just adding myself to this thread.  I have followed Jim on another site and if it is the same Jim, I believe that he has since passed away.

[tmsnyder]

This is based on that article posted above, it's a great read.  Well worth the time to read it and think it over.

I'm planning to set the initial time to 10-12 degrees BTDC .  Right now it's 4.  Haven't done this yet.  4 is a dead giveaway that the SMOG demons have had their hand in the tune.  Every chevy motor I've seen pre-1970 has more like 10-12 degrees of advance timing.   Maybe theres an odd one out there, but standard chevy V8 timing was always around 10. 

This is the vacuum can I installed, Accel 31035.

http://static.summitracing.com/global/images/instructions/ACC-34000V.pdf

As delivered, it starts adding advance at 6.5 inHg and maxes out at 6.75 degrees distibutor timing, 13.5 degrees engine timing, at vacuum more than 12 inHg.  Hooked to manifold vacuum where it belongs

This is the mechanical kit I used, Moroso 72300:

http://www.moroso.com/sites/default/files/instructions/72300_inst.pdf

I put on the medium springs, so it should add zero advance until 1000-1500 rpm, then maxes out at 23 degrees at between 2500-3000 rpm

At crank:  10 degrees
At idle, 10 +13.5 = 23.5
WOT at or above 2500 rpm:  10 + 23 = 33

And cruising with the throttle just cracked and 2000rpm, it will vary based on vacuum but could be as much as 46.5 degrees which is what it needs.

Wish I had some results for you but it's still up on blocks with the tires off so I can get at the engine.  Replacing _all_ the 25 year old hoses and belts and working through some other annoying bugs.  All I'm doing now is sitting in my driveway rev'ing the engine and it sounds a heck of a lot better than it did.

So what are you setting your initial base timing at? How much total vacuum advance will you have and how much does it drop at 5" of vacuum? That is about half throttle. What will the total mechanical advance be at 2500 rpm which is about where they mostly run. You have me interested, now feed me the facts.

[Rickf1985]

Ok, sounds good but just so you know this was all discussed five years ago. I can tell you right now that 46 degrees ain't gonna cut it on today's gas unless you are running premium and that sort of negates any gains. But you are on the right path and I am curious to see how it works out for you. One of our current members also wrote up an article not long ago on using a distributor from a ZZ4 I think, crate motor, which has close to the needed curve in it.

Here is the article from back then.

http://forums.motorhomemagazine.com/Index.cfm/fuseaction/thread/tid/25062446/gotomsg/25077917.cfm

[tmsnyder]

Good to know, thanks Rick! 


46 would only be at the high vacuum / high rpm condition where the throttle is mostly closed.  If there were pinging at partial throttle / light load, then it would be from the vacuum advance, which is adjustable and could be backed off.   Thanks for the warning,  I'll keep an ear open for it!

[tmsnyder]

That looks to be a discussion about increasing the base timing on computer controlled models from 4 degrees to up to 8-10 if possible, until pinging then backing off the timing; but without changing out any of the advance components.  Without changing out the vacuum advance, yes you'll get a bunch of ping because those are set up to run on ported vacuum and add a bunch of timing under partial throttle / low load.  They come in as soon as they see any vacuum.


With the computer controlled stuff you should be able to just maintain those as is and they should work pretty well. 


For recurving the timing, I'm talking about the worst of the worst smog scenario, no computer, dual air pumps, carbon canisters all over the place, 1989 chevy, carburated 454.  Same engine essentially, as any V8 from Chevy from 1955 on except they messed with the timing, big time, after 1971.  If it's got a carburetor and a distributor, then it should be running on a vacuum advance designed for manifold vacuum (not ported) ($25) , about 10 degrees of initial advance ($0) , and about 23 degrees of mechanical advance ($15 kit).  No need to pull the distributor, it's all done from the top, at least on a chevy.

Here is the article from back then.

http://forums.motorhomemagazine.com/Index.cfm/fuseaction/thread/tid/25062446/gotomsg/25077917.cfm

[Rickf1985]

I was talking about the post that is right there when the link opens. This one. Computer controls do not have timing advance in the distributor.


""I have NO idea on your mileage increase would or could be...

When I purchased the 83 Itasca 33'(454) it was one sick puppy with 4.5 MPG would need to climb the hills in 2nd gear and run hot (215*) while climbing those hills.

Since I have been down this road befor with an 87 Winnie 22' it was not hard to figure out...First was tearing it down and replacing the smallish 4 row radiator and placing a 5 blade stock fan with a stock 7 blade along with a better (Stewart water pump) and it's related thermostat....

Next but not last was to dig into the poor ignition setup, Most but not all stock HEIs hand out only 16* of mechanical advance and mine was no different with it's 16* so off to the salvage yards (I was a partial owner of one at the time) and find the correct factory weights and centers that would hand me the 22* of mechanical advance this engine needed and bump up the initial (base) timing to 10* for a 32* max advance without the vacuum advance connected....

Problem is the vacuum advance must be cut WAY BACK from the stock 25* to 10* and so your ignition setup will be "10* base, 22* mechanical and 10* vacuum advance for a grand total of 42* while you lite foot it down the road, Remember stock is 4* base and 16* mechanical and generally 25* for a stock vacuum advance for a grand total of 45* while lite footing it down the slab.....

Problem is when you are hill climbing with a stock setup with near zero vacuum your engine is in the retarded (20*) setup and thats another reason they run HOT, New setup gives you 32* without the vacuum advance in play which is much better all around.

All of the hills in southern calif are now high gear pulls with vacuum to spare most of the time, Remember the vacuum advance starts to drop out around 6 inches of vacuum...

All related ignition componets are factory STOCK and yes I do run the better octane fuel in the heat of summer but can cut back to 89 octane in the cooler winter.....

Best on the road MPG has been 9.25 and the worst (Hills & wind) hand out around 8 MPG but I prefer the power while dragging around an "Chebby HHR" (3120 pounds).

Jim ""

[cncsparky]

Thanks for posting that, Rick.  I followed Jim's advice and set mine up like he did. Info is in my P30 project thread.   


http://www.classicwinnebagos.com/forum/index.php/topic,9139.msg50770.html#msg50770

[tmsnyder]

Yup, that particular post looks about right. 

I like this source better, it has a bit more history and technical stuff : http://www.camaros.org/pdf/timing101.pdf


One part of the post from Jim(?) that doesn't look quite right is where he says the vacuum advance starts to drop out at 6"Hg.  That wouldn't be true unless he kept the ported vacuum advance can, which would be a mistake.  It should be starting to advance timing at about 6 and maxing out at about 10 inHg when on manifold vacuum.  Here's an aftermarket vacuum advance can with a diagram showing the vacuum advance curve: http://documents.holley.com/accel_instructions_vacuum_advance_gm_hei_v8_31035.pdf  Installed as-is, they provide 0 deg advance when vacuum is <6 inHg, and 13 deg advance when vacuum is >10. 


Here's a couple examples of GM vacuum cans and how they worked, from http://www.digitalcorvettes.com/forums/showthread.php?t=75830


Here's a typical pre-smog (good, on manifold vacuum)  Starts at 5.5-8 inHg, max adv 12 distributor degrees (24 engine degrees) @ 14-18 inHg
Here's a typical post-smog (bad, on ported vacuum): Starts at 3-5 inHg, 7.5 max adv distributor degrees (15 engine degrees) @ 9-11 inHg


A stock GM ported vacuum can is going to kick in at a lower vacuum and add less advance.   The low vacuum point is what would cause a problem if someone connected it to manifold vacuum instead of ported, it would add a bunch of advance under a decent engine load and would then cause pinging.   But connecting to ported vacuum would make it worthless, b/c then you're right back to retarded ignition at idle and dumping unburned fuel out the exhaust.  For $25 just throw it away, put in a good aftermarket can and connect it to manifold vacuum where it belongs.


Plus I don't see the point in spending a day digging around in a junk yard for used parts that probably aren't there anymore anyway, when they can be had online for pretty cheap. $15 for the mechanical, and $25 for the adjustable vacuum can, made in USA. That way you know what you're getting too, with instructions.


Funny tidbit, I looked around for info on timing a 1948 8N tractor because that group has a healthy community of hobbyists.  Tractors don't have a vacuum advance, but usually will have mechanical advance.  Guess what the total timing at working rpm was in 1940's?  ~33 degrees!  And it's probably a bit low for safety's sake since they really had some poor fuel to worry about back in that day.


Anyway we arrive at it, for an RV with a distributor and no computer,  8-10deg initial, 33-36deg initial+mechanical, with 8-12deg manifold vacuum advance is going to be a vast improvement over the timing found in the smog engines.  (Probably on the lower end of those ranges just to be safe.)  And it goes back way farther than 1970!

I was talking about the post that is right there when the link opens. This one. Computer controls do not have timing advance in the distributor.


""I have NO idea on your mileage increase would or could be...

When I purchased the 83 Itasca 33'(454) it was one sick puppy with 4.5 MPG would need to climb the hills in 2nd gear and run hot (215*) while climbing those hills.

Since I have been down this road befor with an 87 Winnie 22' it was not hard to figure out...First was tearing it down and replacing the smallish 4 row radiator and placing a 5 blade stock fan with a stock 7 blade along with a better (Stewart water pump) and it's related thermostat....

Next but not last was to dig into the poor ignition setup, Most but not all stock HEIs hand out only 16* of mechanical advance and mine was no different with it's 16* so off to the salvage yards (I was a partial owner of one at the time) and find the correct factory weights and centers that would hand me the 22* of mechanical advance this engine needed and bump up the initial (base) timing to 10* for a 32* max advance without the vacuum advance connected....

Problem is the vacuum advance must be cut WAY BACK from the stock 25* to 10* and so your ignition setup will be "10* base, 22* mechanical and 10* vacuum advance for a grand total of 42* while you lite foot it down the road, Remember stock is 4* base and 16* mechanical and generally 25* for a stock vacuum advance for a grand total of 45* while lite footing it down the slab.....

Problem is when you are hill climbing with a stock setup with near zero vacuum your engine is in the retarded (20*) setup and thats another reason they run HOT, New setup gives you 32* without the vacuum advance in play which is much better all around.

All of the hills in southern calif are now high gear pulls with vacuum to spare most of the time, Remember the vacuum advance starts to drop out around 6 inches of vacuum...

All related ignition componets are factory STOCK and yes I do run the better octane fuel in the heat of summer but can cut back to 89 octane in the cooler winter.....

Best on the road MPG has been 9.25 and the worst (Hills & wind) hand out around 8 MPG but I prefer the power while dragging around an "Chebby HHR" (3120 pounds).

Jim ""

[Rickf1985]

It would probably be fairly easy to modify a stock HEI distributor with a limiting screw to be able to set the amount of vacuum advance using the original vacuum can. Problem solved without spending time or money but for the price of an aftermarket adjustable advance unit I would go that route. So you are doing the same as Jim, we eagerly await your results. I am going to do this to mine if I get time, A lot going on right now so I may or may not get it done.