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tuning HEI for 4spd 350

my kids took the truck

my kids took the truck

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I searched ck5 and don't see much about tuning the HEI - other than a few posts where there were numerous problems beyond the timing. In this case my blazer runs just fine - so we are talking about fine+

While figuring out a fuel evaporation hard start in another thread, one of the ck5 posters suggested tuning the timing. I'd never paid attention to my timing other than the baseline, the cap was clean, everything inside was moving, making sure the wires were not grounding, and there was no ping. My tuning method was to turn it up till it pinged at full throttle then dial it back a bit - usually required dialing back a bit more for part throttle pinging and hot days. I have had a GM Performance HEI on my blazer for the past 20 years. As this post pointed out - anything performance that is tuned for everything from a camero to a dump truck just does not sound right.

The website for the GM Perf HEI has these specs
  • Performance mechanical advance curve (what does that mean?)
  • Vacuum advance canister (there was a chance my HEI was shipped without one of those?)
  • Engineered for high-performance applications (see 'engineered' must be good)
Based on the advice I bought a Crane HEI tuning kit for $40.

As a foundation for comparison - this is my setup
  • '74 K5, 350cid, 4spd SM465 manual, 3.08 gears, and 32" tires, locker (if it matters) - an important consideration with the '75 and earlier K5 is they have no fuel return, so these have 'hot' fuel (I have quite a bit of heat shielding and insulators). Also, the aftermarket fuel pumps may overpressurize the needle and seat causing a rich condition at times - I don't have a pressure regulator installed but will by the end of this timing testing.
  • My main goal is the wife is happy the occasional time she drives the blazer
  • Engine specs: 750 cfm Q-jet with some Cliff parts, dual plane (early '90s design) alum intake with hot plenum crossover, '77 small valve 1.94/1.5 heads (part# tbd), flat top pistons (9:1), cam 204/214/112 (crane 12353917 basically an edel perf plus), 1 5/8" primary tri-y headers, and full length dual exhaust (no cats), mechanical fuel pump, cold compression is good --> solidly rebuilt by a respected shop 40k ago, they are known to do some extra machine work when they have time so you can guess what else they did.
  • The engine idles at 600 rpm and has 21 in-hg vacuum at idle - at freeway cruising the vacuum is usually around 17 in-hg
  • test 1 on this dyno test is close - because of the tri-y headers I have a more aggressive power (20HP) band in the 4,000rpm range that I never use
If you have already tuned the HEI for something comparable and found the right answer - please jump in and share.

In the next post I will review what is already on the web as far as HEI tuning for small blocks, the pages applicable to full size chevys, and then see if I can find anything that mirrors my specific configuration.
 
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-Performance mechanical advance curve--The advance curve is how fast the ignition advance ramps up. Mechanical advance is controlled by a spinning set of weights, and as centrifugal force increases, they advance the timing. You may notice in your tuning kit a variety of springs. These springs control the rate of advance. Heavier springs result in the advance coming on slower, lighter springs cause faster timing advance. Typically, conventional wisdom says that with heavy 4x4s you want a slower advance curve, but it comes down to personal preference and modifications.

-Vacuum advance--This looks like a little canister below the distributor. It is basically responsible for adding additional advance and is powered by engine vacuum (ported or manifold depending on preference). This gadget mainly comes into play on the highway, where it can markedly improve your fuel economy. Some people install a stop to limit the amount of advance that the can will provide, since if you're running much mechanical advance, chances are the total advance with the vacuum will be too much for most SBCs.

-Engineered for high-performance applications--Depends on your definition of high performance, but the HEI was a massive step forward in spark energy and reliability compared to points systems. For the vast majority of truck applications, it will be plenty adequate.

It sounds like you know you to set timing. Personally, when I had an HEI and carb setup, I just used an advance timing light, and set initial and maximum mechanical timing (after making sure my quadrajet was tuned properly), limited vacuum advance to 12 degrees, and played around a little bit with the different springs until I was happy. If you're serious about finding the sweet spot, perhaps a trip to a shop that can dyno tune carbs and HEI might be a worthwhile investment. So much of timing comes down to personal preference and what makes your individual rig run the best.
 
Honestly never got much out of one of these kits that a proper tune on the carb didn't produce better results. But it will give you something to play with, and in some event you might find a small power gain
 
thank you for your advice
  • @paratrooper307 I think your tuning process for timing is more or less what I am following
  • @bent72 The timing seems to have a big effect on my off-idle performance (wife able to drive without complaining performance - she has a light foot and it must have excellent low end drivability) that I cannot replicate with the carb unless I run it rich
Just to post it - my fuel economy before changing the timing: 12.7 mpg

I looked around on the web and found a few sources
  • There are a few CK5 pages on timing
    • This is a chevy truck specific tune - 15 deg initial plus 21 deg mech with light springs all in by 3,000 rpm caused pinging, he found 12 deg initital and the same 21 deg mech but with heavier springs all in by 3,800 worked for his truck
    • Talking about the adj timing kit here; includes an excellent rundown on the initial and mechanical timing components and the optional vacuum advance
    • In this post he is looking at the interaction between timing and carb tuning but really mostly talks carb tuning with wideband; all in by 3,000 rpm and 18 deg from the vacuum advance - previously he had heavier springs and less vacuum advance but got more aggressive with it.
  • This chevytalk page has the Crane spring table.
  • This crankshaftcoalition wiki has good info on the timing process.
  • This 73-87.com site gives real numbers with advice on timing.
  • This page at hotrod.com has fancy pictures but it also seems to have some good timing info relating to cam - if not a lot of extra words - vacuum advance should engage only over 10in-hg, before you hear detonation you already are fighting compressing the expanding combustion (still, time timing to just before the point of detonation), a 220 deg cam typically is combined with less (10-12 deg) initial timing than a more aggressive cam, mechanical timing should not come in below 1,000 rpm.
  • More car talk at stevesnovasite.com with a good thread on just mechanical advance with plates and weights (nothing about springs or vacuum cans) - for now I kept my plate and weights the same so this is just here to be complete.
  • This chevydiy.com page seems to just make it all simple - usually a good sign the author knows their stuff.
  • Found the specs for my GM Performance Distributor
    • This hotrodders.com thread has the GM Perf Dist as the same distributor on the ZZ4 crate engine - the long and short is a retard engineer (prob had an MBA) setup the GM Perf Dist, it has to have a crane kit or similar added to it
    • Over at 67-72chevytrucks.com they really dig into the ZZ4 distributor and the factory specs - says to keep the plate and weights, throw away the springs (starts at 1,400 rpm and all in at 5,500 rpm), they mod the vacuum advance to tune for full vacuum instead of ported vacuum (the root of the problem).
    • The guys at clubhotrod.com figured out the vacuum canister - it starts at 2 in-hg and by 7.5 in-hg it is the full 20 deg of advance; explains the problem I always had with pinging at part throttle (had to retard my base timing). The GM Perf Dist is basically all vacuum advance and no mechanical advance during normal driving (if you never get above 2,000 rpm normaly).
    • The big issue with the GM Performance Distributor is it is intended for ported vacuum - the following two thread have big, big, discussion on that topic
  • As far as the process of tuning, I did not find anything better than what paratrooper307 says (carb tuned first) - the Crane kit instructions are simple, no timing light just tune until it pings then back it off
    • First disconnect the vacuum (no vac advance) and put on the heaviest springs (no mech advance) then turn up the base timing until it pings under load (try hills too).
    • Then start with either heavy springs and work down (until it pings) or light springs and work up (until it stops pinging) depending on your style. I did not play around with plates and weights so I assume like paratrooper307 says you'd play around with max mechanical by changing plates or limiters.
    • Last, after all that connect the vacuum and adjust the vacuum can until there is no ping at light/part (aka high engine vacuum) throttle (I find it a problem on hills). Check high gear at 35 mph and 65 mph on slight grade. The Crane instructions say to play around with vacuum limiters but I did not do that.
    • Retune the carb - I fattened the idle screws a quarter turn to eliminate low speed bucking and get a less sensitive off-idle clutch transition (both relate to my 3.08 gears, SM465, worn trans to t-case adapter splines, and 32" tires).
So there is my review of what is already out there - a few of these links I need to reread and write a better summary of the core contribution of why I included that specific page.
 
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You've obviously got the ambition to work through things like this, what I've never understood is why you couldn't run a minimum of base timing (as close to zero as possible) and a very healthy vac advance to get you to something like 20* at idle.

AFAIK all the GM EFI of the 80's+ run much higher higher advance at idle (right around 20*), and it cleans up idle a bunch.

Maybe there are some mechanical limitations I'm unaware of, but if possible, I would think high vac advance, and healthy mech advance would be somewhat simple to do, if one took the time to play with the various components.

I would *think* that there would be a tradeoff such that you could run, for instance, a vac can that would pull 20* (if such exists) while still having your mechanical ramp up to provide the proper curve for power.

I dunno, stuff like this is why I went injection. There is a LOT to do with carbed setups to make them run better, but it's a lot of mechanical picking and choosing and modifying, whereas it's instantaneous with EFI tuning. Not trying to convince you one way or the other, but I did have aspirations at one point of trying to get a carbed motor to run with a curve more similar to what EFI is able to achieve.

When you throw out emissions testing as a requirement to meet with a tune, I think that opens up more options to making a good all around setup in terms of fueling and timing. GM was saddled with that back in the early 70's and the tech available then, and instead of just going EFI early on in the 80's, chose to bandaid carbs with electrical junk that also had to meet emissions criteria.
 
@dyeager535 you just described the GM Performance distributor setup (20 deg vacuum adv at idle with maniford vac) - they probably expect you to set the Perf dist to the factory 3 deg advance; more or less that is zero.
 
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That seems odd (sorry, can't read through all the links you posted) most every GM carbed setup starts with 8* as I recall. But hey, if that's what it is, at least for idle and cruise the vac advance should be plenty.

If that's the case, mechanical plus base, if any, would be the only thing to mess with as I see it.

Also, not sure if the vac cans are the same as the secondary pulloffs/breaks, but I know GM "tuned" the hole in the nipple on those to allow pressure to effect change more or less quickly, if vac cans were the same way, perhaps they could be tuned as well, if needed.
 
I changed the 20" vacuum can for the CRANE adjustable can - the problem with the GM Perf 20" can is I'd get a part throttle ping and the only option was to back off the base timing which resulted in poor off idle drivability unless I richened the mix.
 
Was that with 0* base timing? How much will the crane pull max? Part throttle ping, could that not be removed by increasing the RPM for the mechanical to be all in?

I assume your idle at 20* was good?
 
yes, the idle was good

The base timing was the most I could get without ping - it has been 20 years since I put a light (exhaust anylizer at community college tuning class) on it but somewhere around 12 deg.

About the springs you are right, but the springs in the GM Performance HEI are heavier than the heaviest Crane kit spring. The ping was at highway cruising 2,100 rpm and the springs just start to come in at 1800 and are not all in until 5500 so at 2100 it must have been just barely contributing any advance. At cruise it was just too much vacuum and created a highspot inbthe curve that then pinged slightly.

When I got into the higher rpm it was always under heavy throttle so the vacuum advance went to zero and the mechanical advance took over.
 
Don't people usually tune for high 40's on the carbed setups with older heads and cruise? Part throttle cruise, pinging, if under 40 some degrees, I would *think* would be a fueling issue.

I know back in the day on my 305 (which were high compression with bad chamber design, so recipe for pinging) as soon as I removed the EGR, no other changes, I started running into a lean surge. Slight richening via the primary side took care of it.

Of course, since the needles are all tapered differently, and the power piston springs were different in the carbs, I'm sure it's all tied together, and part throttle cruise is probably the toughest to get right when everything else is good.
 
good point - the problem first showed with my smogger holly that ran lean. Last year (as you helped advise) I changed to a QJet and solved the part throttle ping with a richened auxiliary power. But, even then I was unable to gain base timing and the off-idle divability was sub optimal.

My guess on total timing with the GM Perf HEI is it was 12 deg base plus 20 deg vacuum for 32 deg total before the mechanical. At 2,100 rpm - maybe a little higher rpm - the mechanical contributed 4 deg for 36 deg total on the highway. It was a warm summer day and I was driving up a steep grade at 60 when it last pinged part throttle - it was about as severe a scenario as there is (without trailer).

You are right - I should not have been pinging yet and I did resolve it with more fuel. But the off idle was still not there.
 
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  • The power piston spring is one Cliff sent http://www.cliffshighperformance.com/
  • no EGR
  • The GM Perf HEI ping w/ factory timing curve
    • Was on a hot day on a steep grade at highway speeds but in traffic so the engine rpm was a little lower than I'd like for powerband (under load) - I solved the ping by richening the idle (1/4 turn) and power piston (1-turn). Though I could not advance the timing or the ping returned.
    • The part throttle ping is resolved - if it was fuel or timing curve related I won't know until it comes back or I get a wideband
  • My wife likes the new timing curve
    • The on ramp to the expressway has a sharp curve that you had to slow way down for then drop it into 2nd gear before accelerating up the ramp and onto the expressway - dropping from 3rd to 2nd for the curve and then immediately back into 3rd to accelerate was annoying
    • She said she can make that same turn now in 3rd
  • After a few tanks I will post the fuel economy and after I have driven more than around the block a few time to vrfiy this setup works for hills and freeway driving in different temperatures I will post my base, mechanical spring, and vacuum can settings. So far I have not played around with limiting the total vacuum advance.
 
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I little more on the lean bucking at idle and just off idle at low speed. This post at cliffshighperformance.com describes the same symptoms and they resolved it with a timing change; they went to manifold vacuum (vac. advance at idle) from ported vacuum (no vacuum adv. at idle). The next post suggests tuning the idle circuit by drilling ports. I think this is what you are talking about @bent72 with tuning the carb in your post #3.

A few more post on low speed bucking and tuning - some of these suggest richening the idle circuit by drilling the passages
After reading these I am seeing how a poor timing curve will cause low speed bucking (unless the carb is rich) and I see the next step is to richen the idle circuit; I guess the timing resolves many problems but it is still a lean condition in the idle circuit.

I am going to try five things one at a time
  • incrementally richen the idle with a 1/4 turn each and see what happens (set idle screws at upper end of vacuum drop, on lower end now)
  • lower the mechanical advance tip-in point from 800 rpm to 600 rpm with silver/silver springs
  • get more aggressive with base timing and 92 octane (fuel prices are dropping)
  • (won't work) more vacuum adv at idle - the Crane has 14 deg advance at 17 in-hg and starts at 5 in-hg - I will try a more aggressive can with 20 deg adv at 17 in-hg with the same start (the GM Perf HEI can is 20 deg adv at 7.5 in-hg starting at 2 in-hg, this is too much)
    • nice vacuum can part # list over at corvette-restoration.com though the most aggressive is 15 deg adv at 15 in-hg, close to the Crane can.
    • 67-72chevytrucks.com thread with comparable canister list and the source of the list - has nice vac advance rule of thumb
      • advance should be all in at not less then 4" less than idle vacuum - my idle is 21" so my vac can should be maxed at 17" - it is maxed at 14"
      • this means the GM Perf HEI with 7.5" is intended for a 11.5" idle vacuum
      • I ran 20 deg at idle without a problem and since I changed to 14 deg at idle I do smell a bit more fuel at idle - I need an adjustable vacuum can with a higher vacuum advance than 14 deg
    • it looks like the GM Perf HEI 20 deg vacuum canister is one of the highest advance cans (and fastest at 7.5 in-hg) available - the rest are comparable to the Crane adjustable can
  • incrementally richen the auxiliary valve 1-turn at a time (should let me lean the base idle at the same time)
 
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Why don't you just raise your APT 1 turn and see what happens, could just be lean just off idle.
 
Typically see issues like the bucking when the power circuit is out of whack. Is the spring under the power piston in the primary circuit? If not, what color is the spring?
 
@bent72 and @blazer74
  • Two turns of the APT did it - with one turn it still bucked; drives really nice, we just cruised El Camino for half an hour it was so nice
  • After recurving the timing the gas mileage is 8.7 mpg; that is the worst I have measured - it was all around town in traffic but I have been getting 12 mpg in the city. I don't smell fuel. I will have a long highway cruise next week for thanksgiving so I will get a better measure.
  • @bent72 what do you mean by "in the primary circuit"
  • I don't remember the color of the spring - it is one Cliff sent
This photo is of the power piston spring, but I think that is the one that came out. According to Cliff I have the right one.
Me: "Cliff, I got one spring for the power valve. You sell a kit with four springs. Based on the information I provided on the application were you able to determine the right spring, or was that spring the standard spring?"
Cliff:"I supplied you the correct spring based on the info you provided, no worries....tks...Cliff"

20150215_163013.jpg

note: to get the thumbnail instead of the large image, change 'ATTACH=full' to 'ATTACH'

20150215_163013.jpg
 
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Light blue or dark blue/black power spring probably.
All that mumbo jumbo, 10-12 initial with another 22-24 mechaninical adv starting at 1100 rpm gives you 32-36 all in around 3k. Add another 15 vac advance makes 47-51 at cruise. At idle would be initial 10-12 plus 15 vac is 25-27. Should work just fine for you.
 
I seem to recall it was a blue spring,

but could also have been green.

Just to put it in here, this nastyz28.com page has a nice dataset on the APT spring opening points http://www.nastyz28.com/forum/showthread.php?t=216888

The z28 thread suggests picking a spring with an opening at 1/2 of idle vacuum. My idle vacuum is 21" so I should have a 10.5" ADP spring. I am probably well under that point since even the Dk Blue spring (heaviest) does not open until vacuum drops to 8.5". Turning a corner without downshifting and when starting from a stop in 1st gear (true 2nd gear) - it at times drops to 7". On the highway driving normally (even on steep grade) it never gets down to 8.5".

The z28 thread relates there is a Brown spring (Cliff discontinued) that opens at 11.5" and that is probably what I need.
 
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@blazer74 I agree - with a few changes
  • 10-12 initial
  • 22-24 mechanical adv starting at 800 rpm to give 32-36 all in around 2,600 rpm (Crane blue/yellow springs)
  • 15 vac advance starting at 5" and all in by 17"
  • Estimated
    • At idle 10-12 base plus 15 vac is 25-27 (if Crane silver/yellow spring mech is 2.5-3 for 28-30)
    • At (60 mph 2,000rpm) cruise 10-12 base plus 15 vac plus 15-16 mechanical for 40-43 (if Crane silver/yellow spring mech is 22-24 for 47-51)
    • At 2,600 rpm (78mph) cruise 10-12 base plus 22-24 mech plus 15 vac for 47-51
    • full throttle 10-12 base plus 22-24 mechanical for 32-36
With this configuration and 87 octane on a 56 deg F day I found a slight detonation at 2,000 rpm under a light acceleration on a light highway grade pulling vacuum down to 10-12" but then on a fairly steep highway grade with no acceleration there was no ping even at 12" so I don't know. I turned back the vacuum advance 1 degree with no change.

I will try adding 1 turn of APT and see if it is a lean spot.

Other than the ping at 10" & 2,000 it drives really nice and fuel economy seems good - just measured 14.5 mpg today and 12.2 mpg yesterday (added fuel pressure regulator this morning).

The nasty z28 thread says the APT is all in by 8.5" but where does it start - is it 14.5"? If 14.5" is that start point then on a flat grade (17") and even slight grade (15") I am over 14.5", so that looks good. Also, how much does the APT adjustment change this, what is the all lean and all rich range?

In the photos are
  • A try at the silver/yellow spring (start 500 rpm and all in at 1,800 rpm), this was too aggressive and I settled on the blue/yellow (start 800 rpm and all in at 2,600 rpm) - I will try the step up (2,800 rpm) and step down (2,200) to verify.
  • The crane cam springs and the stiff GM Perf HEI spring (1,400 rpm to 5,500 rpm)
  • Back of paperbag notes with father-in-law figuring out interaction and tradeoffs between base timing, and mechanical and vacuum timing curves, as well as air/fuel ratio at part throttle - the big unknown is the APT range of adjustment (notes on left) from full lean to full rich in relation to vacuum and degree of enrichment; I will replace these notes with images of tables and charts made in excel.
if you know how to resize these, please let me know

20151101_133358.jpg

20151101_143434.jpg

20151129_131208.jpg
 
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