Home » Public Forums » GMCnet » Relative compression test & Cam timing
| Relative compression test & Cam timing [message #334152] |
Thu, 28 June 2018 23:15  |
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GMC2000
Messages: 193
Registered: March 2018
Location: Georgia
Karma: -3
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cam chains on the Olds 455 seem to have a pretty bad reputation as well as the plastic gear. many GMC folks suggest replacing them at 75 - 80,000 miles.
most of the GMCs Ive looked at have more miles and some the engines do not run at all.
Ive been pricing out common replacement parts and whats involved in the replacement of them. ran across a tidbit about replacing the timing chain with a cloyes racing t-chain set that has a multi slotted lower gear so you can advance or retard the camshaft upon replacement.
someone somewhere (lol) mentioned that someone else (Dick Paterson) said (all hearsay lol) that he recommends using the slot for four degrees advancement on the 455 in the GMC. its actually widely recommended just about everywhere for V8s with stock camshafts but not aftermarket because the 4 deg advance is built into them. many say 2 of those degrees is nullified almost right off the bat upon t-chain break-in.
but generally, advancing the camshaft moves the torque down lower in the rev range which seems like a good idea for a GMC but then I thought is it really a good idea because of the heat generated by the 455 and a 12,000lb load?
so Ive been searching for info about the heat effects of altering the camshaft timing. I havnt found any but it seems to me it would have to? at least the combustion temps anyway? but no luck, nothing about it that I can find but I did find some other very interesting things..
the first one is that you can experiment with the v8 cam timing (if you have an adjustable gear or gear pin) by doing several compression tests starting with no advance, then use the first advance setting, do another compression test, then the first retard setting and a compression test and then depending on which setting had the highest readings (only have to test one and the same cylinder BTW)
go to the next adjustment in that direction and retest and so on until the reading start to drop again. then go back to the cam timing adjustment that gave you the highest compression reading.
I thought about this because Ive never heard about doing this. it seems to me it would be great for RVs and low rev engines but for high rev engines and camshafts with lots of overlap, I dont think it would be a good idea.
anyone here tried this? seems like a good idea if you dont have a dynomometer handy.
also while seeking out info in this area, I discovered the new schools "relative compression testing" I think some of y'all are gonna like this:
https://www.youtube.com/watch?v=BEklqMPkAbQ
a way to test compression (and other things) without removing sparkpugs! check it out.
still havent found any insights into how, if at all, cam timing on a V8 effects combustion temps either way, anyone?
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| Re: [GMCnet] Relative compression test & Cam timing [message #334153 is a reply to message #334152] |
Thu, 28 June 2018 23:24  |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Factory marks are sometimes not very precise. It pays to degree in the
camshaft before any serious attempt to time one.
Jim Hupy
Salem, Or
78 GMC ROYALE
On Thu, Jun 28, 2018, 9:16 PM GMC2000 wrote:
> cam chains on the Olds 455 seem to have a pretty bad reputation as well as
> the plastic gear. many GMC folks suggest replacing them at 75 - 80,000
> miles.
>
> most of the GMCs Ive looked at have more miles and some the engines do not
> run at all.
>
> Ive been pricing out common replacement parts and whats involved in the
> replacement of them. ran across a tidbit about replacing the timing chain
> with
> a cloyes racing t-chain set that has a multi slotted lower gear so you can
> advance or retard the camshaft upon replacement.
>
> someone somewhere (lol) mentioned that someone else (Dick Paterson) said
> (all hearsay lol) that he recommends using the slot for four degrees
> advancement on the 455 in the GMC. its actually widely recommended just
> about everywhere for V8s with stock camshafts but not aftermarket because
> the
> 4 deg advance is built into them. many say 2 of those degrees is nullified
> almost right off the bat upon t-chain break-in.
>
> but generally, advancing the camshaft moves the torque down lower in the
> rev range which seems like a good idea for a GMC but then I thought is it
> really a good idea because of the heat generated by the 455 and a 12,000lb
> load?
>
> so Ive been searching for info about the heat effects of altering the
> camshaft timing. I havnt found any but it seems to me it would have to? at
> least
> the combustion temps anyway? but no luck, nothing about it that I can find
> but I did find some other very interesting things..
>
> the first one is that you can experiment with the v8 cam timing (if you
> have an adjustable gear or gear pin) by doing several compression tests
> starting with no advance, then use the first advance setting, do another
> compression test, then the first retard setting and a compression test and
> then depending on which setting had the highest readings (only have to
> test one and the same cylinder BTW)
>
> go to the next adjustment in that direction and retest and so on until the
> reading start to drop again. then go back to the cam timing adjustment that
> gave you the highest compression reading.
>
> I thought about this because Ive never heard about doing this. it seems to
> me it would be great for RVs and low rev engines but for high rev engines
> and camshafts with lots of overlap, I dont think it would be a good idea.
>
> anyone here tried this? seems like a good idea if you dont have a
> dynomometer handy.
>
> also while seeking out info in this area, I discovered the new schools
> "relative compression testing" I think some of y'all are gonna like this:
>
> https://www.youtube.com/watch?v=BEklqMPkAbQ
>
> a way to test compression (and other things) without removing sparkpugs!
> check it out.
>
> still havent found any insights into how, if at all, cam timing on a V8
> effects combustion temps either way, anyone?
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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| Re: [GMCnet] Relative compression test & Cam timing [message #334156 is a reply to message #334154] |
Thu, 28 June 2018 23:59 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Honda V4 motorcycle engines used 4 cams, chain driven with advance/retard
capabilities, and no reference marks. You HAD TO degree them in. There was
no other way to do it. Took a good long time with no distractions to
accomplish. Lots of experience with those things.
Jim Hupy
On Thu, Jun 28, 2018, 9:52 PM GMC2000 wrote:
> James Hupy wrote on Thu, 28 June 2018 23:24
>> Factory marks are sometimes not very precise. It pays to degree in the
>> camshaft before any serious attempt to time one.
>
>
> yes, thats a given. I should have mentioned it, thanks. Ive seen/worked on
> engines that had four camshafts, spin the opposite direction of the
> crankshaft, and had no timing marks at all! real PITA
>
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| Re: Relative compression test & Cam timing [message #334161 is a reply to message #334152] |
Fri, 29 June 2018 07:18 |
Chris Tyler
Messages: 458
Registered: September 2013
Location: Odessa FL
Karma: 7
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Have played with cam timing on other engines for some time.
You are correct that most aftermarket High performance cams have advance built in. If you check the timing card, the lobe centerline is the number to look at. {Not to be conused with the lobe separation angle.]
Some factory engines [notably 460 fords] have a lot of retard but in. Timing chain slack agrevates it.
Most stock engines will see higher cranking compression and idle vac with the cam advanced. On a low compression engine, This helps low end torque . The loss at the top end wouldnt hurt on our application. Degreeing the cam is always a good idea and veriying TDC marks is a must. With a stock cam, valve cleaence shouldnt be an issue at 4 degrees.Once you start changing the specs, it becomes more critical
An overcammed engine will benifit from being advanced as well. Conversly a very high comression engine will benifit from bleeding off some pressure to avoid detonation, and a n undercamed engine will improve the top end power retarded
Also, be aware that if you change the valve timing the ignition timing will need to be adjusted as well
76 Glenbrook
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| Re: [GMCnet] Relative compression test & Cam timing [message #334219 is a reply to message #334216] |
Fri, 29 June 2018 20:03 |
emerystora
Messages: 4442
Registered: January 2004
Karma: 13
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Senior Member |
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You say that GM calls for 91 octane. You must have read that in a 1973-1978 manual.
The method of setting octane today is quite different. They take the average of two methods. So, today’s 86 octane is equivalent to that eras 91 octane. You can burn 86-87 octane fuel in a 455 or 403 GMC engine just fine.
Also, power has nothing to do with octane number. Higher octane won’t give you more power. It just eliminates pinging (pre-ignition).
I also find it strange that you won’t tell anyone your name or location even when some have asked. Paranoia?
Emery Stora
77 Kingsley
Frederick, CO
> On Jun 29, 2018, at 6:12 PM, GMC2000 wrote:
>
> I think this cranking compression method using an adjustable gear to find the highest compression reading is a good way to go on these. this page
> explains it well http://www.superchevy.com/how-to/81679-tuning-engine-compression/
>
> too much will make more heat but I dont know if thats possible with a stock 455 but it may could get into the range where premium fuel must be used.
> and thats another thing to consider, GM says these MHs should run 91 octane so perhaps with cam timing adjustment alone, you could make it run better
> on lower grade fuels. (sacrificing some power obviously)
>
> seems you could also get it running cooler too so if the intake manifold isnt cracked already, it might help with preventing that for engines that
> have original intakes with active exhaust crossovers.
>
> thats another thing to consider with the modern programable FI systems. if I were gonna experiment with any of those. I would not do it without some
> way to measure combustion temps because getting the most out of the engine that way is going to make the most heat. even with a blocked crossover
> there is still the risk of burning exhaust valves if they are OEM valves and especially high mileage ones.
>
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| Re: [GMCnet] Relative compression test & Cam timing [message #334221 is a reply to message #334219] |
Fri, 29 June 2018 20:30 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Geez, I wonder what the difference might be in BTU comparison between a
heavily laden GMC motorhome at steady state 65 mph and a lightweight
passenger car rolling beside it in the left lane. Also the Octane
requirements. Do you think there might be some differences? I already know
the answers, so don't spend much time figuring it out.
The larger the throttle opening, the more fuel goes through it, and
the more heat is produced. Lightweight vehicles have different requirements
than do heavy vehicles. Cam timing requirements, too.
Jim Hupy
On Fri, Jun 29, 2018, 6:04 PM Emery Stora wrote:
> You say that GM calls for 91 octane. You must have read that in a
> 1973-1978 manual.
> The method of setting octane today is quite different. They take the
> average of two methods. So, today’s 86 octane is equivalent to that eras 91
> octane. You can burn 86-87 octane fuel in a 455 or 403 GMC engine just
> fine.
>
> Also, power has nothing to do with octane number. Higher octane won’t give
> you more power. It just eliminates pinging (pre-ignition).
>
> I also find it strange that you won’t tell anyone your name or location
> even when some have asked. Paranoia?
>
> Emery Stora
> 77 Kingsley
> Frederick, CO
>
>> On Jun 29, 2018, at 6:12 PM, GMC2000 wrote:
>>
>> I think this cranking compression method using an adjustable gear to
> find the highest compression reading is a good way to go on these. this page
>> explains it well
> http://www.superchevy.com/how-to/81679-tuning-engine-compression/
>>
>> too much will make more heat but I dont know if thats possible with a
> stock 455 but it may could get into the range where premium fuel must be
> used.
>> and thats another thing to consider, GM says these MHs should run 91
> octane so perhaps with cam timing adjustment alone, you could make it run
> better
>> on lower grade fuels. (sacrificing some power obviously)
>>
>> seems you could also get it running cooler too so if the intake manifold
> isnt cracked already, it might help with preventing that for engines that
>> have original intakes with active exhaust crossovers.
>>
>> thats another thing to consider with the modern programable FI systems.
> if I were gonna experiment with any of those. I would not do it without some
>> way to measure combustion temps because getting the most out of the
> engine that way is going to make the most heat. even with a blocked
> crossover
>> there is still the risk of burning exhaust valves if they are OEM valves
> and especially high mileage ones.
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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| Re: [GMCnet] Relative compression test & Cam timing [message #334226 is a reply to message #334224] |
Fri, 29 June 2018 21:28 |
C Boyd
Messages: 2629
Registered: April 2006
Karma: 18
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Senior Member |
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Fred: Bet it is the only one with a Black List to.
http://bdub.net/Black_List/GMCAssist.pdf
http://bdub.net/Black_List/
GMC2000 wrote on Fri, 29 June 2018 22:01emerystora wrote on Fri, 29 June 2018 20:03You say that GM calls for 91 octane. You must have read that in a 1973-1978 manual.
The method of setting octane today is quite different. They take the average of two methods. So, today's 86 octane is equivalent to that eras 91 octane. You can burn 86-87 octane fuel in a 455 or 403 GMC engine just fine.
Also, power has nothing to do with octane number. Higher octane won't give you more power. It just eliminates pinging (pre-ignition).
I also find it strange that you won't tell anyone your name or location even when some have asked. Paranoia?
yep, thats where I read it, not sure which year it was now though. the link in my last post will explain it. why do you think racers use race fuel?
I find it strange that out of all the open internet forums Ive been and am involved with, which is probably more than most people, that this is the only one ever that has an extreme target fixation on names and locations.
C. Boyd
76 Crestmont
East Tennessee
[Updated on: Fri, 29 June 2018 21:39] Report message to a moderator |
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| Re: [GMCnet] Relative compression test & Cam timing [message #334238 is a reply to message #334237] |
Sat, 30 June 2018 07:01 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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Ken Henderson wrote on Sat, 30 June 2018 06:38We GMC Dixielanders were lucky that when Joe Mondello "retired", it was to
Crossville, TN, hometown of his wife Mary. Having a GMC motorhome (with a
Mondello engine!) he became very active in the club. We were all amazed at
his knowledge, and willingness to share it. Certainly we miss him greatly.
Ken H.
On Sat, Jun 30, 2018 at 3:22 AM GMC2000 wrote:
> ...
>
> good photo and article of The Man dialing it in:
>
> http://www.hotrod.com/articles/your-turn-remembering-joe-mondello/
>
>
I had a couple a very good discussions with him at our GMC rallies. Then he was gone at much too young.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Relative compression test & Cam timing [message #334242 is a reply to message #334152] |
Sat, 30 June 2018 08:05 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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Listen to JimH. Timing and particularly advance requirements are different from light to heavy. More than one VW engine in a bus died because someone put the wrong (Vacuum advance) distributor in it instead of the rpm only advance distributor.
--johnny
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] Relative compression test & Cam timing [message #334248 is a reply to message #334233] |
Sat, 30 June 2018 09:33 |
Dave Stragand
Messages: 307
Registered: October 2017
Karma: 0
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Yes sir, those are the analyzers. I went through a “collecting professional shop equipment” phase for a while and at one point had 5 of the big analyzers.
In the mid 1980’s, a “complete computerized diagnostic test” was a big upsell at garages. My old Bear ACE analyzer used to run a semi-automatic complete test that took around 10 minutes to run, and would produce long, detailed printed reports for the customer. It had a full complement of on-demand diagnostics and emissions tests as well, and could interface with OBD-1 and EEC-IV (the old Ford proprietary) electronics of the era. It was my favorite, but it was getting hard to find parts for.
I still have two Bear PACE 200’s. One has the complete cabinet and even wheel alignment capabilities. The other Is stripped down and made portable, with a laptop, a “lunchbox” analyzer and the “suitcase” emissions analyzer.
It’s been a while since I used any of those as these days I just plug a matchbox-sized device into the OBD2 port and do diagnostics from my iPhone.
If anyone is interested, I do have a big console Allen DEA (digital engine analyzer) for sale or trade, cheap. Looks awesome in a shop or garage, and it still very useful! (It can be disassembled and fit through the door on a GMC to take it home).
-Dave Stragand, 1978 Transmode, Near Pittsburgh
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1978 Transmode (403)
Pittsburgh, PA
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