Do you think that switching from 3.07 gears to a 3.42 gearing. Could keep this higher compressionengine from pinging at load?
455 with Higher Compression Pistons / Persistent Pinging Under Load
- Thread starter mwilkie
- Start date
It's a possibility that it would help to switch to a lower gear ratio. It would reduce the load on the engine somewhat. The Buick V-6 in my Jeep has 11:1 pistons in it and doesn't ping on premium pump gas. It also weighs less than 2500 lbs and has 5.38 rear end gears.
1. The combustion chamber on the olds is an 'old" design with little quench area so... yes too much compression. I dont think you shoud go above 8.5 to 1. Higher compression means molecures are closer together and requires less timing for a full burn.
2. Back off the timing, disconnect the vacuum advance and set total timing and advance curve so it doesnt ping under load. Hook up the vacuum advance to manifold vacuum so it only advances under light cruise (higher vacuum).
3. Yes, you may need a recurve depending on the results from 1 and 2.
4. Make sure you're not lean and running hot.
5. / 6. Un-necessary 'Bandaid apprach', I wouldn't do it.
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I started the discussion about HHO In post #383 o Page #20 of "Converting to LiPFeO4 battereis and Sailor Man's LiFePO4 Build.
Chuck's link doesn't work anymore but the link on post #395 does for a paper from Alexandria Engineering Journal.
There are a lot of trucking companies with HHO systems installed to get better gas mileage.
One of my neighbors is a commercial fisherman, He sail that one of his buddies installed a HHo system in his truck and it worked great. Unfortunately, he doesn't have that truck anymore.
I may have downloaded Chuck's HHO paper and I will look for it.
I found Chuck's updated and corrected presentation at the GMC Western States Motorhome CLub's Spring 2015 Rally.
It starts with 43% Improvement in MPG at 65 MPH with an HHO System.
It is a 37 page presentation.
Also check out Chuck's posts on his thread
Saving $1.00/mile with my HHO hybrid system
started by Chuck on June 2, 2015.His post #36 is particularly interesting.
Too bad we can't talk about it with Chuck.
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Definitely
Just keeping the RPMs up and not lugging the engine will help avoid it, partial throttle.
And actually using 2nd gear much more will accomplish this as well , that gear is good up to 45-50 mph. If i were pinging I would consider just leaving it in 2nd except to run down the highway.
To follow up, I checked my static timing (@approx 800 rpm, FiTech controls the idle speed, didn't check for bypass on that) and found it to be 4 BTDC. Advanced it to 8 and will check for ping when I drive it next.
The question as to how much compression vs performance gets complex quickly. Here's an article about it: https://www.hotrod.com/how-to/ccrp-1012-maximum-compression-ratio-on-olds-455.
My machinist and I discussed it and went with the general idea of "more compression means more power". I haven't seen a good reason yet to change my mind.
OP mentioned tuliped valves, something I've never seen. Makes me wonder about the quality of the rebuild and what other issues might exist with that engine other than compression ration.
The question as to how much compression vs performance gets complex quickly. Here's an article about it: https://www.hotrod.com/how-to/ccrp-1012-maximum-compression-ratio-on-olds-455.
My machinist and I discussed it and went with the general idea of "more compression means more power". I haven't seen a good reason yet to change my mind.
OP mentioned tuliped valves, something I've never seen. Makes me wonder about the quality of the rebuild and what other issues might exist with that engine other than compression ration.
The pinnacle of engine horsepower was late 60s-1970ish from the big 3, you just have to look at their compression ratings at that time 10 and 11:1 before the smog laws dropped the compression ratios to the 8s.
But those are for hot rods, light cars. Ours are under constant load, about 90 hp just to run down the road at 70mph on the flat.
In medium duty trucks, Kodiaks and Topkicks, pulling 30,000 lbs of dump truck hauling it up mountains at WOT _continuously_ they used tall deck big block chevy engines running 8:1 compression. They tested 'good' at cranking pressures around 120 psi. (and running valve rotators btw, which may have prevented the OPs tulipped valves)
When you get a chance, can you do a cranking test on your engine? You've got the high CR pistons and the Melling MTO-1 cam? That should get you some pretty high cranking psi, I'd love to hear what they ended up at.
'More compression, More power' is a fine disposition to have for a muscle car running around on cruise nights and making hard pulls a 1/4 mile at a time with a high performance exhaust system with a cam that bleeds off dynamic compression. A motorhome has a very diffrent set opperational demands. For example, the muscle car might run hard at WOT for 12 to 14 seconds at high RPM with loose converter. The motorhome might have to pull at full load for 10 minutes or more while climbing a grade. It might have to run at full load for 20 seconds often just to get up to 60 MPH. These two engines are not the same and have to be built and tuned very diffrently.
Many of these aggressive performance attitudes on compression come form the late 60 and early 70 when gas had an Octane rating of 98. That's a far cry from the 87 to 91 octane fuels we have today. Combustion chamber design, aluminum heads, and modern engine managment evolved to compensate but your application has none of those advantages.
Did you confirm you head P/N and chamber size?
They are C heads. Valves appear to be standard size, chamber should be the same size as the J heads. I would have used J heads if they were available at the time I needed them.
My preference is to be in the range of 9 - 9.5. This preference (I must admit) is pretty much based on opinion. I consider higher compression to be 10 - 10.5.
When I saw the amount of material removed from the lower compression pistons, I balked. I built engines full-time in the early 80's for a couple years. Never saw a piston with that deep of a dish and decided to switch to the "high" compression pistons which are flat like pretty much everything else I've ever worked with.
If anyone has a readable reference about heavy duty ICE design such as we have here where the motor is under moderate to heavy load quite a bit of the time, I'd love to see it.
At any rate, so far, I haven't really encountered any issues that I would trace to excessive compression. I had plenty of ping problems with the original engine and then lost a rod bearing or two. At this point, I'm pretty happy that it starts and runs without pinging or banging. Best performance option for me would be an interactive timing control and a higher ratio final drive. Not really looking to perform those high dollar conversions until I finish some (mostly labor intensive) physical repairs and maintenance. Plus use it!!!
My preference is to be in the range of 9 - 9.5. This preference (I must admit) is pretty much based on opinion. I consider higher compression to be 10 - 10.5.
When I saw the amount of material removed from the lower compression pistons, I balked. I built engines full-time in the early 80's for a couple years. Never saw a piston with that deep of a dish and decided to switch to the "high" compression pistons which are flat like pretty much everything else I've ever worked with.
If anyone has a readable reference about heavy duty ICE design such as we have here where the motor is under moderate to heavy load quite a bit of the time, I'd love to see it.
At any rate, so far, I haven't really encountered any issues that I would trace to excessive compression. I had plenty of ping problems with the original engine and then lost a rod bearing or two. At this point, I'm pretty happy that it starts and runs without pinging or banging. Best performance option for me would be an interactive timing control and a higher ratio final drive. Not really looking to perform those high dollar conversions until I finish some (mostly labor intensive) physical repairs and maintenance. Plus use it!!!
Even the 'high' compression 10:1 pistons have a 1/8" deep dish in them. The standard 8.5:1 compression pistons have a 5/16" dish
The real issue with many rebuild pistons is where they put the wrist pin; 0.030" higher on the piston. The compression height is 1.720" instead of the stock 1.750". It assumes that the block has been decked 0.030"
Talk about a compression killer.
One heavy duty ICE design reference I know of is everything written about the tall deck BBC engines made from the 1960s through the 1990s. They share many attributes with our Olds engines; special hard iron alloy, even lower compression ratio 8.0:1 (120psi cranking pressure in the specs), and valve rotators but add in increased cooling, a fourth compression ring, forged crankshaft and 4 bolt mains. They were designed to run WOT indefinitely. I don't know of a book discussing this, we can only look at the results from the GM designers.
I got Silvolite pistons. Used them all the time back in rebuilding days. They are considered a standard replacement item. Sounds like the ones you are talking about come with the fancy price tag.
Don't forget these same motors were/are used in performance boats turning 8K - 9K RPMs. The parts to enable that aren't wanted or needed here.
For all the nitty gritty specifics, I admit I depend on the machinist. That's their bread and butter, after-all. A legitimate concern is the number of machinists familiar with these engines. That number will continue to dwindle.
Don't forget these same motors were/are used in performance boats turning 8K - 9K RPMs. The parts to enable that aren't wanted or needed here.
For all the nitty gritty specifics, I admit I depend on the machinist. That's their bread and butter, after-all. A legitimate concern is the number of machinists familiar with these engines. That number will continue to dwindle.
Here is some information on timing curves for GMC's stock and modified. https://www.gmcmi.com/wp-content/uploads/2015/06/Ignition-System_simmons_lamey.pdf I wonder if the balancer ring has slipped on the crank hub and the mark is not at zero anymore. Here is facebook post about a GMC with a slipped ring https://www.facebook.com/groups/439249376261161/search/?q=balancer slipped
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I'll check that reference - Thanks!
Balancer rubber failure does occur but it's pretty easy to spot. I had the front pulley (vibration dampener) balanced with the engine and verified marks when setting up the chain.
Where did you get this idea from? The olds 455's were used in jet boats becsase they made great low RPM torque needed at move a loaded prop or jet drive through water. They typically maxed out at 4500 to 5000 RPM.
Yes Silvo-lite, No fancy price tag, they are the cheapest rebuild pistons available, they are the ones with the 0.030 lowered compression height which kills the compression ratio unless the block has been decked.
Not sure. My bad on this one. Google search returns the same results. I'm thinking it was from a book on that engine that I downloaded to somewhere. The lower values certainly make a lot of sense giving the weight of the reciprocating assembly.
Thanks!
Well, since mine was decked, I should be fine. Primarily, (as above) because I depend on my machinist. I take him all the parts, discuss my options, and have him do the work.
I do my own assemblies and everything else but transmissions. After they've run for a bunch of miles the constant paranoia of breakdown starts to lift and eventually I trust them. The latest bit of aggravation is getting a decent set of electronic components for the distributor in my 91 F150 (rebuilt the 86 mustang roller cam engine in that years ago) which failed on me 7 miles after installing a brand new, name brand module.
Bottom line on all this engine stuff, I think is this:
The 455 (and many of the others from 60s - 70s) is a tough and reliable motor with great low rpm torque. Used to be, these engines were cheap to rebuild and lots of folks knew their particular quirks and requirements. That is no longer the case due to parts (used and new) availability, soaring labor costs, loss of that knowledge. When costs go up shortcuts become more popular (pretty sure that's what happened to my original motor) and those usually drastically reduce the lifespan.
All I can say is, that's life. It's the way it goes, my friends.
I have uploaded "Rocketing the 455 Oldsmobile Engine into the Future". An article by Joe Mondello regarding rebuilding our favorite engine to Resources. It might already be there somewhere but I couldn't find it.
Haven't seen the original poster on this thread in some time... But if he comes back, some of the best money I've spent on my coach is the addition of an AFM. Honestly, without it you are pretty much just guessing that your tune is correct. Yes, you can read plugs, listen to the engine, note seat of the pants powerchanges, etc. But if you are having tuning issues in only in a small part of the running range, an AFM will make troubleshooting SO much easier.
My first engine had forged pistons that were about 9 to 9.5:1, and I always had to run premium to keep from pinging unless at high altitude... But I was able to get it to run well, once tuned properly. Believe they were SpeedPro L2390Fs, and were installed by the previous owner. Though they were considered "Performance" pistons, they had conventional round dished piston crowns. The AFM was super helpful to get this engine running well.
Current motor has standard compression pistons... It was much easier to tune and of course, you can run moose piss for octane and it runs well... Do I miss the high compression when in the mountains: Yes. At sea level, it's not really noticeable, and I do not miss running gobs of premium though the coach... Especially when towing our jeep.
If you are still having issues with pinging once you confirm your tune, as a bandaid, the cam you are using can be changed to affect "Dynamic" compression ratio. Also as mentioned above, you can look at running thicker head gaskets. I think Cometic makes gaskets all the way up to a .120 for the BBO. Yes, this hurts quench, but our combustion chambers are very basic don't utilize much quench to begin with, so you probably won't lose much there.
If I were to try running a high comp engine again, I would go about it differently than my original engine. Stock heads, with dished pistons pretty much means it will only run well at low compression ratios. There are BBO pistons out there (mahle and Icon come to mind) with a more modern chamber design in the piston crown, that can be coupled with a set of performance aluminum cylinder heads, (with a modern design for the combustion chamber, high quench, valve swirl, etc) that should generate much better performance than a factory olds cylinder head with a conventional dished piston. But from what I've found, those setups end up with CRs of 10:1 or greater, which I would not want on a GMC. If I could find a setup with a modern combustion chamber in the 9.5:1 range, I might consider it, as I think you could get it to run well on 87 with the proper tune.
My first engine had forged pistons that were about 9 to 9.5:1, and I always had to run premium to keep from pinging unless at high altitude... But I was able to get it to run well, once tuned properly. Believe they were SpeedPro L2390Fs, and were installed by the previous owner. Though they were considered "Performance" pistons, they had conventional round dished piston crowns. The AFM was super helpful to get this engine running well.
Current motor has standard compression pistons... It was much easier to tune and of course, you can run moose piss for octane and it runs well... Do I miss the high compression when in the mountains: Yes. At sea level, it's not really noticeable, and I do not miss running gobs of premium though the coach... Especially when towing our jeep.
If you are still having issues with pinging once you confirm your tune, as a bandaid, the cam you are using can be changed to affect "Dynamic" compression ratio. Also as mentioned above, you can look at running thicker head gaskets. I think Cometic makes gaskets all the way up to a .120 for the BBO. Yes, this hurts quench, but our combustion chambers are very basic don't utilize much quench to begin with, so you probably won't lose much there.
If I were to try running a high comp engine again, I would go about it differently than my original engine. Stock heads, with dished pistons pretty much means it will only run well at low compression ratios. There are BBO pistons out there (mahle and Icon come to mind) with a more modern chamber design in the piston crown, that can be coupled with a set of performance aluminum cylinder heads, (with a modern design for the combustion chamber, high quench, valve swirl, etc) that should generate much better performance than a factory olds cylinder head with a conventional dished piston. But from what I've found, those setups end up with CRs of 10:1 or greater, which I would not want on a GMC. If I could find a setup with a modern combustion chamber in the 9.5:1 range, I might consider it, as I think you could get it to run well on 87 with the proper tune.
