Rich mixture and burning valves

[firstgen89sho]

I have been fighting a lack of power in my 93 F350 for some time (well back when it was my parents truck, and now that it is mine)

I was wondering if running a real rich propane mixture would burn either the intake or the exhaust valves to the point of loosing compression?

The truck in question is my 93 F350 w/ 460CI Fuel Injected/Propane Carb. The engine itself has 325,000 kms on it, and has not been rebuilt to my knowledge (It has been in the family for almost 10 years now, and about 130,000 kms)

About 5-6 years ago I had the propane carb off the engine, and when it was re-connected, I had 2 lines mixed up, one was weak vacuum between the mixer and TB (for the Dual Curve computer control mixture solenoid), and a enrichment port on the side of the vaporizer, I had the enrichment port plumbed into the weak vacuum port, creating a real rich mixture (to the tune of 700km to a 400L tank of propane, from 1300kms), I have done a compression test on the engine (warm), and I had come across around 130-140 PSI of pressure, to me that seems low, especially on an engine that has been run primarily on propane most of it's life. I have not preformed a wet compression test or a cylinder leakdown test yet (that is next on the list), but I am trying to get possibilities of what it might be.

If this is the cause (and I think that it might be, just wanted to confirm what I was thinking), I am looking at rebuilding the engine anyways, now for running mostly propane, and running a little gas if need be, what would be a good compression ratio to go? 10:1? 9.5:1? I am guessing that the factory compression ratio is about 8.5:1 ratio.

Thanks, Tom

[Mattelderca]

In my opinion, 130 psi is pretty good for a 460 with over 300,000km's!
As long as pressures are consistant you should be fine, hows oil consumption?
Do the wet test and leakdown and let us know.
On compression ratio, you might find that 460, being fuel injected, is higher than 8.5. It might be 9.1 if memory does not fail me. A good ratio for dual fuel on a big engine, dedicated LPG would be a different matter.

[franz]

I agree with Matt, 130 psi compression is not that bad, especially on an engine with that number of K's. What you should look for is irregular readings. If they are all within 10% of each other, no problem. As for the lack of power, do a wet and dry compression test.

Remove ALL the plugs;
Prop the throttle fully open (if the throttle is closed, the compression reading will be inaccurate);
Do a dry compression test. Count the number of "bumps" you hear during the cranking, all cylinders should be the same number of bumps;
Record the readings;
Do the same compression test, this time, squirt some engine oil into the cylinders and rotate the engine about 10 or 15 times to ensure the cylinder walls are well coated.

If you find more than about a 25% difference between wet and dry compression readings, the cylinders, pistons, and rings are worn.

Lastly, about your primary question, will a rich running engine burn intake and exhaust valves? Yes and no. Yes, a rich engine will burn valves, but not the intake valves, only the exhaust. The intake valves are never open to hot exhaust gases. A dry gas fuel does not have any liquid evaporative qualities as with gasoline, running rich will allow some combustion gases to pass through as the exhaust valve is opened, often causing a torching of the valve face.

Franz.

[firstgen89sho]

Thank you for your reply's, they were all within 10% of each other. I will be preforming a wet compression and leak down test this weekend.

I was discussing this issue with my boss, and he was saying that most likely, my problem, is stemming from a timing issue, not spark timing, but probably cam timing, I had changed the timing chain at one point, and installed a timing chain set from a 69 429 engine, but I have a feeling that the chain has stretched and jumped. The engine has backfired a couple times that I know of since the chain has been installed. I am looking at installing a adjustable timing chain with 4 degrees of advance timing.

[Mattelderca]

Timing chain? Maybe but the computer should pick up that, maybe? It's probably just tired. A little lost compression, weak valve springs, slack lifters, or even worn cam lobes. A combination of these things will add up. The other performance limiter is a dual fuel setup, you can't take advantage of the octane in the lpg. Unless you have some decent and working ignition add ons. If the bottom end tests out OK you might get away with a complete cam swap. Do everything from the cam to the springs. It can be done without too much teardown, hell, you don't even need to pull the heads. But on a motor with that many klicks you might run into more issues after all that work.

[franz]

A stretched timing will cause a gradual loss in engine power. You can put a socket on the crank pulley bolt and rock the crank back and forth. No more than 10 degrees is allowed (!) Personally, if you can feel the slop, its time to change the chain. My experience with timing chains is that if they jump, the engine will never keep running, it will jump too far to stay in sync. In my 40 years of experience, I have NEVER seen a timing chain jump just one tooth. I have however, seen them installed one tooth off.

The early 69 timing set had a crank gear that was indexed 4 degrees from the later cam which was retarded to reduce emissions (passive EGR).

If you install a new camshaft, and one with more lift, be VERY careful that you install new valve seals. The increased lift will cause the original seals to split and fall apart.

Franz

[firstgen89sho]

There will be a full rebuild coming up soon, addressing all of that (rings, bearings, springs, cam, pistons (11-12:1 compression) but I'm just trying to get something together to get more power than what it has right now, which is maybe 100-150 hp (feels like, had an empty 16' tandem axle flatdeck trailer on yesterday, and had to keep it out of overdrive to keep any speed up, but once the RPMS were up, it wasn't as bad (above 3500 RPM), and hills were a killer)

Being a mechanic, I shouldn't be grasping at straws, but something is not right at all..... I'm just trying some quick fixes right now to try to pass the time till I can rebuild the engine

[Steptoe]

Quick fix... with the lower compression wear on cam lobes, the spark could be a couple degrees to retarded.
There is a point where power/economy drops off real fast when retarded...2 or even 5 degrees and/or maybe a slightly faster curve could very well compensate.
Thu engines often increase calculated compression ratio as they get old due to carbon build up on the pistons and heads, compensating a little for blow by wear of guides/rings etc.....So when stripped down you could find more bore wear than anticipated....dont de coke it ..ie the old school 'water down the carb'

300.000 miles..my compliments, an engine regularily serviced.

[firstgen89sho]

I wish that it was 300,000 MI, it's more like 200,000 MI. The engine has been serviced regularly, The timing was set back in December, it was set to 10 BTDC, even up at 14 BTDC, it was still a real dog.

I will be replacing the timing chain on Saturday, I will let you know of how it responds after the repair.

[Mattelderca]

Your rebuild list quotes 11-12:1 compression! You running race fuel?
10.5 is high enough these days with fuel the way it is. Even on dedicated LPG I would think 11 was a bit high for a street/ tow vehicle. Shoot for a lower value here, that 460 will respond a whole lot better down low, which is where you want a truck engine to work. Also IMHO, don't waste your time with a chain swap right now. Just baby it till you do the whole job, you will need a new chainset again when you do.
Just my 2 cents!
Cheers,

[Steptoe]

Your rebuild list quotes 11-12:1 compression! You running race fuel?

That is more for pure propane...pure popane is very expensive and not avalible at the pump.
LPG is a blend og propane, butane, iso butane and a couple other bits and peices that drops the octane a bit
10 or 10.5:1 fine for a dedicated LPG engine.
The issue is when one wants to run petrol, with that high compression,,,and add to that a stump puller cam that has a high dynamic CR at low rpms, the engine will detonate on petrol without dual curve
If duel fuel it would be better to run 9:5 or 10 to lower the risk of detonation, depending on the cam/rpm range

[400HPONGAS]

Steptoe , that why the refer to it as PROPANE, because in the USA/CANADA thats what it is !!! I think there silly spec says 98% propane !!!
Its only in Australia/Nu Zillin that runs a blend down to 50/50 propane/butane , in Europe its a blend and it varies .

Now given that , what would make my enginene deliver more Horsepower ? 100% propane or the 50/50 blend of Butane/Propane.
Be careful here Steptoe !!!!!!!!!!!!!

[Steptoe]

Be careful here Steptoe !!!!!!!!!!!!!

Of what?

Now given that , what would make my enginene deliver more Horsepower ? 100% propane or the 50/50 blend of Butane/Propane.

Well that depends what the engine is used for and how well the 'go' components are selected, and if one requires HP or more torque..In the real world is is very rare one uses all the HP potentual at WoT and cruises at the peck power ranges.

Its only in Australia/Nu Zillin that runs a blend down to 50/50 propane/butane ,

Got a chip on your shoulder m8?

[franz]

Propane has approx 104 octane, butane approx 99 octane. Propane has 2500 btu's/cu ft, butane 3150 cu ft. There is no reason an engine running a blend of autogas, major butane or major propane concentrations should have any less or more than the other. I do notice however, than if I get a batch of LPG that has a higher level of butane than normal, I tend to get a bit of detonation (lower octane).

The LPG sold in the US is rarely much over 95% propane, sometimes as high as 98%, but often in the low to mid 80% range. The balance can be any hodge-podge of N-I butane, propylene, some methane, ethane, carbon dioxide, water, and non-distilled-non soluble heavy compounds.

Franz

[Uglydog56]

factory 460 timing chains are 8 degrees retarded.