454 efficiency - Maximum CR

Propane, Butane, LPG, GPL, C3H8, C4H10
burbfixer
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454 efficiency - Maximum CR

Post by burbfixer » Fri Nov 10, 2017 1:34 pm

Hey everyone. First post to this sight.
My question pertains to a safe maximum CR for a propane engine build. I understand that cam choice factors in heavily. No doubt other things have a bearing too, such as quench.
I built, and drove for several years, a 396 that had 10.25 cr, and a short 204/216 cam. It cranked with 200-205 psi. It ran perfectly like that on propane with a standard 14-14-14 timing curve. Then I added a Paxton supercharger with 6ish psi of boost. It ran great.... then the weather got warmer and I detonated it so badly (and loudly!) that I thought the fan was hitting something. The engine survived, likely thanks to the forged pistons. I reduced the timing a bit and it ran great for several years in that configuration, with the blower on it.
I'm building a 454 now, and my past experience proved to me that even with the short cam, 10.25 cr was certainly way safe if it tolerated boost on top of that. I'm hoping to get a better idea what the safe the limit really is.
How far have some of you pushed the CR envelope successfully in your own propane engine combos?
Thanks in advance!


storm
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Re: 454 efficiency - Maximum CR

Post by storm » Fri Nov 10, 2017 2:04 pm

The Question of compression ratio pops up from time to time, I think a FAQ sticky may be a good idea.

With compression ratio you need to consider a few things
1. Fuel octane
2. Fuel quality,
3. Engine design.
4. Engine build quality.

With regards to fuel octane LPG usually has an octane of 105-110 depending on where you are located. This means that around 11 to 1 compression ratio for an NA engine is about the safe practical limit if the engine is designed to have a long service life. Add forced induction to it and it will need to be lowered. Add alloy heads and you could raise it 1 full point.

With regards to fuel quality the % blend of propane and butane can affect the octane of the LPG. The higher the % of butane the lower the octane and the lower the ideal compression ratio. Some places have different blends for summer and winter.

With regards to engine design the BBC is a great engine. Canted valves, good shaped chambers, good ports all make for an engine that will run great on most fuels. Don't get too caught up on Quench (in my experience most people don't know what it is anyway and get it confused with Squish) as that is taken care of when ordering pistons to suit a particular head.

With regard to build quality if you are building the engine yourself make sure you remove any sharp edges off the combustion chamber and that will drastically reduce the chances of detonation from hot spots. Good quality pistons should already be designed to have smooth shape and contours in the combustion zone. Basic things such as this help protect against detonation. If you intend running a blower again I'd consider some sort of intercooler to help keep the intake air temp charge down as well.
Fuel flow requirements calculations viewtopic.php?f=5&t=1638

gottago
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Re: 454 efficiency - Maximum CR

Post by gottago » Sun Nov 12, 2017 11:46 am

Hey burb direct from speed talk to the propane forum. Good place to find info and even some experts.. The use of propane as an auto fuel seems to be diminishing and thus much of the posting. Still a few hard cores out there though. In regard compression, there have been discussions on that subject here previously but spread around in more than one thread. Not sure all of the questions have ever been answered and a universal formula was a bit elusive. Link to an older thread...
viewtopic.php?f=5&t=1453&hilit=maximum+ ... n&start=15

burbfixer
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Re: 454 efficiency - Maximum CR

Post by burbfixer » Sun Nov 12, 2017 10:37 pm

storm,
Thanks for the info, sounds like you know your stuff! Anyone else care to chime in on the maximum CR you've used sucessfullly?
gottago
Thought you might find this! Super-appreciate your input on the other site. I had actually forgotten about this site, but there seemed to be a bit of activity, so thought I'd test the waters a bit. Clearly a wealth of experience here from others who feel propane is a great motor fuel and very well suited for high performance use. Sounds like Mr Hoffman may very well be the worlds leading authority on the subject, but you and several others here have played with it extensively in their own real world combos and been kind enough share the results.
Agreed, it's sure not what it used to be here in Western Canada. I used to work for a company in northern BC. At our peak, our shop did a dozen conversions a week, and were booked ahead 3-4 months. Almost exclusively pickup trucks, very few cars. My job was tune ups and trouble-shooting during the day, then dyno tuning and road testing the days conversions towards the end of the day. I live in the Okanagan Valley now and my town has at least half a dozen refueling stations and 2 cardlocks, so availability is still very good. Isn't it still fairly popular in other parts of the world? Either way, as long as i can buy it, I'll be using it in my big ol' daily driver. Still saves me $ over using gas, and fewer maintenance headaches.

storm
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Location: NSW, Australia

Re: 454 efficiency - Maximum CR

Post by storm » Sun Nov 12, 2017 11:38 pm

Where's the thread on Speedtalk? I lurk there occasionally but don't post.
Fuel flow requirements calculations viewtopic.php?f=5&t=1638

BigBlockMopar
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Re: 454 efficiency - Maximum CR

Post by BigBlockMopar » Mon Nov 13, 2017 6:38 am

https://www.bigblockmopar.com
'73 Dodge Dart - 318ci - IMPCO E vaporizer / 425 mixer - A518-OD trans - 3.55 gears - 225/50/17" tires.

storm
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Re: 454 efficiency - Maximum CR

Post by storm » Mon Nov 13, 2017 2:21 pm

Just read both threads. It looks like zmechanic knows his stuff.

A couple of comments based on both threads. If you go higher than approximately 11-1 you will need to be extremely careful with timing. LPG is a dry fuel and provides no cooling effect (unless you are using liquid phase injection) on the cylinder/combustion chamber. Petrol (or gas as it is called in North America) does provide a cooling effect. Timing requirements depend heavily on latent heat within the cylinder/combustion chamber, to much heat and you'll detonate very quickly. In my Troopcarrier with Holden V8 I've been toying with the idea of reverse flow cooling system to cool the heads first rather than last.

Cam timing is important, I prefer dual pattern cams with more exhaust timing.

Peanut port heads are for low low rpm engines. If you pull more than 2500 rpm you want oval port heads, good port size great velocity especially if the engine is a larger big block. Unless you're running a high rpm engine there is no need to use rectangular port heads port velocity at low rpm is slow.
Fuel flow requirements calculations viewtopic.php?f=5&t=1638

gottago
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Re: 454 efficiency - Maximum CR

Post by gottago » Mon Nov 13, 2017 11:17 pm

If you go higher than approximately 11-1 you will need to be extremely careful with timing
I think that's the jist of what burb is looking for... a number to work from.. Safe is one thing too safe another.. Ideal compression ratio seems to vary from one engine to another. Head design, modern fast burn chambers, alloys etc all contribute to what compression any engine will take.

The 11:1 number is Ok imo dependant upon what you want the engine to do. 12.5 is OK too if you want to use a longer duration higher revving cam. The same engine that runs well with 11.1 static and maintains a high dcr gets doggy if you just install a bigger cam. You want the static compression to be raised with the bigger longer cam to maintain the same dcr. Perhaps that's the question that needs answered. What is the ideal dcr for a propane bbc? One does have to define their goals quite realistically and look at the pros and cons. Different heads and misc other parts may be required. $$$

Timing does get finicky with more compression too but as long as its in the range its far from impossible. A 377 cu in sbc running 12.5:1 compression with 135psi cranking pressure was timeable. Even got vacuum advance by the end. Not saying this is the way to go, it is pushing it but the power and mileage both responded noticeably to the maximised compression.

Burb, you do have to build for the fuel quality where you are. I think the Okanagan has a fairly good quality supplier more so than some other areas. That too can make a difference. The fellow I mentioned previous that ran a 454 in his tow truck with a tunnel ram is in your area. Cherry wreckers out in Cherryville north Okanagan. He might be worth a call someday..

storm
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Re: 454 efficiency - Maximum CR

Post by storm » Wed Nov 15, 2017 9:19 pm

If you install a bigger cam to bleed off cylinder pressure you also raise the optimum operating rev range of the engine. In a heavy vehicle that hardly, if ever, goes above 3k rpm that is false economy.
Fuel flow requirements calculations viewtopic.php?f=5&t=1638

gottago
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Re: 454 efficiency - Maximum CR

Post by gottago » Wed Nov 15, 2017 11:31 pm

True but as the optimum max rev range increases, so too does the power band including the midrange If compression is maintained. I up the compression when using a larger cam so as not to bleed off pressure. The engine can be built to handle it. You may loose a small bit at very low rpm but make up for it very quickly 2000 rpm and up. If you have enough power to spin the wheels at will with a small cam, your engine could probably pull a slightly larger cam. We aren't talking a huge difference, just a few subtle changes to compression and cam that may make the difference between a low rpm tow motor all done by 4000 rpm at best and a more traditional car like power band. Both are nice in their own way just depends what you want in the end. That's up to burb but a basic engine simulation program could provide some more insight. Just sayin, You aren't stuck with a tow type motor if that isn't what you desire.

ps.. heres a link to an old article on cam selection by Mr. Vizard. All good info even for propane users. Example, spreading the cam lsa when you have higher lift and increased compression. How much /per each.. In the end you may negate the necessity for any decrease in lsa.
http://www.hotrod.com/articles/0607phr-camshaft-basics/
example-
Duration & Lift: Its Effect On Output
Now that we are over the duration hurdle and can look at it in a more realistic fashion, let’s look at exactly what it delivers. Assuming the compression ratio remains constant, longer duration just moves the torque curve up the rpm range. Peak torque itself usually only increases a minor amount. The additional hp comes from the fact that the torque delivered happens at a higher rpm and power is directly proportional to torque times rpm. The graph of duration versus output shows what typically happens as the duration increases while all other factors are held constant. Although it looks like a good before-and-after test showing the longer cam’s value, this test does in fact favor the shorter cam. Because the intake valve closes sooner after passing BDC, the running ratio, or dynamic compression ratio, with a shorter cam is higher. If the compression ratio with the longer cam is raised so that the same cranking pressure as the shorter cam is seen, we find that much of the low-speed torque loss is recovered. In addition to this, the longer cam will deliver a much better top end when an appropriate compression increase is made. This is an important factor, so don’t overlook it. If you don’t feel inclined to run a compression to match a longer cam’s requirement, then stick with a shorter one, as it will produce better results.

burbfixer
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Re: 454 efficiency - Maximum CR

Post by burbfixer » Sat Nov 18, 2017 12:52 am

You both make valid points, and the input is appreciated.
For my purposes, a shorter cam with high 10's or 11:1 compression sounds like the way to go, you two arent that far apart on your opinions there. Since fuel efficiency at very low cruise rpm is a desired result, I wonder if going bigger with more cam and more compression might start to compromise that. It would be a more 'fun' engine with the higher rpm capability, but might come at a cost in unnecessary fuel use, which I'm not willing to pay. I guess my ideal scenario after hearing the input on the forums would be to have great low rpm grunt for the stoplight, around town fun, plus a motor that would run cleanly up to 5000, and not be compromising it's best fuel efficiency to do it all. In other words, an engine that does everything better because it's optimized for the fuel, the vehicle, and the driver. This might sound like I'm asking too much, but actually when it comes to engine performance, I'm a cheap date - it doesn't take much to impress me.
In my 88 Suburban 4x4, the 'stock but better' tbi 350 moves the 5100lb vehicle around town really well imo, and is almost, but not quite, fun at times. Acually it was kinda fun once. Recently I was driving it and realized that since I installed and tuned this engine several months ago I hadn't gone to wot from a stop yet, so at an appropriate green light I did. This vehicle has a 700r4, 3.42 gears and 35" tires. It launched so much harder than I was expecting it startled me. I let off out of mercy for the stock 10 bolt and half worn out ujoints, but the front driver's corner of the vehicle was twisting into the air really hard! With this engine, the party is over very quickly of course, but low, low grunt is kinda impressive for a 350. And on a recent long trip, it averaged just over 18mpg imperial. Where this engine in this vehicle really starts to feel pathetic is at highway speeds, pushing a headwind, and then trying to pass someone. It really struggles. That's why I want a 454. The big, low end grunt, plus more horsepower when needed.
I'm going to seriously consider an 11:1 or so compression ratio, and 2 mixers for flow. The cam I will need to decide on, but likely in the 208-218 range. Tighter lsa is something I'm very curious about, but at this point how exactly it affects fuel economy is not clear to me. The impression I'm getting is that with tighter lsa's you start walking a very fine line with fuel efficiency - everything else has to be perfect for the combo, and that perhaps a wider, or more normal lsa choice is more forgiving.

storm
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Re: 454 efficiency - Maximum CR

Post by storm » Sat Nov 18, 2017 3:06 am

Back in th late 70s and 80s companies like Edelbrock did alot of R&D on very high comp, big cam, and low octane (for compression) engines. The theory was exactly as we are discussing here that if you bleed off pressure all else being equal the engines will perform like a high comp and correct octane (for compression) engine. In the end they gave up because the theory just didn't work in real world conditions. That may have changed with modern head design and modern cam profile design but I am yet to see any published articles that provide verifiable data to support the claims made.
Fuel flow requirements calculations viewtopic.php?f=5&t=1638

gottago
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Re: 454 efficiency - Maximum CR

Post by gottago » Sat Nov 18, 2017 3:26 pm

Ideas are changing with modern heads etc.. Some engines will respond "differently" than others to compression/ cam/ head combinations. Driving characteristics can be tailored to suit individual tastes to some degree. Even within the range where burb is looking, there is quite a difference in driving feel between choices. As he notes, his current engine at much smaller displacement is already a torquer but runs out at highway cruise conditions.
Where this engine in this vehicle really starts to feel pathetic is at highway speeds, pushing a headwind, and then trying to pass someone. It really struggles. That's why I want a 454. The big, low end grunt, plus more horsepower when needed.
The cubes and compression will be able to cure the highway issues with the right cam. The terms we each use can be a bit misleading. As burb describes,
That's why I want a 454. The big, low end grunt, plus more horsepower when needed
I don't think its "low end" that needs any increase, its midrange.. You can actually drive gentler when the torque spreads and carries out a bit further in the rev range. You can get too much low end in the larger cube engines. Having said that, within the cam selection choices burb is looking at, I think there is one that'll work just fine for his purposes. Remember what a racer terms low and midrange may have an entirely different meaning from what Joe average fuel mizer thinks and vice versa...

Just for a matter of interest on the cam/ modern head / mileage thing, I've driven a lot of combinations over the years and did take note when power and mileage seemed better than average. A ford 390 gt 4bbl getting 25 mpg was nothing to sneeze at. Wide lsa and high compression, high geared. Noted the same in other similars. These were near 150 mph vehicles.

Lately a comment by a racer on speedtalk caught my interest.
Some of the FAST racers under cam a big compression motor with wider LSA and retard the camshaft a considerable amount. You would be surprised at the outputs and rpm band the motor will work at. The intake valve has to flow well at low lifts.
Of course this is to the extreme for what we are speaking of here, but note the premise and finer points. A gm vortec style head with great low lift flow and a valve job to max that out might work out really good in a bbc. I doubt anyone wants to experiment just to find out but just saying there are other ideas to work with. I did a similar combo, in a sbc mind you, but have to say the lower rpm fuel mileage is really phenomenal. That's in an engine that will light em up at will and hit 7000 rpm. High compression, medium duration lower lift cam retarded to the max with good breathing low lift flow, fast burn chamber, dual mixers tunnel ram. High rpm can suck a fair bit of fuel, low rpm, the opposite. As said not for everyone but there are a bunch of choices between an all out torquer and something like this. Looking at your current inadequacies and realistically defining your goals is the starting point. Burb, for your situation I do agree with where you are headed.. There are gentle ramp cams and faster ramps.. As you are narrowing it down, post the @50 thou duration as well the seat to seat duration, lift and lsa of each. Maybe someone has a dyno sim program they could run your numbers thru..

The Q..
I wonder if going bigger with more cam and more compression might start to compromise that. It would be a more 'fun' engine with the higher rpm capability, but might come at a cost in unnecessary fuel use, which I'm not willing to pay.
You aren't far from the high compression, smallish cam wide lsa combination...

storm
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Re: 454 efficiency - Maximum CR

Post by storm » Sat Nov 18, 2017 5:50 pm

Just reread this thread again to make sure I'm not getting mixed up because I have been away and haven't really had a chance to post much. My post from Thursday was in between breaks at a teaching course I am doing.

A couple of points need to be made because the thread has gone from a simple question of "How far have some of you pushed the CR envelope successfully in your own propane engine combos?" to answers that are delving heavily into theory and individual R&D.

Static compression must be your first consideration simply because octane ratings are linked to them, albeit by accident. 11-1 works out a comfortable (not to safe) compression for 105-110 octane fuels on traditional style V8 engines.

Dynamic compression (I would argue there is no such thing but lets leave that to another discussion) is dependent on many factors. Engine temp, static compression, cam timing, cam duration, cam overlap, etc etc etc. An engine with an operating temp of 160 will have a different DC to an engine that is up to operating temp of 180 or 195, a new engine will have a different DC compared to an engine that is a 12 month old daily driver. 2 engines that are exactly the same in all aspects except for valve size will have a different DC.

To answer the question simply, which is how I entered this discussion, 11-1 is pretty much spot on. To answer it technically would require alot of data to back up any claim made. Engine analyser programs are great, I use them extensively, but they are limited by the information that is given to them. They also work on generic theories so a basic program will only give you a basic output and a very detailed program will require much more data to provide more detailed output.
Fuel flow requirements calculations viewtopic.php?f=5&t=1638

gottago
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Re: 454 efficiency - Maximum CR

Post by gottago » Sat Nov 18, 2017 10:33 pm

What cam specs would work best for the intended purpose of this weight vehicle starting with a bbc at 11:1 cr? Even the basic dyno programs are way better than nothing for the generation of torque and hp charts. You can then see the changes from differing cam specs reflected on the same charts.

I have noted that burb is better suited to a safe and traditional power curve. No problem.. Even in that range though there are choices that would change the power curve and mileage enough to warrant at least looking at. Don't think he is in too big of a rush..

By the way.. dcr... biggest changes are with ivc point. Why its important the cam should match compression/for the intended fuel no matter the rev range. Try a few basic changes using any cam specs. note changes to the dcr..

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