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Old 12-20-2008, 10:10 PM
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Engine Build spec, how do you get yours

Lurking here a bit, I have seen quite a number of this part or that part of here's a build formula. I'm kinda new here, so maybe this came up before, if it did, I apologize. But here's some thoughts of how I went about the last two motors I did and although very general, I thought would be a place to stir an interesitng conversation. I see too many people doing the internet thing wasting time, money, and energy and not getting what they want out of it.

So...this subject came up in another thread I thought it would be an interesting philosophical discussion about the process about how to get the engine you want. I have seen too many people go to forums, get advice on how to build an engine, and wind up totally disappointed in the result.

In one instance, this "forum recommended motor" was put in the car. My friend spent a lot of money buying the most shiny and cool parts, in the end, it just didn't run. He was getting his *** kicked by "junkyard builds" as he called it. People who spent a lot less money but got it right.

So....I'd like to submit my thoughts for consideration and have your comments. Perhaps we can stimulate some good discussion for future reference....

Anyway, I would think that the first thing to do is consider how you're going to use the car. What the operational range is. A street car will rarely see over 5000 RPMs. You probably want the power down low, etc... A strip car will probably want to see power in the 3500RPM range and up...etc....of course, the higher you spin it, the more power you have, the more exotic parts you will need.

This, in my opinion, is the most important part of any build. The more time spent here, the happier the end result will be.

Short block: Not the most critical point, but can determine operational range, for example, a 377 is going to spin up and is more at home at higher RPMs than a 383. That's about all you can say for a bottom end. There's the debate about dished vs. flat vs. domed pistons, today the trend is dished, but past that...nothing much more.

The first place I'd go after displacement is determined is the cam. The cam determines the characteristics of the motor. How it will behave, operational RPM range, torque curve, HP potential, etc....the entire motor's personality is determined by cam selection. Probably the most critical decision in putting it all together. The cam ramp, an often ignored metric, will have an effect on performance.

Next, I'd have to say rocker ratio. not much to say there, but that will determine how the cam reacts.

Third, the intake. Why? The intake needs to match the cam in terms of application. Is it a low RPM cam and a high RPM intake? Won't work. Also, the intake has an effect on how the car behaves almost as much as the cam.

Fourth, heads. Given the cam operational specs, the bottom end and piston selection, the intake characteristics, and the desired compression ratio based on piston selection heads come into play. Since all you're really doing with heads, in a simplistic description, is passing air from the intake, into the cyl, then out to the exhaust, they are simply a means of moving fuel and air. Given the cam selection and intake selection head selection will stem from the operational characteristics of the cam and intake. Lift, flow, etc...to match the other two. Chrarcteristics include getting the right compression ratio you want by means of chamber size. Two schools of thought there, some people go larger chambers and more piston. Others go smaller chambers and more piston. Then you have flow within the correct range of the the rest. If your cam isn't pushing .600 lift, flow numbers at .600 lift are meaningless. Then there's the whole debate of chamber design and so forth, which is a entire thread by itself.

Fuel delivery. Simply a means of matching injector size or carb size to the rest of the engine. My only comment here is that I see many people on the FI side pick injectors that are too small for the application. There are many injector size tools in the internet, go up a step. If it's 39 go to 42.

Anyway, that's my .02 and it's worth about 1/2 that. Thought it would be interesting to hear your thoughts on the matter.

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Old 12-21-2008, 12:03 AM
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OK, i'll bite. i can't argue with anything you said.

be realistic about the RPM range your engine will ACTUALLY run in and build for MAX torque in that range.

DON'T over cam or over carb your engine (the 2 biggest mistakes made).

pay attention to detail on assembly and do all the math.

check, check, check everything on assembly and don't be satisfied until everything is right on the money.

jsup, i'm a retired mechanic and lifelong hotrodder. i have done hundreds of engines. what is good enough for grandma's DD-POS chevy is not good enough for a performance build.

i just build my own engines for my own cars now (restoring the collection) my current project is a body off complete resto-mod on my 64. i built a 383 stroker for the car. mock up assembly is necessary to do all the measurements and calcs, then i take it apart and correct everything that is not dead on. the third and last assembly was dead on, that means checking everything X3. i find this typical of the engines i do. i learned decades ago that paying attention to the smallest detail will make a car faster than a giant pile of bling-bling parts and cubic cash. a stock engine is built for ease of manufacture to mass produce them, there are literally a thousand things you can do to increase their effecincy-those 1000 things all take time. i have always known cars, in 1971 i bought a 66SS 396, 4 speed, 4.11 12 bolt posi used car off a dealer lot for the obseen price of $1200, that car tought me a lot, it was my rolling laboratory. other than headers it kept all it's stock parts-they were all, and i mean ALL, were just massaged and tuned within an inch of their life. that "stock" car never lost a race aginst bigger engined cars with all the bling-bling racing stuff (including big block vettes). it would litteraly walk away from anything on the street. i owned that car for 20 years. contrary to the fad back in those days, i built it to handle (lowered, koni shocks, trans am sway bars, 8 inch rally wheels and on an on).

thanks for bumming me out.. i miss that car. but i moved on to vettes because->->->

fiberglass doesn't rust...
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Old 12-21-2008, 02:08 AM
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Jsup, in my opinion, everyone starts at the wrong end of the car. Gears and tire size will determine operational rpm range. Then rear suspension. How many times have you read or heard about guys building a hot rod motor and then can't hook the car up and are out of money? What a trajedy.

You need to sit down and be realistic about how you're going to use the vehicle. A car that's used 100% on the freeway might need a final drive (top transmission gear multiplied times differential gear) of somewhere in the high 2's. A car that's used 75% on the freeway and 25% at the strip might use a final drive around 3.25. A car that's used 50/50 might use a 3.73 final drive. A car that's used 25% freeway and 75% drag might use a 4.10 final drive. A 100% drag car would use a final drive that will allow top engine hp going through the traps. That might be 4.56, 5.10 or whatever.

Any dual-purpose car you build will not be ideal for either purpose and you need to realize that up front. That's why I normally suggest using your daily driver as a daily driver and leaving it alone. It's ideal as a daily driver and was engineered at the factory to do that one job to the best of it's ability. Once you start dinkin' with it, everything is a compromise. For Saturday night fun or drag strip action, find the smallest lightest body you can and do an engine swap with the biggest motor you can shove into it. That's where you use the 4.56 or 5.10 or whatever final drive you determine will allow max hp through the traps and you just live with its evil street manners on Saturday night runs to the Sonic to wow the geeks.

Once you have the rear gear and suspension nailed down and know the operational rpm range of the car, you buy the heads based on the displacement of the motor and rpm range. Piston crown configuration, piston deck height, squish, gasket thickness and chamber size will determine static compression ratio. Then intake manifold and headers based on rpm range. The camshaft and converter would be last. You can't cam the motor properly until you know the SCR. And you can't buy the converter until you have nailed down the cam.
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Old 12-21-2008, 07:11 AM
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Quote:
Originally Posted by techron
OK, i'll bite. i can't argue with anything you said.
Ah, nothing to bite, just making conversation
Quote:
be realistic about the RPM range your engine will ACTUALLY run in and build for MAX torque in that range.
Exactly, based on how you want to use it.

Quote:
DON'T over cam or over carb your engine (the 2 biggest mistakes made).

pay attention to detail on assembly and do all the math.

check, check, check everything on assembly and don't be satisfied until everything is right on the money.

jsup, i'm a retired mechanic and lifelong hotrodder. i have done hundreds of engines. what is good enough for grandma's DD-POS chevy is not good enough for a performance build.


Quote:
i just build my own engines for my own cars now (restoring the collection) my current project is a body off complete resto-mod on my 64. i built a 383 stroker for the car. mock up assembly is necessary to do all the measurements and calcs, then i take it apart and correct everything that is not dead on.
I'd love to pick your brain, can you tell us about those calculations and how you arrive at the decisions?
Quote:

the third and last assembly was dead on, that means checking everything X3. i find this typical of the engines i do. i learned decades ago that paying attention to the smallest detail will make a car faster than a giant pile of bling-bling parts and cubic cash.
That's kinda my point. You summed it up nicely. That is my point. If people spent more time doing it right, then buying cool parts, the results would be better, money would be saved, and more people would be doing this stuff, which is good for everyone.

THIS is the difference between a catalog jocky and internet baller, and a hot rodder.

Quote:
a stock engine is built for ease of manufacture to mass produce them, there are literally a thousand things you can do to increase their effecincy-those 1000 things all take time. i have always known cars, in 1971 i bought a 66SS 396, 4 speed, 4.11 12 bolt posi used car off a dealer lot for the obseen price of $1200, that car tought me a lot, it was my rolling laboratory. other than headers it kept all it's stock parts-they were all, and i mean ALL, were just massaged and tuned within an inch of their life. that "stock" car never lost a race aginst bigger engined cars with all the bling-bling racing stuff (including big block vettes). it would litteraly walk away from anything on the street. i owned that car for 20 years. contrary to the fad back in those days, i built it to handle (lowered, koni shocks, trans am sway bars, 8 inch rally wheels and on an on).

thanks for bumming me out.. i miss that car. but i moved on to vettes because->->->

fiberglass doesn't rust...
That was my C3 back in the day. In 1971 I was 3 years old so..... Now I'm trying to make you feel old too!!!! My C4 is now that car for me, learning as I go, figuring out what I think I know, vs. what I really know, etc...

Thanks for the well thought out response.
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Old 12-21-2008, 07:20 AM
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Originally Posted by techinspector1
Jsup, in my opinion, everyone starts at the wrong end of the car. Gears and tire size will determine operational rpm range. Then rear suspension. How many times have you read or heard about guys building a hot rod motor and then can't hook the car up and are out of money? What a trajedy.
OK, that's me. I can break the tires loose at 70MPH. Is it tragic? No. Am I out of money...well, for now. In april I will be doing a rear/suspension upgrade which probably won't be much better. But this is a long term project. A little at a time.
Quote:
You need to sit down and be realistic about how you're going to use the vehicle. A car that's used 100% on the freeway might need a final drive (top transmission gear multiplied times differential gear) of somewhere in the high 2's. A car that's used 75% on the freeway and 25% at the strip might use a final drive around 3.25. A car that's used 50/50 might use a 3.73 final drive. A car that's used 25% freeway and 75% drag might use a 4.10 final drive. A 100% drag car would use a final drive that will allow top engine hp going through the traps. That might be 4.56, 5.10 or whatever.
The most important part I think. I see guys saying they want a specific 1/4 mile times, next day they are talking auto cross, this is for a car that is described as a "street car".
Quote:
Any dual-purpose car you build will not be ideal for either purpose and you need to realize that up front. That's why I normally suggest using your daily driver as a daily driver and leaving it alone. It's ideal as a daily driver and was engineered at the factory to do that one job to the best of it's ability. Once you start dinkin' with it, everything is a compromise. For Saturday night fun or drag strip action, find the smallest lightest body you can and do an engine swap with the biggest motor you can shove into it. That's where you use the 4.56 or 5.10 or whatever final drive you determine will allow max hp through the traps and you just live with its evil street manners on Saturday night runs to the Sonic to wow the geeks.
Yep. all part of the planning.

Quote:
Once you have the rear gear and suspension nailed down and know the operational rpm range of the car, you buy the heads based on the displacement of the motor and rpm range. Piston crown configuration, piston deck height, squish, gasket thickness and chamber size will determine static compression ratio. Then intake manifold and headers based on rpm range. The camshaft and converter would be last. You can't cam the motor properly until you know the SCR. And you can't buy the converter until you have nailed down the cam.
Really? Hmm... because the head runner and chamber size are linked to compression, which is linked to the cam. Overlap can effect DCR so I would think that once you know the overlap, only then can you determine the static based on the dynamic. This will dictate chamber and runner size. Where do you think I have that wrong?
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Old 12-21-2008, 01:18 PM
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Posted by Jsup:
"Really? Hmm... because the head runner and chamber size are linked to compression, which is linked to the cam. Overlap can effect DCR so I would think that once you know the overlap, only then can you determine the static based on the dynamic. This will dictate chamber and runner size. Where do you think I have that wrong?"

Head runner size has nothing to do with compression in the context of figuring SCR or DCR when you're putting the package together. And you don't figure the SCR based on DCR, you figure the DCR based on SCR. The things that dictate runner size are displacement and rpm's.

Overlap has nothing to do with DCR. The closing of the intake valve, some 100 degrees past overlap is one of the determining factors in figuring DCR. The other factors are SCR and rod length.

Bottom line is that it's easier to choose the cam last because there are thousands of cams available where there may be only two choices for chamber size in the heads you want to use. For instance, 65 or 75. So if you choose the cam before the heads, you may find yourself locked into a range of SCR where you don't want to be for the cam you chose.

Last edited by techinspector1; 12-21-2008 at 02:10 PM.
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Old 12-21-2008, 02:38 PM
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Originally Posted by techinspector1
Posted by Jsup:
"Really? Hmm... because the head runner and chamber size are linked to compression, which is linked to the cam. Overlap can effect DCR so I would think that once you know the overlap, only then can you determine the static based on the dynamic. This will dictate chamber and runner size. Where do you think I have that wrong?"

Head runner size has nothing to do with compression in the context of figuring SCR or DCR when you're putting the package together. And you don't figure the SCR based on DCR, you figure the DCR based on SCR. The things that dictate runner size are displacement and rpm's.

Overlap has nothing to do with DCR. The closing of the intake valve, some 100 degrees past overlap is one of the determining factors in figuring DCR. The other factors are SCR and rod length.

Bottom line is that it's easier to choose the cam last because there are thousands of cams available where there may be only two choices for chamber size in the heads you want to use. For instance, 65 or 75. So if you choose the cam before the heads, you may find yourself locked into a range of SCR where you don't want to be for the cam you chose.
Compression plays a role in head selection the higher compression you're running you're going to want larger runners, more flow. Air starts to compress at .6 mach which in an engine, rarely sees that speed, however, if you have high compression and start pushing air through the head more quickly you run into issues of reversion and so forth that will effect air flow and destroy cyl performance. Rather than get into a long diatribe about the fluid dynamics of head design and air flow, I will simply point out that Brodix, arguably the company putting out the heads that win most trophies, uses compression as a condition to select the right head. It is most certainly part of the criteria.

I cite for your perusal:
http://brodix.com/heads/compression.html

As far as static chamber sizes, that's just not right. I have personally ordered heads from Brodix at 55CC, 58CC, 66CC and just about anything I want from Dart, Patriot, and basically any manufacturer you want. The only thing you need to be aware of when doing this is that the intake side of the head needs to be milled so the intake fits correctly and corresponds with the bottom milling. In fact, I'd go as far as to point out that Brodix notes on their website that you have to mill XX per CC of chamber you want to take out. These companies typically charge $200 for this service regardless of the size you want.

In other words, chambers can be had in just about any size you want, making the choices limitless.

Now, heads I know, cams I'm learning. Now, I'm not saying I don't understand the basic functionality of a cam and what duration and lift mean. I am saying that when I wanted to learn heads I spent almost two months and countless hours on the phone, in person, and over email with four engineers with degrees in engineering who use fluid dynamics in their daily life and are avid racers and engine builders in thier own right. One is an aeronautical engineer who designed engines and fighter jets for Lockheed and now makes his living building $70,000 sprint car engines. Along with a few well known engine builders. I have not yet invested in the equivalent time and effort to learn cams as intimately as heads, yet.

That being said....I was always told to determine DCR before SCR that the cam will effect it.

I guess there are different approaches in going about this, and that is what I wanted to learn with this thread. Thanks for the input.

Last edited by Jsup; 12-21-2008 at 02:43 PM.
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Old 12-21-2008, 03:16 PM
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"In other words, chambers can be had in just about any size you want, making the choices limitless."

Tell that to the typical motorhead on this board who buys a set of heads from his buddy or off ebay.

"Now, heads I know, cams I'm learning. Now, I'm not saying I don't understand the basic functionality of a cam and what duration and lift mean. I am saying that when I wanted to learn heads I spent almost two months and countless hours on the phone, in person, and over email with four engineers with degrees in engineering who use fluid dynamics in their daily life and are avid racers and engine builders in thier own right. One is an aeronautical engineer who designed engines and fighter jets for Lockheed and now makes his living building $70,000 sprint car engines. Along with a few well known engine builders. I have not yet invested in the equivalent time and effort to learn cams as intimately as heads, yet."

I'm not so hard-headed that I refuse to learn a new concept and I certainly won't argue with engineers who do this for a living. I will say, however, that in the context of this board where most everyone is building a street motor, that the SCR will be in a very short range, not the 8.5 to 15.0 which you cited from Brodix. I do appreciate the information though and will pay attention to this variable in the future. Thank you.

"That being said....I was always told to determine DCR before SCR that the cam will effect it."

Again, you must know SCR before DCR can be determined. If you can provide a formula that works the other way, I will gracefully yield to your way of thinking. And yes, the cam most certainly does affect DCR. As I said, the intake closing point is one of the values used to determine it.

Now, let me add this. I have, in the past, used the DCR calculator on Keith Black's site to determine the intake closing point of the cam to be used to arrive at a certain DCR value, but you have to have the SCR value to figure it. I have also worked the system backward, using a fixed DCR value and playing with variable SCR's to arrive at the best SCR to use with a certain cam.

Last edited by techinspector1; 12-21-2008 at 03:32 PM.
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Old 12-21-2008, 03:33 PM
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Quote:
Originally Posted by techinspector1
"In other words, chambers can be had in just about any size you want, making the choices limitless."

Tell that to the typical motorhead on this board who buys a set of heads from his buddy or off ebay.

"Now, heads I know, cams I'm learning. Now, I'm not saying I don't understand the basic functionality of a cam and what duration and lift mean. I am saying that when I wanted to learn heads I spent almost two months and countless hours on the phone, in person, and over email with four engineers with degrees in engineering who use fluid dynamics in their daily life and are avid racers and engine builders in thier own right. One is an aeronautical engineer who designed engines and fighter jets for Lockheed and now makes his living building $70,000 sprint car engines. Along with a few well known engine builders. I have not yet invested in the equivalent time and effort to learn cams as intimately as heads, yet."

I'm not so hard-headed that I refuse to learn a new concept and I certainly won't argue with engineers who do this for a living. I will say, however, that in the context of this board where most everyone is building a street motor, that the SCR will be in a very short range, not the 8.5 to 15.0 which you cited from Brodix. I do appreciate the information though and will pay attention to this variable in the future. Thank you.

"That being said....I was always told to determine DCR before SCR that the cam will effect it."

Again, you must know SCR before DCR can be determined. If you can provide a formula that works the other way, I will gracefully yield to your way of thinking. And yes, the cam most certainly does affect DCR. As I said, the intake closing point is one of the values used to determine it.
Thanks. I am trying to wrap my head around where the cam fits into DCR. I mean I get it but I've been so focused on heads and most cam discussion is so high level and simplistic it's basically useless. When I put my mind to learn something, I like to learn it dead cold. Perhaps I have it backwards in my head right now DCR vs. SCR. I went through this when specing my motor and I understood it then, just need to re arrange the chairs in my head.

You can still take an EBAY set of heads to a shop and have them done to any size you want. However, if the goal is to do it on a budget, which I am a fan of, it clearly implicates compromise, the extent of which can be insignificant if done carefully or could be great if haphazard.

Hey, I'm here for three reasons. To get help when I need it, to help where I can, and to shoot the ****. Every day I learn something is a good day. That's the intent of the thread, not to talk about what I know or think I know.
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Old 12-21-2008, 04:01 PM
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Compression cannot begin until the intake valve closes, somewhere past bottom dead center on the intake stroke and expressed as X degrees after bottom dead center. There will normally be two values, one at the advertised duration (could be on the seat or 0.006" or 0.020" depending on the grind and the whims of the grinder) and one at 0.050" tappet lift.

The 0.050" figure was devised for this very reason. It provides a standard across the board between grinders and allows some modicum of common sense to the buyer so he can compare cam to cam.

You will hear some profess that reversion and rump-rump is a result of overlap. Ron Iskenderian claims that there is insufficient cylinder pressure at overlap to push the mixture back up the intake tract and that rump-rump is a result of closing the intake valve later while the piston is on its way up the bore, providing pressure in the cylinder to cause reversion just before closing the valve. Since Mr. Iskenderian has researched and tested a few more cams than I have, I must yield to his conclusion.

Just trying to help you understand camshaft theory a little.
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Old 12-21-2008, 04:06 PM
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You can very easily find your target SCR from DCR, rod length, and ICP. Infact a lot of people do this to get to the exact DCR they think they can get away with on a parricular fuel. I would build in a certian amount of cusion in there to be safe but its one way you can build on the edge.

I pick SCR first (as a result of head and piston selection), then DCR, then the cam. And for the cam I pick the LSA first, the I/E split next (closer to single or a wide dual pattern), and lastly overlap. I know its not the typical "oh i heard that was good" method, but it makes the most sense to me.
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Old 12-21-2008, 04:13 PM
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Might be bass ackward on this but, DCR is what it is. If you have properly matched parts the engine will work.

RPM range, fuel type, vehicle and drivetrain specifics along with the intended use of the engine/vehicle are all a part of determining what combination of parts, and their specifications, will be optimal for the engine. Cam design is not an independent process with specifications to be predetermined by any single aspect of the combination.

That mindset will always result in less power than the engine has the potential for making.



Larry
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Old 12-21-2008, 04:47 PM
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Quote:
Originally Posted by coldknock
Might be bass ackward on this but, DCR is what it is. If you have properly matched parts the engine will work.

RPM range, fuel type, vehicle and drivetrain specifics along with the intended use of the engine/vehicle are all a part of determining what combination of parts, and their specifications, will be optimal for the engine. Cam design is not an independent process with specifications to be predetermined by any single aspect of the combination.

That mindset will always result in less power than the engine has the potential for making.



Larry
Larry, you've hit the nail on the head with your "properly matched parts" statement and determining DCR is part of that process. If your motor will operate on the available fuel with a DCR of 8.0:1 and you build the motor at 7.0:1, you've left a full point of compression on the table and the motor won't operate up to the potential it could have.

You can build a motor with a certain DCR by juggling SCR and cam intake closing point. In other words, you could build 8.0 DCR by using 11.0 SCR and a delayed intake closing point to bleed off some of the compression before the intake valve closes (reversion and rump-rump) or by using 8.0 SCR and an earlier intake closing point to trap more of the mixture and not blowing it up the intake tract.
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Old 12-21-2008, 05:44 PM
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I have to keep reminding myself that not everybody here uses C16 for fuel.



Larry
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