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Old 03-04-2004, 06:18 AM
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I_Thnk_Ford I_Thnk_Ford is offline
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Question High compression vs. low compression.

1)What are the advantages of high compression?

2)What are the advantages of low compression?

3)If I where to build a motor; wanting torque and horse power.
a)how would a higher compression engine serve me?
b)how would a lower compression engine serve me?

4)Regardless of whether this engine was used for street/strip or towing. What the advantages or disadvantages of both?

All input is appreciated.

Robert P.
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Old 03-04-2004, 08:14 AM
jwtaylor jwtaylor is offline
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1) The motor is more effiecent power and mpg wise, the higher the compression you can run without detenation in a naturally aspirated engine the better.

2) You can use cheap gas without detenation concerns.

3) a. If you want torque and horsepower in a naturally aspirated combo, then the higher compression will make the throttle crisper, and increase the efficiency of the motor, the by product would be better hp and tq.

b. A lower compression motor will let you run cheap fuel, but your motor won't be as efficient, less efficient, less power.

4. see above


later
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Old 03-04-2004, 08:35 AM
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What compression you use will be determined by which fuel you want to put in it. Typically an engine can live on 87 octane with a maximum 8.5/1 compression. With an excellent set of heads you could get by with 10-10.5/1 on 93 octane anything more than this and you will have to use racing gasoline. It is not unusual now for big cubic inch engine to make 7-800 hp on pump gas, a feat which was un heard of a few years ago.

To answer your question... either engine will create large amounts of Horsepower and Torque if built correctly. Obviously a high compression engine will make a giving Hp and Tq easier than a lower compression engine (simple thermodynamics).
There is a lower cost associated with operating a low compression engine ($1.60 a gallon vs $4.50 for fuel). High compression (13-14/1) can be harder to start when hot, most will require high torque starters. If the engine is to be driven regularly then pump gas would be a big factor, it's hard to carry around the amount of fuel needed to keep a thirst high compression engine happy for long trips.
If high Hp and Tq are your goals using pump gas then look at a forced induction engine. You can start with 7.5-8/1 compression and let the supercharger/turbo do the rest. High numbers can be achieved at a lower RPM with incredible reliability.

Some of the determining factors with todays technology are:

1. What fuel do you want to use?
2. What are your objectives for HP and Torque?
3. How much am I willing to spend to achieve my goals?
4. Now how am I going to use this? (Make sure the top three fit)

Hope this helps...
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Old 03-04-2004, 04:00 PM
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I think Ford......thanks for starting this thread, I have been curious about it myself, as I listen to a lot of different opinions, my project truck is a 73 f100 4x4 with a dove-a block 429 in it with dove c heads, Ive been told my compression will be about 10.5:1 (although I dont know because it isnt put back together yet) and Im wondering how Im gonna make this truck run...Im also wondering how it ran in the past with that high of a compression, The prev owner sold it to me for $500 becasue he was tearing it down because it was backfiring.....hmm, wonder why and all the different opinions on here are making me wonder if thats what I want?

So im watching this thread with great anticipation.....I just dont know what Im anticipating

Kevin
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Old 03-05-2004, 12:50 AM
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The speed-o-motive has a succinct but informative discussion WRT tuning. It includes a discussion of compression, detonation, octane and combustion chamber size and shape.

Generally speaking, you want the highest compression you can manage. By using heads with compact chambers, you can actually run higher compression for a given octane. An example of a compact chamber is the E6SE 302 head. The chamber is relatively large at 69cc's but it is compact and has a lot of quench. The E7TE head has a smaller chamber, but it is an open chamber with much less quench. The E6SE head is very efficient but the small chamber shrouds the valves, so peak power is less than with the E7TE head. The ultimate head is the F7TE - the GT40p head - small, compact chamber, big valves, centrally located spark plug, and a 'swirl' design.

Speed-o-motive describes the optimum setup as one with a dish in the piston that matches the shape of the combustion chamber. This is actually what the guys with the 400's need; a piston with a dish to match the Oz heads. Reasonable compression ratio, lots of quench, ports that are the right size.

I still don't know why so many truck heads are 'open chamber'. It seems like they would be the ones who run with more throttle for longer periods of time. Maybe the open chambers run cooler.
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Old 03-05-2004, 03:23 PM
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I_Thnk_Ford I_Thnk_Ford is offline
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I started this post for a varity of reasons But, I'm not going to let the cat out of the bag I want to see some of the opions out there on this subject, or an engine I hope to build shortly.
I was told by a local engine builder that if I where to want an engine for towing or highway usage I should think about a low compression piston, like 7:1. He said it would allow my motor to stay in it's torgue range and not be so hard on the engine (because of the lower compression) and it would last longer. He also made mention that if this was for street use I would lose H/P at the top end. Which is what you want for a street driver.
Keep those thoughts coming.



Robert P.
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Old 03-05-2004, 06:41 PM
3eggOmelet 3eggOmelet is offline
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7:1 compression is shamefully low. that would be one weak engine, though like the mechanic said it would definately be easy on the engine and make it last longer. but you would need the extra life to cover the extra time it would take you to tow anything anywhere.

no way in the world should you build anything with less than 9:1 compression. for a true street/strip vehicle you could go as high as 11:1 and still run pump gas (93 octane).

More compression gets you more power, no two ways about it. any dyno program you run will show a direct relationship between compression and horsepower/torque.

The only time when lower compression is better is in a boosted application such as turbo/supercharging. in that case a lower compression is good, because when the compressor kicks in it raises the compression quite a bit. example: a v8 engine running at 8:1 compression with a supercharger pushing 12 pounds of boost at max RPMs will raise the effective compression to over 17:1. This isn't bad for your engine because how often are you going to be hitting 5500+ RPMs for more than a few seconds? So in this case the lower compression of the engine gives you more room for more boost from your compressor.
But even in boosted applications it is good to keep a reasonably high compression so that the engine won't run like a dog at idle or low RPMs when the compressor isn't pushing much.
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Old 03-05-2004, 07:19 PM
pcmenten pcmenten is offline
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Quote:
Originally posted by I_Thnk_Ford
I was told by a local engine builder that if I where to want an engine for towing or highway usage I should think about a low compression piston, like 7:1. He said it would allow my motor to stay in its torgue range and not be so hard on the engine (because of the lower compression) and it would last longer. He also made mention that if this was for street use I would lose H/P at the top end.


Robert P.
That's just crazy talk. As mentioned before, higher compression is higher thermodynamic efficiency. Lower compression is lower; power, economy, engine life.

If you want less power, use a smaller engine. If you want to tune an engine for a particular operating range, get the cam ground for that operating range, and match the intake and exhaust ports and runners to that frequency.

Regardless, you want an efficient combustion chamber, and the highest compression that you can run with the fuel you have locally.

Obviously, the cam's tuning plays a part in this. So does the size and length of the intake runners, the size and length of the exhaust manifold runners/tubes/whatever, the length of the exhaust system. You can acheive greater than 100% VE and that will cause a boost in compression ratio, but that's a pretty tweaked engine.
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Old 03-05-2004, 08:10 PM
jwtaylor jwtaylor is offline
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Only thing I could say now is......don't talk to your engine builder and try and get advice, go elsewhere.

As mentioned before, the higher the compressoin in a naturally aspirated engine the better, no matter what the application. Your only concern is the fuel grade you plan to use, then shoot for a compression ratio that your motor can live at. Best bet, talk to those in your area that have performance engines, see what they are getting away with, fuel quality varies from pump to pump, what may work in my area, may not work in yours. Good luck
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Old 03-06-2004, 10:43 PM
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Let the cat out now. This thread is inefficient if the respondents do not know your motives.
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Old 03-07-2004, 04:15 PM
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I_Thnk_Ford I_Thnk_Ford is offline
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jwtaylor,

Quote:
Only thing I could say now is......don't talk to your engine builder and try and get advice, go elsewhere.
Thats why I asked it here.

Ecuri,
Aren't the above questions simple enough for an answer? Are you a cat lover? Is that what offended you?
For those who are in fear of my cat suffacating. Here's the jizzy. There are two ways to build a motor. Sreet/strip or highway/towing. Most fellas here go for the street/strip in your gut power/torque setup. Which, only if done properly, will yield minimal mpg's; 8 - 12 mpg's. I have been researching cams and intake and carb/injection setups for 6 months now. Truely, comp. ratio is the next check on the list of things to get info about.
Even with all the good ideas that are posted here, there is no way for me to get ALL my questions answered.
Essentually I want the most powerful motor that can tow and still yield 14 - 16 mpg (without a trailer) out of a 360 FE /AOD set-up in a 71' crew cab.

Meow. Here kitty, kitty.

Thanks again for all the worth while posts.
Robert P.
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Last edited by I_Thnk_Ford; 03-07-2004 at 04:17 PM.
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Old 03-08-2004, 12:57 PM
Ecuri Ecuri is offline
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All I wanted to know was where and why you were going with this, so if I knew anything about what you wanted to accomplish I could add something constructive. Beating around the bush is what frustrated me. Sorry if I was not very diplomatic. I do not consider myself highly knowlegable on the FE, so I will bow out of this discussion and wish you good luck.
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Old 03-08-2004, 12:57 PM
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