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Old 02-07-2007, 07:49 AM   #1
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Default Highest Compression

Take for example a Series I Normally Aspirated engine *L27*. Say that you wanted to rebuild an L27 to a different compression ratio by changing the pistons and any other internal parts required. How much could you realistically change the compression ratio without having to run anything more than 91-93 octane fuel?

Now take this same engine, and say along with this compression change, that you wanted to overbore it and/or stroke it. How if at all, would either of these displacement changes affect the realistic limits of the compression ratio.

And third question, if anyone knows, how much power, both in regards to horsepower and torque would the maximum compression ratio change to this example engine'* output?

And also, please give information about what other things would have to be changed along with the compression ratio besides octane level? Rockers? Cam? EMs? Intake? Computer reprogramming? Maximum safe engine rotational speed? Valves?

Also, please include information about what N/A 3800 would benefit most from the compression change, as well as any known info about how the strength of engine internals changes from the LG3-LN3-L27-L36-L26 regarding connecting rods, crankshaft, pistons, etc.
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Old 02-07-2007, 10:39 AM   #2
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I'll throw in what info I can.

If maintaining a stock cylinder head and piston, a longer stroke would increase the compression ratio. I do not know the numbers as I think it depends on the bore. If I recall, larger bores also increase stroke if the piston dish/dome volume remains the same between the different sized pistons. ZZP'* stroker kit jacked the 9.4:1 to 11:1 and said something to the effect that the increase in compression ratio netted a 7% increase in power.

Higher compression allows the engine to ignite the fuel more efficiently. I think it creates a hotter burn, and it actually lowers emissions(until you start adding more fuel and air).

I've been told that some vehicle setups can run extremely high compression on pump gas when coupled with the right cam, but this means you can't really romp on it when you're just driving around(something to do with valve overlap and basically reducing the volume in the cylinder by allowing the piston to push part of the air/fuel out the exhaust before it'* fully closed? Someone correct me here).

Longer stroke engines typically offer more bottom end torque.

In my adventures across the net, I've read that you can run up to 11.5:1 compression ratio on pump gas with cast iron cylinder heads.

The fuel you run depends on the tune of the vehicle. I think the benefits of higher compression are noticed most in the top end of the powerband.

The L36 comes with a cast crank, powder metal rods, and hypereutectic pistons.

I hope this all proves to be accurate and answers part of what you want to know.
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Old 02-07-2007, 11:18 AM   #3
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There is no definite answer to how far you can take the compression ratio. The limit depends on many factors. Some are how well the air and fuel are mixed, how the combustion chamber is shaped (piston and head), the cooling efficiency of the head (no hot spots in the coolant passages or combustion chamber), cam timing and duration, and the general flow of the intake and exhaust.
Things like aluminum heads, implementing swirl and tumble techniques in the combustion chamber(in both the intake runners and design of the piston/head combustion chamber), eliminating hotspots, larger cam overlap, and freer flowing exhausts all allow for a higher static CR.
No one can really say for sure how high you can go with the CR. Rasing it a half a point seems reasonable with supporting mods. FWIW the L36 has been taken to 11.6:1 in an all out race configuration.
The octane level is just as uncertian. It is dependant on much of the same things the CR limit is.
Milling the heads and getting custom pistons/rods will increase the compression ratio. Stroking the engine will also increase the CR depending on which rod you choose.
There are rough estimates on how much CR will effect hp/tq. If you google it you should be able to find calculators.
Max rotational speed also depends on a few things. The rod to stroke ratio and how well the oiling system can keep up are just a couple major items to mention.
What I said is only the tip of the iceberg. The more you learn about this stuff the more you realize you don't know.
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Old 02-07-2007, 12:43 PM   #4
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http://www.rbracing-rsr.com/comprAdvHD.htm

A very good site with a lot of information on what affects dynamic compression ratio.

Valve overlap is not the key, intake closing piont is.

In example, my race rabbit engine with a very radical cam.

Camshaft, Rod Length, Boost and Altitude Correction to Compression

Your engine summary is as follows: Bore 3.27 inches, stroke 3.4 inches, rod c-c length 5.67 inches, with a static compression ratio of 12 :1.
Your camshaft specifications call for an inlet valve closing of 79 degrees ABDC (after bottom dead center).

Your chamber volume is 42.54 cc'*. With this camshaft your dynamic, or effective stroke is 2.28 inches. Your dynamic compression ratio is 8.38 :1 corrected for cam timing, altitude, and rod length.
Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 168.81 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 0 PSI is 8.38 :1.


Now advance the cam 10 degrees.

Camshaft, Rod Length, Boost and Altitude Correction to Compression

Your engine summary is as follows: Bore 3.27 inches, stroke 3.4 inches, rod c-c length 5.67 inches, with a static compression ratio of 12 :1.
Your camshaft specifications call for an inlet valve closing of 69 degrees ABDC (after bottom dead center).

Your chamber volume is 42.54 cc'*. With this camshaft your dynamic, or effective stroke is 2.54 inches. Your dynamic compression ratio is 9.22 :1 corrected for cam timing, altitude, and rod length.
Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 190.86 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 0 PSI is 9.22 :1.


This is a significant difference.

As far as how high you can go on pump gas; the general rule of thumb is 10.5 to 11. A more radical cam will allow you to go higher but will not be very drivable on the street.

The cam on my Rabbit does not reach the power band untill 4000rpm, it will pull to well over 8000rpm. This very high power band works well on a light (<2000lb) car with a close ratio transaxle when driven on the track.
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Old 02-07-2007, 01:18 PM   #5
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That does make sence.
I can't see a larger overlap not helping though.
Less cylinder pressure.
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Old 02-07-2007, 03:58 PM   #6
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You can't build pressure in the cylinder until the intake closes.
The overlap just effects where the charge goes, out the exhaust or reversion through the intake.

Even with a radical 316deg cam with 100deg overlap the exhaust is closed more than 200deg before the intake.
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