Higher Compression Ratio

[blueboss]

I got into a discussion on a local board about compression ratios, the application of boost and cylinder pressures and it got me thinking. Apparently, there are quite a few folks with mod motor mustangs applying a significant amount of boost to an already high static c/r. Something in the neighborhood of 10:1 to 12:1 then putting anywhere from 10 to 20psi of boost on top. According to the posters, these are stock-ish engines, daily driven on a pump gas diet with thousands of miles on them post-forced induction and have yet to consume themselves. I know that one of the benefits of aluminum head technology was that you could raise the cylinder pressures over stock, within reason (and what was deemed "acceptable" for iron head technology) without the fear of detonation. The reasoning was that aluminum is a much better conductor of heat and is able to deal with the higher cylinder temps as a result of increased cylinder pressure. That being said, configured correctly, how high could one take the compression ratio and boost on the Ford 2.3 lima, feed it pump gas and have daily driver reliability? Perhaps with an o-ringed block/head is 10:1 with 15-20psi of boost doable while meeting the above criteria?

[Bob Holmes]

Well, let's lay aside the issue of internet veracity.

Modern engine design, smaller bore, better chamber design, optimized spark plug placement, aluminum construction, significantly better engine management aside; If you were running an aluminum Bo-ported or equivalent head, had top-notch intercooling, and an appropriate stand alone ECU, I think you can make 10:1 work.

I wonder how much the VVT on the mod motors help to make the engines live?

[Raven855]

I did this reply awhile back for another site:
Realistically, it depends on your engine. Certain combustion chamber shapes tolerate higher compression without detonation (ie hemispherical ) than others (ie OHV wedge,or sidevalve). Therefore, higher compression may provide efficiency benefits in some engines without much ping.

I believe in most cases excessive compression or timing are both detrimental to engine durability and efficiency and are not useful tuning methods if taken to excess.
The answer is that the correct combination is what's best. Lets assume that your car is properly tuned for your current combo of compression and 87 octane. The total ignition lead lets say is 38*. For most engines and designs, even if you went to 10.5:1 and 94 octane, the best power will still be found right around 38*. The reason is that the distance from the spark kernel to the piston crown hasn't changed enough to make a big difference. You're still shooting for peak cylinder pressure at the proper crank angle regardless of the rest of the combo. If you add compression and have to retard timing to prevent detonation, you have taken the engine out of optimal tune. You're crutching high compression by retarding timing. Peak cylinder pressures will occur after the sweet spot of crank angle and you'll lose power. Same goes for the other way; if you reduce compression and add timing, you're making too much pressure early and its all over before the sweet spot in the crank's travel.

For best operation, the expanding flame front should be at its peak pressure at about 20-23 degrees ATDC. Increasing the compression or changing the octane doesn't appreciably alter flame front speed, so that is why ignition advance is most efficient at the same place regardless of other states of tune.

Now lets say you drop the compression to 7.5:1. Now you can (in fact need to) advance the timing to compensate for the extra distance and lower pressure before ignition so that you can still get your peak pressure after ignition in the right place on the crank.

Compression doesn't change flame front speed.
Octane doesn't change flame front speed.
Flame front speed is the largest factor in when cylinder pressures peak, so since flame front speed is relatively fixed, ignition timing is the biggest key. The moral of the story is, build your engine in whatever compression you need for your cam, set the timing for peak power, and then run the octane you need to keep detonation away. Anything else is a crutch and will operate with less power and efficiency than if you were running the right combo of parameters.

If that weren't the case, then there would be no need for high octane gas. We could just all run 10.5:1 on 85 octane by just retarding the ignition, but that's not the case.
The way the geometry works out, the same crank angle is where you want peak pressure regardless of compression, rod/stroke ratio, stroke, most anything. The reason is (since ignition happens BTDC) pressures are rising as the piston is still on its upward travel. If you advance too far, you're wasting flame pressure on fighting crank revs in the wrong direction. If you go too late, the piston is falling away faster than the flame pressure can effectively act on the crank and you lose power. That's why you want peak pressure in the same place on the crank. Its also why crutching tune with anything less than the required octane will hurt power.

The retarded timing (if necessary) on higher compression engines has to do more with the reduced distance from the spark plug to the piston crown. Other factors are involved, like piston shapes. If you have achieved your higher compression with domed pistons, you may actually have to advance timing to keep your engine tuned. Domes slow flame fronts and make the peak pressure happen a greater amount of time after ignition. BBCs and Hemis are notorious for this. The typically large open chambers require big domes to get decent compression and it makes them somewhat poor in the detonation tolerance category. A fast-burning chamber design which accelerate flame speed, and you can get by with 87 octane on 10.5:1 because you only need about 30-32* total advance to get you peak pressures in the right place on the crank. BUT... typically a 10.5:1 runs best with the same total advance as an 8.5:1.
Compression ratio does not affect flame speed. It does raise compression temperature which persuades the ignition point to occur at a slightly earlier crank angle. Perhaps even before the spark plug fires. Given a sufficiently high compression ratio we don't really need a spark plug. When the autoignition point of the fuel is reached, it will ignite without a spark.

Flame speed is primarily determined by squish velocity. Squish velocity will determine the turbulent flame speed by agitating the unburned mixture and bringing unburned fractions in contact with the wrinkled flame front. Squish velocity is determined by combustion chamber design, piston to head clearance and piston speed. An engine with higher squish velocity can run on lower octane fuel without detonation but may run the risk of pre-ignition if excessive. An engine running at very high speed can use lower octane fuel.

Its also important to note that octane is not cumulative; that is to say, more doesn't really affect things. If you have a combo that needs 91 octane, anything below 91 will require you to retard timing, but running 104 won't really let you advance it much. That's because it doesn't really change flame front speed. The cylinder pressures will peak at the same amount of time after ignition regardless of octane... its just that if you don't have enough octane, it might ignite before the spark plug says it should Then you get detonation from insufficient octane. Higher octane fuel might control the onset of detonation but it will not prevent pre-ignition due to excessively advanced ignition timing. Therefore, ignition advance timing is much more important than compression ratio. The goal is to produce maximum combustion pressure at the proper crankshaft angle. Pressure rise prior to TDC can destroy pistons and conrods and bottom ends, late pressure rise will reduce power and overheat the exhaust valve and port and pipes and water jacket.

So in answer to your question, I believe that there is not a lot to be gained and the potential for harm is pretty great going over about 9.25:1-9.5:1 compression. With aluminum head add an additional point. Get a BB turbo, do the fuel pump strategy like Pat advised, run some higher rpms and go have some fun.

[65ShelbyClone]

configured correctly,

What does that mean, exactly?

I will point out that even stock 8:1 engines running stock "premium" boost levels often pull timing out in response to knock. You could build the engine with an aluminum head, a lumpy cam, laggy turbo, and put all the power up high in the rev range to do what you're talking about, but it probably wouldn't be a fun daily driver.

One other thing: those boosted NA mod motors do blow up. Many use water and/or methanol injection too, which is comparable to using high octane fuel, so its really just a half-truth to say they're putting 20psi on 10:1 with pump gas.

[MikeFleming]

I will point out that even stock 8:1 engines running stock "premium" boost levels often pull timing out in response to knock. You could build the engine with an aluminum head, a lumpy cam, laggy turbo, and put all the power up high in the rev range to do what you're talking about, but it probably wouldn't be a fun daily driver. You're talking about a 40+ year old engine design with a very poor combustion chamber and the wrong bore/stroke and stroke/rod ratios. It was *never* *designed* to be boosted - all Ford did was change pistons, rings, ignition and fuel system. Same head, same ports, same rods, same crank, ...

Take a more modern engine designed specifically for turbocharging: the 2008+ Mitsu 4B11T engine. 86x86mm bore/stroke, LONG rods, 4-valves with central spark plug, 9:1 static CR running to 23 PSIG boost pressure. Stock. 2.0L (14% smaller displacement than the SVO engine) and making 50% more power. DEFINITELY a fun car to DD.

Contrast to the 2001-2003 Prius engine using a static 13:1 CR happily running on 87 octane. Some Mazda DI engines are running 14:1 static CR.

Engine design makes a BIG difference.

Just sayin'

[blueboss]

What does that mean, exactly?

Put it like this, "I" wouldn't attempt this without the best forged rotating assembly (pistons, rods, crank) which has been dynamically balanced to within .2 oz/inches, combustion chamber cc-ing on nothing but an aluminum (either volvo or 2.3 aftermarket) head, machine shop discipline that would make Jack Roush jealous, liquid to air intercooling and a top notch engine management system. That's what configured correctly means.


I will point out that even stock 8:1 engines running stock "premium" boost levels often pull timing out in response to knock.

If you have a stock 2.3 pulling timing at stock boost levels, something is wrong somewhere.


You could build the engine with an aluminum head, a lumpy cam, laggy turbo, and put all the power up high in the rev range to do what you're talking about, but it probably wouldn't be a fun daily driver.

"Lumpy" cams and "laggy" turbo's are an outdated, archaic concept. With the advancements in turbo, camshaft and engine management technology I don't need all that 60's hot rod mentality to have a perfectly capable and fun to drive daily driver that doesn't have a powerband 2000 rpm wide above 3500. There are new ways to skin that cat.


One other thing: those boosted NA mod motors do blow up. Many use water and/or methanol injection too, which is comparable to using high octane fuel, so its really just a half-truth to say they're putting 20psi on 10:1 with pump gas.

That may be true. However, the local guys I've talked to are not running any type of water/meth injection or any intercooling for that matter. And the engines are fed 87-93 octane and driven accordingly. Apparently, the stock modular engine configuration is perfectly capable of handling 15 to 20 psi and live a long life. Admittedly, they're not doing sustained runs at that boost level but still.....

[65ShelbyClone]

Contrast to the 2001-2003 Prius engine using a static 13:1 CR happily running on 87 octane.

They use an Atkinson cycle, though. As an extreme example of a regular four-stroke, the current 600cc crotch rockets do the same thing. Exactly as you said: all in the design.


1.) Put it like this, "I" wouldn't attempt this without the best forged rotating assembly (pistons, rods, crank) which has been dynamically balanced to within .2 oz/inches, combustion chamber cc-ing on nothing but an aluminum (either volvo or 2.3 aftermarket) head, machine shop discipline that would make Jack Roush jealous, liquid to air intercooling and a top notch engine management system. That's what configured correctly means.

2.) If you have a stock 2.3 pulling timing at stock boost levels, something is wrong somewhere.

3.) "Lumpy" cams and "laggy" turbo's are an outdated, archaic concept. With the advancements in turbo, camshaft and engine management technology I don't need all that 60's hot rod mentality to have a perfectly capable and fun to drive daily driver that doesn't have a powerband 2000 rpm wide above 3500. There are new ways to skin that cat.

1.) That's why I asked; my definition is a lot different than yours. Throwing the Volvo head in there almost makes this two separate discussions.

2.) Yeah....the induction, the head, the EFI, the intercooling....

3.) I know all of that, but it's applicable when you're still talking about using a 1960s engine design. I think that by the time you got a Lima to do what a 40-years-newer engine does now, there would be little of the original package remaining and your definition of "properly configured" aligns with that. My point is that the answer to your question of "what would it take?" is "a ground-up redesign." SVO was already doing that when the program was canceled.

[blueboss]

:like3:

[MustangRacer18]

I would like to clear up a few things, since engines/racing are my background in engineering, with no disrespect to Raven855...there's just a few points that are a little off. Most detonation in spark engines occurs after the spark and flame have started. If it occurs before you got bigger issues. Its the end gas out at the outter edges of the cylinder that pop off before the flame front gets to it that is 99.99999% of detonation in spark engines. First off CR is great....it helps not only thermal efficiency of an engine but volumetric too. The problem is it can cause greater temps and pressures in the cylinder which cause detonation (since intial conditions before the burn are increased the end gases see higher temps and pres as the flame comes to it). In order for a AF mix to auto-ignite several conditions must be met but the main two are that the AF mix must be above a certain temp cause the auto-ignition temp (AIT) AND it must be held above that temp for a certain amount of time called the ignition delay or induction period. You always want to run the lowest octane you can without detonation...why? Octane does indeed slow the speed of the flame across the bore. This is bad news for detonation as you want a faster flame to pop everything off before it has time to auto-ign. However...octane greatly increases the time required above the AIT hence why higher octane prevents detonation (it gets into chemical combustion kinetics that I wont get into yet). So essentially if you get to pick the CR you want to run the highest octane you can (or wallet can afford) to allow you to run the biggest CR to help out in vol and thermal efficiencies. CR also does increase turbulance in the clyinder which is good for flame speed. You want as fast a flame speed as you can get to have as close to volume=constant combustion. Since CR is total volume at BDC (swept and clearance) divided by clearance....decreasing clearance volume (head chamber volume) means you increase CR and you are squezzing air into a smaller volume. This creates a much higher squish velocity that Raven mentioned which is what you want. You really want peak pressure at about 7-15 deg ATDC. Greatest instantaneous torq occurs slightly later...due to geometry. Flame speed is also driven by temps...higher the temps the faster the speed (again due to chemical kinetics). So in a nut shell:

Higher CR does increase flame speed due to higher turbulance in cylinder and higher temps

Higher octane slows the speed of the flame but more greatly increase the induction period so end effect is it reduces the chances for knock

Always run the lowest octane your engine can handle...running higher hurts flame speed and thus efficiency and you will get worse performance and mpg.

Looking at another post water injection is done to keep temps in check if you can't run higher octane but since it displaces air...and an engine is nothing but an air pump....it really is a crutch and is a result of poor design..it should never be done. Methanol on the other hand is very good :D Not only does it cool the incomming charge when it evaporates from liq to gaseous but its the only std fuel that cools it so much that it will draw in more air. (Increase volume due to state change is less then what it shrinks due to temp cooling). Methanol injection is very good!

Hope this clears some things up...hit me up if you have any questions.