Compression Ratio

phybertek · Post by **phybertek** » Thu Aug 30, 2001 9:18 am

Is it best a the safest to stay with factory default compression ratio ranges. I'm building at 2270cc engine. I live in NYC. No where above nor below sea level. Should i do 7.0:1 or 7.3:1. Also, could you explain why someone would like to increase the ratio in the first place? Is it to increase the 14.7:1 air/gas intake at a hire rate. Where does the octane needed gas come in?

thanx in advance.

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Thu Aug 30, 2001 10:00 am

DO NOT BE FOOLED WITH LOW CR MYTHS, especialy those claiming to be the cure for wevery heat problem with an aircooled engine...

With that said, I can say we run up tom 8.5:1, with a DTM shroud, and up to 10:1 on street engines with a 911 system and a smaller displacement engine.

our 2270 burns clean at 8.5:1 and head temps stay at about 350 all the time, with a DTM...

The trick is to have an efficient design, with CAM, Heads, Valve sizes and Induction..

Low CR engines are the tuners nightmare as they respond lazily to jet changes, and most of the time run terriably.

------------------
Jake Raby
Raby's Aircooled Technology
www.aircooledtechnology.com

neilca · Post by **neilca** » Thu Aug 30, 2001 12:36 pm

Bravo Shad. Very well put. I would only add that the higher the octane rating the slower the burn of the fuel. There is no extra energy available in higher octanes.

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Thu Aug 30, 2001 12:53 pm

By no means did I try to implicate that street cars should try to run excessive CR, without some modifications, sorry if it was taken that way..

I simply meant that there is no reason to cam an engine, and give it some good heads, only to kill the power with low CR.

I set my street engines to run on premium gas, and they all do it well, even with 8.5:1 and mild cam/heads

------------------
Jake Raby
Raby's Aircooled Technology
www.aircooledtechnology.com

ray greenwood · Post by **ray greenwood** » Thu Aug 30, 2001 2:28 pm

A little note to go along with that. If you are running stock injection on the type 4, too low of a c.r. will greatly hinder the ability of the fuel to ignite properly the way it is mixed.This will be even more evident on the FI engine than the stock carb engine. Both the pressure senor (D-type) and the air vane (L-type) are not sensitive enough in their leaning out adjustments to compensate for lower vacume and less efficient combustion encountered with lower compression ratio. With that said, my experience has shown that with two FI engines of the same cylinder diameter, one with a flat top piston and one with the small dome...the dome always seems to produce more even combustion and more tunability. I have found the very best tuning results on the FI type 4, at compression ratios between 8.0:1 and 8.8:1 . Any lower and the way the fuel and air is mixed and burned seems to be affected greatly and can't adequately be adjusted for. Ray

Dave_Darling · Post by **Dave_Darling** » Thu Aug 30, 2001 8:04 pm

Stock 1.7 L 914s ran 8.2:1 compression, on high-octane (~92 "pump" octane) fuel.

Stock US-spec 2.0 L 914s ran 7.6:1 compression, on regular-grade (~87 "pump" octane) guel.

Stock European-spec 2.0 L 914s ran 8.0:1 compression on mid-grade fuel. (~89 octane.)

This was with the stock "pancake" cooling system, in a lightweight car, with the stock mild cam. And with the factory-engineered safety margins built in. I know people who run 8.5:1 2.0 L 914s on high-test all the time. The 9.0:1 guys have told me that it's a bit "iffy" with everything else stock.

Big cams with lots of overlap will decrease the compression pressures, which has effects similar to decreasing the compression ratio. You can (and often, should) bump up the compression of an engine that is using a really big lumpy cam.

--DD

------------------
1974 VW-Porsche 914; 2.0 liter Type IV motor

Paul Illick · Post by **Paul Illick** » Thu Aug 30, 2001 8:37 pm

Shad: "Since diesel engines don't inject fuel until they want it to ignite, they can get insane compression ratios, almost always over 20:1." Huh? It has to be up around 20:1 anyway, so what does when the fuel's injected have to do with a diesel's compression ratio?

Shad Laws · Post by **Shad Laws** » Thu Aug 30, 2001 11:08 pm

Re: flat top versus "dome" piston, do you mean dish Ray? I think you do... which makes perfect sense: to get the same CR, your deck height is lower. The closer to zero you get (with a tolerance for stretch, of course), the better burn you have. (which, BTW, is why I'm custom-making all my pistons for a 0.040" deck

Re: big cams and CR, the reason is simple: Static CR is just that, a static number. What really gives power is dynamic CR, which with other stuff points to BMEP (brake mean effective pressure), which in turn points directly at torque curve, which with engine speed points directly at horsepower. Dynamic compression ratio has to do with the actual fluid mechanics of the engine at WOT. If you have an infitinely slow engine and a 180 degree cam, then you'll get a dynamic compression ratio equal to the static one. But, get stuff going fast and it changes!

All other things equal, "Torquey" cams get torque due to a higher dynamic compression ratio at a certain place in the RPM band. "Power" cams get less torque, but the peak is much higher in the RPM band, yielding more power. It's not quite that simple, but you get the idea... So, to get the same dynamic CR with a hot cam, you need a higher static CR to "compensate."

Re: CR in diesels, I was just trying to make a point since the original poster didn't seem to understand what CR does, that's all. LPG and natural gas have a higher octane rating, so they can get a higher CR, too... which helps to offset the lower energy to A/F mixture volume ratio they have (i.e. with all other things equal, you can't get enough LPG burned in the cylinder to equal gas's power).

Take care,
Shad

Shad Laws · Post by **Shad Laws** » Thu Aug 30, 2001 11:31 pm

Static compression ratio refers to the ratio of volume of a cylinder at BTDC to the volume of the cylinder at TDC. The higher the compression ratio, the higher the thermal efficiency. This means that for X amount of gas burned, you get MORE power and LESS waste heat (whether it be in the exhaust gases or in the parts surrounding the combustion chamber). So, to a degree, higher CR = better gas mileage and greater power.

However, the peak temperatures in the cylinder increase with higher CR. This can lead to autoignition (a.k.a. knock) of gasoline, which means that it burns before the spark plug says to, giving a net effect similar to too much ignition advance. Octane rating is an anti-knock rating. The higher the rating, the more resilient to knocking it is. So, higher octane fuels can withstand higher temperatures without knocking than lower octane fuels. Since diesel engines don't inject fuel until they want it to ignite, they can get insane compression ratios, almost always over 20:1

.

There are other problems, too. At full load, high CR means more stress due to higher peak temperatures and higher pressures. And, because a stock aircooled VW engine is notorious for having a high mean operating temperature, they must run at a lower CR to compensate. Add that to the pathetic Mexican fuels of today and you get new Mexican bugs at a CR of 6.6:1

. But, in Europe, VW has offered a CR as high as 8.6:1.

Long story short: high CR = more power and better gas mileage to a point, then decreased engine life beyond that point. For a typical T4 engine running at a low load (light cars), I like 8.5:1. But, if you can drop the operating temperature a bit and help the weak-as-stock seal between the head and cylinders, then a higher CR is in order...

Take care,
Shad

Paul Illick · Post by **Paul Illick** » Fri Aug 31, 2001 5:14 am

Shad: "so they can get a higher CR, too"? It's not a question of octane letting you 'get' a higher ratio. In a diesel it's the compression itself that ignites the charge, which happens up around 20:1. That's why the compression's that high, because it has to be. Diesel's a different animal, not Otto, so it's probably not the best example for your explanation.

Have you met Dan? He's a Berg-er and has some pretty strong feelings about CR's.

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Fri Aug 31, 2001 6:21 am

Hey Paul,
Lets rephrase that last part...

"Dan has alot of friends with strong feelings about CR" LOL LOL

Tom Perso · Post by **Tom Perso** » Fri Aug 31, 2001 6:25 am

There is a great thread over in the drag race forum concerning C/R. Dan from Ocean St., John C. and some others got involved in it, some flames thrown around, but lots of info. Go check it out... Plus, everyone knows how much certian people around here love Dan...

Jake, don't give him such a hard time, for he knows no better...

Hahahhaha...

FWIW, I am running 9:1 in my 2270 in a lightweight 61 sedan for pleasure use only. I have 48x38 1.8L Bus heads, ported, with the split duration Webcam bumpstick. For the limited use this car will be seeing (autocross and just for fun, not daily driver) I felt the bump in compression would not be an issue. Plus, if I have to run race fuel (or a mixture) it's no big deal in my book.

I'll be honest, it took a long time to figure out what C/R to run, I toyed with a bunch of different numbers and deck height combos, but I think (hope at least) this will work. I'll let everyone know once I get the motor together.

Later,
Tom http://www.qtm.net/~persoj

[This message has been edited by Tom Perso (edited 08-31-2001).]

Spyke · Post by **Spyke** » Fri Aug 31, 2001 9:15 am

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by neilca:
I would only add that the higher the octane rating the slower the burn of the fuel.<HR></BLOCKQUOTE>

No flame intended, but I've heard this said a billion times, yet no one ever has any proof or a reasonable explanation. Can anyone show me tech info that shows 88 octane gasoline burns appreciably faster than 92 octane gasoline?

It makes no sense to me that a slower-burning mixture would prevent knock. In fact, if the mixture burns slower it should knock WORSE...

A cylinder doesn't knock because the flame front is too fast. The cylinder knocks because the mixture autoignites, and it autoignites because of time spent at high temp and pressure. The faster the mixture burns, the less time it's exposed to the conditions that make it autoignite. So, burn the mixture faster, and you have less tendancy to knock. Burn the mixture faster, and when it finally DOES knock, there is less mixture there to autoignite and the knock isn't as severe.

Think about how many performance improvements (for lack of a better term) either directly or indirectly promote burning the mixture faster. Designing the ports to promote swirl in the charge. Adding another plug to a large bore. Designing the chamber/piston crown shapes so as not to impede the travel of the flame front... where does burning the mixture more slowly figure in to all this?

-Craig
www.teamyikes.com

kdanie · Post by **kdanie** » Fri Aug 31, 2001 10:17 am

Many years ago when I was in a vocational auto mechanics course in my local college, we had a rep. from Chevron come in and talk about fuels/octane ratings. He did a demonstration that I found very interesting... He had a pyrex tube 2.5" in dia. about 3.5' long, a rubber ball that fit inside the tube and 2 samples of gas, one was about 90 octane and the other was 102 octane if I remember correctly. He squirted a small amount of the 90 octane into the tube, placed the ball in the tube and rolled it back and forth a couple of times to mix the air in the tube with the fuel, then lit the mixture from the bottom of the tube...the result was a violent explosion out both ends of the tube. He went through the same procedure with the 102 octane but when he lit the mixture the result was a nice even flame that burned from the lower opening to the top opening....it was a very rapid burn but no explosion and only a little whooshing sound. My point is that your engine needs higher octane when the mixture explodes before the piston reaches TDC causing the explosion to push back on the piston (that's the pinging you hear!) Higher octate causes a slower more controled burn so the cylinder pressure is still rising as the piston reaches TDC and peaks shortly after causing the force to the piston just as it starts down. Higher compression ratios increase the intensity of the explosion and advanced timing gives the explosion more time to build pressure. There is no extra power (BTUs) in higher octane fuel and using a higher octane than needed will actually loose power because the piston is already on the way down when the max burn is taking place causing cylinder pressure to be lower. This is straight from the Chevron rep.
I long for the days of 104 octane at the local pump for less than .50 cents a gallon and I could run 13-1 compression on the street!
ken

ray greenwood · Post by **ray greenwood** » Fri Aug 31, 2001 10:54 am

Shad, Nope...I mean domes. There were 3 distinct pistons available in the 411/412 models of 1.7 l. All FI 1.7's came with domes...except for a mysterious few with flat-tops from the factory. The dish piston..whose dish exactly mirrors the domes, was used only on the carbed version. If you find one residing in a 411/412 with 1.7l and it has dish pistons...it was either carbed...or , if it is FI...it is probably not the original engine. I will try and drop the part# in here later. These were superb piston and cyl. kits. Kolbenschmidt pistons with mahle liners. The Cofap kits (TRW of Brazil) still use license built Kolbenschmidt pistons with TRW cylinders. The difference appears to be how the dome keeps the mixture "stirred" as it compresses, right before ignition. In the experiment I spoke about, I used a flat top 1.7 piston, with the barrel shims adjusted to give the same 8.2:1 that I was running with the domes. Of course there could always be running differences between the two sets due to break-in. tolerance etc...but I have run several sets of flat tops compared to domes with the D-type injection, and it isn't really that the domes run that much better than the flat-tops...it just makes the adjustment of the pressure sensor easier and more accurate. Less prone to change in temperature and barometer...at least that what I observed. Spyke...about compression and octane, a better way to explain....88 octane compared to 92. 88 will have a higher propensity to detonate in dvance of the correct point. Without trying to remember the formulas...88 reflects the amount of times out of 100...percentage wise that the mixture...in a lab test cylinder...ignites on time at the preset deck and compression. The formulas of calculating manufacturers octance number(MON) and recommended octane numer (RON)....and the average of the two...coupled with lab results in the cylinder is what gives us #'s over 100 for percentage results, with racing fuels.It is more of a quantitative expression of the ultra stability ofthese fuels...or so I hav read and been told. The 88 octane pre-ignites...at a lower compression (remember compression causes heat) and therefore has absorbed less heat....and will have less of a gas expansion speed because of this lower heat. Because of the overall lower velocity it actually burns slower...but it ignites faster...usually faster than it should. Higher octane fuel is harder to ignite. It is able to be compressed farther than lower octane fuel...because it has a higher vapor pressure and flash point...so will have more heat...from the compression...and will have higher velocity movement and energy when it ignites. The logical mind would ask...could you not design a fuel that ignites at the lower temperature potential/octane range...but that has combustion/compression energy similar to the higher octanes?...of course! but those fuels are not gasoline..hee hee. For those dreaming of strange fuels...you should check out the fuel they used in the 16 shot afterburner of the SR-71 spyplane (tri-methyl borane) an octane rating of about 1...it is pyrophoric!...ignites on contact with oxygen...burns blue green...kickin! Ray