Low compression pistons

[rice_eater]

I've been told that low compression pistons like little or no timing advance...is this true? The reason i ask is because i took the car out on some empty highways today and while it was pulling very good, after 5500RPM it would start to fall on it's face...now 160 in fourth at 5500RPM from a neon is pretty good considering i have 2000RPM left till redline but my redline might as well not be there if the rest of the band is useless. I have a mopar computer that already advances the spark, and my cam is advanced only two degrees. The engine has 8.8 compression. Here's all the mods...

96 SOHC with stripped interior CAI, pacesetter header, muffler, no cat, #12 crane cam & gear, magnum head & springs, mopar ECU, 60mm TB, deyeme front mount, EGR block, engine completely rebuilt for turbo

[rice_eater]

no one here knows??? Are exhaust and intakes the only thing people know about here?

[^SkylinE^]

With lower compression pistons your gonna make less power at the higher RPM GET THAT TURBO IN THERE!

[stockEK]

All things being equal, you should be able to run more timing with less compression. Within limits of course.


Sounds more like a breathing problem. I'm not familiar with dodge stuff and the mods you have specifically, but I would venture a guess that your single cam head is the bottleneck.

[rice_eater]

well...from 49mm TB to 60 mm Tb and CAI it should in theory have more than enough air going in....the duration in the cam is 250/250 up from the pathetic 180/190 (roughly i'm not 100% on stock numbers). I didnt go any higher and i picked no interference because that will hurt the turbo. Skyline...why do low compressions make less power at high rpm? I got two really borring classes today so i'll give it some thought too

[alloroc]

I know I'm full of crap, (yeah right), but no one else is helping and it sounds like he is going to have a kickass car. I would hate to see all that work implode so here are some tips.


I just want to make sure I read you post right, your running low compression but no turbo (yet) correct?

For now (no Turbo)

Lower compression engines can run better with more timing advance, that is you gain more HP at the top end and lose some at the bottom. Its all about drivability and things can only be tuned so much, don't forget that a lower compression normally aspirated car runs with way less HP.

When you decide to go forced induction (Turbo),

The higher the compression, and lower the fuel octane, the less advanced the timing must be to be to avoid detonation. (operating at peak efficiency)

I would be extremely concerned about the fuel mapping/delivery and the timing. Obviously the timing is the easiest one to check. You can't always hear pre-ignition and running with too much advance could be a disaster, on the other end of the scale, I don't know the programming of the mopar ecu, and it might not advance the timing enough, safer but annoying non the less.

Another potential problem could be fuel. (I understand you have probably taken these things into consideration and will add them when the turbo is installed) I read shitloads of mods that will increase airflow but didn't see many regarding fuel delivery, FMU injectors etc. Running lean is just as bad, if not worse than running with knock as things get REALLY hot REALLY fast rings and shit starts to melt, also leading to preignition, that's bad, and cooking those nice new pistons and rings would be a waste.

When you do convert to the dark side (add the turbo) give serious consideration to spending a couple hundred and get it tuned it on a dyno with a wide band o2 sensor setup.


Good luck.

[alloroc]

N/A lower compression engines make less power throughout the whole band for many reasons. Probably the most important is that the closer to the critical pressure, point of spontaneous ignition, the more 'explosive' the burn is. As you lower the compression you eventually reach a point where the burn becomes so slow it doesn't complete by the time the piston reaches 3/4 the way down it's stroke, useless. The intent is to burn most of the fuel air mixture reasonably close to the top of the stroke to allow the heat of the burn, <-and chemical changes) time to attempt to expand within the combustion chamber and have that force the piston down. It is the expansion of gases etc that gives most of the power not the boom. This is why with low compression engines advanced timing gives more HP at higher RPM, you are igniting the mixture sooner and because of the slow burn it is not putting excess pressure on the piston dome (detonation)

[rice_eater]

i'm gonna sit down and digest all this slowly...the only thing i'm gonna say now is that the stock fuel delivery is good up to 180whp so as far as i know from the neon people i should be fine now...Thanx for the info and i'll most likely drop a longer post in a couple hours...maybe i should dyno the car now rather than later...or maybe not since i'm getting ready to pull the engine since cold weather is coming and my forget pistons bitch at me every morning, eventhough the car sits in the garage...

[rice_eater]

some ideas from the neon guys...
http://www.neons.org/forums/viewtopi...=866136#866136

[stockEK]

From the neon board:
/QUOTE
High compression NA cars generally run high octane and lots of advance. Low compression NA cars generally run low octane and very little advance. /QUOTE



This is backwards. Higher compression motors do not use more advance (relatively speaking)

They are also interchanging alot of variables which kind of confuse the issue, and making comparisons equates to an apples/oranges situation.

Higher compression, boost, or intake temps will require less timing or higher octane fuel to compensate.

[alloroc]

Frank95 has it right.

"I thought that a lower compression generally requires more spark advance because of the time and control required for proper/full burning of the fuel/air mixture. "Flame propagation" and "efficient fuel charge burning" are the issues. Too little advance for the low compression, the flame front doesn't have enough time to initiate and maintain effective burning. Poor performance is the result. The engines may also run hotter than normal, especially exhaust valves. I'm avoiding such factors as cylindar head design, spark plug placement and heat range, head/piston materials and design, gasoline quality/rating, number of valves, and intake system design, among others. These also impact the optimal spark advance for an engine. "

Same thing I said.

Getting a Dyno run done now would be very interesting indeed. If you have the cash .. go for it.

You would obtain a very interesting baseline for comparision between stock N/A (standard compression - you can get those numbers anywhere) Your current setup .. low compression N/A and lastly low compression with forced induction.

There is no need to go with high octane fuel (at the moment. With low compression. Octane is not a measure of the burn rate nearly as much as it is a measure of the fuels flashpoint. You don't have to worry about compression induced pre-ignition with 8.8:1 pistons.

Simply put the compression is a measure of how much the air in the combustion chamber is squeezed at the top of the stroke. Higher compression pistons squeeze the air into a smaller volume than the low compression pistons. Even though the stroke is the same length the size of the combustion chamber at TDC is significantly different between the two designs. Enough to make a measureable difference in air pressure - and performance.

[rice_eater]

no but see he contradicts himself in the next post and agrees with the other guy that i should RETARD the timing on the cam as a first step and if that helps switch back to the stock computer (no timing advance) and regular octane...

[alloroc]

He's saying to put the *cam timing* back to the original and advance the *ignition timing*.

Two different things.
Cam timing, when the valves open. Keep this stock.
Ignition timing, when the spark plugs are fired. Advance a few degrees.

It sounds like you are a wee bit over you head on this at the moment. Take it in to a shop you trust and get a mechanic to tune it, else something bad may happen.

[hjr]

well alz i know is that i run 10.0:1 and im almost fully advanced. My car runs like absolute crap if its not advanced alot. I had it retarded about 45* from where it is now (book spec) and it was soo bad i could bairly get up to 50. Not only that, but my mani and downpipe were glowing red. I advanced the distributer almost to the end with consistenly better results each time i advanced it. Now what this means to you? i guess advace the sucker and take it for a run. If its better, advance again, if not retard. keep going until optimum power is achieved.

[rice_eater]

Originally posted by alloroc
He's saying to put the *cam timing* back to the original and advance the *ignition timing*.

Two different things.
Cam timing, when the valves open. Keep this stock.
Ignition timing, when the spark plugs are fired. Advance a few degrees.

oopsie...now it makes sense...well timing i was just gonna go set it back to zero. Ignition is advanced by the mopar ECU by a fair bit and at the moment there isn't much i can do about it...but if i were to switch back to 87 octane like the guy said or mayb 89 in the first step wouldn't that be somewhat like advancing the ignition? SInce lower octane burns faster...

[alloroc]

No it does not burn faster. (at least noticeably) All that running the higher octane does is waste your money.

Octane is a measurement of the flashpoint if you are not getting pre ignition you don't need a higher octane.

[rice_eater]

the reason i use it is because my ECU has a big sticker on it that says MINIMUM 92 octane! If i go back to the regular ECU i will not have the timing advance that you all said i needed...if i use 87 with the mopar ECU wouldn't i get knock? I love engines! It's always a balancing act

[alloroc]

92 octane at what compression ratio? 8.8:1 or 10:1

[rice_eater]

true...8.8 from regular 9.8

[rice_eater]

put the timing to zero on the cam gear and wow!!!BIG BIG improvement...gonna try the 87 octane tomorrow as my tank is almost empty gonna try with a couple bucks worth and if it runs good i'll stick to it Thanx for the help guys!