Lower compression ratio?

[iliketocrash]

I've been doin a decent amount of reading lately and i've noticed a lot of really high powered turbo cars have a rather low compression ratio, like 7.5:1 and such. Something about lowering the chance for detonation and fiddling with the timing to achieve sometimes rediculous numbers. At the same time it also doesn't appear that the motors last very long pumping out so much power. I was wondering if i might be able to achieve any gains (even small) from dropping the compression ratio to 8:1. I figure i'll also be sacraficing some low end power and increase turbo lag but i know everything is a compromise. I'm wondering if anyone has any knowledge on this topic and could possibly lend some input. sorry if this is a stupid question but i'm just curious.

[c-rad]

You would probably need a standalone because it would throw off everything else as well. Second, your off-boost response would suffer.

[biosehnsucht]

no point unless you plan to be a dyno queen.

sure, you can get crazy #'s, but its going to be useless for most forms of racing.. and you'll have a hard time keeping it from blowing itself up still.

in most forms of racing its more about the area under the curve not the peak of it.

[float_6969]

lowering compression allows you to potentially run more boost and timing with the same quality of fuel (ie. 91 octane), but that is only "streetable" up to a certain point. The only time dropping the compression down that low is good is if you are building a drag car and only need peak HP, or as was stated earlier, you are building a dyno queen.

I actually increased my compression on my car to improve the off boost response and decrease turbo lag. The problem with doing this (and I'm still currently fighting it) is that you aren't able to run as much total timing on pump gas. I'm hoping that with a more effecient intercooler setup and perhapse some water injection, I'll be able to get more timing out of it.

[float_6969]

To rehash a slightly old thread, I just read something that Corky Bell said, that stated that to never lower the compression below 8:1 on a street car.

[c-rad]

To rehash a slightly old thread, I just read something that Corky Bell said, that stated that to never lower the compression below 8:1 on a street car.

Factory compression on my DSM was 7.8:1

[float_6969]

Was that a conquest? Mitsubishi likes really low compression for some reason....

[r34 gtr]

this reminds me, has anyone tried running this new ethanol 85 in their CA? 110 octane doesnt kid around, and i know my fuel system is alcohol tolerant.

- tim

[sideways danny]

Ford used a 7.5:1 on the YB with a .48 T3 . Nasty engine but the can make a lot of power in the right hands.

I've calculated my CR as 8.17:1 with the squish pads removed and a 1.3mm head gasket and 1mm oversize istons

[iliketocrash]

isn't the e85 the higher ethanol conten blended gas? i think they have at least two grades of it. but anyway, you sure your fuel system is alcohol tolerant? because i read a writeup once about someone converting an older car to accept the blended gas and it talked about needing special rubber compounds for the hoses, o-rings and such because normal rubber degrades quickly because of the ethanol. but i digress. good to hear more info on the compression thing. thanks guys =)

[teddy]

isn't the e85 the higher ethanol conten blended gas? i think they have at least two grades of it. but anyway, you sure your fuel system is alcohol tolerant? because i read a writeup once about someone converting an older car to accept the blended gas and it talked about needing special rubber compounds for the hoses, o-rings and such because normal rubber degrades quickly because of the ethanol. but i digress. good to hear more info on the compression thing. thanks guys =)

That same concept about hoses and o-rings already holds true to our current engines. Typical rubber hoses and o-rings will decompose(? for lack of a better word) when in contact with gasoline for a sustained amount of time. Which is why when you buy fuel lines, you ask specifically for fuel hose, which is not only resistant to high pressure, but treated for gasoline as well.

This is also the reason I had to go to the honda dealership and pay $4.00 for a new FPR o-ring. A normal rubber one would have been eaten away quickly.

[iliketocrash]

i know that fuel components are specially treated for gas but i'm talking about something different. a different rubber compound is needed due to the addition of the ethanol. or so i hear.

[Swedish Mike]

i know that fuel components are specially treated for gas but i'm talking about something different. a different rubber compound is needed due to the addition of the ethanol. or so i hear.

I´ve used E85 on most of my cars for years now and no problem yet.One bad thing is aluminum and E85 together but only in places where the fuel is stored a long time, like fuel tank.

My daily driver right now got plastic tank, steel fuel lines, alu fuelrail and short rubber hoses on the return side. No problem yet, runs like a dream.

/Mike

[Coldspawn]

increase turbo lag

No, it does not. The "turbo lag" is the same. The higher compressor does give it a fatter midrange (in boost).

Most people see that and think it is lag.

To rehash a slightly old thread, I just read something that Corky Bell said, that stated that to never lower the compression below 8:1 on a street car.

Cocky Bell generalizes things. You can twist what is a "street" car, etc. The fact of the matter, you build the motor for what you are looking for. I am not talking about dyno queens.

I actually increased my compression on my car to improve the off boost response and decrease turbo lag. The problem with doing this (and I'm still currently fighting it) is that you aren't able to run as much total timing on pump gas. I'm hoping that with a more effecient intercooler setup and perhapse some water injection, I'll be able to get more timing out of it.

Increasing (or the willing to increase) timing to make power is silly. Get your intercooler set-up, then dyno it, you will see.

[AZhitman]

E85 is also very hygroscopic, which means it's only a matter of time before you have injector issues from corrosion.

Even on new cars (E85 Tauruses), there were issues less than 2 years into their use.

[float_6969]

No, it does not. The "turbo lag" is the same. The higher compressor does give it a fatter midrange (in boost).

Most people see that and think it is lag.

How many high compression turbo CA's have you built? How many have I built?

From EXPERIENCE, my car spools up AT LEAST 200-300rpm's quicker than it used to. I'm also making boost as low as 2000-2200 rpm (1-3psi). I'm fully spooled before 3000rpm. This is on an S15 T28.

Cocky Bell generalizes things. You can twist what is a "street" car, etc. The fact of the matter, you build the motor for what you are looking for. I am not talking about dyno queens.

I see you now think you are more educated than the renowned engineer Corky Bell?

Increasing (or the willing to increase) timing to make power is silly. Get your intercooler set-up, then dyno it, you will see.

Please refrain from trying to educate me on tuning principals. Especially if you're not fully aware of the current tune of my motor.

I'm currently HAVING to run about 10-12degrees of total timing under boost (3psi+) to prevent detonation. Better intercooling is going to help that some, but 92 octane can only do so much.

I'm fully aware that advancing timing doesn't nessicarily make power, but when you are running as LITTLE advance as I am on pump gas, and have EXPERIENCED the difference more timing makes with high octane fuel, I KNOW that I am loosing power.

I am also contemplating running my car on E85 for the octane benefits. I'm still researching fuel pump, fuel tank, and FPR compatability.

[Coldspawn]

Jesus, calm down.

How many high compression turbo CA's have you built? How many have I built?

I do not see how it is in any importance.

From EXPERIENCE, my car spools up AT LEAST 200-300rpm's quicker than it used to. I'm also making boost as low as 2000-2200 rpm (1-3psi). I'm fully spooled before 3000rpm. This is on an S15 T28.

Any data to back that up? I have dynoed the same set up with different compression. I am not saying you are wrong, but I have seen very different.

I see you now think you are more educated than the renowned engineer Corky Bell?

Nope, read it again. I have talked to him before, he is very smart. Glad to see you are reading Maximum Boost.

Please refrain from trying to educate me on tuning principals. Especially if you're not fully aware of the current tune of my motor.

If your set-up is different then what you have posted here, then no, I do not. Do not be soo closed minded. I never said you had to accept everything I say.

Float, what timing are you running and why you think it is soo little?

[float_6969]

I previously stated that I was running 10-12 degrees of total timing under full boost. (full boost is a relative term as I'm still fighting boost creep)But after monitoring it a little closer today in a higher gear (3rd) I'm actually more like 9-11 degrees advanced.

I generally consider myself relatively open minded. But I'm well aware that 9 degrees of total timing is low no matter how you look at it, and when running higher octane, I've advanced the timing and felt power gains. I've not been able to go to the dyno and prove it, so I guess that technically my experience is circumstantial, but regardless, it very noticable.

[Coldspawn]

I generally consider myself relatively open minded. But I'm well aware that 9 degrees of total timing is low no matter how you look at it, and when running higher octane, I've advanced the timing and felt power gains.

As you stated, you need a few things to help your setup. I think your air temp going into the motor is on the highside, you are running a cold air intake out of the engine bay? What are you ECTs and IATs?

But, do not think because they are "low" that you would need more timing to make power. You know you have to fix a few things, so do that and see what happens.

[float_6969]

I don't think my air temps are the problem, but will openly admit that IAT's can't (for all practical purposes) be too low.

CAI feeding from the fenderwell infront of the drivers side tire. It's getting incoming air from the opening in the silvia bumper that fed the stock SMIC.

I'm running a 160 degree thermostat, 85% Distilled water, 15% Antifreeze, and one bottle of watter wetter. Generally my ET's are actually about 150 degrees right now since ambient isn't getting much over 70 degrees. If I get on the highway or sit in traffic it will get up to 160 degrees, and if it idles for a long time will get as high as 180. This motor has never seen more than 180 degrees since it's been rebuilt.

IAT'S are generally running about 10% higher than ambient off boost and may get as much as 20% over ambient on boost, but I've only seen it get that high when the ambient temps are 80 degrees +.

And lastly, the point that you keep missing is that I have tested more advance on higher octane fuel and had evident power gains.

I'm NOT saying that you have to advance timing to make power.

I AM saying that I am severly retarding the timing to prevent detonation. As a general rule, you don't want to run much less than 15 degrees of total timing. This is is due to the fact that under this much total timing, the PCP starts to happen too late in the pistons' stroke to apply the most torque to the crankshaft.

[sideways danny]

I'm surprised your EGT's aren't massive and you're getting terrible response with timing retared that much Float. I had trouble with timing a while back and couldn't get more than 12deg, it felt flat and nasty to drive.

My personal preference is to run a little lower than standard comp with soft chambers to make it harder for det to occur, plenty of advanceoff boost to make the response better and as much advance as is safe on boost. Mapping the ignition is more important that the fueling (which still needs doing) in order to get decent power

[Coldspawn]

I would change the thermostat out. The car needs to run hotter then that.

You have an acutal log of your IAT vs ECT? I would like to see it.

And lastly, the point that you keep missing is that I have tested more advance on higher octane fuel and had evident power gains.

I'm NOT saying that you have to advance timing to make power.

Of course, You will be able to run more timing with a higher octane fuel. All octane rating is, is the resistance of detonation.

I AM saying that I am severly retarding the timing to prevent detonation. As a general rule, you don't want to run much less than 15 degrees of total timing. This is is due to the fact that under this much total timing, the PCP starts to happen too late in the pistons' stroke to apply the most torque to the crankshaft.

It matters the motor, porsches run almost no timing.

You about the burn (or force) to happen at about 16-20 ATDC.