Boost effects on low compression engines vs high compression engines

[DrGonzo]

I have only ever run 93. No race gas around me and the few times I went to the track they only had leaded which I don't want to run.

I'm really trying to see what she is capable of on pump gas. So staying away from the 100+ & E85.

AFR's tuned to about 11.0 with the meth on full.

Once I get the 450's installed I'll have to do a full re-tune and mess with the timing more to squeeze the last power out of it.

[IPD]

Don't want to hijack Blown's post but out of curiosity did you try any high octane fuels (racing fuel)? What type of Air fuel ratios (if you monitored) did you hit at 16psi? Very nice to have a fellow that has actual experience in this matter.

lots of people have 10:1 tt experience. Gonzo's just gone a bit further with meth, etc--whereas most everyone else throws in the towel and goes 8:1 when they hit the wall way below 400whp.

i don't think ethanol/race gas is relevant. compare pump+meth numbers are available on several dr750 cars. the gap between thier whp and gonzo's on a 10:1 is almost 300whp. no need to compare multiple fuels when 93+meth already shows the contrast nicely.

http://www.3sgto.org/f2/dyno-results...750s-2102.html

now i'm assuming that gonzo is running 9b's. even so, if he upgraded to, say, 13t's...the limit would be the compression ratio of the engine, not the capability of the turbos (and we know that 9b's start to fall down after 14-15psi). my hypothesis is that on a good day (50 degree temps) and pump + meth-on 13t's--gonzo might be able to scratch 400whp...but that's it; larger turbos wouldn't add any additional benefit. and 400whp can be done without breaking a sweat on basic turbo upgrades for an 8:1.

[Keyan]

You're starting to catch on....

There are two completely different factors at play. A motor can be envisioned as simply an air pump... The more air you can pump, the more power you can make. The secondary factor is how much energy(power) you can extract from the mixture you have. The more you compress it, the more energy will be released during combustion. This is not only mechanical compression ratio in play, but also camshaft profile and cylinder head design. There is always a limit to the amount of cylinder pressure any given motor can withstand on the fuel used.... The higher you go with the octane rating of the fuel, (burns slower with higher flash point) the more pressure you can get away with...

Boyle's law states that gas volume and pressure are inverse at the same temperature. As one doubles the ether halves for a fixed amount of gas... That can also be interpreted by if you double the amount of gas in a fixed volume, you double the pressure. Let's imagine you have a perfect intercooler, and you get no temperature or pressure drop as you compress the air: If you increase the the amount of air by one atmosphere(14.7 lbs/sq.in) you have doubled the air charge in the cylinder, and doubled the pressure.

At 10:1 ratio, you decrease the volume by 90%, so your pressure rise on a 100% efficient N/A, with no thermal expansion calculated, would net 147 psi.

At 8:1 compression you decrease the volume by 78%, so your two atmospheres of boost would net you a pressure rise to 134 psi.

At the same boost level (two atmospheres) a 10:1 motor would net 291 psi... Seeing a problem with this picture?

Of course those are all theoretical numbers that would be complete bullshit in the real world when you factor in thermal expansion, pressure drop, and fuel mass... But I think it makes the point.

You make a very good point, but leave out what the pressure actually does in the cylinder. It is (mostly) not the pressure that makes the gas preignite, but the temperature. By severly increasing the cylinder pressure, you exponentially increase the temperature of the air charge. Which is basically what an intercooler tries to prevent. By chilling the air, you make it denser. Which increases the in-cylinder pressures....but at the same time, significanly reduces the temperature that can be generated FROM that pressure (if the air had 0 energy, compressing it wouldnt create any....but there was energy, compression causes all the molecules to bounce around in a smaller space, which causes friction and an increase in temperature. there is a simple equation for how pressure and temperature relates but i can't seem to remember it right now.) Basically, the lower the temperature going in, the less hot it can get, which reduces pre-ignition potential.
All race gas does is resist pre-ignition at a higher temperature than an equivelent fuel at a lower octane. So it can be safely injected into a higher temp environment with no worries (or less worries) of preignition.

It is finding the balance of compression ratio / intake air temp / octane used that is the key to turbocharged, supercharged, and even naturally aspirated very high compression engines.

[DrGonzo]

Yeah I'm running stock 9b's. There is no need to upgrade turbo's as the limit of the 9b's is matched to the limit of the 10:1 from what I have done. I have a good feeling that I have hit my max with the 16psi not only with the 9b's but with what the 10:1 can handle. I still want to see if I can get to the 18psi mark even though I know he turbo's will not hold it to redline....

I'm currently building a stock TT block and once I have it completed I'll be swapping out the 10:1 block. Then I start tuning it with the 9b's, get it stable and maxed, then probably look at dr650's. It will probably be 2 years before I get to the dr650's as Iam in no rush to build the engine up.

[DrGonzo]

You make a very good point, but leave out what the pressure actually does in the cylinder. It is (mostly) not the pressure that makes the gas preignite, but the temperature. By severly increasing the cylinder pressure, you exponentially increase the temperature of the air charge. Which is basically what an intercooler tries to prevent. By chilling the air, you make it denser. Which increases the in-cylinder pressures....but at the same time, significanly reduces the temperature that can be generated FROM that pressure (if the air had 0 energy, compressing it wouldnt create any....but there was energy, compression causes all the molecules to bounce around in a smaller space, which causes friction and an increase in temperature. there is a simple equation for how pressure and temperature relates but i can't seem to remember it right now.) Basically, the lower the temperature going in, the less hot it can get, which reduces pre-ignition potential.
All race gas does is resist pre-ignition at a higher temperature than an equivelent fuel at a lower octane. So it can be safely injected into a higher temp environment with no worries (or less worries) of preignition.

It is finding the balance of compression ratio / intake air temp / octane used that is the key to turbocharged, supercharged, and even naturally aspirated very high compression engines.

That statement is completely true and it was one of the walls I hit multiple times. Hence the reason for having to add the large FMIC and the water/meth injection. At a certain point, it was all about keeping the air intake charge cool with the high compression. The stock SMIC are just not efficient enough to keep the charge cool. Even just regular driving if I got on it I would get knock. Just not enough area in the SMIC to keep the charge cool enough.

Keep in mind that I have enough bolt on's to properly support a 600hp TT "Minus turbos" and I am not to the 400hp mark. But I also know that all of those bolt on's will be moved over and help support the next 8:1 engine I am building so the money spent was not in vain. After I finish the block my only cost will be the DR650's since I have all of the supporting mods already.

[Blown3000]

I really appreciated the numerous points you have all made. From your inputs forced induction on a NA, in my case supercharging, has its benefits although somewhat limited by comparison to motors using lower compression ratios. From what I've read and your posts, combining a higher compression engine with forced induction gives you better low end throttle, which as a street engine and a daily driver is where I live. From what I gather, my combination was a good choice and I'm happy to have learned that.

[ibsorgn]

I really appreciated the numerous points you have all made. From your inputs forced induction on a NA, in my case supercharging, has its benefits although somewhat limited by comparison to motors using lower compression ratios. From what I've read and your posts, combining a higher compression engine with forced induction gives you better low end throttle, which as a street engine and a daily driver is where I live. From what I gather, my combination was a good choice and I'm happy to have learned that.

Glad to see you probed this issue. You will have no regrets supercharging your NA. I've been running my Magnuson MP90 for 4 years without a hitch. These little motors respond well with the low rpm boost provided by the supercharger, and your right about the power being provided in our normal low throttle DD rpm range. I'm can be at full boost at 2500.

[Blown3000]

Glad to see you probed this issue. You will have no regrets supercharging your NA. I've been running my Magnuson MP90 for 4 years without a hitch. These little motors respond well with the low rpm boost provided by the supercharger, and your right about the power being provided in our normal low throttle DD rpm range. I'm can be at full boost at 2500.

Long time getting back to this forum. 4 years problem free! That statement really knocks a hole in those that nay say supercharging. I've been into my mods for about a year and haven't experienced any problems to speak of. I just don't see any drawbacks on supercharging these little motors other than it's not the normal power adder route.

[retrorocked]

I am a bit late to this thread, but perhaps this will help the OP. There are a couple of things to keep in mind when comparing two engines with differing static compression ratios and boost.

Temperature and pressure are both factors and they are interrelated but it is not necessarily as straight forward as it may seem. Doubling the air/ fuel charge in the chamber (through boost) results in an increase in peak cylinder pressure by approximately 20%, but average cylinder pressure throughout the cycle is much higher, resulting in significant power increases. The static compression ratio will raise peak pressure as well, but will result in a much smaller increase in power due to much lower average cylinder pressure throughout the stroke.

Compressing the air prior to entering the cylinder through forced induction creates heat, the amount of additional heat is determined through various factors, one of which is the efficiency of the device used to compress it. Intercoolers are used to reduce the temperature of the incoming charge, thereby increasing the air density and allowing higher boost levels. Reducing the charge temperature through proper intercooling significantly reduces the chance of detonation, which is what allows you to run slightly higher boost.

Because the higher static compression ratio increases peak cylinder pressure without the corresponding significant increase in average cylinder pressure (which is where the real power is made), you give up some power because you have to give some boost-- assuming of course that all else remains the same.

[donniekak]

Part of the problem in this thread is that most people in the 3s community have not ever written a timing map. A 9:1, and 8:1 engine can run the same boost, but the 9:1 engine will need less ignition timing, and the room for error between mbt, and detonation will be much smaller. The new BMW turbo's run 10.2:1 compression, and can make plenty of power.