Relationship of Comp ratio to HP

[John Coffey]

You will probably want to back off the timing a bit. You can buy octane booster at any auto parts store around here if needed.

[conedodger]

John, maybe you can speak to this; what is the relationship between static and dynamic compression ratio? A cam with lots of overlap seems to mitigate a higher static CR...

[LeonV]

John, maybe you can speak to this; what is the relationship between static and dynamic compression ratio? A cam with lots of overlap seems to mitigate a higher static CR...

Overlap isn't the right parameter to talk about. It's the IVC (Intake Valve Closure) point that matters. The later the intake valve closes, the less cylinder pressure you'll have at low RPM. The thing that "hotter" cams do is build cylinder pressure at higher RPM than a stock cam. The reason why a more aggressive cam makes you less prone to detonation is that at high RPM, there is less time for detonation to happen. However, if detonation does happen at high RPM (and high load) then boom goes the dynamite...

As for kensval: 10:1 comp, stock cam, and stock EFI DO NOT mix with CA gas. I would bet on you experiencing noticeable pinging if you're currently at MBT timing (not knock-limited). If you don't want to retard timing, there are some stations that still sell 100 AKI ("octane") gas in CA. You can look this up online. Mix the tank 50/50 with 100 and 91 and you get 95 octane. Expect to pay a little extra for 100 octane...

[bartsscooterservice]

Hey all, I'm getting ready to start a 280 NA street build, and am wondering how to pick a compression ratio. Is it worth it to target, say 10:1 (head/piston swap), over the stock 9:1, all other things being equal? I know that going too high will cause problems, but aren't most high output na motors around 10.5:1?

thanks

Eric

Compression ratio matters, BMW straight 6 engins run around 10.5 to 1 ratio for instance. But I don't think it will matter alot of hp, the horsepower gain in NA is to make it breath and the delivery towards the gearbox as easy as possible, a heavier flywheel will reduce top rpm hp.

[240260280z]

Isn't timing partly related to how close the plug is to the piston? So for lower compression motors, the chamber is bigger and the plug generally farther away so it takes longer for the flame/pressure front to build up and also to reach the piston surface thus it can handle more advance than a high compression motor?

[conedodger]

I am preparing a new engine and although I originally considered building it myself, I spoke with Dave Rebello and told him my project, a mostly street engine that does track days and autocrosses when I can. I told Dave that I had two good heads that could be used for the project, an E32 that had been prepared by Geralomy in the late 1980's and a P90 from an auto-trans L28 turbo. He said that he had customers who live at my altitude (4500') who run CR in the 10:1 range without detonation. It was at that point that I decided Rebello would be assembling this. I don't want to make a mistake with that kind of CR.

[coop]

Rob,

My ITS engine runs 10:1 comp with no problems although I've never run a tank with less than 100 octane. I'm not sure if 93 octane would work or not.

[Wade Nelson]

It all depends on the fuel you want to buy for the rest of the time you own the vehicle. Do you really want to constantly have to add octane booster, etc? Go no higher than what the locally available premium will support.

[conedodger]

Dave Rebello says pump gas. As I said, the static compression with the ITM pistons is in the 10:1 range. He has to be doing something with timing and the cam to have that running without detonation on pump gas at 4500'.

[John Coffey]

the horsepower gain in NA is to make it breath and the delivery towards the gearbox as easy as possible, a heavier flywheel will reduce top rpm hp.

A higher compression ration increases cylinder pressure which is the reason internal combustion engines make any power at all. And flywheel weight has nothing to do with horsepower.

[jmortensen]

Less air pressure at 4500' means that you can raise the compression ratio safely. In some areas you'll find that octane ratings at the pump is lower at higher elevations, because higher octane isn't needed there. I remember buying 85 octane gas at a station in the mountains on a trip once. The high octane was 89 or 90. Forced induction cars can also run more boost at altitude too. Take it down to sea level and you might find that you have issues. If it is going to live and die in Reno, or if you don't mind adding some octane booster (not the stuff in the bottles at the gas station) no problem. IMO there isn't a huge power difference between a 9:1 motor and a 10:1 motor at sea level. If you're building it to the limit of the rulebook, then you need to maximize and run on the ragged edge. For most people building a street motor, it makes little difference and the dangers of running the higher compression or the solution of backing off the timing to prevent pinging aren't worth it. Better to run the compression a little lower and be able to optimize the timing.

John, I disagree that flywheel weight has nothing to do with hp. It has nothing to do with MAKING hp, but it can have a lot to do with keeping it.

How a lightweight flywheel works

[LeonV]

Less air pressure at 4500' means that you can raise the compression ratio safely. In some areas you'll find that octane ratings at the pump is lower at higher elevations, because higher octane isn't needed there. I remember buying 85 octane gas at a station in the mountains on a trip once. The high octane was 89 or 90. Forced induction cars can also run more boost at altitude too. Take it down to sea level and you might find that you have issues. If it is going to live and die in Reno, or if you don't mind adding some octane booster (not the stuff in the bottles at the gas station) no problem. IMO there isn't a huge power difference between a 9:1 motor and a 10:1 motor at sea level. If you're building it to the limit of the rulebook, then you need to maximize and run on the ragged edge. For most people building a street motor, it makes little difference and the dangers of running the higher compression or the solution of backing off the timing to prevent pinging aren't worth it. Better to run the compression a little lower and be able to optimize the timing.

John, I disagree that flywheel weight has nothing to do with hp. It has nothing to do with MAKING hp, but it can have a lot to do with keeping it.

How a lightweight flywheel works

Jon pretty much said what I was going to say. Higher elevation = less risk of detonation (less cylinder pressure because of thinner atmosphere) and raising compression from 9:1 to 10:1 doesn't do a whole lot in and of itself (~3% gain according to "Internal Combustion Engine Fundamentals", John B. Heywood).

As far as lightened flywheels go, we've had this disussion on the board before. The link Jon posted is actually the best I've seen regarding the effects of flywheels (and inertia in general), and is exactly what I was talking about in the prior discussion. A lighter flywheel changes the "effective" mass of the vehicle, and moreso in 1st gear than 5th because of "effective" inertia due to gearing.

We (doradox and I) eventually settled the disussion saying that we're both essentially saying the same thing, but in different ways. After a long conversation with an engineering-minded cousin of mine, I gained a deeper understanding of the true differences in views (makes more power vs. removes weight).

This all comes down to "free-body diagrams" and Newton's 2nd Law (F=ma).

If you isolate the car as whole, lower flywheel inertia is represented as a loss in overall weight. Since motive force stays equal (flywheels don't change engine output), in order for acceleration to increase the mass must proportionally decrease. Since the flywheel rotates, taking 10lb off the flywheel is not the same as 10lb off a non-rotating component, thus the effective weight.

Now, if we isolate the drive-wheels in a free-body diagram, we see a different story. The wheels themselves don't change in mass, but acceleration increases. Thus, to the wheels, it looks like more power is being made (more force available to drive the wheels).

As you can see, both are valid points of view, if we understand that there are differences in what we're talking about (the whole car vs. just the drive-wheels). Personally, I think it makes more sense to view it form an "effective weight" perspective (the whole car) since that is the actual derived quantity. As you can see in the link, the "effective power" increase is just an approximation of how much weight loss increases acceleration vs. an effectively equal gain in power. It's the same as pulling out the seats of your car and calling it a power gain.

It just makes more sense to me, but for those that want to look at isolated drive-wheels, have at it!