mdb4879

Hey all, I've done quite a bit of searching on this topic and have come up with a few questions. I originally posted them in another forum, but have yet to get a clarifying response. I'm just going to copy and paste them, so if they seem a little out of context, I'm sorry.

This is all in reference to a B-series VTEC motor. Differences may vary between using a GSR head, B16 head, and ITR head:

"Yes, I'm aware higher octane fuel itself doesn't make more power, but if the compression is too high and you have to retard timing to keep the motor "safe" then using higher octane fuel will provide opportunity to benefit from timing advance and the higher compression.

I just wasn't sure if the "undercammed" effect mentioned was due to this, or if it had in some weird way to do with how the air flows through the motor?

More compression should make more power given all other factors remain constant. Unless, of course, the compression is so high that you have to run a more conservative tune, such that you actually lose performance. In stating this, I think I realized undercamming is due to the limit in the fuel and not airflow (idk why I was thinking that earlier).

But still, back to my other questions:

Does static compression play a role in detonation, or only dynamic compression?

Are there any concerns with running at the general dynamic compression ratio limit (10.6:1) as calculated by the VTEC lobe specs when not in VTEC? (non-VTEC cam profile is more mild, so while one would have a 10.6:1 DCR in VTEC, out of VTEC the DCR would be considerably higher, so are there any issues here?)

Does anybody have an idea of what the ideal DCR on our motors would be when running E85?"

unusual71

I think that a big factor in the DCR is the rod stretch. this won't be a factor at lower speeds so the non-VTEC conditions probably aren't an issue as the actual DCR will be lower relatively

mdb4879

I'm sure at high rpm's rod stretch could play a (very minor) role in dynamic compression, but a different cam profile would play a much larger role. I just don't know how having the separate cam profile affects the tuning a power while out of VTEC (my concern is if it affects it adversely)

josephcmiller2

Cylinder pressure and temperature is what causes detonation. Just saying that you have a DCR of 10.6:1 doesn't make your cylinder pressure constant. Intake close point has a dramatic effect on the amount of air that makes it into the cylinder for different parts of the RPM range.

Following the torque curve on a dyno will demonstrate that an engine has different flow characteristics all across the RPM band. In places where flow is inefficient you'll have a lower risk of detonation than where flow is greater.

This flow is highly dependent on the cam configuration, intake manifold, and exhaust setup. It's a system. Focusing on just one point such as dynamic compression doesn't tell you what you want to know.

Your question seems to be one about concept. If you really want to know about a particular application, post your setup and ask the question then.

mdb4879

Well, right now my setup is a basically stock GSR with I/H/E (cheap ebay crap from the previous owner), so what I'm asking doesn't really apply yet. I'm just doing research for if/when I decide to build a motor so I can know the technical aspects of the "design" (I guess what parts would work well with others and what compression).

So basically, what I got from what you said is even though the theoretical DCR (on a basic level) when out if VTEC is higher than with the VTEC profile, in practice detonation is most likely to occur at peak torque. Which, due to scavenging and cylinder filling is at a higher rpm. Effectively the DCR is higher in VTEC than out of it. So dynamic compression of the VTEC profile is the only thing that needs concern when choosing parts and doing the math.

Well, that's one question knocked down, and I can only assume that for the first question static compression has nothing to do with detonation, only dynamic compression. So I guess that just leaves what the general upper limit of DCR with E85 is, before being "undercammed"?

Thanks

josephcmiller2

I understand the questions but I think you're focusing on the compression too much. You need to design a complete system. Determine your power goals and your desired power band. Only then can you design a system around it.


Posted from Honda-tech.com App for Android

josephcmiller2

For the compression question you need to study up on MBT. Hard-core. If you're serious about learning tuning you need to know a lot about this.


Posted from Honda-tech.com App for Android

Kozy.

I started a similar thread on this, it didn't get much interest though.

https://honda-tech.com/forums/all-motor-naturally-aspirated-44/does-cranking-pressure-matter-3070111/

I did a bit of research into the effects of compression and I am not entirely convinced by it's ability to make more power. It is said that a high CR will increase the thermal efficiency of an engine (up to a point), but I am not sure that in itself equals more power. What it will do is increase cylinder pressures (and thus torque) at partial throttle openings, which will undoubtedly make the car feel much faster in daily driving. The trouble is, at full throttle, you're still limited by your fuel, and the amount of timing you have to pull to offset the effects of increased compression in reaching the charge's autoignition temperature will reduce the torque output, possibly back to the amount it was at a lower CR with MBT timing.

This is synonymous with the effects of bigger front brakes in your road car. You get more braking force at lower pedal inputs during day to day driving so they feel much better, but ask them to do a full on emergency stop and the tyres are still the limiting factor, so you can't actually stop any faster than you could before.

Here's some plots for simulated pressure/temperature over the power stroke and the output torque over the cycle.



Engine 1 is running 11:1 and 36° advance.
Engine 2 is running 13:1 and 36° advance.

Engine 1 produces 169lbft and has a peak cylinder pressure of 700psi
Engine 2 produces 182lbft and has a PCP of 853psi

If the fuel used has a knock limit higher than the PCP, then great, your increased compression has netted you a useful 13lbft gain. However, consider this as one of those JDM engines (an H22A in this particular case) that are designed to run on 100RON. If the standard mapping is considered to be just on the safe side of the knock limit, then assuming the best fuel you can get is 100RON, that 13:1 is going to need the timing pulled back to get the PCP under 700psi.

Here's how it looks with Engine 2's ingition pulled back to 28°.



PCP is now 700psi and the torque is 170lbft, virtually no gain.

I know this is all very simplistic in comparison to the real processes inside a cylinder, but it does rather prompt the question; Why such high CRs in Honda's where a low calculated cranking pressure (DCR) is really of little consequence?

mdb4879

I guess it may seem like I'm focusing on just the compression, will would be a valid observation. But i'ts not entirely all I'm thinking about. I think I just feel I need to fully understand this concept before I can apply it with any other. The same goes for all other parts of the equation. I need to understand each part individually before I can apply them together.

As for determining a power goal, I hate to say it but I'm probably going to sound like a typical ricer here (which for what it's worth I am far from). I'm think I would like at least 200whp as far as number goes, but it may not be enough. I would love to be able to take a sh** on an S2000 (even on a stock one would do) and make power to "over 9000" rpm. I'm going to keep full interior with some added bracing and keep A/C, P/S, and probably even cruise (I hope that gives a basic idea of my what my weight and power loss will be. My car is a 1995 Integra GSR with cloth interior, btw). And this is my daily so I want it streetable. (Of course this may have a different meaning to me than it may to others. I came from driving a 70hp car that weighed over 2400lbs w/o driver. So it doesn't take a lot of bottom end to impress me, lol.) I think if I could maintain at least stock torque or more from 2500rpm up I would be plenty happy (from 2k up would be even better).

With all that in mind, I really don't have too much of an idea just how feasible this is at this point. I've never really been a Honda guy until now, and I must say I am VERY impressed with the potential of these motors, in any trim. I use to rag on them for not having any torque and being slow POS's, and I still do but now I'm a little less serious about it, lol. From what I've seen, stock GSR's make about 150whp on a dyno, and 200whp seems fairly easily doable. Even for a mild build using mainly OEM parts.

My problem is I think about where I want my powerband (the strong one, not thinking about bottom end right here) and with a cam profile that would make power at 9k and have a system that works well with itself and is synergetic I would overshoot 200whp by a long shot.

So I'm still hunting around trying to figure out what setups can produce which results and what would suit me most. Btw, I deliver pizza, so speed is key

mdb4879

I was typing my other post while you were posting that, Kozy.

That is exactly the limit I am trying to find. Just as you said, higher compression equals more torque. But if the higher compression causes you to pull timing and even add fuel to fight detonation/preignition, then the gains could easily be lost and may even hinder performance. So I'm trying to figure out roughly where that point is.

Idk if I'm suppose to make links to or refer to info on other forums, but I was over on Team-Integra and found a very in depth thread about this topic (hence where some of my questions arose). I think he was on here, so some may know who I'm talking about here. Michael Delaney went very in depth on the topic and said that while many factors (such as burn efficiency, port design, combustion chamber design, etc.) affect the maximum beneficial compression, from what he had seen based on other setups and doing his own math from their parts 10.6:1-10.8:1 DCR was about where people made the most power before being undercammed. IIRC he also said that about a 5% decrease in compression will result in being overcammed and 15% higher will begin to be undercammed. That's typically, though. I believe he stated that depending on factors I mentioned earlier undercamming may not take effect until a 25% increase in compression over stock.

Also, I think I may have come to a conclusion earlier, but I just would like some confirmation. Is this upper limit of DCR due to fuel/octane restrictions, or is it something to do with how the air flows through the motor and is combusted at extremely high temperature and pressure.

Kozy.

Improved head design promoting swirl, good quench areas etc will improve things, but ultimately it's the knock limit of the fuel reigning the fun in.

Quite common over here for people to take the JDM engines, crank up the comrpession into the 13s and run them on RON 2points lower than they left the factory mapped for, and expect it to make good power.

"It's all in the tune maan, you need to have a good tuner"

So far as I can see, you just need someone who can dial the advance back until it stops detonating to cover up his crappy advice on how much compression to run. That's not a good tuner, that's a bullshitter. Even relatively big cams seldom need much beyond 12:1 to compensate for lost cranking pressure, and then we're right back at the question of whether that even matters on a VTEC engine...

I dunno, I don't have any real world experience on this only an unhealthy obsession with needing to know as much as possible about as much possible with no money to do anything. Maybe it does all work but like the brake analogy, people tend see and feel things and interpret them wrongly.