A converstion about how much compression is too much for pump gas

[jmortensen]

Not so sure abot those earlier comments regarding octane boost. I do believe the Industry gets a bad rap & a lot of it is self inflicted, but do your research pick a reputable brand that has been 'independently' tested to raise octaine ~ 2.0 units & used on the right Engine (ie L28 10:1 CR !) the improvements can be profound.

Gnosez is right. If you look at the bottle of octane boost, it will tell you what is in it. Usually xylene or tolulene or some combo of the two. It's been some years since I messed with this stuff, but tolulene is 114 octane if I recall, and xylene is 118. From there you can do the math. If you have 5 gallons of 92 octane and add 1 gallon of 118 octane, you'll end up with 96.3 octane. (5*92) + (1*118) / 6 total gallons = 96.3 octane. Tolulene gives 95.6. If you search "home brew octane boost" you'll see lots of sites and they talk about adding mineral spirits and ATF to the mix for top end lubrication. There just isn't any way for 8 oz of tolulene or xylene to affect the octane that much. It is plain ol false advertising IMO to say that it gives a 2 point boost and mean two decimal point boost, but that's really what they're doing.

I screwed around with this on my 11:1 L28/E31 combo (shaved and ported and all that) and found that for my build I needed about 95 octane in order to run advance in the mid 30s. If I were to build a hot street motor I would always choose to reduce the compression and I would definitely go for a bigger cam than I did previoiusly (.490/280 is what I had). Mixing your own octane boost isn't fun, and it's damn expensive. A gallon of tolulene was about $10 when I was screwing with it, I think it's closer to $20 now.

[bajcsi]

Hey Lazeum -> I guess I should have been a little more forth coming. I'm running an unshaved E31 head with larger valves cut and a bit of headwork. My pistons are set of JE Pistons that I had them build to my spec. They are flat tops with valve reliefs. I'm running distributorless with an Electromotive HPX. I am currently running it without the MAP sensor though it's absolutely on my list of things to do in the near term. DCOE 40-151 carbs.

Regarding timing, I've erroring on the side of extreme caution until I had the motor broken in as I don't really have a lot experience in that department. That combined with not having a knocksensor yet, I didn't feel comfortable with upping it much more. I'd like to plumb for the map sensor in the next week or two. With the HPX, I know you can hookup a multimeter to a couple of pins to get the current timing. Any suggestions on monitoring in car while driving? I'd like to have more of an understanding of how the MAP sensor is affecting timing at given load and RPM.

Thanks!

Your timing seems not appropriate to me.

I'm running a much more conservative setup with CR at 9.5:1, quite aggressive cam from Rebello for the street with some overlap (don't have numbers, Rebello doesn't share data, you have to measure if you want to know), L28 with F54 & ported P79.

You're not saying which head you're running, since combustion chamber has a major impact on detonation & max timing to get max output, it's important to know.

Anyway, I'm running 20° at idle (with no load, obviously) whereas your setup is much more aggressive so you should be in that range too.

32-36° is the range where max hp is reached so you should be good there.

Regarding knock, it usually happens at high load, mid rpm range (where max torque is). You're likely running classic distributor and I believe this is why you run such timing at idle. You should check what timing you need at idle and get your dizzy recurved. Running such low timing at idle means also you're running very low timing at cruize, fuel consumption is probably very bad and you're not getting the torque the engine is capable of.

To check knock, you can also build your own detcan with some noise cancellation helmet at $7 from hardware stores, some fuel hoses and a copper pipe. Total cost should be $20 max.

Once more, with proper timing management, you could most likely run your engine at proper timing at WOT & manage to reduce timing only where it needs to be reduced to avoid knock.

At cruize, you're running between 11 & 33° (most likely around 20° at 2000rpm) whereas I'm running 50° with my setup. It makes a huge difference. I've switched from dizzy to distributorless system with engine load input last winter, I cannot be happier.

[240260280z]

They have 94 octane at the pumps here in Montreal... F1 city it is!

[Lazeum]

Hey Lazeum -> I guess I should have been a little more forth coming. I'm running an unshaved E31 head with larger valves cut and a bit of headwork. My pistons are set of JE Pistons that I had them build to my spec. They are flat tops with valve reliefs. I'm running distributorless with an Electromotive HPX. I am currently running it without the MAP sensor though it's absolutely on my list of things to do in the near term. DCOE 40-151 carbs.

Regarding timing, I've erroring on the side of extreme caution until I had the motor broken in as I don't really have a lot experience in that department. That combined with not having a knocksensor yet, I didn't feel comfortable with upping it much more. I'd like to plumb for the map sensor in the next week or two. With the HPX, I know you can hookup a multimeter to a couple of pins to get the current timing. Any suggestions on monitoring in car while driving? I'd like to have more of an understanding of how the MAP sensor is affecting timing at given load and RPM.

Thanks!

So you've got what's needed to make your engine run properly

Getting low timing would not help, especially if you're breaking in the engine. Give it what it needs. Regarding knock, it happens only if you have too much pressure in your cylinders, this is not going to happen at low load where you do not fill them up sufficiently.

You should definitely plug the MAP sensor on your build, it will help you a lot & will make your engine runs cleaner. I'm running a TPS since it was much easier to setup & to install but outcome would be similar (MAP is better since it gives you more resolution on your map, drawback is low vacuum with huge cams).

My L28 seems to be outbreathing my 40DCOE with 32mm chokes, your L31 will most likely follow the same path.

I wrote "seems" because I have a fuel supply issue, I'm running lean at high rpm - not sure yet if it is because of fuel pump or carbs...

If you need some guidance regarding how to tune your ignition, I've made a thread on Autosportlabs forum that could help you if you'd like.

Datsun 240z with L28 & 3x 40 DCOE, MJLJ w/ TPS

Edited September 13, 2013 by Lazeum

[mdec]

Gnosez is right. If you look at the bottle of octane boost, it will tell you what is in it. Usually xylene or tolulene or some combo of the two. It's been some years since I messed with this stuff, but tolulene is 114 octane if I recall, and xylene is 118. From there you can do the math. If you have 5 gallons of 92 octane and add 1 gallon of 118 octane, you'll end up with 96.3 octane. (5*92) + (1*118) / 6 total gallons = 96.3 octane. Tolulene gives 95.6. If you search "home brew octane boost" you'll see lots of sites and they talk about adding mineral spirits and ATF to the mix for top end lubrication. There just isn't any way for 8 oz of tolulene or xylene to affect the octane that much. It is plain ol false advertising IMO to say that it gives a 2 point boost and mean two decimal point boost, but that's really what they're doing.

I screwed around with this on my 11:1 L28/E31 combo (shaved and ported and all that) and found that for my build I needed about 95 octane in order to run advance in the mid 30s. If I were to build a hot street motor I would always choose to reduce the compression and I would definitely go for a bigger cam than I did previoiusly (.490/280 is what I had). Mixing your own octane boost isn't fun, and it's damn expensive. A gallon of tolulene was about $10 when I was screwing with it, I think it's closer to $20 now

=================================

Yes - there are 'bad' octane boost Products out there that do not do what they claim & with the benefit of hindsight going with a lower CR ratio is the way to go. With that said I do not have the time or cash to swop my engine & the octane boost I use is allowing me to run the Motor to close to it's full (timing) potential - no question. I also am not driving 10-15k a year in my Car so the additional running cost is tolerable. My advice to the guy's who are running higher CR engines with retarded timing is to try one of the more reputable brands - if it does'nt work they lose what - $10 ?. FWIW advancing the timing by ~ 3 deg in my Car increases Power by close to 20hp - I have the dyno data to confirm this.

[Zedyone_kenobi]

Just a note, I took my car out for a drive after she sat for over a month, I have had the same tank of gas in the car for 3 months. I know sad, but I have been busy with my baby boy. This is the same tank I reported with one bottle of octane boost in it that solved my problems.

I currently run the following advance curve:

10.5 degrees at 900 rpm

20 degrees at 1680 rpm

30 degrees at 2500 rpm

33 degrees max at 3000 rpm

I drove around for about 30 minutes yesturday marveling at how smooth my car drives. Loved it. On the way home in 100 degree heat I mashed the throttle wide open from low rpm in second and got the familiar sound of pinging. I put it in third and floored it and again got the familiar sound. It was isolated to when get over 50% throttle opening.

I have done zero rejetting on the main jets and emulsion tubes since I put the triples on the new L28...I am thinking that I may be leaning out a bit thus promoting some pinging. I am going to rejet the mains to a larger Air corrector and a larger main jet.

More pieces to the puzzle.

[jmortensen]

I have no doubt that the last few degrees of timing could be worth 20 hp. That's why for others building motors I'd suggest cutting back just a little on the compression to enable running the correct amount of timing. If your motor just needs a few tenths more to keep from pinging that's great, but a more reputable brand of tolulene or xylene isn't going to be fundamentally different in its ability to prevent pinging. So long as they're selling tolulene or xylene, the math is the math.

[mdec]

jon,

I was using this fairly well known internet article Octane Boosters | Fuel Tech Experts as a point of reference for my OB selection. FWIW both the AMSOIL & NOS brands are purported to up the octane by ~ 2.0 units..'independentaly tested'. This review did include some brands which did only fractionaly raise the Octane rating as you & others have noted. But whether its +2.0 or 0.2, my only point is that the additon made an obvious difference for my Motor & may be worth trying for others who are in the same bind. At this point I'm hoping that a series of small incremental changes (cooler plugs etc) can provide the margin I need to run with the timing I want. My motor is not way out there on the CR, it's around 10:1 so I'm hoping it will not take anything to radical.

I have heard others mentoin lower temp thermostats, but I'm wondering whether waterless (Evans) Cooling systems may make more sense as they claim to have the ability to reduce the occurence of hot spots that could be sites for the pinging/detonation - your thougts ?

[jmortensen]

It would appear that they are using different chemicals now, so I stand corrected. If the bottle says tolulene or xylene though, it's not really doing much of anything.

[bajcsi]

So you've got what's needed to make your engine run properly

Getting low timing would not help, especially if you're breaking in the engine. Give it what it needs. Regarding knock, it happens only if you have too much pressure in your cylinders, this is not going to happen at low load where you do not fill them up sufficiently.

You should definitely plug the MAP sensor on your build, it will help you a lot & will make your engine runs cleaner. I'm running a TPS since it was much easier to setup & to install but outcome would be similar (MAP is better since it gives you more resolution on your map, drawback is low vacuum with huge cams).

My L28 seems to be outbreathing my 40DCOE with 32mm chokes, your L31 will most likely follow the same path.

I wrote "seems" because I have a fuel supply issue, I'm running lean at high rpm - not sure yet if it is because of fuel pump or carbs...

If you need some guidance regarding how to tune your ignition, I've made a thread on Autosportlabs forum that could help you if you'd like.

Datsun 240z with L28 & 3x 40 DCOE, MJLJ w/ TPS

Hey Lazeum,

I finally got away from my 8 month old twins long enough to install my MAP sensor and Knock Sense (Engine knock detection and indication for all vehicles)... I reset my timing and went with the following:

8 degrees initial

+18 @ 3000 (26 total)

+2 @ 8000 (28 total).

The MAP sensor allows the HPX to adjust the timing a maximum +15 degrees at idle/no load cruising. Still pretty conservative and there is room to play. One thing I don't like is that I'm not able to configure how much MAX timing at idle. That's why I went so low with initial because with adding 15 degrees, that's still 23 degrees at idle. I'm going to lower it a little more and then bump up what is added @3000 so that I can get down closer to 18 degrees at idle. When the car first starts, it idles fine around 1100. After driving around, it sits at 1800 for a minute or so before dropping back down.

I've got no knocking so far although I need to adjust the sensitivity my knock sensor a bit more. With no load around 3000 it starts flashing like crazy. I have a hard time believing there is any knocking because I can't hear it and I drove next to a jersey wall both at full load and no load and didn't hear anything remotely like pinging. I had to mount the knock sensor on the head so it could be picking up drive train noise. My valves REALLY need readjusting - I can hear them rolling down the road. Ooops... I'd really like to get an Electromotive XDI2.

I've been running Dynolicious v3 on an iPhone 5 (glorified butt dyno) to monitor performance changes after my tuning changes. I know the numbers aren't entirely accurate, but it's still good for showing performance trends. On September 5, I pulled multiple 150 RWHP runs. Last night after the changes were made, I pulled a 189 and a 191 RWHP with a best 0-60 of 6.01 and best 1/4 of 14.62…. I recognize those will vary by a couple percent in either direction from reality, but the ~25% improvement isn't too shabby at all.

I feel there is still quite a bit of room to play. I've got timing, fuel AF/r, valve timing and general valve adjustment to start really digging into. I'm also running a pretty conservative cam - one of the MSA (Schneider 270-80F - 270/280 .460" lift)...

Relative to this thread by the way, my car runs ~12.5:1 on 93 with the NOS Octane booster (which appears to be of limited use)... Also, thanks a bunch for the link to your timing discussion on the other board. It helped alot.

Thanks,

Bajcsi

Edited September 27, 2013 by bajcsi

[Lazeum]

I'm glad to read that

With 0 load or very little (like feathering the throttle pedal), you should have no knock. Most likely some noise has triggered the sensor: valves, exhaust leak, etc.

A friend of mine had his knock sensor going crazy, it lasted months before he found the issue: he forgot a screwdriver that got stuck in between motor and gearbox

To set timing at idle, I check while engine is running the timing to get max idle RPM. this would be your idle timing.

Regarding max timing, you could try to get mid range timing (at 3000rpm) low and get max timing (with 8000rpm setting) at little higher. You will need to try it out. Not sure it is the right time if you're breaking in the engine for now.

Edited September 27, 2013 by Lazeum

[Davey G]

Finally got my 280Z back home. I'm in the break-in period, so no WOT stuff yet, but up to 3500 rpm no detonation. Much more torque than I'd expect from a .485/.270 cam with flat tops, even more than stock. 1000 rpm to 3500 rpm feels great, big improvement. I'll report more as I'm able to open 'er up more and more.