Need more octane

[leecheater]

Hello all,

  I am a new 240z owner. My Z had engine rebuilt by former owner. It has the 2.8 L-92 engine with E88 head and N42 block. It has Venolia pistons with ? head work, 280 cams, arizona racing exhaust manifold/headers, holley carb, MSD ignition.

 

Short video of the Z

 

 

  When I bought her I wanted to be sure I had the correct A/F mixture. The guys at the shop did a compression test, average 265 per cylinder. There is around 3K on the rebuild now. They recommended race gas. If I remember correctly, they estimated over 13-1 compression. Not sure how to figure it but I do know the Z loves race gas. I am concerned about detonation.

 

  Here in MN e-85 is cheap. I can have carb converted to be set for e-85 for around $400

 

http://marksullense85carburetors.com/contact.html   

 

This guy does the conversion on a lot of upper midwest cars.  My other thought was water/meth injection. They make kits that are around the same $400-500 price.

 

  After doing a search, it seems there is a lot of controversy about this. All I know for sure is at $8 a gallon, it won't take long to be at that $400 mark

 

  Please let me know your thoughts. Links appreciated. Anyone in my situation??

[djwarner]

Do you have any indications of detonation? Pinging? What is your timing set at?

 

If you want to up your octane a couple of points, add 4 ounces of acetone to a tank full. Be aware there is little or no benefit to increasing this amount of acetone. 

 

For the record, many people think that high octane fuel has more energy, this is false. High octane impedes the speed of the flame front and resists dieseling. Detonation is when the A/F mixture explodes rather than burning.

 

The intent is to burn rather than explode. Burning takes time. That is why the timing is set ahead of top dead center. It takes time to build the pressure. Pinging occurs when burning A/F reaches peak pressure before the piston reaches TDC. Pinging can be controlled by retarding the timing so the peak pressure comes later.

 

Detonation is another matter and is a function of the compression, chamber temperature and chemical make up of the fuel. I'm no expert on E85 but is does have less energy per pound of fuel. Even though E85 is supposed to be 105 octane, a quick google search came up with a Corvette with 13.25/1 compression experiencing detonation on a dyno.  

[NewZed]

265 psi is very very high pressure.  With a good gauge, 170-180 psi is where most stock engines seem to come in. 

 

The simplest "fix" might be to install a head with a bigger combustion chamber.  You can do that in the driveway.  Otherwise your car's range is limited to where the right fuel is available.

 

Sounds cool though.

[djwarner]

Another alternative would be to drop the compression ratio by using a 2mm Nismo head gasket.

[skirkland1980]

Car sounds amazing but there is no need for 13:1 compression on the street. For track only the race fuel can be justified. I don't know how meth/h2o injection would work with a carb unless you had a knock sensing system to trigger it. With a turbo it's controlled by boost pressure. Good luck on your project

[TimZ]

I'm no expert on E85 but is does have less energy per pound of fuel. Even though E85 is supposed to be 105 octane, a quick google search came up with a Corvette with 13.25/1 compression experiencing detonation on a dyno.  

E85 has ~30% less energy per pound of fuel, but this is largely irrelevant, since at max power you generally burn 40-50% more fuel per combusition cycle.  All else equal you should make more power over gas, but have lower mpg.

 

The Corvette example you noted is inconclusive - the guy asked why he would experience detonation on the dyno, but then offered no details on his setup aside from his CR, or how it was tuned (if at all).  For all we know he might have just filled up with E85 and wondered why it was running funny.

[leecheater]

I was told by mechanic (these guys are really experienced) that it was pinging. I do have experience with e-85 as I run it in my MKIII 2jz zwap. It has AEM engine management so much easier to controll fuel.

 

  It would be nice to know how much adding a thicker headgasket would drop it? I honestly did the math tonight while out bow hunting. At $8 a gallon for race gas and say 15 mpg, it wouldn't take long to save enough to either buy the meth kit or have Holley carb converted. 

 

   Now that I think about it, pulling the head might help me have insight as to what was actually done to the motor since the previous owner didn't have paper work to show what exactly was done.

[leecheater]

Snow performance  Stage 2 system (#20020MC) includes a 4150 carb plate and a controller that references vacuum as well as an rpm signal to command injection.

 

http://www.snowperformance.net/stage-2-boost-cooler-muscle-car-n-a.html

[djwarner]

Do you know where your timing is set? Timing specs have changed for various models over the years. Easiest test would be to fill up with premium and add 4 ounces of acetone. Costs you almost nothing.

 

Obviously your engine configuration does not exist in the manual's timing specs. If the acetone doesn't help, the next step would be to retard the timing 5 degrees.

 

Both of these can be done before opening up the engine.

 

BTW for grins and giggles, pull a plug and inspect the piston top for carbon build up. At 3K miles you shouldn't see any, but if you do, it could explain the super high compression ratio.

[leecheater]

djwarner, I am going to check on the timing degree. I do know they mentioned it when I had the compression done. I think they backed it off a few degrees. Still a bit unsure about the acetone. I will do some more reading on this. I don't want to open up the engine but if I had to I would want to go with a different head all together. I know the one I have is not considered the best.  I am fine with the performance, she is pretty fun in 3rd and 4th gears. I just don't want to shorten the life of the engine by not proving enough octane.

[djwarner]

re-reading my last post I realized I wasn't very detailed when I talked about the timing changes over the life of the design. In reality, our engine timing varies with vacuum advance and centrifugal advance as well as when we twist the distributor during a tune-up. For the two dynamic advance have been adjusted over the years.

 

For example, we normally set the distributor with the vacuum disconnected. When it is reconnected, at idle the timing changes. As we open the throttle the vacuum changes. How much and how fast the vacuum changes the timing is actually a mechanical program. Similarly, there is another mechanical program for the centrifugal advance which is determined by engine RPM.

 

Over the life of these cars, programs have been tweaked several times as can be verified by the distributor part number changes over the various engine changes.

 

Unfortunately, your particular engine configuration has probably never had the benefit of comprehensive analysis by a factory engineer who would design a custom distributor with unique programs for vacuum and centrifugal advance. This leave us twisting the distributor wondering what is enough.

 

Now on top of this, you want to mess around with the fuel performance.

 

As for the acetone, in the small percentages (4 ounces in 16 gallons) you aren't going to hurt your engine and it will boost your octane 3 or 4 points. If your engine is on the edge of running right, this may nudge it in the right direction. The goal for now seems to be eliminate the pinging during idle and normal steady state driving on pump gas. If you get that far, you may still experience pinging when lugging the engine or higher rpms. (vacuum and centrifugal advance issues). 

 

Mazda's Skyactive technology runs engines in this compression range and gets fantastic mileage to boot. But they use an engine computer to dynamically change the fuel/air mixture and timing.

 

All that said, after adjusting the timing and perking up the fuel, it may still not be enough to eliminate the pinging or you may end up with a car that doesn't drive all that well. But you haven't done anything that can't be easily undone.

 

Over the years, I've learned the unless you determine the cause of a problem, it is likely to be there after a major change. Only this time you are further into the boonies.

 

What I am suggesting are logical steps to determine what doesn't work. Sort of like the bass fisherman quickly moving from area to area eliminating the areas where the fish aren't. This so he can spend time fishing where the fish are.

 

In the long run, I would expect a carbureted, high compression engine with todays fuels will not be happy as a daily driver. That is why a 2mm head gasket may be in your future. 

Edited October 9, 2013 by djwarner

[NewZed]

The basics aren't being addressed, it seems way premature to be planning a new head gasket, or an E85 conversion.  Sticking the E85 nozzle in the tank isn't really experience.  No offense but you don't even know what your static timing is and the engine has been described as "pinging".  Damage is occurring every time it's driven.  At least buy a decent timing light and take a measurement before starting a discussion about modifying the engine.

Edited October 9, 2013 by NewZed

[leecheater]

NewZed:    No need to buy a timing light, we have several of them in our shop.The timing is 18 degrees with vacumn line pulled off by my testing. When I reved her up she retatrds to about 32-34. I don't know how many rpm's it was but I will guess 2500, was alone in the shop, couldn't see the tach.  I fully realize that my stong suit is not auto mechanics. That's why I am on here trying to learn as much as I can. I bought the 240 this way, no idea the compression was this high.

 

  I also pulled a spark plug & tried to take a pic but with my old camera couldn't get it to turn out. The top of the pistons have just a light brown color to them. I can see plenty of the metal as well. I would say carbon build up is not the issue.

[NewZed]

Now you know a little bit more about what you have.  From the initial purpose of the thread timing would be one of the most important pieces of information.  You still have the major problem of owning a sensitive, possibly high-performance engine, but with very little knowledge of the basics of how an engine works.  No offense, I'm no expert myself, but that seems to be the situation.

 

The timing that you measured is 32-34 degrees advanced.  If the engine is still pinging while you're driving, adjust your timing back to 10 degrees or less while you're trying to figure out what to do.  I would make a friend of someone who knows these engines and learn while he/she works on it.  It will save you money and if you pick the right friend you'll be able to drive the car while getting it right.

 

What kind of shop do you work at, and which area do you work in?  Curious, since you're surrounded by timing lights.

 

 

Edit - curious also as to why someone would build such a high compression ratio L6 engine.  It's not one of those Datsun Parts LLC engines is it?

Edited October 11, 2013 by NewZed

[leecheater]

When I run the high octane gas, I don't think it is pinging. The timing was set by the guys at Midwest Motor sports. They work on all the import cars around here. I trust their advice. They felt with race gas I should be OK. I have been driving her back and forth to the clinic and to some of the outreach clinics, not pushing her hard. With Minnesota weather, it's only a couple more weeks until I park both her and the Supra until early April.

 

  The "shop" I work at daily specializes in vintage homo sapiens with pain issues and opioid dependence. The big shop out behind my house is a 30 x 50 building with 12 foot wallls and a hoist. I can store most of the toys in there and it's a great way to relax, even if it's just changing oil or rotating tires, or shooting the bull with friends. Nephew is usually hanging around. Thankfully he's a better than average back yard mechanic. Right now we have a 72 and 73 Toyota Landcruiser in the shop. Sean is restoring the 72 (only 26K original miles) and doing a v8 swap into his old Volvo. His day job is at Fat Joe's Racecraft. They do pretty much anything to do with drag cars. He's a wiz at both tig and mig welding and fabrication.

 

  No idea why the previous owner had it put together with such high compression. I think he had the machine work done by a shop in Duluth and assembled it himself. If I would have known, (should have done a compression test before buying) I probably would have bought the red 280Z vith V8 swap that he also had for sale. I didn't like it as much because it was an old carbed 350 with a 3 speed automatic. If it had an LS engine and a 6 speed, the choice would have been easy.

[NewZed]

Sounds like your nephew could figure out what's up with your race engine.  You should get it dyno'ed with race gas just to see where you're starting.

 

Should be a good winter project.  Good luck.

[djwarner]

The timing is 18 degrees with vacumn line pulled off by my testing. When I reved her up she retatrds to about 32-34. I don't know how many rpm's it was but I will guess 2500, was alone in the shop, couldn't see the tach.

 

When you rev an engine, timing advances not retards. This is due to centrifugal advance.

 

There is no Datsun spec to set timing to 18 before top dead center that I know of. I would not assume that a pinging engine set in advance of factory settings was set correctly by your shop. Most sources I have say timing should be set to 10 degrees BTDC not 18.

 

If the engine was set up for racing with racing fuel, one could possibly get away with 18 BTDC, but not for street use. Retarding the setting to factory spec of 10 BTDC will do absolutely no harm to your engine and will improve your situation.

 

While you are there, write down the model number on the distributor, the number is located on the metal casting on the aft side. from this we can determine the vacuum advance and centrifugal advance specs. 

[leecheater]

dj, I had hoped to get over to talk to the mechanics but it's a Friday and clinics are always busy on them. I did get the #s off the distributor however.

 

 D67403 

22100N4301

 

 

there are several really small #s at the end of the top #, but die to the position, am unable to read them.

Edited October 12, 2013 by leecheater

[djwarner]

OK that is the standard Nissan distributor for a 75 thru 78 280Z. Timing should be set to 7 degrees BTDC for a 75 or 76 non California engine. Otherwise 10 degrees BTDC. The only exception would be for a dual reluctor models ( automatic transmissions?).

 

Sounds like a very good next step.

[Xnke]

Pull a spark plug and using a wire or something, check for domed pistons. At TDC, you should not be able to push the wire past the end of the spark plug hole very far if you have domed pistons; if they are dished you will be able to slide it across the top of the piston about 73mm.

 

Let us know the head casting number. Bet it's an E31 or E88, *possibly* an N42.

 

Venolia made a couple types of stock-item pistons for the L6, both flat-topped and domed. I don't think they make any dished pistons as a stock item. If you know which pistons are in the engine, we can get a better grip on possible compression ratios.

 

Do you have the cam specifications? Or who cut the cam? A lot of bigger cams require more idle advance than stock just to crank over and run.