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How to De-Tune a Z


Zrush

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I read this information here: http://suncoastzcarclub.homestead.com/club_carz_Moschettos.html

and while it is not particularly thorough, it sounds like the car should not be running anything too extreme in terms of compression. Flat top pistons, not domed, .040" oversize, shaved head. Doesn't say what head is being used. Do you know which head is on it?

What the website info did tell me is that most of the compression is in the head because the flat top pistons are not REALLY high compression pistons. So rather than getting a new shortblock like hls30.com suggested, a new head would be the way to mellow out your engine. Either the P79 or P90 heads should drop your compression down to the high 8 to 1 range, and at that compression you can probably run 87 octane.

I would expect compression for your engine to be in the 10 to 11:1 range. I think my own engine is at the high end of possible ratios for flat top pistons since I have an E31 head which has one of the smallest chambers. Mine is about 11:1, and my engine requires about 95 octane to keep from pinging. I trial and errored it out until I got the minimum octane that I could use with full timing advance and not ping.

If you bought one gallon of the 109 octane and mixed it with 5 gallons of 92 octane, that would net you 94.89 octane. That should be pretty close to getting it to run without pinging. If that didn't quite cut it you could go with a 4:1 ratio for 95.4 octane. The lower your octane the more power you can get from the gas, so you should always run as low octane fuel as you can with the appropriate amount of timing for max power and not ping. I believe Dan Baldwin proved this on the dyno way back when. Upped the octane and lost hp, tried to increase timing, lost more hp.

Anyway I hope that helps some. The more info you give us the more closely we can try to figure it out.

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If it's bog and hesitation that is your problem and not pinging under load or light loads, then I might think the issue with pump-gas -v- race-gas may be more in formulation. Many of the competition fuels are formulated to 'older' specifications with specific gravity more suited to carburetted use. ALL pump gases now are formulated for fuel injection so they lack the light aromatics that make carburetted fuel work in those systems.

I had a smiilar problem here on the left coast when they went to reformulated gases in the early 90's. My high compression Corvair didn't necessarily ping, but it wouldn't cold start worth a damn, and stumbled and bucked like crazy unless I ran Chevron Gasoline in it. Come to find out talking with some engineers at Unocal in Brea, as well as some people at Chevron Technical Assistance that indeed the Chevron Premium I was finding ran so well still had the old formulation...and they told me a specific date that it would no longer be available. Right to the week they said, the car started stumbling on Chevron as well. I went to Union76 Racing fuel of the lowest octane I could get (I think it was 95) that was set up for carburetted race series, and the car would run like a top! I found that blending the race fuel by about 10-20% (depending on what brand I was pumping)was all I needed to make the car operate correctly with it's NOS Rochesters. And that was with 87 or 91 octane. I found with the stumbling and bucking and hesitation (which sounds like exactly what you are experiencing!) it was not so much Octane as formulation of the fuel. See if they have a lower octane race fuel that is specifically set up for carburetted racing series. I know the 104 I get at the pump (VP Racing Fuel) is set up for Fuel Injected Imports, and haven't really tried it in anything carburetted but my pit minibike, and occasionally the lawnmower if I'm getting a lot of pressure to cut the grass!

If you're not pinging, and it's more a drivability problem, I think you are going to need to search for proper formulation, rather than higher octane. The 109 Octane you are running may be formulated for a carburetted racing series, and that is why it runs so well, not necessarily the Octane.

Oh, and that's about highway robbery for that fuel! If you are buying it in a five gallon pail...and it's going to sound bad...buy it by the drum! Our driver Dave Richardson buys his Race Fuel by the drum, and gets it for about a DOLLAR LESS per gallon than what I can buy it for at the pump! We save because we have access to drums so we don't get tagged for the deposit. We sell the gas off to friends for a bit of a profit to defray the costs, but the more you buy, the cheaper it is, and that $9.50 a gallon is highway robbery!

You also have to be able to store a drum of fuel....it's not a 'garage in the house' commodity, needless to say. Shop around, you should be able to get a better price than that, and like JM says, mixing is a viable option to really decrease your costs. With 11 or 12:1 compression you probably only need octane in the low 90's, and if your setup is like my Vair, you may get away with mixing 20% race fuel to standard pump gas to get enough aromatics in the gas to clear out the stumbles caused by the syrup they sell at the pump these days.

Good Luck!

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Thanks for the many suggestions, keep them comming.

Anyway, I went to the strip and ran the car down to a half of tank of 109 octane. Now, should I go the the gas staion and get premium 95 or regular at 87.

If the car runs fine on half and half that will save some scratch.

Thanking all my Z Freinds in advance.

Vicky

PS she ran 7:00 et in the 1/8th. Remember this is me driving, a more experienced driver could more than likely get a lower trap time.

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1/2 and 1/2 with 87 should give you 98 octane. Figure 16 gallon tank so 1/2 tank = 8 gallons. 8*87 = 696 + 8*109 = 872. Add those two together and divide by the number of gallons, 1568/16 = 98.

That's enough for 12:1 compression in all likelihood. I still think you could get down to 95 or so and be OK, based on the info we have.

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No Math Please

I'm running up to the staiton now and get 87. The track is open until 2:30, so that gives mew time to test.

If it still bogs on the half and half mixture what do I do then?

Vicky

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Outlaw or other octane booster!

Will

Octane booster is Tolulene or Xylene or a mixture of both. You can buy a gallon from the hardware store (it's also used as paint thinner) for ~$12, or you can buy a 16 oz bottle for $8. Tolulene is 114 octane and Xylene is 118 octane. When the bottle says it raises octane one point, that actually means .1 so it will take 92 octane to 92.1 octane. You can do the math in oz as shown in my previous post to see what the actual effect is, and it's basically nil.

There are site's with homebrew octane booster recipes, and I think it's been talked about here before. If not it's come up a couple times on Hybrid Z if you want to learn more.

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As your know I ran the Z down to a half of tank of 109 and went and bought 95. Seems OK until you down shift for more hp.

What now, air mixture adjustment?

Bogging is so embarassing. Please help. Plus black smoke came from the tailpipe wereas with 109 she was Greenpeace clean.

Help

Vicky

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Put simply, that don't make no sense. I would suggest at this point that you take it to a mechanic who knows Z's. Bogging shouldn't have to do with octane. Pinging should have to do with octane. 109 octane is ridiculously high for what you have there. Since your Z still apparently runs stock FI, I doubt that the issue is the specific gravity of the fuel. I think it has some other problem with the FI that's causing your hesitation.

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What can I say that's what happened when i did that. Actually this Zrush is a pain in the AZZ. Honestly race fuel is expensive here but pump gas boggs down the car. You dow know what bogging down means yes? That was a term we used in the 70's for crappy prerformance. Gas/Fuel can make all the difference, for any hi-po car.

I would appreciate any help with makeing this Z a daily driver.

PS no one really told me the difference between running rich and running lean. Which is what we want for our Z cars.

Thanking anyone in advance for help.

Vicky

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What can I say that's what happened when i did that. Actually this Zrush is a pain in the AZZ. Honestly race fuel is expensive here but pump gas boggs down the car. You dow know what bogging down means yes? That was a term we used in the 70's for crappy prerformance. Gas/Fuel can make all the difference, for any hi-po car.

If you mixed 50/50 109 octane, I can't see any way possible that the formulation of the gas could make the car bog. It would have plenty of the aromatics that Tony D referred to at that point and the octane should be sufficient for 12:1 compression, which I don't think you have based on the info that was on that website I linked to. That's why I think you have some other problem going on.

I would appreciate any help with makeing this Z a daily driver.

You could put a stock head on the engine. That would pretty much return everything back to normal as far as the engine is concerned. Then you'd have to figure out what kind of mods were done to the fuel injection to make it work in the racey configuration. Do those two things and it should run on 87.

PS no one really told me the difference between running rich and running lean. Which is what we want for our Z cars.

At cruise you want stoichiometric ratio, which is 14.7:1 air to fuel. At WOT (wide open throttle) for NA you want about 12.8 to 13.4:1 for max power.

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