zz3 engine spec questions?

[bravo1218]

I just purchaced an airboat with a zz3 small block chevy engine. I have a couple of questions I can't seem to find answers to.
What octane fuel should I use in the engine. The listed compression ratio is 10:1. I can find 89 at the local marinas but it is hard to get 92 except at gas stations.
Does anyone have the total spark advance for the engine? I have the stock HEI distributer with a vacuum advance. The engine has a belt drive so crusing rpm is 3000ish.
Thanks in advance for any help
Tim

 

[Anonymous]

What water temp does it run? I need to know this before I can offer any advice. Static compression ratio, octane, cylinder temp, rod length, chamber design blah blah blah, all affect the overall timing curve and fuel requirement..But water temp is probably the most critical, or should I say commonly overlooked. IMO

Felber

 

[marshmaster pat]

From my experience, those ZZ3 & ZZ4 motors need high octane to stop from dieseling when you shut the engine off. Otherwise they keep dieseling and often will back spin when it finally dies.

Keep that engine cool. Running that engine around 190 can cause problems with head warping if you shut the engine down. Let the engine hit above 200 degrees and you will probably warp those head.

Great engine otherwise, good luck.

 

[basketcase0302]

Bravo1218,

Agreed with Marshmaster pat, My ZZ4 would diesel even with 93 octane when ran hard, (dieseling is pretty scary on an airboat, prop turning when ignition shut off is not good)!

Waterthunder preaches to let the engine idle for a little after running to prevent dieseling. It works for me as my 454 has only done it once when I forgot to idle after running.

Basketcase

 

[bravo1218]

Right now the boat runs between 120 and 130 deg. It has a mechanical gauge and I think it is correct. I don't think it has a t-stat. I can hold my hand on the t-stat housing and its not hot enough to burn me. I replaced the radiator before I ever ran the boat. Nest time I go out I am taking an infared thermometer out to check the actual temp. Besides that as far as I know the engine is stock.
One question I don't have an answer for yet is my compression. When I got the boat one side of the carb was plugged and it was not running right. I did a compression test and one side of the engine has about 30 psi more compression than the the other side. All 4 cylinders. They are even on each side 2,4,6,8 and 1,3,5,7, but left side is 30 psi more. The old owner said that the engine only has about 100 hrs on it and that looks to be true. It also doesnt look to have ever been apart. I checked the casting #'s on the heads and they are the same and are the corect part # for the zz3 according to gm. The engine hadent been run for a long time before I did the compression test. Now that I have a few hours of good running on it I am going to do a compression test again on it.
Thanks again for all your replys.
Tim

 

[Anonymous]

If the engine runs that cool, the zz3 should run fine with a relatively agressive timing curve. Now no one mistake timing with engine run-on/"dieseling"..Engine run-on is mainly caused by the throttle blades being too open, and when combined with low octane fuel becomes a "diesel". On an airboat the load of the prop causes the need for idle circuit metering that no carb manufacturers sell on the mass market. When the throttle blades are too opened to obtain a "good" idle, it signals the secondary/transfer circuit as well as the main circuit.....So when you go to shut it off...It still gets a good amount of fuel and air, and with poor quality fuel..It will diesel as long as there is enough heat in the combustion chamber (Higher static compression ratios make this worse). This is where Waterthunder gives excellent advice...Letting it run will cool chamber temps after a hard run....But ultimitely in my opinion to cure it would be a properly designed carburetor, or a "dashpot"/fast idle solenoid..

Now as for the compression....I think at least one head has been pulled off before and milled, or the engine has some heads that are one-off. You have 2 different static compression ratios in the engine for both sides to be off on compression on a consistent pattern like that. Even if the carb fuel-washed 4 cylinders, they still wouldn't be that consistent on compression, and a dual plane intake sends the fuel mixture to two cylinders on either side from each primary. I honestly think you just have two different heads as far as chamber cc's go...Or maybe a different thickness head gasket etc....Either way, I would bet my last nickel it has had at least one head off of it.

Bottom line on timing and fuel octane...IF the cylinder temps/compression were equal based on your coolant temp...For performance only, I would suggest 93 and 36-38 total advance with all ignition lead in by 1500 RPM. For a ground pounder/ride boat, and 89...I would tend to go 34-36 and have full advance in by 2500. Of course every engine is different, and the load/driving style as well.....I would say that should be pretty safe #s to start by with an aluminum head...It's all a dance...I'm sure Waterthunder, myself, or many other people here will be willing to help you out.

Felber