Mixture adjustment required?

[halz]

Here is a hypothetical question for anyone with some insight into carburetion and engine performance:

Suppose I have an L24 engine which is in good working order with stock Hitachi SU-style carbs and a compression ratio of 8.76:1 (eg E88 head). Exhaust gas analysis and seat-of-the-pants tell me that the carbs are well tuned.

If I then only change the compression ratio to 9.13:1 (eg E31 head) but make absolutely no other changes, will I need to adjust the mixture of the carbs?

Possible answers:

(a) Yes - richer. The new higer compression ratio will effectively mean each cylinder will generate proportionally more heat. A richer mixture will help cool the combustion chamber.

( Yes - leaner. The smaller combustion chamber volume at TDC will mean that the previous (low compresion) mixture will be too rich.

© No. Correct mixture for low compression = correct mixture for high compression.

Your thoughts ladies and gentlemen...?

[halz]

anyone game to venture an opinion...?

[2ManyZs]

Is there a D: All of the above answer?

I'd guess since the compression has changed such a minimal amount, that you'd be pretty close with what you have now. If anything, I'd say it would go a bit lean, since you'll be having higher cylinder temps due to the higher compression, promoting a faster and more complete burn, which would mean you might need to richen it up just a bit.

Of course, the combustion chamber shape may not work as well with the exhaust set up you have now, which could move the mixture the other way as well.....

It would be another of those things you won't know the answer to until you do it.....there are so many other variable such as ignition strenght, plug heat range, etc that will play into the equation of the chamber volume and shape that I doubt there would be a hard fast rule as to which way it would go, especially if you use a different head instead of just shaving the one you are using now.

Of course, there's always the possibility that all you would need to do is advance the timing a degree or two and then you'd be OK...

[Walter Moore]

Is there a D: All of the above answer?

...

Of course, the combustion chamber shape may not work as well with the exhaust set up you have now, which could move the mixture the other way as well.....

So, 2Many... how does one go about matching the combustion chamber shape to the exaust setup?

I only ask because... well read my signature line. :cheeky:

[2ManyZs]

Well Walter, the only people that would know that would be some of the race engine builders with the tools and testing equipment (flow bench) that can answer that question with any authority (I'm just a "shade tree" mechanic:rolleyes: ).

Every head is going to flow with different characteristics, and only someone that has tried all or most of the combinations will know the true story.

One head may flow better with a 6 into 1 while others may do better with a 6-2 into 1, and the different header manufactures may use differing primary tube sizes/lengths that will make a huge difference as well.

Someone like Phred would probably have the best opinions on which header to use the E-31/early E-88 head due to his racing engine prep experience.

One of these days, maybe we can put together a list of the different heads that came on the Z's and list which header types and/or header manufacturers work the best in the real world or through flow testing.

[halz]

One of these days, maybe we can put together a list of the different heads that came on the Z's and list which header types and/or header manufacturers work the best in the real world or through flow testing.

Now that would be useful. As you have probably guessed, my current engine is the one described in the hypothetical above. It is stock in all respects except for the E31 head. I guess that my car would benefit from a set of headers more than the same engine with an E88..?

The reason for all of this is that I am presently chasing down a hesitation in the engine off idle and from part throttle while cruising. Exhaust gas analysis at idle indicates the mixture is fine and the car pulls well when warm if the choke is on. I'm wondering if the jets may have been changed in the past or if the car has always been poorly tuned since having its head swapped...

Its off to a Carb specialist next week so I will post the latest developments then.

[beandip]

Give this a try, get some thicker oil in the dampers of the carbs. If you are using ATF or Mistery oil it is too thin and causes a momentary lean condition when the throttle is first depressed . I have geen using what was recommended by Datsun , 20 wt. and it eliminated the same problem for me. It's worth a try. Gary

[halz]

How do I drain the existing oil out of the carbs?

[biggels_7]

Just pull the piston out and dip it upside down and the oil will just run out

cheers

Richard

[240ZMan]

FWIW, I did something kind of like what halz asked. When I swapped my L24 for my L28 I kept the E31 head and round top carbs. I also added headers. When I first fired the engine up I hadn't changed any carb settings. Actually ran pretty well. You would have thought with the bigger bottom end and addition of a header it would run lean. But the L28 has dished pistons which lowered the CR. Also, my L24 rings were shot.

In the end I am about 1/2 to 3/4 turns farther out. Also switched to SM needles .o1 smaller than stock given I live at altitude. Not sure what this teaches us though :stupid: