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


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Looking at initial timing and advance in FSM's and engine manuals

1970 17 @ 650 _6.0 mech 5.5 vac and 5 @ 750 12.0 mech 9.0 vac

1971 17 @ 650 _6.0 mech 5.5 vac and 5 @ 750 12.0 mech 9.0 vac

1972 17 @ 650 _6.0 mech 5.5 vac

1973 _5 @ 750 12.0 mech 7.0 vac

1974 _8 @ 750 13.0 mech 6.0 vac

1975 13 @ 800 10.0 mech 7.5 vac

1976 13 @ 800 10.0 mech 7.5 vac

1977 10 @ 800 _8.5 mech 9.0 vac

1978 10 @ 800 _8.5 mech 9.0 vac

1982 _8 @ 800 _8.5 mech 15.0 vac

Edited by Blue
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Those 71 numbers are for the non emissions cars Blue, as you know the 71 US emissions dizzy equipped ran 5 degrees initial advance.

Yup, that is shown beside it.

Now I don;t feel bad with 20 degrees at idle with no vac on L24 with triple webers. It is less than stock. I also get 37 at WOT using the ZX distributor.

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I have scheduled some dyno time on Friday morning to see what my car puts down. I will bring some jets and a timing light with me! Should be fun, but I am dealing with a shop full of Brotatoes who only work on LSX engines and Vipers.

Out of curiosity what are your reservations?

... ?

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Yup, that is shown beside it.

Now I don;t feel bad with 20 degrees at idle with no vac on L24 with triple webers. It is less than stock. I also get 37 at WOT using the ZX distributor.

Sounds like you running the same exact set up as I .

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I took the '78 280Z on a 75mi test drive on Saturday. Everything was fine, but I kept it 3000 rpm and less during this break-in period. With the low-rpm-only driving I'm not sure if I have any detonation, but I did run 93 octane Shell premium for the test. The higher compression, cam, headers and 2.5" exhaust will not be utilized much until revving past 3000. More tweaking of timing, fuel pressure, etc. this week, as well as a/c. I may have her back home by this weekend. :laugh:

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  • 2 weeks later...
congrats on the maiden voyage... keep her rolling!

I'm to pick up my 280 Thursday or Friday of this week. At <3000 rpm there have been no signs of detonation. I'll see if that holds true above that, as the 485/270 cam will let the engine easily rev to 6500 rpm now.

I may need to come to League City to let you have a look at her.

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I just had a conversation with a teacher about bright boys and girls. Boys can be 'enthusiastic' and this is a little hard for teachers to handle sometimes. I suspect this is a reason why the diagnosis ADHD has managed to multiply so dramatically in recent years.

Childhood is not a mental illness. Let's leave the pills in the bottle!

Now about those dyno results...

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I would definitely like to see the dyno results as well, Zedyone... My engine build is a little silly in that I'm at about 12.5:1 compression according to the calculations but it might be a little less. They look something like below (when using the calculator posted above). Unfortunately, I also have DCOE 40's which most say will be fairly limiting with my motor. I'd actually prefer to run ITB's but that's not really in the budget just now. Anyhow, I have yet to get it on a dyno as I'm just finishing the break in process also. I run 93 oct + the NOS Octane Booster in every tank.

As for timing, I'm running around 15 @ 1100 and 33 @ 3k. I had to dial it back a little bit because of pinging but right now the engine seems pretty content with that - at least from the perspective of detonation. I've ordered a Knocksense so that I can get a better handle on it as well.

Block Height= 207.87 mm

Deck Height= 0.63 mm

Chamber Volume= 42.4 cc

Cylinder Volume= 516.35 cc

Gasket Volume= 6.5 cc

Dish Volume= 0 cc

Deck Volume= -3.92 cc

Uncompressed Volume= 565.26 cc

Compressed Volume= 44.98 cc

Compression ratio= 12.57:1

Total displacement= 3098.13 cc

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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.

Edited by Lazeum
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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.

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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.

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