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

[Zedyone_kenobi]

Let us all have a talk about compression, and timing. How much is too much, how to deal with it. And how to achieve the max amount of advance you can while not pinging.

Enter the subject. My Datsun Spirit STR2.8 liter engine.

L28 N42 Engine Block, N42 Cylinder Head base

87mm (bore) x 79mm (stroke) = 2817.8cc

Combustion Chamber Volume: appx 43.5cc

Piston above deck: appx 0.25~0.30mm

Compression ratio: 10.5 : 1

Camshaft: STR274F

Duration 274

Max Valve lift: 12.00~12.19mm

Lobe: IN: 106, EX: 110, Lobe Center: 108

I have driven this engine hard for 2000 miles or so since it was installed. It has run beautifully. During 1000 mile rally the car ran without a hitch, but I purposely dialed back my timing to take into account for iffy gas as small mom and pop gas stations I may encounter. After the rally I crept back up to 34 deg BTDC at 2800 rpm and drove her. At low RPM in 3rd and 4th when the engine is under heavy load I noticed copious amounts of pinging. I run 93 octane only locally.

Long story short, I cannot run 34 deg of max advance without dropping a bottle of good quality octane boost in my car. With the mileage I drive, that means about 5 bottles a year.

Correct timing is so important to getting the max HP out of your engine. Every degree of timing can yield substantial HP changes. So what is the max timing a street engine can run. Naturally I have cast my stone. And I find with my particular set up, 34 degrees is about all I can run with 93 octane and a bottle of NOS octane booster (don’t laugh, it was ranked 3rd best out of the top 10 in an independent study, plus I like the blue bottle)

Can the veterans of the Z world chime in on this. What steps can you do after the engine is built to help avoid pinging. I have not tried some BP7ES plugs which are colder ( I have them in my garage though) . I currently run a 190 deg thermostat. Would a 160 help?

I love how my car runs and the extra 65 dollars a year in octane boost really does not bug me that much considering how well the car responds to it, but for others building an engine, what should be the max limit on93 octane gas. I would have to say 10.5 is too much.

Granted the camshaft over lap and lift all have an effect on this. But lets get a good conversation going.

[LeonV]

Two very important things need to be mentioned here:

1. There is no specific compression ratio-to octane conversion, since knock-resistance is dependent upon much more than just static compression. Some key factors are combustion chamber design (including spark plug location), RPM, AFR, and valve timing profile.

2. Max timing DOES NOT equal max power. The optimal timing map for an engine is found during controlled dyno testing.

There are a many reasons that can cause engine knock, as well as different types of knock. Fundamentally, knock is when a part of the mixture ignites before the flame front from the spark plug gets there. In your case, I'd be thinking about things getting too hot in the combustion chamber. This usually comes from three things: spark plug, exhaust valve, glowing deposits. Going to a cooler plug and/or dropping coolant temperatures could help, but not guaranteed.

Spark plugs are easy enough to try out. However, if I were you, I'd pour in some 100 octane fuel and go to the dyno to see what YOUR engine really needs as far as timing goes. Come up with a "good" dyno testing strategy, i.e. minimize variables.

Just keep in mind, knock happens for many reasons and is also dependent on ambient conditions. This is why there's a safety net built in on factory spark maps.

[madkaw]

Might want to add piston to head clearance and HG specs also. The get ready for more opinions then raw facts;)

[Zedyone_kenobi]

Leon, well said!!

I agree with your assessment. My BP7ES are going in this weekend, but I am not sure it will make that big a difference, but it certainly is in the direction of goodness. I have been wanting to get the engine on a dyno for a while, since it is totally broken in. I just need to find some time.

I also echo'ed your opinion about all the variables and knobs that can be turned. Right now the problem is not really that bad. But I am liking the idea of dropping some 1000 octane and playing with timing until I get 'the ideal settings', then try to figure out how much I am losing with 93. According to independent lab reports NOS octane booster increases the octane by right at 1.8-2 points per tank. So figure i start with 93 at about 95 all my pinging stops and the car runs very well with 14 degrees at 900 rpm, and 34 degrees max at 2800 rpm. I have not advanced it beyond that as it starts up, runs and keeps AFR dead nuts on all the time. In fact it runs perfect.

This is where my engine build got a bit off track. We wanted to use OEM Datsun L28 flat tops in the build. But we could not get any .005" over bore. OR rather Eiji could not find any and I came up empty as well. But being the impatient person I am I ordered some ROSS forged pistons (completely overkill) that were 0.005" overbore. However they were a few thousands taller. We had to choose smaller lift cam because of this as we had valve clearance, but not the recommended amount. So we put a slightly smaller lift cam in. This is also the source of my extra compression. I think we were shooting for around high 9's or 10:1. Add a taller piston and blame, 10.5+:1 and the need for uber fuel now exists.

I do not want to come across as angry or mad. In fact I LOVE how my car runs. It is strooooong. In fact my only real complaint is that I should have bought 45 DCOE's instead of 40's.

The purpose of my post was to warn others and to ask now that I have taken this path of high compression, is there anything I can do other than add octane boost or retard timing to ease the situation?

[Zedyone_kenobi]

Thanks Madkaw, the opinions form you fine fellows are always welcome!

I have thought about finding another head a P79 perhaps or something with a bigger combustion chamber and having some serious port work done to it, and then one day swap out my hardware with it. But that would be a bit of an undertaking. I would however get me down to closer to 10:1 and at the same time get me some FLOW...

[240260280z]

Squish of a Maxima N47 head is nice. When I looked at the Maxima head, it looked like an E31 all welded up and shaped like the racers did in the 70's. I think the racer experiments were adapted in the Maxima N47 head.

[siteunseen]

The purpose of my post was to warn others and to ask now that I have taken this path of high compression, is there anything I can do other than add octane boost or retard timing to ease the situation?

Thank You. I've got the F54 turbo block with dished pistons & an E31 head with hopes of running pump gas.

[LeonV]

The purpose of my post was to warn others and to ask now that I have taken this path of high compression, is there anything I can do other than add octane boost or retard timing to ease the situation?

If colder plugs and cooler coolant doesn't help at all, I'd really consider going with a more aggressive cam or at the least retarding your cam. My hypothesis would be that the high heat and pressure caused by your current valve timing profile and static compression is causing engine knock. Knock is more likely to happen at lower RPM because it has more time to develop while waiting for the flame front to reach the hot spot. Therefore, you want to decrease cylinder pressure within the zone where knock happens. As far as valve timing goes, this is mainly done by delaying the intake valve closing point.

I have thought about finding another head a P79 perhaps or something with a bigger combustion chamber and having some serious port work done to it, and then one day swap out my hardware with it. But that would be a bit of an undertaking. I would however get me down to closer to 10:1 and at the same time get me some FLOW...

That's a good plan for the future, IMO. An unshaven P79 or P90/P90A with flat-top pistons will get you to ~8.5-9:1. The shaved P90 that's going on my flat-top L28 has 44cc chambers and should give me approximately 10:1 compression. I was hoping for a bit lower but I'll probably be close to the edge of what I can run on pump gas, especially in CA where we get up to 91 at most pumps. I do have access to 100 octane if I need it, but I REALLY don't want to resort to that...

Thank You. I've got the F54 turbo block with dished pistons & an E31 head with hopes of running pump gas.

You'll be fine, probably even on regular gas. That combo is ~8.5:1.

[Zedyone_kenobi]

So running a tigher lash on the intake valve would keep it open a hair longer.

That is something to think about. I will try the cooler plugs this weekend. And I have to find a good quality 160 thermostat.

[Stanley]

The Nissan parts man in Tucson had the 160's but talked me into a 180 thermostat.

My engine seems to make more power when it's well warmed up. Always check my revs vs. mph on the freeway, it consistently goes faster at 3700 on a warm afternoon than in the cool morning. A 160 might run too cold in the winter.

[Zedyone_kenobi]

there is no such thing as too cold in houston.

[LeonV]

So running a tigher lash on the intake valve would keep it open a hair longer.

That is something to think about. I will try the cooler plugs this weekend. And I have to find a good quality 160 thermostat.

Sure, tighter lash would leave it open a bit longer but it's not exactly what I was recommending. You can try but I'd go with the lash settings given by the cam grinder. I was recommending retarding the cam (as long as you have ample valve to piston clearance) to drop low-rpm cylinder pressure. You may need an adjustable cam sprocket for this depending on where your cam is currently set. If it's on the #2 hole, set it to #1. That will retard the cam by 4 degrees and may be enough to help. You'll lose a bit of low-end but make up for it in the high-end, if you notice anything at all.

Always check my revs vs. mph on the freeway, it consistently goes faster at 3700 on a warm afternoon than in the cool morning. A 160 might run too cold in the winter.

That makes absolutely no sense to me. Engine RPM to MPH is controlled by gear ratios, i.e. solid meshing gears that don't have magical temperature-sensitive gear ratios.

Could you clarify what you mean here?