My Datsun Spirit L28 Build.

[Zedyone_kenobi]

It is a chromoly steel flywheel designed to be 9 lbs.

NO idea who the vendor is. Eiji has a vendor he likes. Again, he is running it on two of his cars, so obviously if it was good enough for him, it is good enough for me.

[LeonV]

Should be a really fun engine, although I would have some concerns about detonation with such high compression. You might be ok on 93 octane, depending on your advance curve. Basically, you won't get the engine's entire potential without advance being around 32-35 degrees (empirically determined by dyno testing). The open chamber heads (e.g. N42) tend to be at the higher end of that spectrum.

If you have to dial down spark advance because of detonation or preignition, you're losing power. The cam and advance curve have to be really well-matched to the engine in order to take advantage of the high compression. Hopefully you don't run into issues, but don't be surprised if you do. Good luck!

[Zed Head]

Compare the duration and lift of his cam to a stock cam. It's probably why there was initially some piston-valve conflict. Cylinder pressures probably a little lower than if he had 10.5:1 on a stock cam.

Will there be dyno tuning?

[LeonV]

Compare the duration and lift of his cam to a stock cam. It's probably why there was initially some piston-valve conflict. Cylinder pressures probably a little lower than if he had 10.5:1 on a stock cam.

Will there be dyno tuning?

With the longer-duration cam, cylinder pressure at low rpm will probably be lower. However, at high rpm, cylinder pressure will definitely be higher, otherwise you wouldn't be making any more power over a stock cam!

What a longer-duration cam does is it moves the peak cylinder pressure point to a higher rpm (where intake inertia and exhaust scavenging effects are stronger). The longer duration and overlap periods make for more reversion at lower rpm, thus dropping cylinder pressure where there is not enough intake inertia or exhaust scavenging to keep the mixture flowing into the cylinder.

EDIT: This relates to detonation and preignition because of the shift of peak cylinder pressures. By shifting peak pressure to higher rpm, you give the mixture less time to self-ignite. This is why longer-duration cams can be effective in suppressing detonation when compared to a stock cam at the same compression ratio.

Higher compression increases cylinder pressures at all points and the longer-duration cam drops cylinder pressures at low rpm. If the superposition of these is not much higher than the stock cam/compression combo (at "low" rpm), then you might be okay. This is starting to get into the details but I wanted to shine some light as to the physics of the phenomenon we're discussing.

Edited June 12, 2012 by LeonV

[Zed Head]

So, with 10.5:1 calculated CR would you rather have a long duration or a short duration cam, to avoid detonation?

[Zedyone_kenobi]

Yes, in the simplest of terms. But another thing critical thing to look at is lobe separation and valve overlap. A longer overlap will also decrease cylinder pressure since you spend more time with both valves open. This all has effects on when the cam is in the sweet spot.

To be honest, I have to go with what Eiji recommends on this. He thinks I will be fine on 93 octane. I have no reason NOT to trust his word.

I was planning on running 10-15 deg initial advance and 33 max as a starting point.

The only dyno tuning will be with me on the road with my O2 sensor. Better than seat of the pants, but I may look for a local dyno shop near me. Most guys at dyno shops near me only want to deal with late model muscle cars with EFI.

Edited June 12, 2012 by Zedyone_kenobi

[Zed Head]

I was just trying to get off the simple high CR needs less timing theme and get in to some details.

A good tune on the dynamometer would be interesting after the seat of the pants, to see how good your seat is, and just for some good information on a Datsun Spirit motor. Seems like one session might be worthwhile, considering the investment. On the other hand, it would be money that could be spent elsewhere.

[Zedyone_kenobi]

Well I will definitely call around to try to find an hour or two of dyno time.

But I have to be honest with you. If my AFR come in spot on, and it drives fine, I will be hard pressed to care. Sure I may be leaving some power on the table, but if temp is good, AFR is good, power feels good, not sure it will matter to me. I cannot emphasize how much I want drivability over peak HP. I think dyno tuning can drive you crazy trying to track down that last few HP. I have a feeling I am going to be quite happy.

But my only worry is what LeonV said. 10.5 is quite a bit of squish. So we will see what happens.

I will say this...If I get it and it just feels weak and my AFR is spot on, I will want to know what is going on.

[madkaw]

For the small investment-compared to what he has spent-I would think it mandatory to spend the 150-250$ on dyno tune.

I learned so much about my motor in one 85$ session. All the timing talk gets verified on the dyno.

I was running too much advance according to the dyno. The last 3-4 degrees got me nothing for power, but probably benefits me on cruise considering I don't run a vacuum advance. I also learned that the butt dyno sucks and is very decieving. Like running SM needles on my car, when the stock would have made more power and used less fuel.

Once I get my Mikuni's dialed in I plan to make another trip to the dyno for comparison. My wideband should get me close, but these 200$ widebands aren't as good as the 2000$ ones they use at the dyno. The race car driver/dyno operator I worked with said that the widebands are frequently a point off, which could be leaving power on the table.

I was just trying to get off the simple high CR needs less timing theme and get in to some details.

A good tune on the dynamometer would be interesting after the seat of the pants, to see how good your seat is, and just for some good information on a Datsun Spirit motor. Seems like one session might be worthwhile, considering the investment. On the other hand, it would be money that could be spent elsewhere.

[Zedyone_kenobi]

Fair enough, I hear what your saying. It is not lack of outright desire to know, more of availability. Most of these new dyno shops around me only know EFI late model mustangs and camaros. I will try to find one though when it is all broken in.

[madkaw]

You make the comment about a lot of squish; makes me think that's why Eli went with reworking your cam instead of messing with the pistons. He was looking for that ideal squish zone for these motors and the piston to quench pad was there to help with detonation, so he opted to increase clearance in other ways.

Well I will definitely call around to try to find an hour or two of dyno time.

But I have to be honest with you. If my AFR come in spot on, and it drives fine, I will be hard pressed to care. Sure I may be leaving some power on the table, but if temp is good, AFR is good, power feels good, not sure it will matter to me. I cannot emphasize how much I want drivability over peak HP. I think dyno tuning can drive you crazy trying to track down that last few HP. I have a feeling I am going to be quite happy.

But my only worry is what LeonV said. 10.5 is quite a bit of squish. So we will see what happens.

I will say this...If I get it and it just feels weak and my AFR is spot on, I will want to know what is going on.

[madkaw]

I was just expressing my opinion, so please take it that way. I hear what your saying too, if your happy with the engine the way it runs, idles and performs and that's all the further you need to know then cool! I guess there is a point you get obsessed with this stuff-my wife thinks Im possessed.

Fair enough, I hear what your saying. It is not lack of outright desire to know, more of availability. Most of these new dyno shops around me only know EFI late model mustangs and camaros. I will try to find one though when it is all broken in.