emripires Posted February 18, 2019 Report Share Posted February 18, 2019 Making a thread instead of messaging random people on their experiences. Kind of just thinking of things, need input.. L20B block U67 head open 86mm flat tops.... Standard rods L20B crank The way things are going together, it seems I will be running 10.6:1 CR. My local pumps run MAX 94 octane. I kind of want to run pump gas... What do you guys think, has anyone run a similar or identical setup? I am open to running domed 280zx turbo pistons, which will drop the CR quite a bit. Quote Link to comment
datzenmike Posted February 18, 2019 Report Share Posted February 18, 2019 L20B, 86mm flattops is only 10.11 compression. This leaves you plenty of room to unshroud around the valves and grind away any edges, sharp or otherwise, and open up the combustion chambers. Remove just 5cc and you're down to 9.33 compression. There's a 'how to cc your combustion chambers for free' Click the arrow in the upper right The U67 head has a 45.3cc combustion chamber, but check it because it might have been milled down to level it, so try for around 50cc for about 9.33. Quote Link to comment
emripires Posted February 18, 2019 Author Report Share Posted February 18, 2019 Would 10.1:1 not be a good Cr to run 94 with tho Quote Link to comment
datzenmike Posted February 18, 2019 Report Share Posted February 18, 2019 Don't know. Higher compression will burn a little faster so the timing will likely drop down some. If it pings, you can always retard it. The higher the compression the more efficient the combustion assuming you have control of any pre-ignition. You gain (maybe) about 4% per point of compression, so going from stock 8.4 to 10 would be about 5 hp. More if you are making more HP to begin with. If you run a low octane gas and have to retard the ignition you can't take advantage of the increase in compression. I had a muscle car back in the early '70s and ran premium octane for it's10.5 compression. I have no idea what the octane rating for premium was back then. If it was 94 you'd be fine. I did drive a company EFI van long distances and ran regular grade gas in it. The EFI automatically adjusted the timing. I tied the highest grade. What a surprise! It actually went farther per tank and the extra mileage paid for the increase in cost!!! I did about a half dozen tank fulls and it was the same. However it felt like I was spending more of the company's than needed and didn't want them asking why the increase is fueling the van even though I could show it was actually saving money. I just went back to the lower. . Quote Link to comment
Stoffregen Motorsports Posted February 18, 2019 Report Share Posted February 18, 2019 I used to build Datsun engines for a living. Back then, 10:1 for a street motor was the target CR. Sure, you can run a little more, but you will have to tune around it. Maybe slightly less timing, or a slightly larger cam. Either way, when getting to the 10:1 CR range, you will want to have your distributor re-curved. Search here on Ratsun for more info on this procedure. You can do it yourself, but it would be less hassle to have a pro do it. Rebello Racing in CA or Advanced Distributors in MN can do this for you for very little cash. 1 Quote Link to comment
emripires Posted February 18, 2019 Author Report Share Posted February 18, 2019 Thanks! 1 Quote Link to comment
2wheel-lee Posted February 19, 2019 Report Share Posted February 19, 2019 What are they basically doing to recurve a distributor? Not that I would necessarily do it myself, but I'm just curious what they're actually doing with it. Is the vacuum advance still used? If so, are they basically playing with the sensitivity of when the vacuum advance activates? 1 Quote Link to comment
Stoffregen Motorsports Posted February 19, 2019 Report Share Posted February 19, 2019 In theory, the mechanical advance is limited and the springs that hold back the weights are adjusted. No vacuum advance is used after this is done. While vacuum advance is good for mileage, it is not "safe" for a performance tune, as it is hard to account for the dynamics that go on inside the distributor during on-off throttle situations. Datsun L motors tend to like a quick influx of timing. Simply removing one of the springs allows this, but needs to be checked and verified. If there is not enough spring tension on the weights, the timing can float around at idle. Initial timing at idle is set at around 12 degrees advance. Next, the total mechanical advance has to limit the timing so that it never goes over roughly 32 degrees total timing advance. This is done by limiting the travel of the distributor cam. The slots in the cam are welded shut slightly and then filed smooth. Nissan Motorsports used to sell a solid breaker plate and a "SSS" distributor cam that would take care of two out of three of the mods. I don't think they are available anymore. 3 Quote Link to comment
2wheel-lee Posted February 19, 2019 Report Share Posted February 19, 2019 Thank you! Great explanation! In a previous life, I was heavily involved with race Jet Ski 2-stroke engines. Ignition timing was handled just the opposite: Lots of low-RPM advance, then retard as timing goes up. Just the same, the amount varies with carburetion, compression, combustion chamber shape, port timing, exhaust tuning, etc. Even that seemed so long ago that I've forgotten most everything I once thought I knew. Quote Link to comment
scooter Posted February 20, 2019 Report Share Posted February 20, 2019 10 hours ago, Stoffregen Motorsports said: In theory, the mechanical advance is limited and the springs that hold back the weights are adjusted. No vacuum advance is used after this is done. While vacuum advance is good for mileage, it is not "safe" for a performance tune, as it is hard to account for the dynamics that go on inside the distributor during on-off throttle situations. Datsun L motors tend to like a quick influx of timing. Simply removing one of the springs allows this, but needs to be checked and verified. If there is not enough spring tension on the weights, the timing can float around at idle. Initial timing at idle is set at around 12 degrees advance. Next, the total mechanical advance has to limit the timing so that it never goes over roughly 32 degrees total timing advance. This is done by limiting the travel of the distributor cam. The slots in the cam are welded shut slightly and then filed smooth. Nissan Motorsports used to sell a solid breaker plate and a "SSS" distributor cam that would take care of two out of three of the mods. I don't think they are available anymore. you can jb weld the slots if you dont have a welder. the next piece of electronics i want to add is a cb performance black box, so i can build a 3d timing map to get best power at all rpms. couple this with a locked dizzy and migrate to crank trigger. cant get much better other than going to coil packs. overall you should be okay. just try to make sure theres no sharp edges in or around the chambers so you lessen your chances of preignition 1 Quote Link to comment
Stoffregen Motorsports Posted February 20, 2019 Report Share Posted February 20, 2019 Who doesn't have a welder these days...? ? Harbor freight sells them for less than $200. Never thought to use JB weld for that. Good idea. 1 1 Quote Link to comment
john510 Posted February 20, 2019 Report Share Posted February 20, 2019 I guess i'll throw my two cents in.I have an L-20B with 86mm L-18 4cc dish pistons,an A87 closed chamber ported head.I'm running 44 Mikuni's and a stock matchbox distributor.I can only get 91 octane here in California.I don't have any pinging issues with timing set at 12* at idle.I believe in another thread some years back Datzenmike calculated the CR at about 10 to 1 for my motor. 2 Quote Link to comment
Tedman Posted February 22, 2019 Report Share Posted February 22, 2019 Those L18 small dish pistons are scarce. How long ago and where did you find them? Quote Link to comment
datzenmike Posted February 22, 2019 Report Share Posted February 22, 2019 L18 86mm, 4.38cc dish, 1.2mm crushed gasket and 41cc closed chamber head is 9.158. If head was ever milled to level it or thinner gasket it would be much higher. I may also have included your compression if an L16 head was used which would have been over 9.5 1 Quote Link to comment
Tedman Posted February 23, 2019 Report Share Posted February 23, 2019 I'm getting a full point higher. Swept Volume = 499.6 cc Compressed Volume = 41 + 4.4 + 7.0 + 2.6 = 55 cc CR = (SV/CV)+1 = (499.6/55)+1 = 10.1 Quote Link to comment
datzenmike Posted February 23, 2019 Report Share Posted February 23, 2019 An 86mm over bore L20B is 86 X 86 +499.3 Deck height volume is 3.19cc Gasket thickness volume (assuming 1.2mm crushed) is 6.96 cc Dish .00cc Open U67 is 45.2cc 499.3 +3.19 + 6.96 + 45.2 = 554.65 / 3.19 +6.96 + 45.2 = 55.35 ............ 10.02 A41cc closed chamber head would be 10.76 Quote Link to comment
Tedman Posted February 23, 2019 Report Share Posted February 23, 2019 Agreed, but the config in question was 86mm bore and stroke with 41cc peanut head and 4.4cc dished L18 pistons. Using your numbers and rounding to tenths: (499.3+41+4.4+7+3.2)/(41+4.4+7+3.2) = 10.0, not 9.158. 1 Quote Link to comment
datzenmike Posted February 23, 2019 Report Share Posted February 23, 2019 Christ loosing track. ? 10.5 hour day working in the bush. The first post was for flattops, 86mm x 86mm, and closed chamber head = 10.87 (deck is 0.45mm below so 2.61cc) (crushed gasket 1.2mm so 6.96cc) Same as above... but with L18 pistons with 4.36 cc dish would be 10.08 or rounded to 10.1 This is theoretical compression based on the piston compressing all the air from BDC to TDC which it most certainly does not. Quote Link to comment
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