burninator Posted July 2, 2009 Share Posted July 2, 2009 (edited) Over the last few days I've been trying to dial in my car on the dyno. I was doing most of my testing without the air-box on, so no horns attached to the carbs. After I got the car about where I wanted it I attached the air-box back plate (with the horns on it) and I lost power starting at about 5000 rpm. At 6000 rpm it had 9 hp less with the horns than without them. I thought the horns were supposed to help airflow not hurt it. There were no significant gains anywhere in the curve so I'm kinda confused. I think I'm going to remove the horns from the airbox unless anyone can think of a reason not to. Has anyone else done any testing and found different results? Here is the dyno chart: Edited July 2, 2009 by burninator Added Graph Quote Link to comment Share on other sites More sharing options...
MAG58 Posted July 2, 2009 Share Posted July 2, 2009 (edited) Reading an old datsun performance manual I have, they recommended against using both the air horns AND an Air box, as the air horns are supposed to straighten and direct from a relatively open area, while if you add air horns to the air box, the rearward cylinders basically can only get air from the far side from the air box. Take the Air box off and leave the air horns on and you should see your gains. ~Bob Edited July 2, 2009 by MAG58 grammar Quote Link to comment Share on other sites More sharing options...
burninator Posted July 2, 2009 Author Share Posted July 2, 2009 When I did the run only the plate with the air horns was attached. The rest of air box was still off the car. Quote Link to comment Share on other sites More sharing options...
johnc Posted July 2, 2009 Share Posted July 2, 2009 Try tuning the car with the air horns installed. Quote Link to comment Share on other sites More sharing options...
burninator Posted July 2, 2009 Author Share Posted July 2, 2009 I'm probly not the best SU tuner in the world, but during my tuning attempt it seems like the adjusters on the SUs make almost no difference in the air/fuel ratio at wide open throttle and the air/fuel ratio makes almost no difference in power. Maybe I just wasn't able to adjust it enough to notice a difference (I didn't even try running it if I though the mixture at idle was too lean). I was considering getting some new needles or nozzles to try, but over the next winter I plan on installing an L28 and using these carbs on it so I didn't want to bother with ordering parts and such for this engine just to run it for the summer. Quote Link to comment Share on other sites More sharing options...
MAG58 Posted July 3, 2009 Share Posted July 3, 2009 Ah, see when i first read this i originally though you had triple sidedrafts, I was obviously wrong. However, from the look of your AF ratio graph, I'd recommend trying some stiffer Dash-pot springs. This may even improve your mileage, as it holds your dash-pots down longer, but it should also be beneficial as the air-horns flow substantially more air than the stock box. Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 3, 2009 Share Posted July 3, 2009 This is the stock airbox backplate? Or Aftermarket backplate / horns? Quote Link to comment Share on other sites More sharing options...
Drax240z Posted July 3, 2009 Share Posted July 3, 2009 Did you just have 1 run to confirm these issues? could also be a heat soak issue as I see your later run was the one with the horns... Quote Link to comment Share on other sites More sharing options...
1 fast z Posted July 4, 2009 Share Posted July 4, 2009 Ok, its like saying. I turned my boost up from 10 lbs to 20 lbs. I thought more boost was supposed to make more power. Dont tune it for 10 lbs then turn up the boost to 20 lbs and expect to get more power! Tune it with air horns! It will make more power with air horns, even with the air box. Quote Link to comment Share on other sites More sharing options...
srgunz Posted July 4, 2009 Share Posted July 4, 2009 IMO. When posting a question like this you have to give people information to help you. Like what carburation you are using. What back plate. What air horns. Otherwise you just run people around in circles. I thought you may have tripples too. Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 5, 2009 Share Posted July 5, 2009 I gotta admit, when the phrase 'air horns' and 'air box' comes up, the LAST thing I think of is the Stock SU's. Then it's 'Air Cleaner Assembly' or there is a reference to removing the air cleaner assembly and substituting Aftermarket SU Tuned Ram Horns or some such nonsense. Heck, we haven't heard if it's the stock stuff he was running, or aftermarket... Quote Link to comment Share on other sites More sharing options...
burninator Posted July 6, 2009 Author Share Posted July 6, 2009 Sorry I guess looking back I should have made it more clear. I am running SUs, stock air box, and stock horns. The loss is backed up by multiple runs and is not due to heat soak. I appreciate all the advice, I am the one asking the question afterall, I just didn't think the AFR was different enough with or without the airbox to say a lack of tuning caused it (especially where the power dip occurs). I beleive I have the car as lean as it will go at my altitude (~5000 ft) without getting new needles and/or nozzles I guess I might have to get some to see enough difference in the air-fuel ratio to see a diffence in power. As far as stiffer springs in the dashpots, I was thought it was the other way around. Heavier springs may cause the needle to sit further in the the nozzle, but the air is moving faster and the venturi affect causes it to pull more fuel and actually richens it up. Thats what I read, and I have a few runs to back up this theory. I started out with no oil in the dampers and stock springs, now I'm acutally running it with no springs and ATF in the dampers. I did quite a bit of testing and it did improve the AFR. Thanks for the input. Quote Link to comment Share on other sites More sharing options...
MAG58 Posted July 6, 2009 Share Posted July 6, 2009 you're correct, my apologies. Been too long since i've tuned su's Quote Link to comment Share on other sites More sharing options...
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