stav2201 Posted February 5, 2012 Share Posted February 5, 2012 Ive decided to step away from my 83 turbo swap on my 1975 280z and move to carbs. Ive done some research on what Milimeter carbs to get and it seems for my needs 40mm would be the best choice. (I would like a decent power build that is also very reliable and somewhat gas efficient). I have my p90 head that is rebuilt that i would like to use but im afraid the lower compression would kill some power, or maybe i can switch to a L28 block with non dished pistons and a thinner head gasket? Im having a hard time trying to find a thread with a very thorough build information regarding this, could anyone direct me to a link? Quote Link to comment Share on other sites More sharing options...
Blue Posted February 5, 2012 Share Posted February 5, 2012 (edited) If you'renot planning on porting, putting a mid-range to top end cam ... that is "hot" or "hotter" .... than the tripple 40mm's will serve you well for a cruiser and a touch of scooting. If it were me ... i would acquire a good condition ... F54 block (flat top piston oem) ,,, with a p79 head. (F54/p79) head. Then i would proceed to install tripple 40 or 45mm webber carburetors .... buy the tuning tool ... unless someone has jetted them for a hot modified head/valvetrain ... they are pretty easy to tune/clean/modify ... but follow webbers instructions to the last period. Fuel pressure is critical (buy the correct pump) , run them before you rebuild them ( this way you can identify if something you did caused an issue ) ... keep them clean! !! .... i love the sidedraft design, roller bearing throttle shafts, way easy to open up and adjust/work on. Just follow the instructions, all of them , and you will be satisfied . This will give you factory 9:1 compression ... not to low to want more ... not high enough to have to turn the static ignition timing back to avoid pres ignition .... reliable, fun, and economical/somewhat ... but reliable .... add a late zx 5pd and 3.90 diff for some more fun hehe. Your current turbo head with dished piston block is not going to run the way you wish with the low compression ratio required ... in fact ... i would switch out the head or block Or raise the compression up somehow before attempting to run anything but a turbo at all. I currently run a 4bbl 390cfm Holley on my F54/p79 head ... i love it! One carb to tune .... always fires right up .... i have it dialed in per Holley specs, especially fuel pump sensitivity, brass floats adjusted correctly, correct (good) power valve, a slightly hotter accelerator pump cam, ngk plugs, secondarys begin to open around 2,700 rpm ... just a thought ... i don't understand why there is do much clack against these as i get great mileage with the 3.54 diff, and late zx 5spd ... Dual webber downdrafts should work just fine for economy. Roller bearings for throttle shafts and jetting kits/accelerator pump nozzles are readily available. Easy to work on! . Dual SU carbs ... good carbsThere is also the issue of the throttle shafts being worn (ztherapy or o-ring/teflon fix) that needs to be addressed .... and if there as a worn needle you might as well repair/rebuild them (its a good sign)... don't get me wrong i love them and run them on l24 and l28.... when they are good condition ... hard to beat for the application (if correct air flow application) Hope that helps! Edited February 8, 2012 by Blue Quote Link to comment Share on other sites More sharing options...
BluDestiny Posted February 5, 2012 Share Posted February 5, 2012 F54 flattop with a p79 is not 9:1 its more like 8.5 to 1. Which is a stock motor. You'd want to use en earlier cam if available to have a better powerband. Blue your assumption is wrong about the Dual SU's. They provide more than enough air. At the MSA show a few years back the one guy who was running tuned SU's put down more power than 90% of the guys with Triple webers. He made 197 to the wheels while most the guys with high comp cam'd webers were doing 170-180hp. He had a stock motor with a big cam. Quote Link to comment Share on other sites More sharing options...
Pyro Posted February 5, 2012 Share Posted February 5, 2012 Why not do the turbo conversion? The power from the turbo is worth the work. You could just turbo charge the 75 engine. The money you spend on carb, you could junk the stock efi and do a mega squirt system. Flat tops with a P79 is 8.8:1 cr. The stock piston sticks out of the block around 0.010". Most people forget to include this in the cr calculations. Quote Link to comment Share on other sites More sharing options...
stav2201 Posted February 5, 2012 Author Share Posted February 5, 2012 Why not do the turbo conversion? The power from the turbo is worth the work. You could just turbo charge the 75 engine. The money you spend on carb, you could junk the stock efi and do a mega squirt system. Flat tops with a P79 is 8.8:1 cr. The stock piston sticks out of the block around 0.010". Most people forget to include this in the cr calculations. Ive thought about it alot, i have mega squirt laying around in the garage but i dont have the time or patience for that. Every problem i have with my build so far is electrical. Im not concerned with putting down alot of power i would rather have the least amount of electrical possible. Im going for something very very dependable since i have to commute with my car as well. (Yes i know mega squirt is dependable and gas mileage/power is awesome, i just feel that it will take too long to tune and i will run into way more problems in the long run than i will with carbs), But that is just my opinion.... Quote Link to comment Share on other sites More sharing options...
stav2201 Posted February 5, 2012 Author Share Posted February 5, 2012 it seems like a f54 block with flat top pistons will work, i read that someone on here used a p90 head and a thinner head gasket to raise his compression ratio. I dont want to do an aftermarket cam so i can save money, but im worried that ill lose alot of power using the p90 instead of and n42 or n47. what would you recommend for decent power, reliability and lower chances of detonation? Quote Link to comment Share on other sites More sharing options...
Blue Posted February 7, 2012 Share Posted February 7, 2012 (edited) F54 flattop with a p79 is not 9:1 its more like 8.5 to 1. Which is a stock motor. You'd want to use en earlier cam if available to have a better powerband. Blue your assumption is wrong about the Dual SU's. They provide more than enough air. At the MSA show a few years back the one guy who was running tuned SU's put down more power than 90% of the guys with Triple webers. He made 197 to the wheels while most the guys with high comp cam'd webers were doing 170-180hp. He had a stock motor with a big cam. Alright then. Thankyou for your input. Would you please care to clarify who this "one guy" at the MSA show as i am not aware ... and exact engine specifications on what he was running? Exact engine specifications on the other "webber" set ups of "other guys" ? Stock motor with a big cam? ( no mention of engine ?) Dyno slip/sheet? hmmmmm i will be researching today Thankyou ... pm me the details if you wish to keep thread clean Edited February 7, 2012 by Blue Quote Link to comment Share on other sites More sharing options...
BluDestiny Posted February 7, 2012 Share Posted February 7, 2012 (edited) Not the person I was talking about, but here is another using SU's closing in on 200hp. I'm talking about Scott B. Comments have what he was running. This guys head was ported, but the SU's clearly allow enough air flow if the head can flow it. When I say other guys I'm talking about people slapping together a webber setup, a big cam and mess with the timing till it runs "good" as they say. General assumption so you got me there, although there are plenty of MSA dyno vids showing a weber setups making disappointing numbers I'm actually not sure if Scott is a member on this site. This is what a completely stock engine with a set of triples put down. Sorry to use your vid black240z, And this was when one of his carbs had a bent throttle plate. There is also a video on youtube of someone with a stroker that has webers making 180hp Edited February 7, 2012 by BluDestiny Quote Link to comment Share on other sites More sharing options...
cockerstar Posted February 7, 2012 Share Posted February 7, 2012 Dont worry about loosing power with the p90 head. It will give the same compression ratio as the p79. Flat top f54 block Felpro head gasket P90 head Pick a cam from MSA (most people under cam big time) Headers/exhaust Properly jetted and tuned 40mm webers or SU carbs with bigger needles The above combo will give you something that's extremely simple to work on and be pretty reliable. I'd expect around 150hp from it, which will still let you have some fun around town Quote Link to comment Share on other sites More sharing options...
Blue Posted February 8, 2012 Share Posted February 8, 2012 (edited) Thankyou BluDestiny ... I see the direction.... I am a stickler for specs . Please disregard my statement about SU carbs earlier original poster. I have since edited my post. Edited February 8, 2012 by Blue Quote Link to comment Share on other sites More sharing options...
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