zredbaron

i couldn't agree more! this thread has been the main source of direction for my engine, and i'm quite pleased with its evolution! thanks again to all who contributed. and by that i mean i now curse my 'tiny' tires... update-- i finally purchased some of that vp race gas, and man... it makes more of a difference than i had hoped for. i am getting a dramatically more complete (and faster) burn, and it hasn't even been tuned! not at all! i couldn't lay down the power anywhere, which was frustrating, but yet satisfying at the same time. that is, after all, proof that all of the time and money put into my engine has paid off! i plan on getting to a dyno sometime in december hopefully. i'll report the data when i've got it, but in the meantime, here's a video from today's race: (note the watch in high quality link) as you may or may not be able to make out, i'm having a hard time with the power, both in terms of keeping the rear end hooked up and taming the beast. i'm really, really having a hard time being smooth. manual steering is a whole lot more varsity now. question for you racers out there... the pulsing of rpms that you hear when i'm under load in a turn and going over a bump (ie the tires beginning to break loose and me having to be ginger with it)... to me, i think its the result of perhaps not the stiffest suspension (225# springs over illuminas) combined with my quaiffe hunting for where to put the power. in my case, is the quaiffe doing me more harm than good, or is this just the challenge of having more power than you can use? (if in the case the answer isn't so simple, i'll shift this to a new thread) thanks!

after writing this, i was thinking that i thought it was between the f16 and the f2, not the f11. here's the table that i was recalling: Cylinder capacity.......................Suggested tube 250-325 .................................F11 275-400 (L24) ........................F15 350-475 (L24, L26, L28) ..........F9, F16 450-575 (L28, 3.1 stroker) .......F2 [source is NOT the weber book, but this common guide: http://members.aol.com/dvandrews/webers.htm] so instantly, i'm thinking oh, man am i running the f11 still? sure am! with a 3.1, i couldn't be any farther off! i've never touched the emulsion tubes because most sources say that its rare that you need to change it. again, i don't know for sure yet, but i find this promising. i'll be ordering some f2s soon and i'll report my findings. (anyone try the f2?) "The emulsion tube controls when the air correction jet becomes significant and the rate at which its significance increases." (weber book) unfortunately, the weber definition above sounds to me like a higher rpm influence rather than a lower rpm influence that might be responsible for our studder. in short, i'm still guessing like everyone else...

your jets seem like reasonable starting points. you might *try* running a slightly leaner idle jet (smaller). that may or may not help your studder. again, experimentation is ALWAYS in order. if i had to offer a wild guess, i'd say try 135 mains and 175 airs and see how it likes that. but again, its a WAG. as for your 1.5 turns, did you tighten them or loosen them? that's a pretty big change. usually 1/4 turn can noticeably affect the idle. (also, have you counted the number of turns it takes to tighten them to the bottom seat, and averaging this number and backing them all out to be the same? this *could* be the cause of your fouled plugs) no need to apologize for being new. unfortunately, i don't see anything that blatantly jumps out (maybe someone else does?), so it seems youll just have to start experimenting! if you buy one size smaller and one size larger of each jet, then you can have yourself a good tuning experiment. you might be able to benefit from bumping up your venturis (aka chokes) up to a 30 or 32, but doing so would require new idles, mains and airs. i'd probably say the biggest complaint/trend that i've heard across the board is studder/hesitation until about 2500-3500 rpm or so (variance depends on cam i suspect). i have this hesitation, too. throughout my car's history, below that magic rpm i have to throttle the carbs just right (ie dont floor it, but finesse it) to get the best accelleration. i have two theories for this, and i intend to test both via my butt dyno and a chassis dyno within the next 4-6 weeks. #1: vacuum advance and #2: emulsion tubes. 1) i don't know about you or anyone else that complains of this hesitation, but my iginition does not yet incorporate a vacuum advance. optimal driveability and power is obtained by varying advance based on engine load (measured by manifold vacuum). most cars accomplish this by a vacuum line running to the distributor. i don't yet have a MAP sensor for my direct ignition. its possible that timing has been partially responsible for my hesitation. 2) based on reading about our dcoes, i've always thought that our application is been on the fence between the f11 and f2 emulsion tubes, and that perhaps the higher performance L6s could benefit from the switch to the f2. like i said, i'm going to test these both very soon, but i'd love to hear anyone's thoughts or experience. anyone have any input?

akumazeto, no one has replied yet because you aren't helping us help you. what kind of motor do you have? displacement? cam? mods? application? which carbs!?! venturi size? if you have read this thread at all, you would already know that you can't be advised without this information. second, either this guy is confusing your carb with another or you didn't interpret his instructions correctly. the main and airs *might* be a reasonable starting place *if* we knew your application. however, the other two don't exist in our weber dcoes. this *is* the kind of carb you're using, right? the only thing i can think of by 'accell' is that he means the pump jet, but those are usually around the 40 range. not 140... i have no idea what he wants or means by 'bleed' so again, you have no responses because no one has any idea how to help you. you might as well have asked an art instructor why you cant take the square root of a word. it just invokes the word "HUH!?" ... which is the response the shop gave you when you asked for these jets. however, if you asked for 50f9 idle jets and they don't know what that is, then call a different shop. i've got some in my tackle box. so by all means, get back to us with your application, and you'll likely get some advice.

well, i do have this collection in any reasonable sense. the problem is upgrades. if you follow the mains alone, the stock L24 used 125s i think, my poorly running 3.1 used 135-140s, my new head and high perf cam used 155 or so, and my forged pistons/race compression/race cam uses 160-165. that's four different rounds. not to mention i've gone through 3 rounds of venturis. shelling out $150 for tuning is very, very cheap horsepower. negligible by comparison of the upgrades that took you there. your ITB setup will cost $5k. $3k for the ITBs, and $2k for a computer to control it. if you want cheap horsepower, turbo or small block are definitely the way to go. also, i seem to be maximizing the performance (ie at the upper flow limit) of the 40dcoe with 36mm venturis, and i have found curious results. i want to prove this at a dyno shop sometime, but i'm running 160-165 mains and 185-190 air correctors. not the usual difference. this oddity has caused me to unknowingly purchase in the wrong direction. fuel selection also matters. i'm running 100 octane and i'm about to switch to 109. tempurature, elevation (specifically barometric pressure), humidity, fuel selection and timing all play significant roles in your mixture and subsequent burn rates. lazeum, you have to physically remove the venturis to see the number stamped on them. it's very easy to do (just look in the dcoe section in your book). i wouldn't go any further until you find out what youre starting with. a wideband is a very useful tool. i use one myself.

yeah its not cheap. as much as it pains me to admit it, i'd say after venturis and jets, i've probably invested a little over $1000. again, this is after 10 yrs and about 3 separate rounds of engine upgrades. each round of upgrades results in the need for a new set of jets to play with, +/- an adjustment on each (idle, main, air corrector). if i ever do go to the 45dcoes, i'll likely have to rinse and repeat a few more times. looking forward to it.

http://www.teglerizer.com/dcoe/rasorcom.htm i just re-found this link. lots of great diagrams that the weber book doesnt have. this explains the idle circuit, and looking at the pics, looks like the fuel ALWAYS goes through the idle jets, even when the throttle plates are closed. i thought it had two separate cored passageways. a picture is worth a thousand words!

I had assumed that by "choke fully on" you were mis-speaking and really meant the throttle plates were wide open (which would break your engine if not under load). my mistake. as for the HPBooks Weber book (published 1988) at a minimum, read pages 34-43, 99. lots of good info in 115-135 (modifications chapter). big picture concepts help you realize what you're asking your engine to do and why. there's more to it than simply increase jet size to richen. you can cause fuel leaks by overtightening very easily because of what is in between your carbs and your manifold. each manifold runner has a piece of plastic sandwiched between two rubber o-rings. thats it. if you tighten too hard, you will damage the o-rings or crack the plastic or both. also, don't replace those o-rings with any old o-rings. they are specifically designed to handle fuel and not deteriorate. the design is for this 'gasket' to absorb vibrations AND keep a good seal in both low temps (winter startup) and high heat (fully warmed up). its a very good design, but not foolproof. with regard to the idle jets, the weber book details the following. you are correct, they do not affect idle mixture as the name would indicate. the mixture screws do. thats what the books says. however, the book also says that fuel doesn't go through the main circuit until above a certain rpm. so, until then, fuel is drawn through some very small cored passageways that cannot be adjusted, but DO need the throttle plate to be in the right spot. the adjustment, is the needle at the bottom of the mixture screw, which determines how much fuel is allowed to spray into the carb with the throttle plates closed. thats the theory. the fact of the matter is, in practice, if i tune my car to idle smoothly then change my idle jets, i will absolutely have to re-tune my mixture screws to find out where the engine wants it. the reason is, the throttle plates arent completely closed. they're ALMOST closed! when they are closed, its all mixture screws and cored passageways. however, when you use the idle screws to set the right rpm (900-1000 with a decent street cam, don't remember stock sorry), you are opening the throttle plates a very slight amount, and this is why the idle circuit is in use, which is why it affects idle. fuel is being drawn through BOTH the cored passageways/mixture screws AND the idle jets. the mixture screws then allow you to refine where your idle jets put you. make sense? oh, i get it. you literally mean the chokes used for richening to warm up the car. i forgot we even have those! lol. again, this is why i hate calling venturis chokes. they're venturis! don't ever use the chokes when tuning your car. the chokes have their own circuit entirely, as the weber book refers to them as a "carburetor within a carburator." they are disconnected on my car, and are always shut via their spring. for street use, it's a good idea to use them to warm up the car. a very good idea. yes, the mixture screws have screws and are fairly close to the manifold. one per barrel / cylinder. you'll know youve got the right ones if, while the engine is at idle, if you screw them all the way in, the engine dies. you want to screw them all in to the bottom, then back them out exactly 1.5 turns. thats a starting point. increase/decrease by 1/2 turns and ultimately 1/4 turns to figure out what the engine likes the most (highest rpm and smoothest). then back down the idle speed screws to get the right rpm. the idle speed screws are one per linkage arm (therefore one per carb). located on the side of the carb. don't ever tune your engine until it has fully reached normal operating temperature. likewise, don't tune it if the engine is hotter than normal, either (carb/manifold temp... as somewhat indicated by water temp). it matters. also, taking a look at your jet sizes again, your mains might be too small for use with the 36mm chokes (again, i recommend 32 for you). as a *starting point* the rule of thumb for mains are venturi size x 4. in this case, 36x4= 144, so perhaps 140 or 145 mains would be better. more air from the venturis means more fuel needed. problem is, your head/cam can't flow that much volume, so you can't use all that it's trying to send. 140 is probably a good starter. that said, *again*, i'd like to see you with 32mm venturis, which would be about 130 mains for starters. understand that the 140 estimate is to match the venturi, not your engine. it's helping it run better, not best! air correctors rule of thumb is mains + 60, but i find this a bit too much for our cars. i've also found that the more upgrades i do, the smaller the difference between mains and airs gets. i bet 185-190 would work for starters. i use increments of 10 when messing with air correctors (5 for mains of course). air correctors don't kick in until above 4000 rpm. or so. ish. (depends on engine and other things i know nothing about.) what jets do you have avail? i have a large set, and after my upgrades, many are no longer useable for me. if you or anyone is interested, send me a private message so i don't hijack the intent of this thread. and you're quite welcome, lazeum. it's taken me 10 years to feel like i at least have an idea of what i'm doing with the webers. i have yet to meet an expert and watch what they do. it truly is a lost art, so stabbing around in the dark eventually yields a limited amount of experience, especially when you go back to re-read chapters you've already read. *cough!*

lots of questions, many of which are incomplete. the term ticking has to be defined. you have to *find* where its coming from. is something loose and rattling? is the engine knocking (which can sound like loose marbles under load)? in the case of knocking, try retarding your ignition a little to see if the noise goes away. i doubt that your compression is high enough to warrant higher octane than 91. are you using 87? 55 idles are too big for you. i have a 3.1 with a full race cam that idles at 1500 and i use 55-60 depending on the season. richening up the idle circuit (no such thing as the transition circuit), may feel smoother up to a point, but it will actually make less power/torque. i find that in my car, the pedal becomes overly sensitive when the circuit is just right. for you, i would think 45-50 depending on the time of year. to test out a good starting point for driving, try turning all of your mixture screws all the way in, then backing them out 1 and 1/2 turns. then richen or lean all of them by 1/2 turn increments. if, when you go richer, the engine idles at a higher rpm, then you *might* want to put the next size idles in (i say might because you *might* be on the fence). and vice versa. this is a starting point, so you must road test with various throttle positions in the 1000-2250 rpm range. (after this rpm the idle circuit is no longer in use) your venturis are too big, too. your car is having a hard time at low rpm because your engine simply isn't drawing in enough air until higher rpms. i would recommend 32 mm for you, based on the charts in my weber book. yes, you WILL make more power (wider, stronger, smoother power band) if you put in a smaller venturi for your application. if and when you spend A LOT of money on a VERY GOOD head job and cam, then i'd go with 34s, maybe 36s depending on how your car is used (34 for street use). save the 36s for the stroker. you have to have the chokes fully open to idle at 1000? if that were true, then you couldnt accellerate because the pedal was already floored. are you sure that you've found the idle speed screws? they will be immediately to the side of where your linkage attaches to your carbs. you have to start all of this KNOWING that the carbs are synched on the linkage side. if they are not, then you are wasting your time. get a good flowmeter (like the one top end performance sells) for about $40. back ALL of your idle speed screws out, then hold your pedal at a constant 2500-3000 rpm. then adjust your linkage until they are all matched, and tighten them. MEASURE THEM AFTER YOUVE TIGHTENED, as sometimes it gets thrown off. be precise. then, you can tinker with the jets, and mixture and idle screws. to be frank, you haven't read much about webers at all. perhaps you have been reading peoples opinions. what you need is the book called Weber Carburetors (HPBooks). I found mine at a local barnes and noble. that will detail the relationships between jets, circuits, and will detail how to tune them. one last thing, don't over tighten the bolts between your manifold and carbs, or you will get fuel leaks. they might seem loose, but the washer is actually a spring. it isn't meant to be tightened until it is completely flush. tighten them until the spring washer collapses just a tad more than half way. good luck!

i'm skeptical about the claim, too. i have no doubt that race gas will outperform the AvGas (the race gas is oxygenated, and the flame front of the race gas moves faster --> more complete burn), but a number like 6-7% sounds outlandish. i'd buy a claim of 10 hp. (then again, 6% is about 15hp assuming a base of 250hp) the thing that i'm banking on, is the driveablity factor. i'm willing to bet that avgas feels sluggish by comparison, especially in the less than WOT positions. as for testing it out, i couldn't just swap fuels and note the power difference. i mean, i suppose i could, but it wouldn't be the test results we really want. the car will want bigger jets, no doubt (oxygenation, plus the fuel expert said to run it on the richer side of the stoichiometric range of my a/f ratio). it might even want a tad more advance. unfortunately a truly accurate test would require me to truly dial in avgas on the dyno, then swap fuels, then dial in the race gas on the dyno, all on the same day. that would be at least one extra hour of dyno time, and at 100-150 an hour that just isn't quite worth it. what i was planning on doing, was swapping to race gas and doing my butt dyno tune session on the same day. it won't have numbers, but if it truly were 6-7% i would feel a *significant* performance jump. if it simply *ran better*, then the ~10hp + driveability factor theory would be more accurate. unless you or anyone else has any ideas how this could be tested?

very good, will do! just wanted an affirmation from someone who knows our L6s. thanks, john. i'll report my findings, but that won't be for a while yet. still have a few gals of 100LL to burn through...

a reasonable idea. i gave VP Racing Fuels a call, and spoke to their resident fuel selection expert. he wants to see me run their Motorsport 109 unleaded fuel, which is highly oxygenated (and would likely help my under-carb'd issue). he basically guaranteed a 6-7% gain at the wheels from avgas. quite a claim!

thanks for the info, palmettoz. and welcome to hybridz! well, the car is 100% up and running. here are some more pics... here's my makeshift heatshield. the other side of it is asbestos, which supposedly has very impressive heat shield qualities. here's hoping i don't get header cancer. you can also see the steel tubing and fittings that i routed between my two manifold vacuum draw points. the idea was to pull symmetrically from the two points, even though all cyls are connected. worst case scenario, it doesn't matter at all, but it looks better than just plugging one! had a chance to clean up the carbs. they look almost new again. (they're actually 10 years old with > 20,000 miles!) after this part of the assembly, i broke in the cam and sync'd my carbs with my flow meter. i of course just *had* to take it for a gentle spin around the block, and man... i've never heard such a beautiful sound. no, seriously. i'm considering taking off my air box and taking my hood off permanently, and just racing around with screens on the air horns. the beautiful sound went away when i put the air box and hood on... i had a local machine shop weld on a new plate on my strut brace that i could bolt up to my camber plates. they had to do some custom machine shop magic so i could get my socket over the nut. they did a damn good job, i'd say... the final product from the driver's side. and also from the passenger side. i did some carb tuning and found some rather curious results. according to my weber book, 36mm venturis are a tad small (should be 38-40), so my 40 dcoe's leave the engine a little under-carb'd. in the end, the car wanted more gas and less air. 160 mains and 180 air correctors. i know, it sounds crazy (40-50 above main is nominal starting place for air correctors), but i *swear* the car liked it! and the a/f ratio agreed. my guess is that the cam was sucking a lot more air than before, and this crazy jet combo happened to feed the 'small' venturis better? then again, no doubt the 'small' venturis are responsible for my incredible torque snap / instantaneous throttle response. still can't figure out the idle circuit though. the car has *always* just choked if i ever suddenly floor it below 3000 rpm. anything below and i have to feather the throttle. it's still torquey as hell... the smallest pedal touch jerks the car forward, but still, i would think i could floor it. pump jet perhaps? regardless, the results are promising so early on. it pulls hard up to 6500-7000 or so (i thought the power was going to last longer), but i also didn't expect it to still have so much torque down low. very good autox motor. looking forward to a dyno tune. as for timing, i have it linearly raising up to about 32-33 deg or so according to my timing control knob. does that sound about right for my setup? all of this was tuned with my calibrated butt and wind-noise dyno. it's guessing 254 rwhp. just a WAG. hey, i gotta throw out some kind of prediction before i go to a real dyno! hah. here's a very disappointing video of my autocross this past sunday, but at least it's something. my dumbass only recorded one run, and i zoomed in on my dash, so you really can't see the course very well. to top it all off, there was a crosswind, so when the car is driving toward the water, all you hear is wind. i'll get a better one eventually. here it is: http://www.youtube.com/watch?v=ElwuZNmNGf4 in other news, i've been reading a lot about fuel online. from what i read, AvGas is a really bad idea in terms of automotive performance. yes, it resists knock, and that's what the 100 octane is providing, but it's engineered for a different purpose entirely. it's engineered to be run in high compression engines that run at low rpm (~3000rpm) and *at altitude* (ie very thin air). it isn't designed for high rpm (and therefore doesn't have a very fast flame front propagation) and doesn't have the same energy content (BTU or calories per gallon) as automotive fuel, more specifically racing gas. i've read a lot about mixing your own with pump gas, even from using your own brew such as mixing with toluene (117 octane). although better than avgas, it doesn't always mix linearly like other fuels. also, there's a recommended limit total present in your brew (arguably around 40% of so-called aromatic hydrocarbons such as toluene and xylene) when mixing. unfortunately there's no way to know how much your pump gas has already, so you may unknowingly create an octane lower than you've calculated. (that's why it doesn't always mix linearly, because the fuel your using might have more aromatic hydrocarbons than you know -- it reaches a point where adding more doesn't improve as much). the better octane references i found: http://www.offroaders.com/tech/octane.htm http://www.elektro.com/~audi/audi/toluene.html http://www.idavette.net/hib/fuel/page2.htm the bottom line is, again, you aren't going to out-engineer the engineers. take the race gas for what it's worth. i may only need 95 octane to prevent knocking. i *may* make more power with 100, but there's no way to tell unless i experiment on a dyno with real, no BS race gas. i really want to dial in my jetting and timing on the dyno, too, but it would be prudent to wait until i use up the last of my avgas and move on to a better fuel. so, for the next topic in this very thorough thread, what have the racers out there experimented with? what do our L6s like when running 12:1 compression via the 3.1 stroker?

yeah, i had noticed that the machine shop had scratched the skirts. i wasn't very happy about that, but their reliefs were good, and some 00 steel wool did the trick. well, i've got the block back, but it never goes as smoothly as you think it will, does it? perhaps when i'm a little more seasoned i won't be quite so excited (and therefore more patient) and tell myself hey, i got it all here. 2 weeks, not 2 days. anyways, here are some more pics. note the small amount of water at the bottom of the bag. draped in plastic and an oversize tarp, and water STILL found its way inside! damn hurricane... gettin' ready to go in. and damn, can i say that i had one *hell* of a time mating the tranny and engine together far enough to get the tranny bolts threaded. whew. evidently i really suck at that part. so close, yet so far... really took my time wrapping the headers and customizing my spark plug wires. hah. they're thicker than my batter terminal cables btw. go figure. in other news i spent an embarrassing amount of time on a project that i *thought* was a great idea. hah. so, i have been pulling vacuum off of two cylinders, and i thought hey, why not all six and share the load and stop messing up the mixture for those cyls? so i went to about 10 hardware specialty stores and finally ended up ordering some fittings (they would have looked really sharp i must say), brought my intake to a machine shop, and was ready to go. i went home tonight to take the carbs off of the manifold and... doh. what i *thought* was structural support between the manifold runners was in fact *also* a hollow tube that connected all manifold runners. so basically, porting vacuum off of one cyl was drawing vacuum from all six after all. man, i'm dumb. it *would* have been a good idea though, right?!?

update time again. well, with any luck my new engine will come to life this weekend! then again, i've thought that for a few weeks now... regardless, here's a couple more pics of the new parts. i didn't get a pic of the .6mm gasket, but i couldn't believe how thin it was. like 5 pcs of paper i'd say. i was also really impressed with the damper from rebello racing. to my suprprise, they also seem to be proponents of electromotive ignition, so i went ahead with their magnetic wheel that they've designed to go with this damper. here they are: as expected, i was also impressed with the je forged pistons' craftmanship. as per the sideskirt coating discussion, they came with a wear-in texture on the side thats specifically designed either for coating or for wearing gently in with the piston cyl walls. the pictures don't show it, but it was some seriously precision machining: i ended up not going with the sideskirt coating simply because it was a tight enough fit already that any coating would require the pistons be slightly reduced in diameter to allow for the coating. i decided that perhaps a future rebuild when the walls are honed again or the pistons have worn a little. if they do...? and of course, here are the valve reliefs. they weren't as deep as i thought they'd be with that cam i've got. ...which were one of the reasons for the delays. they WEREN'T deep enough. my machine shop clayed the reliefs and turned it over and basically the clearance was ridiculously tight. calculation error prob from the combination of the head being shaved at some point and that crazy .6mm gasket. we went ahead and had another shop cut them .080" deeper. now they really do look like they're going in a race car... my master mechanic buddy thinks i'm going to see a 100 rwhp gain. me, i think he's got a rather lofty number in his head, but hey. we'll see. 12:1 compression and an aggressive cam certainly won't hurt. regardless of the numbers, it's going to be a little bit on the nasty side and i'm anticipating it's going to be REALLY hard to tame on 225/50R16 DOTs. and therefore insanely fun. i'll try and post a video or two on youtube next week if all goes according to plan. hopefully one idling and one racing this sunday!

first, that book is for all out race cars. it recommends things like tube framing, tack welding your diff, etc etc. your table says *road racing.* sounds to me like you drive your car on the street (since you are "breaking in" your engine). am i right? if so, you DEFINITELY want to undersize peak hp venturi calculations, whether using the hot rod your datsun book or the weber book. your fuel economy and driveability depend on it. i think jon mortenson is right, and i'll expand on what he's talking about. running a carb without a venturi is a very uncomfortable idea for me. yes, you are somewhat correct in that the piston's downward motion 'sucks' the fuel and air in. (technically, the lower pressure in the cylinder allows atmospheric air to PUSH through the carb). as this air passes through the venturi, the venturi speeds up this air (since the diameter tapers). this causes a localized low pressure to be felt, which allows fuel from the carb's float to be pushed (or sucked if you prefer that concept) through the main fuel circuit (ie jets etc) and mix with the air flowing through the venturi. this mixture is immediately passed into your manifold runners. also note that the 'no venturi' line is under the 4 cylinder section and not the 6 cylinder section in your image. very odd. if you remove the venturi, you are using the carb in an unintended (and un-engineered) manner. the phrase "you aren't going to out-engineer the engineer" comes to mind. with no venturi, you are using the engine vacuum to draw in fuel, vice the incoming air doing the work via the venturi. not only that, in my weber dcoes, the walls are somewhat sharp and have jagged little niches, which are completely smoothed out by the venturi (again, by design!). turbulent air won't flow as well, which inhibits how much fuel/air is in the engine, which inhibits power production. (you want your head smooth, don't you? same concept!) i don't see how it could be remotely efficient in any way. that said, as you mentioned you can try it without consequence. the worst that could happen is it won't start or would run like crap. i predict it would start, though it may be hard to find the sweet spot with the pedal. it's *possible* it may even make more power at wide open throttle (if your current venturis are way too small), but i also predict that mid-throttle positions would be poor, and transitions from the idle circuit to the main circuit would be very very poor. it would likely be very very rough. also of significance is how you intend to race, if that is indeed what you're doing. if your racing is high rpm wide open throttle (drag racing, most road racing), then yes, follow the book and go for max hp. however, if you plan on using different pedal positions more than WOT, you should use a slightly smaller venturi in favor of power band / driveability. for instance, my 40mm dcoes are limited to 36mm venturis which is significantly inhibited according to the weber book formula, but my application is autocross. i use 2000 to 7500 rpms and i need power there at all times. sure, my peak hp is a little lower than otherwise, but my low end torque and throttle response are extremely stout. i rarely get to use full pedal. i've found smaller venturis easier to tune. they're just more responsive to changes, whether from jets or from your foot. and yes, i do think 36mm are too small for me, even for my application. i'd experiment with 38 and 40, but hey, 36 is what i've got for now. perhaps i'll give rebello a call for these 38s, but i don't see how its possible in the 40dcoe. the 36s are extremely thin.

i took a look at my nissan catalog, and i have the price of $1800 penciled in the margin next to the 3.1L kit, not 3000. now that i've paid for the replacement parts, i'd say that was a damn fine deal since it included rods, pins, bearings, rings and the LD28 crank. and you guys are right... i'd say i got my money's worth. quick update: the block was honed and is ok. so that's good. all parts have been received. i ended up with a set of custom forged JE pistons spec'd by dave rebello. i intended to work with sunbelt, but i have to say that i've been quite frustrated with them/jim. ordering the cam was extremely painful and i never once was called back when promised (well over a dozen times i'd say). dave rebello on the other hand was very prompt and to be honest seemed more active/experienced with our L6s. anyways, i was thinking about sending the pistons out to get a teflon coating on the sideskirts of the pistons. anyone have experience with this? opinions? regardless, i have a new competition dampener as well. obviously everything else is being replaced: coated bearings, new valves, springs, the cam of course, timing chain etc etc. going to 12:1 compression. side note, evidently the 1mm gasket is unavailable. (according to rebello, anyway). i'll be using a .6mm gasket. (!) hah. got a good kick out of this. [case in point...]

absolutely. pretty much any upgrade(s) will show 'decent gains.' its all relative anyway! obviously headwork will show a larger gain than exhaust, but as long as you realize the relationships and how upgrading one area of an engine inevitable shifts your weakest link to another area of the engine, then by all means go for it! your questions seem to have gone away from the scope of this thread, which obviously is intended to discuss triple carbs only. i'd recommend you do some reading and searching, there are a LOT of threads with the exact same questions asked. if you still feel a little unsure, start a thread in the L6 forum if you feel you need to. that said, as per your hot motor to swap out between a few cars, i agree with jon. your proposal of head work and carbs should do just fine for what seem to be your intentions. don't forget to open up your exhaust a little to keep up with your head/cam/intake. i personally like headers with a magnaflow universal 2.5" muffler, but hey, that's me. good luck out there, i hope you get what you're hoping for. [again something that doesn't seem to exist. damn pursuit is so damn addictive though... hah.]

no such thing...

yes, upgrading carbs can produce more power than stock intake. [assuming of course that you tune them!] if they are too far out of tune, your power can DROP. problem is, if you have too much carb and not enough engine, your driveability will suffer. sure, your peak hp might be a tad higher, but your accelleration (ie power band) might be worse. with a stock setup, 45mm is too big. also be aware that the venturis inside the carb matter more than the external number (40 or 45mm etc). the external number determines what range of venturis will fit, and what shape they are (with the weber dcoe, anyway). as for your other questions, you can do whatever you want. the question is, what makes the most sense for you personally, and what is the most bang for the buck? we can't advise you unless we know your intentions for the car (racing? cool street car?) and your budget. everyone wants more power, but to what extent and how much are you willing to pay for it? NA power is raw and very appealing, but it's also very expensive...

oh, man. reminds me of an electrical fire i had in a '71. never did learn what started it. melted harness bad. and uh... seriously... why the *hell* do you have two bumpers!? doubling up doesn't give you any more protection! haha.

great points. i admit i think i'm being a little harsh. a kit is by definition designed to work with itself, not to be customized further. and it did have over 10k on it. used to be my daily driver. i'll post my findings on pistons. thanks. any other advice out there? i can't wait to hear it either. here's how it sounded about 6 mths before being disemboweled. highlight is at about halfway through. http://www.markhaag.com/media/cold%20start.mp3

power? well if you follow the history, i didn't make much at all for years. over the last year or two though, its realized much more, about 225 rwhp until it started acting up a few months ago. the valve tapping cant happen suddenly (can it?), so i'm assuming it's been that way (perhaps even leaking) since the head was installed. the miles finally did the cyl. wall in. that's my theory, anyway. that said, it was almost always the fastest straight line car at my autox events (only about 50 cars). only the Z06 that showed up occasionally could hang. ...i thought we ALL had the fastest Z? as for cheap piston package, this is horse crap. i can't disagree with your assesment at this early stage... but this kit was bought from Nissan Competition directly, back when it was still known as Nissan Competition. one would think that a $3000 kit from Nissan Competition would *NOT* be cheap. thanks for the info on rings. the more info the merrier.

good info. i think i'll stick to the tried and true re: johnc. 3" pipe after the 2.5" collectors sounds good. probably sounds good, too. whee. everybody likes pics, right? not good, but good. the cam from sunbelt arrived. man, that thing's rough: can't wait to hear it turn over. [and burn my starter motor out!] pulled the engine... i think i found the problem! hah. fred flinstone racing anyone? been setting up shop here the past few days. just had to share this setup i've got with my trailer. and yes... thats an evaporative cooler at the back of the trailer behind my engine stand! hot and humid. or dark and raining... i can be out there if i want to be! opened up the engine and took a look see. several problems... to start with, this is the intake valve that failed the leakdown test miserably (#6). you can make out that half of it isn't seating properly. cyl. #5 failed the leakdown test too, only this one leaked past the piston rings. pulled the piston and found this: one of the rings sheared! still havent found the missing piece. i went digging around in my oil pan for it and found... A SPRING!!??!?! my only guess is that its from the check valve in the oil pump but who knows? the damn thing is like 2.5" long and .5" in diameter. HUGE metal piece to be just hanging out in my friggin oil pan! holy hell, batman. so yeah, it only gets worse! back to the block, the #5 cyl. wall is badly scratched. not sure if honing the wall will be good enough. i'm dreading the machine shop will utter the word bore. sigh. as i'm sure you noticed, my valves have been tapping the pistons as well. it's a damn wonder that i've been RACING on an engine that is in this condition... as you can see, all of the cyls have been hitting. so now i'm wondering if this is the kind of power i've been making with an extremely overworked engine, what the HELL is it going to be like when its tight, let alone tuned, let alone upgraded?!? too bad its going to cost so damn much. not good. but good. good to find the problem(s). so, on to the solutions. obviously i'm ordering a complete set of valves since these are all bent slightly. check. as for the pistons, one of them at a minimum is getting tossed. it has scratched grooves going down both sides. additionally, the pistons (part of the nissan 3.1 kit) seem cast aluminum, and isnt all that thick. if i cut the calculated .170 out, thats uncomfortably thin. so now i'm thinking i need a whole new set of pistons. can pre-cut pistons be ordered for our L6s? if so, where? forged, of course. the plan is to take all the parts to a machine shop, assuming they say honing the cyl wall is good to go and that boring isnt necessary.... any advice on how i should proceed would be greatly appreciated... thanks.

thanks for the info, john. i plan on investing in exhaust next season. one question: why not take the two 2.5" collectors and run two 2.5" pipes all the way back? i know there isn't much room, but it looks like it can almost be done. ever tried this or know anyone who has? ...or does the straight six run better with a single pipe due to the pressure waves from each combustion helping to pull the next exhaust stroke gases more efficiently (waves are tighter)? that probably could have been worded better. anyone know the term for this effect? with regard to the valve clearances, jim advised the following minimum valve clearances: .070 - .080 for intake, .090 for exhaust. his reason for the exhaust clearance needing to be higher was when you remove load (ie lift or push in the clutch) at higher rpms. he said the engine trying to spool down would have a transient that would lag slightly and that the exhaust valve would be most likely to float a little. perhaps the high temp / elongation theory is a different explanation for the same phenomenon of the exhaust valve needing more clearance. i'd have to say i find the valve floating theory a little more plausible; if the exhaust valve elongated enough to cause clearance issues, i imagine it would also cause issues with not getting a good seal when closed. but hey, that's me and i honestly have no idea what i'm talking about. i just look like i do on TV.