zredbaron

The accel jet is mechanical, therefore spurts metered fuel (based on the throttle change) regardless of rpm. As such, this will have more (or less) of an effect depending on the RPM you are spurting fuel. Tuning carbs is more of an art than a science (the latter only providing a starting point for the former), meaning that "feeling out" the difference is worth more than a calculation or WAG based on a split-second AFR change. Personally, I think there is too much lag / reading error (human and sensor) associated with "instantaneous" AFR changes and readings to quantifiably estimate a jet change. In my personal opinion, the AFR is only a useful tool for constant pedal positions that have been held for at least a second or two (whether idle, half throttle or WOT). In constant pedal conditions, it's a very nice tool indeed! (In your case it might be telling you that you're going rich, but that's all you need to know.) (Again, I test the idle jet by blipping the throttle at lower RPMs and seeing which *feels* better. Who cares what the gauge says, I want the engine to respond to the pedal the way I intend it to. But hey, that's me. Technique only.) Bottom line, change out the accel jet and see which one you like better! Test, test, test!

Agreed, AFR and dyno are critical! There are several variables here, fluid density (air being less dense at higher altitudes) and speed (grossly metered by venturi size). The combination of the two dictate both the volume of air mass ingested by the motor and also how laminar or turbulent the air/fuel mixture is as it enters the intake ports. Even worse, if we increase a volume of air that is less dense... are we getting more or less O2? Depends! Enter the AFR. Testing is worth 5x as much as theorizing. Laminar flow is more dense. With a slight amount of turbulence (particularly on the edges of the runners) flow increases slightly but then quickly drops off and diminishes flow. This is a critical step of engine buidling; ensuring the head work / cam selection is also paired with intake runner shapes, lengths, diameters, etc. That's beyond our Weber thread, but it is a significant factor to how they perform. The engine is a complete system. Changing one variable requires other variables to be changed, too. Unfortunately for simplicity's sake, air is compressible, so changing areas and shapes have more complicated effects than our generalizations that "area goes up, speed goes down." Our generalizations should also account for the fact that as compressible fluids are metered into smaller diameters, they encounter more friction, and both mass flow rate and volumetric flow rate are both decreased (the latter more significantly, the former being what we care about) *relative* to that of an incompressible fluid (they still speed up, just not as much as incompressible fluids). Sipping out of a smaller straw is harder to do, therefore sipping a compressible fluid out of smaller straw results in decreased amounts of fluid that makes it through the straw per unit time. And now we also decrease the density of the fluid by going to higher altitudes (making it even more compressible), which helps the fluid go through the straw more readily than the denser fluid, but since compressible fluid incurs losses with smaller diameters, this [small part of the equation] is even more exaggerated since the fluid is more compressible. Overall, more dense fluid encounters more friction of course, but we're talking about combining two variables at the same time -- density and diameter. You will always sip more mass through a straw if it's more dense (up to a point, of course molasses would not be a good example). It doesn't matter how much easier it is, just compare mass sipped from a glass of water to mass sipped from air. It doesn't matter if if the air is flowing faster, you got less mass. To maintain less dense air at an equivalent speed, you need *slightly* larger chokes (increasing to the next increment would likely be too much). The air is less dense, therefore it encounters less friction, and travels *slightly* faster through the venturi. Air speed is not total O2 delivery, which is what we truly care about. The speed *does* however help our carbs mix fuel. Decreased air density does not. Which one wins for combustion engines? Density. We always want dense air, without exception. Since we don't have it at altitude, we need to open up the choke size to deliver the same amount of actual O2 that might have been delivered via a smaller choke size at sea level. Confusing enough? Sound like we're talking in circles? We are! That's half the point. We can theorize all day, but in the end all that matters is what does the car like? Does the AFR indicate it's getting enough air? If your dyno graph shows your AFR go uncontrollably rich at higher rpms, that's because your chokes produce too much resistance to flow to support the O2 delivery required at that RPM. Dynos have an SAE correction factor. It takes into account humidity, temperature and pressure altitude and makes a theoretical "standard day" (25 deg Celsius, sea level, no humidity) hp rating. A car making 200hp at altitude might have made 225 on a standard day, so the SAE shows the latter. You're at altitude, so essentially you're trying to minimize your handicap. Sorry, bud. And all of this is part of why "how do you intend to use the car?" is always the first question asked in engine building and tuning threads. This case isn't different from any other; if you want driveability you want smaller chokes, if you want power, you want bigger chokes. I personally always preferred power over driveability. I can feather the throttle to keep the carbs happy(ish) at awkward RPMS, but if my chokes are too small (which they are) then I can't do anything about it short of an upgrade.

The best answer is always to determine it experimentally, to try both (and tune!) and see which better suits your intended driving conditions. I'll assume your choke selection is already "ideal." All things being equal, if you go up in altitude, air density goes down, necessitating larger chokes for the same O2 intake. Unfortunately, the "air" volume will be moving at a slower speed due to the larger diameter, which means you will sacrifice driveability for the same power output that you had at a lower altitude. Bottom line, altitude is bad and you will lose performance. The question you need to determine is, which performance do you want: driveability or peak power? You can't have both, not even at sea level. I personally would not recommend going down in chokes with higher altitude. You need higher volume air, not lower. The choice at hand is to stay the same or go larger, not to go smaller. Then again, I haven't ever tried to tune for high altitude, and therefore this is theoretical. Keep in mind you must adjust and tune all jets to compensate for the choke changes. Only then can you compare the change.

I don't put many miles on my car, so I only do maintenance before the first race of the season. This includes new plugs, valve adjustment, and synch. I only touch timing when the motor or fuel changes. I go to the dyno for this. My ignition utilizes solid-state electronics, so I pretty much assume it remains at the settings it's been adjusted to. WOT is what I tune for, but driveability matters a lot for autocross. When dialed in (always wiggle room with mechanical devices such as carbs), you'll find that two mains will perform pretty much the same way AFR-wise (assuming increment of 5 with the mains). I use my butt-dyno to distinguish the two (the butt dyno is worth more than a number to me; what I need is the car to be forgiving of throttle changes). One of the two main jets that perform similarly will be more forgiving of partial throttle before the cam kicks in, and that's the one I go with. The idle jet only affects driveability at low rpm, and the main takes over around 1500 with my motor. The more your engine flows (displacement, porting, cam, etc), the earlier the main circuit takes over from the idle circuit. Most cars probably still are affected by the idle jet closer to at least 2000 or so, so you'll have to experiment with where your motor transitions. My tune procedure is pretty simple. I accept that I can only tune my carbs for about a 4k range of rpm at most. Beyond that, something is compromised. I make a run and check the AFR in the desired rpm band (coincided with my cam), and target 12.5-13.0. Keep in mind that the air corrector also plays a role in conjunction with the mains. My motor is affected above 4500 more by the air corrector than the mains. You will have different results as to where this happens, but to test, do a run and watch the AFR and note how it progresses relative to RPM. Then swap out an air corrector with one that's waaay too small (rich, to be safe). When the AFR *starts* to drop below your previous run's profile, that's where the air corrector kicks in. I personally will make no more than two main jet adjustments in a row, since they are too closely tied with the airs. Then I will adjust the airs up to two increments and go back to the mains, assuming of course that the car is still going in the right direction. Do not skip jets, do not change more than one variable at a time. If you adjust too much of an increment, you may pass the peak and not know it. Typically, from weekend to weekend I won't see more of an increment of 5 mains or 5-10 airs (if at all, sometimes I'll do one run and realize no change is necessary). Usually the idles only change once or twice a year (seasonally, depending on severity of your seasons). There's probably a better system out there, but this is what works for me. Hope that helps.

Mostly, yes. When you tune your carbs it is for those conditions only. Temperature, humidity, and barometric pressure are the biggest factors in actual O2 delivered per unit volume of atmospheric "air." Pollution, particulate (dust, etc) and fuel inconsistency are also factors, though less significant. Even with the same temperature, weather fronts (or lack thereof) will noticeably affect your carb tune. Also, mph matters. You will also find that 5k in 2nd performs differently than 5k in 4th. You should tune the gear you drive the most (or the one you want to pull the hardest). I tune for 2nd gear, but that's because I autocross. The inundation with changing variables is why most people are content with getting the main jets right and calling it a season. The more aggressive your engine is, the more sensitive it is to changes in O2 density. Me personally, I tune my carbs the day before race day. This gets me "close enough" since I can't realistically tune my carbs the day of. The day before my race I tune them for the time of day I expect to race. I usually only change the mains by 5 and the airs by 5-10 (if the car wants any changes at all, sometimes it doesn't depending on time of year). It's always experimentally determined. I never did make some master spreadsheet for my car, mainly because I upgraded parameters too often. As stated throughout this thread, *never* change more than one variable at a time, and *never* bump your changes up by much. If you overshoot the peak, you may never know. This is why cars are all fuel injected now. Tune it once (fuel delivery / timing) on the dyno, and the computer automatically adjusts for changes in O2 from then on. And that's also why triple carburetors inspire the nostalgic "awe" factor when they have a full song. Some people are aware of that when they run right, someone spent a lot of time enabling them to do so. Carbs are more of a hobby than a performance part in my book. A fun hobby, at that...

Hmmm... I bet you're right. I thought I had the XDI2, but clearly my memory is wrong. Oops? Mine is definitely not laptop programmable, but I didn't know you can swap the MAP sensor with a TPS, thanks! I'll definitely look into that before I visit the dyno! Luckily their manuals are quite thorough. Concur that the manual indicates that the MAP sensor may be replaced with anything that outputs 0-5V...

Also, one thing I always thought would help a lot (I'm personally putting it off for my ITB project) would be a throttle cable conversion. As we all know, the bellcranks and pushrods make it really hard to open the throttle plates "just a little bit" / smoothly, especially in conditions like autocross where the car (+ driver!) is being thrown about and you're trying to be smooth with the pedal. A throttle cable setup would reduce much of the friction, and ideally it would be non-linear so you can gradually open the first 25% of throttle with say 50% of pedal, since that's where most of the driveability challenges occur. Just my two cents... if you're going to install a TPS, you might as well go with a throttle cable while you're at it!

You're right, I doesn't produce much vacuum, therefore MAP utilization is minimal. Cam isn't happy until about 4750, so the carbs obviously hate to be in "normal driving" rpm ranges. I completely agree that a TPS is worth more with aggressive cams. The XDI series doesn't offer an input for a TPS, but the TEC series do. The TEC series is laptop programmable in as much detail (ie very fine data point intervals, interpolating between tabled settings) as anyone would be willing to program/tune. The XDI series is literally 4 knobs: a rev limit, initial advance, 3k advance, and 8k advance (varying linearly between the data points). XDI works wonderfully compared to a mechanical distributor (in terms of optimization, spark accuracy and of course spark energy), but the TEC series is just as much of a step up if not more. Add the EFI portion, and well, it's as good as an NA tune can get as far as I know. In addition to air intake temp, MAP and TPS inputs, you can individualize fuel delivery and timing for each cylinder! Crazy potential. I know there are other products out there that do all of this, but like I said, I'm partial to Electromotive. That said, it's expensive as heck. It's the Apple product of ignition(/fuel delivery) in my book. You get what you pay for... They should be, and they are. Ask any ASE Master, and they'll agree. We experience tune-ability problems because we're asking more of it than the engine system *as a whole* is capable of adjusting to. (i.e. tuning carbs cannot make up for exhaust or intake runners that don't support the head flow, etc.) As mentioned throughout this thread, the more streetable the head is, the easier it is to tune our Webers. Upgrading one component just causes a choke point somewhere else, and a choke point always manifests a poor driveability range in addition to capping max power output. When we start getting upgrading the head porting and cam (ie anything other than stock), then we need fancy ignitions, fancy intakes and fancy exhausts to help make up for the motor's tendency to have an rpm range that it runs poorly in. We eventually need fancy fuel, too. I still haven't forgotten how well tuned a stock 2.4L with a solid mechanical dizzy, SUs and street headers was. Crazy bang for the buck over OEM. Ever since that stage, I've been chasing around varying degrees of less than stellar driveability. (Well worth it, of course.)

Off the charts good, if you ask me!

Having a programmable advance curve will work wonders with getting the most out of carbs. Tuning is tuning, whether AFR or timing, flow matching, etc. It all has to work together of course. You could say it was a mechanical EFI if you are referring to the ignition half EFI, so sure, I'd agree with that. I've been with Electromotive since the HPV-1 in 1999. The only improvement I've actually noticed as their technology has evolved has been easier starts, smoother rev limits, and more reliable sensors (the early model would crap out after about 2 years). They claim better spark energy since then, but as the driver, I didn't noticed that until I went to race fuel, and it's hard to separate the two variables. (Using both high end spark and fuel, it will burn no matter what the AFR is!) I personally am unwilling to consider any other brand of EFI controller; I've been that pleased with Electromotive's design and spark energy. Unless you're a carb purist, I agree it's a better investment to go with ITBs from the start. Electromotive's TEC-3r / TEC-GT products + ITBs (such as Extrudabody, which I've been eyeing) will likely be about a $5000 setup. I have as much cash in my ignition, carbs, jets, manifold porting, etc., which is absurd when you compare the amount of tuning possible with ITBs to that of a mechanical device (carbs). Vendors of Electromotive's products also supply the appropriate crank trigger wheel, and some offer an adapter plate (2 bolts) to mount the sensor itself to the block. Top End and Rebello are two vendors I know of that are familiar selling/using these products with L6s (I personally find Top End to be incompetent salesmen). Rebello makes their own custom (weight relieved) crank trigger wheel to mount with their high end dampers. If you want performance (i.e. are paying for Electromotive), do NOT go cheap on the damper. Call Rebello and buy their parts. If you want it to rev, then go have the damper, crank, and flywheel assembly balanced, otherwise I'd recommend setting your rev limit no higher than 7k. It's only money, after all....

Yep: 326/315 @ .565". You may be able to call Integral Solutions and order it as the "Sunbelt Race Cam #2." That's what my cam spec sheet said, if memory serves. That said, it was specifically profiled for the flow characteristics of Jim's port work... Also, it was suggested to me that I post this recent tuning video to follow up on the headers build. That, and it will be six months before I'm with the car again to properly dyno it. (It's fairly lame from a performance driving perspective, but you can hear the motor, at least. It's more tuned at the end, of course.)

I am running Elecromotive's XDI2 direct ignition. I ran direct ignition with no MAP sensor for years. I later dyno tuned my race motor (oxygenated fuel) utilizing a MAP sensor that altered the programmed advance proportional to the MAP sensor's output. After tuning the motor, I didn't particularly notice a difference in mid-throttle positions, particularly in the trouble areas below 4k rpm. WOT loved the tune, but mid-throttle didn't make a difference. I even unplugged the MAP sensor to see what a difference it would make. It didn't make much of one. I could tell, sure, but it wasn't enough of a difference to affect my standings in an autocross race. Did I not get much of a difference because of my cam's overlap? Is it because of my oxygenated fuel? Perhaps, but in my case I didn't notice much difference. I still run with a MAP sensor just for the hell of it... [EDIT - I believe having a programmable advance curve (i.e. Electromotive ignition) is the most significant factor as to why the MAP sensor doesn't do much for me. With a mechanical distributor, you don't have the varied timing to begin with, so any input that varies the timing will have a much more dramatic effect.] Not a typical setup, but that's my experience. For what it's worth.

I didn't expect it to be so dramatic, either. Then again, I expected more of a dramatic change in 2008 when I went to 12.5:1, race cam, etc, but that iteration only bumped up the mains by about 10 and the air correctors by about 20. I guess that's what happens when you chase around choke points and then finally open up a big one... My tackle box of jets is about 12 of each type (main, airs, idles). It fell short this time, which really surprised me. That's the problem with triples; if you upgrade the motor enough times, the cost of a diverse jet selection costs as much as the carbs themselves. And that's just silly.

My jet selection is definitely atypical. First, my cam is literally profiled to exactly match the peak flow characteristics of my head. Second, my fuel (VP 109) is oxygenated, meaning O2 is delivered in liquid form via the jets (in addition to the air drawn normally through the air filter). Therefore, the mains are large to send O2 along with the fuel (less dense as a result) and also the air correctors are large because AFR graphs often richen up above 5000 rpm since the venturi usually cannot support the demand. Air correctors help. It should be noted that my previous engine iteration (before tuned race headers) used about 140 mains and 175 airs (with the same fuel) last time I tuned it (about 20F colder, about 1300' elevation lower) with the same oxygenated fuel and same camshaft / head. I believe the dramatic difference to be caused from exhaust scavenging leaving the cylinders cleaner (less remaining exhaust gases) when the exhaust valve closes.

Thanks guys! Only took 13 years to get here... haha. Agreed. It was easier to tune the power band of this iteration of my motor than any previous version. I credit the headers. (My butt-dyno insists that the scavenging so effectively cleared remaining exhaust gases from the cylinders that varying intake mixtures were more dramatic/noticeable.) That said, before the power band, with such a cam overlap -- below 4k does not flow well at all; the air will be slow regardless of venturi diameter. In this case (in my opinion) it's more a question of 'how slow' when comparing venturi sizes. (i.e. how crappy vice crappy or not crappy).

Sort of. The bog / "fall flat on it's face" aspect is presumably circumvented by a combination of tune, cam overlap and exhaust scavenging from tuned race headers. In all fairness, most of the video is above 4k rpm, which is past typical street driving rpm and into the air correction circuit of the carburetors. This same cam used to bog fairly dramatically before I went to the race headers. I also used to be able to break the tires loose by stomping on the gas whenever in 1st or 2nd, and now I can't; I have to let the power do it at higher rpm vice the torque being able to do it whenever. This surprised me. It's more squirrely, but at less predictable times, making it harder to utilize the power efficiently. The engine is not built for under 4k. It's been balanced for 10k, though I will set my rev limit to 7.6k (the video's rev limit was at 7k until I get it dynoed). There are a few points in the video where I offer a little pedal before the cam kicks in, 4:57 is a decent example of it. It sounds like the familiar unhappy race motor / retarded gurgling sound (not in the same league of course, but akin to F1 cars when they need to downshift). It does not hunt and surge like my car used to years ago, but feels more like a steady pull right before it pulls hard. I can floor it, but nothing happens; it neither falls flat nor pulls harder. You can also see a few seconds before that point (at 4:51) in the video that I can also blip the throttle from idle to 3k without having to be ginger with the pedal. It responds quite nicely, but once in gear it's another matter. The motor as a whole simply doesn't want to be under load at that rpm. This isn't a fair motor to compare. I'm sharing more in the sense of "look how far the smaller 40 DCOEs can go" than anything else. And to demonstrate that even with an extreme motor, the engine can run clean with no popping, even outside it's power band. Keep in mind that with carburetors, you can either have low-rpm driveability, or higher rpm power. You cannot have both until you go fuel injection, and even then, you give up one for the other (just not as much as you would with carbs). In my case, I have tuned the carbs for high rpm power, but the venturis are so small that I get some extra driveability down low. How much, I'll never know... but I do know that with ITBs I can minimize the tradeoff.

I'm not enough of an expert on the other stuff to advise, but I can speak to the ARBs. I raced a full season with 1" up front and nothing in the rear. This allowed me to put power down earlier and more aggressively, but at the expense of turn-in, which the S30 already struggles with as it is. My S30 is about 2300 lbs with fuel and driver, and I still could not get it to turn in. Absolute pig. I had to either slow down waaayyy too much, or tap the brakes while I was still on neutral gas pedal to get it to turn in. It was fastest time-wise (around cones at low speeds) if I allowed the rear to slip slightly and let the torque contribute to the turn. I've since gone fully adjustable front and rear hoping to remedy my setup; the driver shouldn't have to be the fix to a poor suspension setup! Can't report on it yet, as the project isn't done and I haven't raced it yet, but I believe it's a step in the right direction...

Been awhile since I've visited the Weber thread. I wanted to chime in again for the purpose of sharing an extreme example; over the years many of us have accepted poor tunes as a consequence of triple carburetion, myself included. It took years for me to assemble the requisite knowledge and collection of jets for a proper tune, but in the end it took me about five years to realize that I didn't have to accept a tune where the carbs fall flat on their face if you didn't work the pedal properly. Most say that the 40 DCOEs aren't enough carburetors for 2.8L+ engines. The book agrees, and I'm of that opinion as well. The difference is, the 40 DCOE will still run just fine, you'll just cap your high rpm flow / power production. My 3.1L race motor, for example, at this point is more or less all out in every aspect of the motor EXCEPT my 40DCOEs which severely restrict flow. The point is that with the right fuel, the right ignition and the right tune, even a severely restricted intake can be smooth and produce a ton of power. A video is worth a thousand words. It's not exciting from a driving perspective per se, but it demonstrates my point just fine. Tuned via butt-dyno (vice AFR), too. It can be done. [For those new to Webers, do *NOT* attempt to duplicate my jet selection as indicated in the video; this is not a typical setup. This is just a demo.] Unfortunately the car is now in storage, so I won't be able to dyno the car to compare numbers, AFR, etc. For now, this is all I've got...but I'm happy to bring back the data at some point. Cheers, Mark

I fixated on your use of the word "pop" which I interpret as excess fuel igniting as it passes through headers at temperature. Growl and snarl are of course subjective. If you're happy, that's what matters!

I disagree. If I can get my race motor to sing and decel without any popping, then anyone's motor can too. [EDIT - Assuming the ports are reasonably consistent and all is in good working order, of course.] I'll be uploading a recent tuning session onto the Weber sticky soon to make this very point. We (Weber users) tend to forgive ill tunes and write them off as the limitations of carburetion when in fact its the limit of the tuner and/or their arsenal of jets. I accepted a poor tune for years, until I finally figured out how to properly tune my own carbs. Strong ignition and quality fuel can make all the difference in the world. Where I feel the carbs cannot compete with FI / ITBs is that odd rpm range around 3000-4000 rpm (depending on motor). And of course driveability since a good FI setup accounts for both throttle position and vacuum. WOT, my Webers have served me quite well...

Those are gorgeous. Saved the pic of the crossmember. Thanks for making my mounts seem amateur! I *used* to be proud of them.... Just kidding, of course.

It's a realistic goal for a daily driver, just probably more expensive than most daily drivers are willing to spend. My yester-year numbers support that 200hp is attainable for a street motor on pump gas (well below 7000 rpm). For a 3.1L, anyway. The peak numbers were there, but the curves aren't particularly flat. Felt far more flat as a driver than my race motor does, though. So, further VE data points below. Always a 3.1L with Electromotive ignition, always with the same 40 DCOEs and street headers. In 2003, crappy head job with a moderate street cam, pump gas. 10.5:1 or so? (E31+stroker) Flowmaster muffler (ugh, what was I thinking...?): 157 whp @ 5250, 176 ft-lbs @ 4500 (DynoJet). Per liter: 51 hp/L, 57 ft-lbs/L Theoretical at crank (again, 15%): 185 hp / 207 ft-lbs Theoretical per liter: 60 hp/L, 67 ft-lbs/L This motor was quick, but it wasn't fast. The torque was fun, though. In 2004, Sunbelt head with restricted cam, AvGas (bad idea IMO) (same compression as above motor). Magnaflow muffler (perfect sound in my book): 197 whp @ 6250, 186 ft-lbs @ 4750 (DynoJet). Per liter: 64 hp/L, 60 ft-lbs/L Theoretical at crank: 232 hp / 219 ft-lbs Theoretical per liter: 75 hp/L, 71 ft-lbs/L This motor was pretty fast on the street, and sounded great (loved the muffler). Enough power/torque to get squirrelly on street tires if I drove like a jackass. These motors were in my daily driver. The dyno numbers were with the AvGas, I later re-tuned it for 93 octane and it actually ran better. (I didn't understand how octane works at the time and blindly put it in the car thinking it would make more power. It ran better on 93 octane partly because of how octane works and partly because AvGas is inappropriate for performance cars that vary RPMs -- slow flame front propagation.) From my tires and also my butt dyno's perspective, 65-70 ft-lbs/L was enough for the street (both motors). And although torque may be a metric of the engine, I agree that it's relationship with RPM does matter, because well, the 2003 motor was pretty weak in comparison and the acceleration just wasn't enough. The 2004 motor didn't ever leave me feeling like I lacked for power, therefore 75 hp/L was a hot street motor in my opinion. I autocrossed for a couple seasons with this motor and had more than I could put down on street tires. (I'm talking about the theoretical crankshaft numbers of course, because that's what I thought was being mentioned.) It's cheaper to get good headers (better than MSA or Monza ), proper sized carbs (new), and Electromotive ignition (combined) than it was to build a Sunbelt / similarly prepared head. (Even after JohnC paid for all the R&D, specifically.) NA costs $... [EDIT - Seems to me Rebello can produce significantly beyond these numbers via SU carbs / pump gas. Probably the cheapest/easiest/fastest route out there, not that I've ever heard a price. I'm surprised he doesn't seem to have any competition...?] FWIW...

Tony D's PM, posted with his encouragement: This is EXACTLY what I told Scott B to discuss with Ron at Isky when he was having Slover's port his head. He was making 218cfm flow on the intake at some specific lift, and curiously that is what Ron capped his lift at.... And given the engineering requirements of TDC and BDC, piston acceleration, etc... It's all been well tested and documented. I simply can't fathom why guys buy a camshaft off the shelf when if they have head flow numbers, bore and stroke information, they can simply call Isky and get a cam that will make KILLER HP without anything else being done to the bottom end. The guy doesn't get it. Scott was really impressed at the power he got, far beyond what he expected. And his budget fell short as the head and cam was setup for 44 Mikunis. I said it would make 20HP more for the 200HP REAR WHEEL number, it was at the SAME power peak that Scott has---of course it will make MORE and a higher rpm...but Scott was making 182 at something like 6250 or some odd number. If the Mikunis added 3-500 rpms to the powerband, I would expect 200 at 6250, and something like 225-235 at 65-6700. All WELL below 7500 rpms. Without forged slugs, and with torque so strong at 3000 he has to watch it because the back end gets squirrely and stays that way till almost 5000 when torque/horsepower cross and taper off (hp rises)... Indeed, it's all about matching. Truthfully he could have spent a LOT more for a Schneider Cam, forged pistons, and like you said "a list of parts from MSA" and ended up with less drivability, less power, and likely would have STILL been happy---but also perpetuated the myths surrounding 'loss of bottom end' etc etc etc... This is the difference between people who have DONE and people who read a lot and then make a plan without LISTENING to real world experience. No matter WHAT an engineer's calculations say, when it goes onto the test floor and does something different---YOU CHANGE THE SETUP! (At least a good engineer does. Of course, the Germans will blame the guy running the test, and the guy who did the setup...but that is another story altogether!)

Tony, in my opinion your PM belongs in this thread. I don't think anyone minds the back-and-forth banter, as we're all still reading! Your comments helped me confirm and understand why I've reached my conclusions. I didn't learn anything new per se, but it's great to have my somewhat fuzzy conclusions confirmed with similar examples and hear a different phrasing of "why." I think your explanation would be very beneficial for silent readers that are on the fence regarding their budget size for head work and cam selection. ~2500 thread views, after all. I honestly don't know what's more impressive: That the [NA] L6, such a rudimentary OHC engine by modern engine standards, is capable of producing such impressive hp/L or ft-lbs/L. That engine builders can modify such a basic 2-valve/cyl engine to acheive such an end. Both are truly remarkable! I go back and forth as to which is the bigger wow factor. Doesn't really matter, but it's part of my addiction to the L6. For the record, this is not a reply to toot my own horn, but rather to emphasize the impact of high-end head work and it's alignment of a *properly* designed cam (vice a catalog cam). And the potential to hear some comments on the matter from people who know far more than I. I feel very strongly that I haven't realized my engine's potential (not even close). I'm of course also wary of that being my personal fantasy since my "numbers" are already stout, having been burned with this same fantasy of bigger numbers in my previous engine builds. Preface aside, I'm quite encouraged by my current output since from what I know, it's still severely restricted. Tale of the Tape: 2008 dyno: 265whp @ 6000, 250 ft-lbs @ 5000. (DynoJet.) 232/153cfm in/ex (w/o plug?) and 209/134 cfm in/ex with plug installed (anyone know why the 'w/o plug' would be useful? i.e. why do I have this data to begin with?) Intake: 40mm DCOEs with 36mm venturis. The Weber book indicates about 44mm venturis (not to be confused with Mikuni 44 bodies of course) would be appropriate for the engine "to breathe sufficiently" to my springs' soft limit of 7600. This of course doesn't necessarily mean it's what I would choose, I need low-rpm driveability, too. (Enter fuel injection!) Exhaust: Monza headers from 1999 (from Victoria British / Black Dragon), 2.5" kinked pipe going back to an open muffler Electromotive ignition 12.5:1, VP 109 race fuel Per Liter: At the wheels: 85.4 hp/L and 80.7 ft-lbs/L Theoretical crank (using 15% drivetrain loss): 100.5 hp/L and 94.9 ft-lbs/L The point of this, is that my intake and exhaust were "streetable" and the rest of the motor is a no-holds-barred autox race motor (12.5:1 is the only compromise; chosen for longevity). The only things I didn't do is polish the rockers and coat the friction parts (not sold on the latter). Even with significant restrictions, the VE of the head seems to take control and presumably maxes out the potential of the intake/exhaust, whichever is the weaker link. Both reduce the potential, but no doubt one is maxed out and the other is not. Looking at my A/F graph and the weber chart, I'd say it's the carbs hands-down. Bottom line: if you don't build everything to work together from the start, you end up chasing around choke points and rebuilding unnecessarily. That's obviously what I've done and am still doing. Don't be like me! [End contribution, begin requests for knowledge.] The thing that I'm wondering now that I have proper race headers / merge collector, is how my torque curve will shift. I'm predicting the bell shape will shift downward in the RPM range (I don't have the largest primaries available since I plan to use the entire RPM range), but that the curve will also shift up and grow wider / more flat. [EDIT - The "shift left" prediction is based on me leaving the restrictive intake on the car; I predict it will go both directions once I go ITBs, but perhaps that's fantasy? Low rpm due to scavenging, high rpm due to feeding more air? Peak hp will no doubt shift above 6000 at that point?] It feels *so* much more full at lower rpms / mid throttle, but the pull's transition is deceivingly smooth and my butt dyno recognizes its limitations / the illusions of sound. To me, the race exhaust has "unlocked" the remaining potential of the head, but it of course won't be realized until I go to ITBs with proper diameters (or proper carbs, which I won't be doing). I'm also curious if the exhaust scavenging will help the A/F ratio or exacerbate it. I predict both, at different RPM ranges. [EDIT - Nevermind, that was retarded. I think it can only help.] Very curious, indeed! Dyno is probably 9 months out since I'm bouncing around for the next six months. I'm mostly encouraged at having a flat A/F line with inputs for throttle position, vacuum, etc. The power under the curve is obviously what I'm going for with autocross. That said, more power is always appealing! (Intoxicating, in the words of JohnC.)

Haha, that was the nice way to summarize my / our hot air. I may or may not have had to look up bandied....