RossD wrote: For throttle response, you need to linearize the opening of the valve to the flowrate.If you want the linear line and the throttle bodies give you the 'quick opening' line, you need the input from the 'equal percentage' line. If you were doing drive by wire, you could just program it, but if your using a cable you need a progressive throttle quadrant.
A lever instead of a pulley works well for achieving that progressive actuation to linearize the butterfly flow.
In reply to Ian F:
I believe for multiple carbs yes they do need to be from a smaller motor but for a single they can be from a similar sized engined motorcycle. For Example my CV carb is from a Harley Big Twin so it is for a range of 1000cc to 1800cc engines and they work really well on the 1300cc and 1600cc suzuki engines. The rev range of the harley v twin is similar to the g13/g16 so the cv carb works really well for them.
RXBeetle wrote:RossD wrote: For throttle response, you need to linearize the opening of the valve to the flowrate.A lever instead of a pulley works well for achieving that progressive actuation to linearize the butterfly flow.
Linearization of the actuation of a butterfly is not equal to linearization of the airflow.
A flow bench will show that there is a very large inrush at the beginning of the butterfly rotation that increase in flow dramacially tapers off the further the butterfly opens.
While you have it on the flow bench remove the blade and shaft and you will note a 20% increase in air flow! If this occurs in an ITB application it is in opposition to the airflow rules that should be applied to an intake port. Air flow needs to be turned at the entrance of the port and smoothly accelerated to high velocity aimed at the back of the valve and turned slightly to flow in the natural direction of swirl. If you have a blade and shaft obstructing the flow to the tune of a massive 20% the natural inclination is to increase the bore to get the flow back. However what has happened is airflow is essentially braked at the blade and shaft by the flow restriction by forcing the air to quickly speed up and then slow down past the obstruction. If the diameter of the blade is increased (to compensate for flow loss) this slows the airflow even more by increasing the area further braking the velocity of the flow.
If you truely want to benefit from ITB you need either a slide valve or a barrel valve so you have a smooth bore of the smallest area to meet the flow requirements while providing the highest velocity to enhance cylinder fill and swirl. Anything else will not provide gains significant enough to offset the high maintenance of ITBs.
For the street a single air valve with a plenum feeding long runners is the most practical solution with individual injectors close to and pointed right at the back of the valves. This provides excellent mixture control between the cylinders, as Fuel distribution is not affected significantly and airflow can be tailored by plenum baffles or shape, and the fuel sprayed at the back of the valve will rinse off any PCV gack that will naturally accumulate.
A cam shaped pulley for a cable throttle or various arm lengths and angles can be used to alter the throttle feel for the driver. The early VWs used a cam against a roller on the throttle shaft to open the blade very slowly at first to flatten out airflow change.
For a butterfly the blue line would represent the airflow and the red line would represent actuation to linearize the response. Depending on the performance of your motor (a zillion tuning factors) drivers feel is what really matters. Can he effectively and smoothly modulate the throttle in conjunction with clutch action. Is the throttle choppy or laggy? In the mid throttle area can he effectively modulate the throttle to control wheel spin? This is particularly critical on limited traction surfaces (dirt). Or monentum racing where you want to let out of the throttle just a little very smoothly to not upset the car.
bentwrench wrote: Anything else will not provide gains significant enough to offset the high maintenance of ITBs.
Wait, what's high maintenance about ITBs?
bentwrench wrote: Why would you want to install 6 carbs on something that can be fuel injected with OEM parts and a MegaSquirt?
The sound. It's insta-grin.
In reply to Knurled:
The sound is one of the bigger reasons.
I have run a set of 39mm Keihin FCR fault slide carbs on my Datsun 1200 for several years now. The car is powered by a 1508cc motor turning 8200 rpms. Note on Keihin's website there is a chart that relates carb size to power output, my car is pumping out about 110hp to the rear wheels and 39mm is a common size for 750cc bikes.
The advantage of flat slide carbs are flow, mainly because there is no throttle shaft obstructing the airflow.
Now some urban myth to get out of the way; any one who says that these carbs won't work at low RPMs or that you can't just wack the throttle open has no clue about tuning them. Any drive ability issues are going to be down to things like cam shafts with huge overlap etc.
My car is jetted for cooler months of the year as my actual racing is done then. Because I don't feel like rejetting the car for track days, I leave it be, there is a 30 degree difference and the car still runs fine. Sure I'm losing power with a car jetted for 75F when it's 105F out but it doesn't suddenly start popping and stuttering.
I also have a Beta 520RS which also uses an FCR carb; I ride the bike summer & winter and the trails we ride vary between 2000 & 7000ft. I've never changed jets and the bike runs flawlessly (read the bike test and all of the magazine testers rave about how well the carb works). The bike lug along at 1500 RPMs or buzz to 10,000.
I have no idea where this unreliability or driveabilty comes from; yes absolutely EFI is more flexible, yes it won't require minor jet changes to get the absolute last few horsepower from it but that's because EFI adjust on the fly and yes it will get better gas mileage.
Take cost out of the decision and I'd do ITBs for a street car but I want a slide valve type (not sure if anyone makes them anymore) I paid $250 for my set of FCR carbs (came off run offs winning DSR), I got them cheap because everyone went to modern bike motors, which have EFI.
Finally yes the intake roar is quite unique and does sound the business. Also note the FCR carbs where good for a 10% increase in power.
Tom
Take it from a guy that built an L28 with n42 head, Schneider cam, and triple mikunis. Go with THREE, two barrel carbs. It's not harder to sync, it's simple in design, it hauls ass at any rpm, and cheap to run. No megasquirt to mess with. My old 260z would walk "evos" and TT z32's all day. I LOVED that car! Also sounds mean as hell.
In reply to Tom1200:
When did you buy your carbs? A quick search on eBay for "39mm Keihin FCR" brings up some rather eye-watering results, price-wise. 
In reply to Trackmouse:
This is the back up plan for sure.
Also, diesel maxima crank with "silver dollar" Pistons (aka flat tops) will get you 11:1 comp and a 3.1 stroke. That engine actually used the carbs!
In reply to Trackmouse:
Ive been looking into building a stroker. Seems like a good way to spend alot of money.
It's wasn't much. I think total I was into it about 10k. But it was a naturally aspirated motor that wouldn't quit, was reliable as a mule is stubborn, and like I said, walked any car I went up against.
Ill probably go stroker eventually. Ive been a passenger in a 240z with a stroker on the track a few times. Its quite exciting.
Y'know, speaking from outside the community, for the effort and expense of building a stroker L-series, I'd just as soon buy a nonturbo RB engine (they made 'em up to 3.0, yeah?) and throw the carbs on that.
Nonturbo RB should be cheap since most swap guys go for the turbo engines.
OTOH my dream 240Z swap is an RB20DE because poor man's Z432.
Yeah that OS Giken head is a true unicorn part. There was a guy that somehow mashed two ka24de heads together and made a dohc head for the L series. Sounded expensive too. Although the Neo-vvl rb26 was TT and had ITBs from the factory. Crazy expensive engine. My rb20det s13 was a riot. I loved that car too, but 11mpg in city sucked.
Im tempted to swap an RB into ot down the road but building up old engines to beat up newer stuff os fun in a weird way.
@ Ian F I got the carbs in 2010ish and did indeed get a smoking deal, they even came with a custom manifold.
FCR carbs tend to run from $600 used to $1500 new for a set of 4, but if you troll various websites you kind find them at a decent price. The carbs are modular in that once you have a pair with a center pull cable you can add carbs either side of them. For a set of six I would expect to pony up a $900 to $1500 depending on luck. It's still cheaper than ITBs and no more than DCOE carbs. There is also another bonus; on left hand drive Datsuns the brake master cylinder is in the way of DCOEs, the FCR carbs angle up and clear without issue. The FCRs flow more as well. There is a reason many a rule book says "no motorcycle carbs" in classes for road cars. I am totally biased when it comes to bike motors versus car motors, more revs and way better gearbox. Somewhere on the net is some guy who installed two sets of flat slide carbs on a V8, very cool looking.
Tom
In reply to Tom1200:
Do you have photos of your setup that you would be willing to share?
The rule may be due to fire safety!
Fire safety? F600 F1000 P1 P2 are all using motorcycle engines, some of these are still carbureted and so I'm not sure why bike vs auto crabs would factor in.
In reply to Tom1200:
That looks awesome
The carbs are absolute works of art; the slides, unlike traditional slides, ride up and down on a set of sealed bearings so there is no chance of them sticking.
Everything is adjustable, beyond the normal high and low speed jets, air jets, needles etc. You can adjust the accelerator pump output and the slides themselves are two piece so you can change the slide cutaways. There are even 4 different sizes of float needle valves as well as adjustable air jets.
The bodies even have "Keihin" milled into the sides. They are probably the most precisely made carb ever but it's what you expect from Honda's engineers.
Tom
In reply to Tom1200:
Wow. Thats alot more adjustability than I thought these carbs would have. Any idea what bike they came on?
http://sudco.com/carburetors.html
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