carb size vs performance?

[barnbiketom]

geesh keep it stock



If you want to go faster, buy a different bike.

[Nessism]

i have heard of people putting the 29mm smooth bores on the gs1000's instead of the 26mm....

would there be much of an improvement in performance in that scenario?

I found a set for $50

29mm smoothbores are worth between $600 - $800+ depending on condition. You best grab them regardless...

Oh, and yes, they were a very popular upgrade for the early 1000.

[Gregory]

geesh keep it stock

If you want to go faster, buy a different bike.

I probably will keep it stock, I was just curious since i ran across the 29's and had read in an earlier thread about them being worth about $600... (also mentioned again here in this thread)

It just got me to wondering how much extra "umph" you could get out of a bike with them. For me it would have to be significant to pony up that kind of money.

[tkent02]

On a stock engine, not much improvement. The limiting factor in airflow is the size of the valves, how far and for what time they open, and the shape of the head. Just bolting on a bigger carb won't do much. If you have opened up the head so it can breath with big valves and a cam larger carburetors will make a huge difference.

[Gregory]

That definitely sounds like wrenchinhg WAY ABOVE my pay grade. Thanks for all the info guys.

The carbs are on ebay right now and I am the high bidder at $50. Someone please out bid me, I have enough money making projects right now and someone here will be able to make a better go of it. (there is another set on for $389)

[gearheadE30]

Carbs are sized to an engine based on the amount of air that each cylinder can flow. As has been said, carbs require a pressure drop to function properly, and this is done by reducing the venturi size in the carb. Bernoulli's equation tells us that pressure drops as velocity increases, and since the mass flow rate through the carb is unchaged when changing carbs, going to a bigger carb reduces flow velocity, which reduces the pressure drop. Having a big step in the intake further reduces the the pressure drop due to what's called a major loss in what is effectively a pipe-flow problem. That's not to say a smaller carb is better, because the stockers on our bikes are actually sized very well. At high rpm you can gain power because the increased mass flow increases intake velocity, putting the carb back into its operating range. The loss of low end response is again due to too slow a flow through the carb. Combined with pods (less restriction = less velocity at low revs) and most 4-1 systems (loss of low-rev scavenging) that many people put on bikes, you could possibly end up with a bike that is a dog around town.

Granted a lot of that is for more extreme cases and there are ways around the issues, but that's a rough idea of how it works.

[Big T]

This guy has done his resaerch and testing to develop his services, Yeah, it's an ad from Ebay

Stock 26mm carbs flow 70cfm (cubic feet per minute) at 10” H20 vacuum.
30.5mm MAXI-BORE carbs flow 105cfm.
29mm Smoothbores only flow 98cfm.

So, the total flow increase in your four carbs is a whopping 140cfm!!! That’s like adding 2 more 26mm carbs!
Think you understand airflow? Here are some questions that might let you know where you stand on airflow.

Q1: If 26mm Mikuni carbs flow 70cfm @ 10” H2O, which alternative gives you the most HP/$ spent:
A) 29mm Smoothbores that flow 98cfm, keeping engine stock & 26mm carb boots.
B) Port your cylinder heads, keeping stock OEM 26mm carbs.
C) Port your cylinder heads after installing oversized valves, keeping stock OEM 26mm carbs.
D) Install high lift long duration cams in your stock engine, keeping engine stock & 26mm carbs.
E) Install big bore kit, keeping all else stock.
F) Bore your 26mm carbs, leaving all else stock.

Ans: F) Bore your 26mm carbs, leaving all else stock. Why? Your engine is an energy conversion device; it converts thermal energy (burned fuel) into mechanical energy (torque X rpm = HP). If you don’t increase the air/fuel (A/F) burned, it can NOT produce more power output.
A) 29mm Smoothbores, used, have recently sold for over $800 on eBay. They may flow 98cfm, but NOT with 26mm OEM carb boots. They may flow 75cfm. So, you’re paying about $160/cfm improvement.
B) Your stock head flows about 68cfm. Ported, about 79.9. Cost: $631.45 ($320 port + $311.45 valve job/guides/seals plus cost of gaskets & labor). $53.06/cfm for head work alone.
C) Your stock head flows about 68cfm. Ported with over sized valves, about 86.7cfm. Cost: $1032.00 ($640 port + $192 O.S. valves + $200.00 to open seats up) plus the cost of gaskets & labor. $55.19/cfm for head work alone.
D) This is a trick question. Cams can NOT increase air flow at all rpms. Long duration cams simply lose low end torque (due to A/F reversion) and may add some top end torque (if lighter retainers and stronger springs are also installed). High lift cams sound good, but on a stock head generally do NOT increase airflow, because any valve lift over .375” does NOTHING to increase air flow. In fact, the intake valve can be removed from the head and a stock port will flow no more than when the valve is opened .375”. So, either way you look at it, big, long duration cams may do no good in terms of making more HP. Big lift may do nothing but cost more for springs and retainers. Big duration my lose low end, to add a little bit on top end. Because there are only 720 degrees available in a 4 cycle engine, adding more intake duration will only increase intake reversion (aka fuel stand off), and increased exhaust duration can only reduce the power stroke, reducing your engine’s thermodynamic efficiency. No matter how you slice it, traditional high lift, long duration cams are of little help in the real world, because they offer almost no overall air flow increase. (Hint: there are more modern types of camshafts that I am NOT discussing here.)
E) This is a trick question. Big pistons can NOT increase air flow. Big pistons can increase low end torque (due to increased bore area), BUT CAN NOT improve top end power (because the intake tract will run out of air fuel at a lower rpm with a bigger engine, thereby reducing top end power). But, there is NO air flow increase because of a big bore kit.
F) MAXI-BOREtm carbs flow 105cfm & 26mm boots air flowed also. 35cfm/carb flow increase means your paying only $14.28/cfm.

When I built my little D Modified drag race GS750 Suzuki back in 1977, I did it one step at a time to see what really made a difference. First, I put in a 850cc high compression piston kit. No change. I then added Yosh road race cams, valve springs & aluminum retainers. A very small improvement. Installed a UFO (Universal Four into One). I lost low end, to gain very little on top end. Installed bigger carbs. Yes. More power was on tap. From there, I ported the head and bored the carbs to become competitive. The magazines just want to sell you something so that their advertisers will buy bigger ads. Magazine celebrities don’t know anything about engines, math, science, chemistry, or any other science, they know only word games.

Q2: If your stock engine head intake flows 68cfm, and your stock 26mm carbs flow 70cfm, with no air cleaners, what amount of air/fuel flows through both when connected together with the OEM 26mm boot?
A) 68cfm.
B) 70cfm
C) 138cfm
D) 35cfm
E) 34cfm

Answer: E) 34.49cfm This is the comparative air fuel each cylinder can burn. Comparative because the intake valve is always either opening or closing, reducing real air flow through the system, so let’s keep it simple and just look at the components to air flow.

Q3: If your stock engine head intake flows 68cfm, and your stock 26mm carbs flow 70cfm, with no air cleaners, what amount of air/fuel flows through both, when you install MAXI-BOREtm carbs together with the MAXI-BOREtm flowed 26mm boot, what air flow comparison should you see?
A) 68cfm.
B) 70cfm
C) 41cfm
D) 35cfm
E) 34cfm

Answer: C) 41.27cfm This is the comparative air fuel each cylinder can burn with MAXI=BOREtm carbs.

Q4: If your ported head intake flows 79.9cfm, and your stock 26mm carbs flow 70cfm, with no air cleaners, what amount of air/fuel flows through both, when you install MAXI-BOREtm carbs together with the MAXI-BOREtm flowed 26mm boot, what air flow comparison should you see?
A) 68cfm.
B) 45cfm
C) 41cfm
D) 35cfm
E) 34cfm

Answer: B) 45.37cfm This is the comparative air fuel each cylinder can burn with MAXI=BOREtm carbs.

Q5: If your ported head, with over sized intake valves, flows 86.4cfm, and your stock 26mm carbs flow 70cfm, with no air cleaners, what amount of air/fuel flows through both, when you install MAXI-BOREtm carbs together with the MAXI-BOREtm flowed 26mm boot, what air flow comparison should you see?
A) 47cfm.
B) 45cfm
C) 41cfm
D) 35cfm
E) 34cfm

Answer: A) 47.40cfm This is the comparative air fuel each cylinder can burn with MAXI=BOREtm carbs.

Q6: Now, let’s work this backward for comparison. If your ported head, with over sized intake valves, flows 86.4cfm, and your stock 26mm carbs flow 70cfm, with no air cleaners, what amount of air/fuel flows through both, (no MAXI-BOREtm carbs nor flowed 26mm boots), what air flow comparison should you see?
A) 47cfm.
B) 45cfm
C) 41cfm
D) 35cfm
E) 38cfm

Answer: E) 38.73cfm You would gain 22% more air/fuel by having your 26mm carbs and boots MAXI-BOREDtm.


But, it takes more than the best flowing carbs in the world to make the MAXI-BORE package! Something even Mikuni didn’t think about when they sold the 29mm Smoothbore as a slip on replacement for OEM 26mm carbs. Buyers installed their new 29s on their engines to find marginal performance improvement and often ran into jetting problems. Why? If your motorcycle came with 26mm carbs, isn’t it likely that it also has 26mm carb boots? Absolutely! What happens when you install 29mm Smoothbores on 26mm boots? The limit to top end air flow is still the 26mm boots. Those boots block 20% of the available air/fuel provided by the Smoothbores. The top end limited airflow created by the 26mm boots distorts the fuel delivery curve, making the Smoothbores extremely difficult to jet, the last 20% of the throttle opening does little to increase airflow or power.
So, Mikuni got it wrong!!

Mikuni made another mistake with their 29mm smoothbores. Remember the smoothbore insert that supposedly allowed more air to flow through the carb, than the OEM 26mm carbs. Well, it turns out that the insert actually killed low speed jet signals, reducing low and mid range performance. Some people would call it a flat spot. In our drag strip tests we found that an OEM 28mm carb (Z-1 or Z-1R) actually outperformed the 29s at the drag strip. The 29s flowed more wide open, but the 28s has better low and mid range mixture control, power, and that is what wins races.

[Chuck78]

Wow, that was a lot to wrap my brain around! I know another member was having trouble doing a very thorough tune/jet job on a set of 29's, and thought about reverting back to the VM26's...

So Maxi-Bore tm a product/service this guy one ebay offers?????? I have heard of others boring out their carbs. I wondered what affect that has on the velocity stack effect of the carb's throat, changing the bore but not having a larger throat opening.

So theoretically speaking, would a $$$$$ set of VM29's with Maxi-Bore 26mm boots flow basically the same as an affordable pair of VM26's bored out larger to 29 or even 30.5 as you say???

[jimb12]

If by 'bigger' you mean higher flow, then yes they can make a difference. On an internally-stock 1100ET that was old and tired, going from the 34CVs with a Stage 3 and pods to 34RS carbs gained .2sec/4mph in the 1/4 mile. Even though the venturi size was the same, the smoothbores flow more air than an equivalent-sized CV. Obviously even the stock engine was able to use more air than the stock carbs could give in this case.

[Big T]

Wow, that was a lot to wrap my brain around! I know another member was having trouble doing a very thorough tune/jet job on a set of 29's, and thought about reverting back to the VM26's...

So Maxi-Bore tm a product/service this guy one ebay offers?????? I have heard of others boring out their carbs. I wondered what affect that has on the velocity stack effect of the carb's throat, changing the bore but not having a larger throat opening.

So theoretically speaking, would a $$$$$ set of VM29's with Maxi-Bore 26mm boots flow basically the same as an affordable pair of VM26's bored out larger to 29 or even 30.5 as you say???

Maxi Bore has been around forever - I remember him from the early 80s

He seems to have done his homework