Pros/cons of adding velocity stack on single downdraft carburator

[isa1023]

http://en.wikipedia.org/wiki/Velocity_stack





Questions:

• Pwede ba to sa stock single downdraft carburator?
• Any practical pros aside from smooth air entry?
• Any practical cons aside from increased fuel consumption?
• San makakabili nito? I tried calling A&M Carburators sa Kamuning wala daw silang ganyan

TIA

[duskylim]

http://en.wikipedia.org/wiki/Velocity_stack

Questions:

• Pwede ba to sa stock single downdraft carburator?
• Any practical pros aside from smooth air entry?
• Any practical cons aside from increased fuel consumption?
• San makakabili nito? I tried calling A&M Carburators sa Kamuning wala daw silang ganyan

TIA

I used to have a belt-driven Saturn engine (1600 cc 4G32) on a Mitsubishi Galant Sigma with twin-down Mikuni Kogyo Solex carbs, high-compression pistons (10.5:1), sport cam (cam 6, 24-64/67-21 timing), big valves (intake from 38 mm to 41 mm and exhaust from 31 mm to 33 mm) and 4-2-1 headers from SMT.

I got a set of locally-made velocity stacks from SMT (Sison-Mercado Trading of Mascardo St, Makati (near Arte Espanol and Vito Cruz ext) - look for Mang Tony Sison or his son, Mon) of Makati.

They were about 3 inches long and made from 2 to 2 1/2 inch B.I. pipe flared at the top (with about a 1/2 inch radius of curvature) and had a hex nut welded into a central bar for attachment to the carb's top.

I painted them flat black and they looked real cool.

Unfortunately they pretty much upset the carburation.

The carbs became balky and the top-end became choked. The low and mid-range power improved the most but nothing really spectacular.

Fuel economy for the most part stayed the same.

Overall I judged the benefits of stacks as marginal at best and sometimes even worse than stock filters.

Of course that could also be due to the fact that I feel they may have been too long for my engine - not enough testing of various lengths of stacks.

Maybe you can continue the testing.

Good luck.

Regards,

Dusky Lim

PS - the best performance and economy boost for my car came from the high-compression and a very good set of 4-2-1 headers of the right size (small pipes!) with the correct mufflers installed.

Big mufflers = Big Noise, Big Cost, Big Fuel Consumption, Poor acceleration (mahina humatak), Bat tune (tends to burn rich due to insensitivity).

[isa1023]

Thanks for the reply...

I basically just needed a lead on where to buy it and what results/reactions should I expect.

I plan to add it to a stock single down carburator of a basic, bonestock Mazda SOHC engine with no head and exhaust modifications.

First thing I should inquire with them is if it would actually fit in due to the clearance on the hood cover (you see Astinas have really low hoods I wonder if it would fit knowing it's downdraft carb).

When it's good fit and everything's a go, here's what I actually have in mind...

Right now my top-end power is choked probably because the previous owner had one of the jets choked. When I add the velostack, I might ask the mechanic to dischoke the said jet (then I expect increased fuel consumption from doing that alone) ...

I'm actually into improving the low and mid-end power without choking the top-end that much (because I barely use it now). By improving the low and mids, I expect gains in torque to somehow negate the additional fuel consumption when all jet work as normal. Or maybe the gas save is wishful thinking at this point.

I get your point regarding the length of the stack so I might discuss ideal length with them. I just hope those exhaust specialists would know something about such intake issue.

Also, installing the stack practically removes the air cleaner. My main concern is that if the velocity stack would be a permanent fixture or can be a quick release pack, as I only plan to use it on weekend night runs - air cleaner/filter on weekdays.

No worries about having to frequently adjust the air/fuel inlet screws I think I can do that.

[duskylim]

Dear isa0123:

One trick I tried before was to hydraulically press the velocity stacks into the base of a set of foam-type air cleaners, I had to drive to the province and there was a lot of dust.

That way you could have the stack and still some air filtration.

The only caveat was that the filters projected up about 2 inches from the carbs.

Buti na lang malaki ang hood clearance ng Sigma, kaya nag-kasya.

Regarding 'choking' the carburetor's secondary, you will get little or no benefit in terms of fuel economy that way.

Basically you sometimes need the power that the secondary of the carb produces, especially if you're passing on the highway.

If you kill the secondary, you will try to make power with the primary and end up stepping hard on the accelerator for a longer period of time - thereby negating any possible savings disabling the secondary gave you.

Furthermore there are definitely situations in which you need that boost of power from the secondary and I for one feel that its downright dangerous not to have it.

The best way to save gas aside from keeping your vehicle well-maintained and in good condition is to control your right foot.

I think my friend Bugoy Hagad said it best - "Bali wala kung 1000 cc yung makina kung DOS LITROS yung kanan paa, matakaw pa din ang sasakyan nya!".

Best Regards,

Dusky Lim

[duskylim]

Dear isa0123:

One trick I tried before was to hydraulically press the velocity stacks into the base of a set of foam-type air cleaners, I had to drive to the province and there was a lot of dust.

That way you could have the stack and still some air filtration.

The only caveat was that the filters projected up about 2 inches from the carbs.

Buti na lang malaki ang hood clearance ng Sigma, kaya nag-kasya.

Regarding 'choking' the carburetor's secondary, you will get little or no benefit in terms of fuel economy that way.

Basically you sometimes need the power that the secondary of the carb produces, especially if you're passing on the highway.

If you kill the secondary, you will try to make power with the primary and end up stepping hard on the accelerator for a longer period of time - thereby negating any possible savings disabling the secondary gave you.

Furthermore there are definitely situations in which you need that boost of power from the secondary and I for one feel that its downright dangerous not to have it.

The best way to save gas aside from keeping your vehicle well-maintained and in good condition is to control your right foot.

I think my friend Bugoy Hagad said it best - "Bali wala kung 1000 cc yung makina kung DOS LITROS yung kanan paa, matakaw pa din ang sasakyan nya!".

Best Regards,

Dusky Lim

[isa1023]

Thanks for the reply. I'll have a carb mechanic look at the secondary and ask him to "make it usable" for high rpm.

I'm used to EFI engines before and I'm just beginning to enjoy the benefits of the carb (stable idling, easy to adjust air-fuel intake by just tweaking screws, etc).

Yep I agree the underpower is really (and terribly felt) especially on SLEx.

[isa1023]

[double post]

[duskylim]

Dear isa0123:

A well-tuned, well-made carburetted engine is a joy to use and drive, the twin-downdraft Mikuni's were very reliable and simple to maintain.

Later on I changed them to twin down Weber DGAV's (32mm primary/36mm secondaries).

There was NO NEED to adapt the twin down manifold - FCC sold a special aluminum adapter plate that bolts on to the carb and manifold.

The Webers are even easier to tune than the Mikuni's and give great top end power.

The advantage of twin down Webers was that they were high performance carbs that had an excellent metering design - as long as you understood carburetion, (I recommend reading John Passini's "Weber Carburators") they were easy to tune.

The key to getting the most out of an EFI system is tunability and instrumentation.

That is, the system must be easily programmable and the set-up must have sufficient sensors to provide the data for the EFI.

One of the best systems available is the MegaSquirt system (by Bowling and Grippo).

The code is open sourced and is accessed using Windows software.

The kit is available at a reasonable price and pre-assembled.

The real key is to provide all the necessary sensors - i.e. manifold absolute pressure, Mass air-flow, inlet air temperature, camshaft and crankshaft position sensors, throttle position sensors, wide-band oxygen sensors, etc.

Then you can modify the fuel/air map to suite your engine.

The control you have with such a system is unmatchable with carbs.

But the carbs are definitely simpler.

Best Regards,

Dusky Lim