26 replies to this topic

[PACINO]

Hi guys! Reading a old post trying to get an information to improve my carbs I see this; "You have proved a single HS2 with a modified head produces virtually the same as the Twin HS2's". What do they mean with "head" ? (I'm sorry but english is only my third languaje).

Definitely, and after writing rivers of ink, is it really possible to improve the performance of a double Hs2 by following the Vizard method or not?

Regards, Luis

[nicklouse]

He is talking about the cylinder head. (Not part of the carb).

 

having Visarded a pair of HS4s I would get the best inlet and use use a single HIF44.

Edited by nicklouse, 12 July 2020 - 07:52 PM.

[Pigeonto]

He is talking about the cylinder head. (Not part of the carb).

 

having Visarded a pair of HS4s I would get the best inlet and use use a single HIF44.

Blasphemy Nick !! 

Maybe the OP is like me and just finds twins so sexy 

[PACINO]

Thanks Nicklause. It already seemed to me that they could be referring to the cylinder head.
But since we are with this topic, someone can give a definitive opinion on whether with the Vizard method with S.u. some improvement is achieved.

And if so, who does the modification to your carb or does it sell ready-made?

Regards

[Turbo Phil]

Modifying the SU as suggested by Vizard makes a huge difference to airflow. Whether you’ll make any more power will depend on if you’re engine requires the extra airflow. If the current carb setup is adequate you’ll gain little if anything.

Phil.

[timmy850]

Remember back in the day there wasn’t cheap carbs in bigger sizes like you can get today. In his first book he said the best option available was twin HS2’s on the factory manifold. These days twin HS4 (if you want twins) or a single HIF44 will provide more than enough power for a tuned 1275+

I’d rather just buy a bigger carb than try and modify them like he suggested.

[luismx123]

I have modified an HS4 for use on my highly tuned 998. Whether or not it has increased performance is questionable as I have never had it on a Rolling road. One thing to take into consideration before doing things like these is that SU has spent many years developing and tweeking their carbs until they worked correctly. Changing things up like doing vizards tuning influences the balance of the carb and you WILL have more problems getting everything set up correctly... for example my idle quality is really poor with the modified vs a standard hs4 (I have swapped back and forth to test this and checking with an AFR gauge). Another issue I have found is the rising of the dashpot piston being out of sync with the engine properties. I have installed a camera in the intake to see the dashpot piston height relative to engine load and I am struggling with overfueling from 2000-5000rpm as the piston is in the fully open position during full load applications. We swapped back to a standard hs4 and this phenomenon has reduced substantially. 

Im no expert, but I speak from experience and I will not be tweeking any of my carbs going forward. Being forced to accelerate at 75% throttle position to stop overfueling will reduce flow more than the ~15% gain made from vizarding a carb.

 

Edit: Before someone tells me to make sure I have enough oil in the dashpot or try a different piston spring...I have. We have tried every oil weight under the sun, every HS carb spring AND HS combined with HIF springs to limit the piston position during constant load tests....

Edited by luismx123, 13 July 2020 - 08:29 AM.

[timmy850]

Yes, there is more to a road car than just full throttle performance

And flow bench numbers don’t tell the whole story

[Pete649]

Just stating the obvious but going with a single carb also does away with the need to balance a twin carb setup.

[luismx123]

As timmy850 says... There is more to a road car than just full throttle, and this is where the vizard modifications come in handy, and where they may mess with part throttle driving!

[sonscar]

I thought Vizards mods were mainly for Motorsport when the carb was not allowed to be changed?Steve..

[petey81]

I have a set of modified twins. Needles profiled by AC and carbs have the wizard treatment! I have posted my bhp increase on here a few years back (several other modifications on my 998)if done correctly it will increase flow and performance. If not then I would start looking at other areas on the car....

[Turbo Phil]

It will only increase performance if the engine can make use of the extra airflow. If the carb already provides adequate airflow the engine isn’t restricted by the carb so won’t necessarily make any more power.
What modifying the carb does do is allow you to use a carb/carbs that are smaller than you would usually find suitable as the smaller carb now flows like a bigger one. The smaller carb will have a higher velocity through it improving throttle response and improving torque to better VE.

Phil.

[PACINO]

Thanks to everyone who has intervened, but I have several questions.

What I want is for the engine to go further as it happens years ago, when I stepped in fourth gear and the engine stretched and stretched .. 4000 ... 4,500 ... 5000 ... 5,500 ... 6000rpm and that's why I asked by the Vizard method to my Twin S.u. Hs2.

If I'm not going to improve this, the method doesn't interest me at all. Perhaps the best option is to put a double Hs4.

Reading information on how to tune a carburetor speaks about reducing the size of the spindle, the butterfly and cutting the screws. Petey81, could you tell me what other improvements did you get to you carb? the needles.. and what else?

Cheers!

[nicklouse]

Thanks to everyone who has intervened, but I have several questions.

What I want is for the engine to go further as it happens years ago, when I stepped in fourth gear and the engine stretched and stretched .. 4000 ... 4,500 ... 5000 ... 5,500 ... 6000rpm and that's why I asked by the Vizard method to my Twin S.u. Hs2.

If I'm not going to improve this, the method doesn't interest me at all. Perhaps the best option is to put a double Hs4.

Reading information on how to tune a carburetor speaks about reducing the size of the spindle, the butterfly and cutting the screws. Petey81, could you tell me what other improvements did you get to you carb? the needles.. and what else?

Cheers!

what is your engine spec?

 

pointless trying to get it to rev more than it can. my old 1275GT would hit 6.5 easily on a single standard HS4 friends engine recently revs to 7 quite happily to 7k.

 

my old 1380 would rev to the same with twin HS4s or a single Delotro 40 or a weber 45. the Carb makes no difference unless it is restricting the engine.