Good stuff as usual. I forget what VW tuner tested CFM on a bunch of Weber carbs on the flow bench, but he found that the carb with the larger butterfly flows more CFM, even if the choke size is identical, and if anything the larger carb running a smaller choke is more beneficial for throttle response as it provides a more positive signal to the carb from a more dramatic venturi effect. So a 48IDA with 40mm choke will absolutely flow more than a 44IDA with 40mm choke, but the 48IDA with 40mm choke also provides better response.
when i was a rallyboy i would always be above 4k revs its the only way my pinto would take the next gear without bogging so looking at that graph that engine would be perfect for rallying ,its making good power that engine
that comparision doesnt really work, if you put a 38mm choke in a 50mm carb it kills airflow through the carb, so it will make a lot less power than a smaller carb with same size choke, like wise put a 40/42mm choke in a 45's you end up with poor control over the fueling as you end up needing really big main jets due to the loss of venturi effect
I wonder if the increase in torque with the 50's was down to hitting a sweet spot of resonant tuning, as its quite a dramatic difference - nearly 12/13 ftlbs !
ive done a lot of cam timing swinging on the dyno, moving it doesnt really change the power curve much it just rocks it around a bit. generally if you retard the cam you pick up a bit of top end power advancing it improves low speed torque, but that doesnt always follow, i recently ran up a Burton BF 322 it lost power and torque if you either adavanced or retarded it
Sorry, I said wrong. I meant to say you had already the optimum valve events for that combination (carbs, in and ex manifolds, cylinder head)...that is what Vizard often talking. Your torque and power decrease when changing the cam timing. When you dont have the optimum valve events to that combination you normally get more torque or power when changing timing...
Could it be the 50’s with the 42mm chokes are better in the lower RPM due to the difference in the stronger vacuum signal to the auxiliary venturis compared to the 48’s with 42mm chokes
the 50's clearly won imho, only a sligthly loss at the 5k range probably due to the fueling but definitely I'd go with them. Another thing is what do we got under the 4k revs but if it's not a street or rally car and it's built to be in the track, it doesn't really matter.
I saw a comparison with throttle bodies $ webers & the tbs lost 9 hp with alloy bell mouth trumpets the guy didn’t have a comparable trumpet to the Weber so he adapted the Weber one onto the tb & gained the hp so trumpet design & length plays a part when tuning these carbs
It’s not unusual for a carb if it’s big enough to show a little more power than throttle bodies, that can be down to the fact injection uses pressure to atomise the fuel where as carbs vaporise it, which cools down the incoming air and makes it denser
great content
Its so good to see the Bestek distributor providing a strong spark at 8k.
Good stuff as usual. I forget what VW tuner tested CFM on a bunch of Weber carbs on the flow bench, but he found that the carb with the larger butterfly flows more CFM, even if the choke size is identical, and if anything the larger carb running a smaller choke is more beneficial for throttle response as it provides a more positive signal to the carb from a more dramatic venturi effect. So a 48IDA with 40mm choke will absolutely flow more than a 44IDA with 40mm choke, but the 48IDA with 40mm choke also provides better response.
Great info as usual, it would be interesting to see if playing with the intake length could bring back some of the loss in the mid range on the 50's?
the small bit around 5,000 rpm where the 50's lost out is tiny, you would never notice it, its boardering on run to run difference
For a track racecar: 50's
For a rallycar: 48's
Thax for educating all round good stuff.
when i was a rallyboy i would always be above 4k revs its the only way my pinto would take the next gear without bogging so looking at that graph that engine would be perfect for rallying ,its making good power that engine
I agree, apart from when it's tight, wet and muddy, then a tractor type power can be very helpful.
I'd like to see a direct back to back comparison of 48DCOE & 48IDF carburettors with the same venturi sizes, is this possible Graham?
Not unless someone will lend me a pair of IDFs
Would be interesting to see comparison of 45/48/50 with the same size Chokes
that comparision doesnt really work, if you put a 38mm choke in a 50mm carb it kills airflow through the carb, so it will make a lot less power than a smaller carb with same size choke, like wise put a 40/42mm choke in a 45's you end up with poor control over the fueling as you end up needing really big main jets due to the loss of venturi effect
Any difference in Main jet, Emulsion tube selection between both sets of carbs? Great content keep it coming!!
They weren’t identical but weren’t much different either
good output on both, predictable outcome [ish]
any news on the valve failure motor
Gathering up bits to rebuild
For the algoritm!
Great video again Graham but what chokes were fitted …..42 in 48’s and 42 in 50’s ? Or bigger?
Thanks
The 50’s as supplied had huge 46mm chokes, but I had reduced them down to 42mm with no power loss
Great content, thank you. On 50 itb's, 2.2 16v race motor, would say 45 venturis be beneficial? Many tks.
I wonder if the increase in torque with the 50's was down to hitting a sweet spot of resonant tuning, as its quite a dramatic difference - nearly 12/13 ftlbs !
Yes I quite agree
Very nice comparison!
Would you be consider doing one on cam timing?
It'd be interesting to see how much impact that has on the powerband
ive done a lot of cam timing swinging on the dyno, moving it doesnt really change the power curve much it just rocks it around a bit. generally if you retard the cam you pick up a bit of top end power advancing it improves low speed torque, but that doesnt always follow, i recently ran up a Burton BF 322 it lost power and torque if you either adavanced or retarded it
So we can say, you had already optimum cam timing in that combination...very good video, thank you!
yes it was already optomised
Sorry, I said wrong. I meant to say you had already the optimum valve events for that combination (carbs, in and ex manifolds, cylinder head)...that is what Vizard often talking. Your torque and power decrease when changing the cam timing. When you dont have the optimum valve events to that combination you normally get more torque or power when changing timing...
Could it be the 50’s with the 42mm chokes are better in the lower RPM due to the difference in the stronger vacuum signal to the auxiliary venturis compared to the 48’s with 42mm chokes
Possibly, it might just be the 50’s hit a sweet spot at the same point the 48’s did the opposite
Cheers.
the 50's clearly won imho, only a sligthly loss at the 5k range probably due to the fueling but definitely I'd go with them. Another thing is what do we got under the 4k revs but if it's not a street or rally car and it's built to be in the track, it doesn't really matter.
Yes Trying to full throttle test on huge carbs and a GTS4 down at 3000rpm is a bit pointless
I saw a comparison with throttle bodies $ webers & the tbs lost 9 hp with alloy bell mouth trumpets the guy didn’t have a comparable trumpet to the Weber so he adapted the Weber one onto the tb & gained the hp so trumpet design & length plays a part when tuning these carbs
It’s not unusual for a carb if it’s big enough to show a little more power than throttle bodies, that can be down to the fact injection uses pressure to atomise the fuel where as carbs vaporise it, which cools down the incoming air and makes it denser
Landscap for me please.
How does the power compare to the last run you did 8 months ago? As they both are making 207HP. ua-cam.com/video/0hIWk4X8w2w/v-deo.html
same, this engine repeats really well, the engine hasnt be run inbetween it was just pulled off dyno whilst i did some other stuff then put back on