Amazon: Electronic Ignition

[Bracpan]

Not sure how we got onto carbs but i like Twin SU's not only for the performance gains but also the looks.
I have these ready and a spare head i can work on. I will collect the rest of the parts when i see for sale at a good price.. I'm not in any mad rush.

[Burdekin]

Originally Posted by Ron Kwas

B;

If the "piston" you are referring to is the Dashpot, they you are not understanding the function or principle of the CV carb...and "cross sectional area either side of the piston" is irrelivant, only cross-sectional area a Venturi vs. mass of ingested air is relevant...Dashpot vertical position is variable, and adjust all by itself, to keep velocity unchanged at Venturi!

Ref: https://www.youtube.com/watch?v=7GRAcqDySog

Cheers

I understand how they work. The two larger carbs air velocity will be slower than single carb due to the volume being larger in the twin carbs. Are you saying this is incorrect and the air velocity is the same for both setups?

My take is at lower RPM the higher velocity of the single carb due to the smaller volume will fill the engine combustion chambers more quickly resulting in more torque but it will get to a certain RPM that the single carb’s volume is the restitution and no longer provide the quantity of air that the twin SUs can.

[Derek UK]

The pistons are raised by the vacuum generated by the engine at a given throttle opening. The engine is just a pump. Cam design is a major factor in the amount of vacuum that the carbs see. Higher lift cams generate less vacuum. Full race cams generate even less and is one of the reasons that you have to keep the revs up when setting the idle. Also the reason that racers keep blipping the throttle when sitting on the line due to the variation of mixture at low vacuums. This problem is much reduced with fuel injection as you can have sensors that modify the mixture at all times. You can't do that with carbs.

[Splinelube]

Thanks for the explanation Derek. Often wondered about that business of blipping the throttle and now understand the principle. Russ

[Splinelube]

Thanks for the explanation Derek. Often wondered about that business of blipping the throttle and now understand the principle. Russ

[Bracpan]

Ok back on ignitions
How many different ignition maps come with the standard123 Dizzy...( not the blue tooth version)
My car at the moment is a low compression B18 with an ugly single Zenith carb.
As i said I will be running twin SU,s new high C/R around 9.5-1 unleaded, 4-2-1 exhaust and a free flow system.. I might change the cam as well.
Will the normal 123 have a suitable map installed ?
I have a good traditional Rolling Road not to far away who knows how to work on carbs so setting it up should not be a problem..I'm not after out and out power just a bit more and better torque...its a cruiser not a racer.

[Burdekin]

Originally Posted by Bracpan

Ok back on ignitions
How many different ignition maps come with the standard123 Dizzy...( not the blue tooth version)
My car at the moment is a low compression B18 with an ugly single Zenith carb.
As i said I will be running twin SU,s new high C/R around 9.5-1 unleaded, 4-2-1 exhaust and a free flow system.. I might change the cam as well.
Will the normal 123 have a suitable map installed ?
I have a good traditional Rolling Road not to far away who knows how to work on carbs so setting it up should not be a problem..I'm not after out and out power just a bit more and better torque...its a cruiser not a racer.

I think the restriction with the B18 is the head valve sizes and the combustion chamber isn't suited for larger valves. You could fit a B20 head. Ben at Tinus Tuning recommended I use my B20A head with the 0.85mm head gasket to raise CR. He said a 9 :1 should be my goal, 9.5 : 1 max otherwise I risk pinging or engine knock with the TT3 cam due to it's very good filling capacity which translates to good torque.

I don't know off hand how many maps are on the standard. Amazon cars have a map installed for them which I think they use on all the B18 and B20 engines. It is supposed to be better suited to unleaded fuel and has a static timing of 17deg. I think their map is based on the B20B map. The difference in the maps really just comes down to smoother running at low and mid range or increased torque. You aren't doing anything crazy so it should run fine on any of them I would say. If you go for a B20B map that I think advances earlier in the rev range so you get better low to mid rpm torque but not as smooth running. You won't need a bespoke map, there really won't be any real gains for you going down that route. Have a chat with Amazon Cars and I'd buy from them, they are a bit more expensive but you get their knowledge and support as well so worth it..

[Burdekin]

Also worth asking Rob about the manifolds and header etc. If going for SUs you can get the separate SU intake manifold and then get the good flowing separate exhaust manifold rather than a header.

https://www.sw-em.com/manifolding_notes.htm

[Burdekin]

Copied from other forums so can't guarantee the accuracy. Combustion chamber and bore volumes from B18 to B20 will be different so not sure what is typically done to get a B20 head on a B18 with decent CR.

B18A (1) 1962-1964, head height 88.0 mm, compr. 8,5 : 1
B18A (2) 1965-1968, head height 88.0 mm, compr. 8,7 : 1

B18B (1) 1961-1963, head height 87.0 mm, compr. 9,5 : 1
B18B (2) 1964-1966, head height 86.2 mm, compr. 10,0 : 1
B18B (3) 1967-1968, head height 86.2 mm, compr. 10,0 : 1

B18D (1) 1961-1963, head height 88.0 mm, compr. 8,5 : 1
B18D (2) 1964-1966, head height 88.0 mm, compr. 8,7 : 1
B18D (3) 1967-19.., head height 88.0 mm, compr. 8,7 : 1

[Derek UK]

Don't use a B20 head on a B18. Yes it will run smoothly but that will be down to low compression unless the B20 head is skimmed quite a bit. B20 compressions are adjusted by head gasket thickness whereas the B18 CR is done by head height. There are different gaskets but Volvo mostly just used one. B20 on B18 gives a mismatch in the larger combustion chamber size on a smaller B18 bore. You then have to pick either a B18 or 20 gasket and neither will match the head to block joint around the bore. Any spec that ends up at 9.0 to 9.5 CR will much prefer best pump fuel. The engine feels noticeably smoother and a bit more lively. Anything over that i.e. the 10.1 engines needs the best, they were designed for 100 octane. B18a and B20a with standard compression can get by on 95 but better on 97, Twin carbs and higher CR best on Premium 99 IMO even at the extra cost.