Peter Cobbold Posted May 2, 2015 Report Share Posted May 2, 2015 (edited) To complete a blog on the fluid dynamics of SU carbs I would like to include calculations of choke tube depression, piston lift etc. for a carb'd TR2 to 4 in stock trim. But I dont have measurements on the carb. Does anyone have a dismantled carb and can supply: HS type length of jet to the elbow bore of jet full bore of the choke tube the depth of the bridge measured at the centre the weight of the piston the outer diameter of the piston rim ( =bore of suction chamber), and inner diameter of the rim the rate of the spring or colour code the max horse power anticipated and does the mouth of the carb at the airbox have any form of bellmouth or was it sharp-edged If anyone has measured piston lift with engine flat out at max hp I'd be very interested to know. Peter Edited May 2, 2015 by Peter Cobbold Quote Link to post Share on other sites
Lebro Posted May 3, 2015 Report Share Posted May 3, 2015 I'm afraid both my HS6's are fully mantled & on the car ! Bob. Quote Link to post Share on other sites
BlueTR3A-5EKT Posted May 3, 2015 Report Share Posted May 3, 2015 To complete a blog on the fluid dynamics of SU carbs I would like to include calculations of choke tube depression, piston lift etc. for a carb'd TR2 to 4 in stock trim. But I dont have measurements on the carb. Does anyone have a dismantled carb and can supply: Peter I can partly fill in this request now and when I have done some measuring will supply some more. Carb Fitment TR2 H4 1991cc engine TR3-3A-3B-4 H6 1991cc TR3B-4 H6 2138cc TR4 Stromberg 175 2138cc TR4A HS4 2138cc TR4A Stromberg 175 2138 cc I have samples of H6 and HS6 HS type length of jet to the elbow What is this? Is this the distance from where the fuel enters the jet from the float chamber to the outlet orifice in the bridge? bore of jet ( 0.9 on H4 ) 0 .100 on H6 & HS6 full bore of the choke tube To be measured the depth of the bridge measured at the centre To be measured the weight of the piston To be measured the outer diameter of the piston rim ( =bore of suction chamber), and inner diameter of the rim To be measured the rate of the spring or colour code Red springs on all SU 8ozs the max horse power anticipated factory book says 90BHP@4800 for TR2, 95 BHP for TR3/3A/3B/4 all with 1991 cc engine and does the mouth of the carb at the airbox have any form of bellmouth or was it sharp-edged All sharp edged from my memory If anyone has measured piston lift with engine flat out at max hp I'd be very interested to know. Peter Cheers Peter W Quote Link to post Share on other sites
Peter Cobbold Posted May 3, 2015 Author Report Share Posted May 3, 2015 (edited) Hi Peter, I realised after posting that the mouth treatment of the carb is going to have big effect on piston lift. I'll post the blog later today, usig Helay 3 litre as an exmple. I hope to do another blog specifically on the HS6 TR4. So need: Would 105hp be about right, in stock trim? The jet length is from its open end to the right angle bend where the nylon hose attaches.-the straight brass tube Full bore 1 3/4inch?? The bridge height , rim area, weight are needed. Did the stock TR4 air box/filter have any form of bellmouthing where it bolts to the SU mouth? or is the entry to the SU mouth form the filter housing a flat surface? thanks Peter Its fascinating. I've 'reverse enginered' the fluid mechanics of a 2" SU and can see the volume air flow, depression, piston lift and fuel flow all coming togther in the numbers to give the right mixture. What was particularly startling was the effect on piston lift of changing the shape of the carb mouth. If the pressure drop at the mouth of the carb is ignored then the air flow will,in the standard twin carb application I've run through ( Healey 3 litre), be insufficient even at wot and max rpm to lift the piston more a than half up. When a square-edged mouth is 'fitted' to the model the pressure drop there helps lift the piston more, at the same air flow. It follows that replacing a square-edged factory fitting with a ram pipe will abolish most of the mouth pressure drop, and the piston will not lift as much. The needle ends up lower in jet than in the factory spec and the mxiture goes lean. The leaning effect gets worse as the air flow rises: small at normal road loads but large at max horse power. Mess with the factory mouth shape and the stock needle wont work well. Peter Anyone know how the air filter/air box is mated to the SUs in a Healey 3 litre ? - it the enry bellmouthed? Edited May 3, 2015 by Peter Cobbold Quote Link to post Share on other sites
stuart Posted May 3, 2015 Report Share Posted May 3, 2015 Peter the Healey 3000`s have very similar air filters to TR2/3/3a bolted flat edged to the carb. Stuart. Quote Link to post Share on other sites
Peter Cobbold Posted May 3, 2015 Author Report Share Posted May 3, 2015 Peter the Healey 3000`s have very similar air filters to TR2/3/3a bolted flat edged to the carb. Stuart. Stuart, Many thanks. I can amend the blog. That flat entry means there will be quite a large depression created that that will persist to the butterlfy and help lift the piston more than if there were a rounded entry. So anyone fitting a ram pipe will find that at higher rpm and wot - when the mouth psi drop is greatest- the piston wont lift as much as normal and the mixture will lean. Peter In theory. Anyone got measurements of piston lift with and wthout ram pipes at full chat? Quote Link to post Share on other sites
BlueTR3A-5EKT Posted May 3, 2015 Report Share Posted May 3, 2015 (edited) Hi Peter, I realised after posting that the mouth treatment of the carb is going to have big effect on piston lift. I'll post the blog later today, usig Helay 3 litre as an exmple. I hope to do another blog specifically on the HS6 TR4. So need: Would 105hp be about right, in stock trim? Yes The jet length is from its open end to the right angle bend where the nylon hose attaches.-the straight brass tube That is 55.7mm for both HS4 and HS6 with the tube. The H type used a jet fed with an internal bore from the float chamber. Full bore 1 3/4inch?? Yes that is correct for HS6 and 1.1/2" on HS4 The bridge height , HS6 (1 3/4") 9.1mm bridge to choke & 6.3mm to atmosphere side. On HS4 they are 6.25 mm & 4.3mm respectively, Not easy to measure in truth. rim area, Does this mean the capacity of the inside of the piston - it is a stepped cup and is irregular internally so difficult to measure - External for both is 66.5 mm and internal rim 61.5 mm the rim is 9.3 mm tall internally. The lower internal piston cup from the step down which is the bit that fills the choke hole is roughly 31.7mm deep and 38.2 mm dia & 32 mm diam for HS6 & HS4 respectively. weight are needed. AUC1332 which is HS6 with needle 168 grm. AUC1331 which is HS4 with needle 143 grm Did the stock TR4 air box/filter have any form of bellmouthing where it bolts to the SU mouth? or is the entry to the SU mouth form the filter housing a flat surface? Note on inlet mounting face for the air cleaner - on the NOS HS4 I have it has a sharp edge, the NOS HS6 has a very slight angle 1mm max on one and sharp on the other. Cheers Peter W. thanks Peter Its fascinating. I've 'reverse enginered' the fluid mechanics of a 2" SU and can see the volume air flow, depression, piston lift and fuel flow all coming togther in the numbers to give the right mixture. What was particularly startling was the effect on piston lift of changing the shape of the carb mouth. If the pressure drop at the mouth of the carb is ignored then the air flow will,in the standard twin carb application I've run through ( Healey 3 litre), be insufficient even at wot and max rpm to lift the piston more a than half up. When a square-edged mouth is 'fitted' to the model the pressure drop there helps lift the piston more, at the same air flow. It follows that replacing a square-edged factory fitting with a ram pipe will abolish most of the mouth pressure drop, and the piston will not lift as much. The needle ends up lower in jet than in the factory spec and the mxiture goes lean. The leaning effect gets worse as the air flow rises: small at normal road loads but large at max horse power. Mess with the factory mouth shape and the stock needle wont work well. Peter Anyone know how the air filter/air box is mated to the SUs in a Healey 3 litre ? - it the enry bellmouthed? Which Healey 3000 twin or triple carb? Neither look as though there is any bellmouth arrangement. from the AC pancake filters. Healey 3000 twin carb arrangement https://www.google.co.uk/search?q=image+underbonnet+healey+3000&tbm=isch&imgil=NVaLEeMlbvwynM%253A%253B4uKW8avFUbqPbM%253Bhttp%25253A%25252F%25252Fwww.conceptcarz.com%25252Fvehicle%25252Fz7306%25252FAustin-Healey-3000.aspx&source=iu&pf=m&fir=NVaLEeMlbvwynM%253A%252C4uKW8avFUbqPbM%252C_&usg=__UeD-g1KyWDQHvGioQhqINfza-jw%3D&biw=1280&bih=836&ved=0CCsQyjc&ei=GYZGVdXHEKiy7QaDt4AQ#imgrc=eByS4w8WviFayM%253A%3B4oWq1J7hzd8abM%3Bhttp%253A%252F%252Fwww.theukmotorsports.com%252Fwp-content%252Fuploads%252F2013%252F11%252FP1010573.jpg%3Bhttp%253A%252F%252Fwww.theukmotorsports.com%252Fpage%252F7%252F%3B640%3B480 Tricarb arrangement https://www.google.co.uk/search?q=image+underbonnet+healey+3000&tbm=isch&imgil=NVaLEeMlbvwynM%253A%253B4uKW8avFUbqPbM%253Bhttp%25253A%25252F%25252Fwww.conceptcarz.com%25252Fvehicle%25252Fz7306%25252FAustin-Healey-3000.aspx&source=iu&pf=m&fir=NVaLEeMlbvwynM%253A%252C4uKW8avFUbqPbM%252C_&usg=__UeD-g1KyWDQHvGioQhqINfza-jw%3D&biw=1280&bih=836&ved=0CCsQyjc&ei=GYZGVdXHEKiy7QaDt4AQ#imgrc=wROMTryON4HHUM%253A%3B2G4reOu7MjErnM%3Bhttp%253A%252F%252Fmembers.home.nl%252Frgubbels%252Fengine3carb.jpg%3Bhttp%253A%252F%252Fmembers.home.nl%252Frgubbels%252Fhistory.htm%3B512%3B384 Cheers Peter W Edited May 3, 2015 by BlueTR3A-5EKT Quote Link to post Share on other sites
Peter Cobbold Posted May 3, 2015 Author Report Share Posted May 3, 2015 (edited) Pete, I used the 3 litre with twin HS8s - 'cause I had an HS8 on the coffee table... Thanks for the links. Still trying after a couple of hours to post the pdf on the blog site. Wish Id never heard of Wordsuppress Peter Edited May 3, 2015 by Peter Cobbold Quote Link to post Share on other sites
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