mike3md Posted October 4, 2015 Report Share Posted October 4, 2015 (edited) Dear All, Has anybody any experience of using SUs on a reworked head where the inlet valves have been enlarged? I ask this as it appears that I may have to consider doing this on a 4A head when it goes for unleading and some gasflowing work. I was going to mate this with a longer duration cam ( 280deg ), plus 89mm liners, as well as the usual bottom end improvements. My experience with such cams in combination with gas flowed heads where the valve sizes have not been increased on SUs has been favourable, with good fast touring characteristics and reasonably lively performance. I normally limit revs to 5000, with occasional 5500 bursts. A search on here hasn't turned up much info about this (SUs and larger inlets) combination Please note I am NOT trying to restart the SU/Weber debate, there plenty enough of that already! Mike Edited October 4, 2015 by mike3md Quote Link to post Share on other sites
Peter Cobbold Posted October 4, 2015 Report Share Posted October 4, 2015 (edited) Mike, If you can guesstimate the new horsepower** then this graph show SUs' capacity: http://www.sw-em.com/SU_vs_disp.jpg Two HS6s 1 3/4" should deliver 150bhp OK. The piston will lift a bit more at those rpm where the breathing has improved. But it will be lifted to positions on the needle where it should deliver a decent AFR without retuning. Peter ** not easy - flow through inlet valves goes through three stages: http://staff.fit.ac.cy/eng.cca/auto401_lecture08_2009.pdf Edited October 4, 2015 by Peter Cobbold Quote Link to post Share on other sites
Motorsport Mickey Posted October 4, 2015 Report Share Posted October 4, 2015 (edited) Dear All, Has anybody any experience of using SUs on a reworked head where the inlet valves have been enlarged? I ask this as it appears that I may have to consider doing this on a 4A head when it goes for unleading and some gasflowing work. I was going to mate this with a longer duration cam ( 280deg ), plus 89mm liners, as well as the usual bottom end improvements. My experience with such cams in combination with gas flowed heads where the valve sizes have not been increased on SUs has been favourable, with good fast touring characteristics and reasonably lively performance. I normally limit revs to 5000, with occasional 5500 bursts. A search on here hasn't turned up much info about this (SUs and larger inlets) combination Please note I am NOT trying to restart the SU/Weber debate, there plenty enough of that already! Mike Yes, lots of experience. Peter Burgess carried out an exhaustive program of SU testing on his flow bench back in the 80s, (which is when I came across him) proving or disproving what carb mods worked to best effect. We then utilised the information along with heavily reworked cylinder heads and oversized valves all reworked by Peters then fledgling Automotive engineering firm to obtain excellent results with the SUs for the TR Register Race championship. Since then Peters business has supplied heads to many of the specialist TR restoration firms and still does.He's definitely the man to either advise or rework the head for you giving you whatever characteristics you want, and he has a whizz bang rolling road to try your new head out on and measure the differences offered. http://www.burgesstuning.free-online.co.uk/ I've no connection with his business or him (although he's a thoroughly nice guy) with a disarming and sometimes alarming demeanour for being entirely truthful about what can be achieved. I've used him over the last 25 years for heads on 4 cylinder and Rover V8 engines and he doesn't disappoint. Mick Richards Edited October 4, 2015 by Motorsport Mickey Quote Link to post Share on other sites
stuart Posted October 4, 2015 Report Share Posted October 4, 2015 Two 1 3/4" SU`s will happily supply a 3.8 Jaguar engine up to 210 HP so apart from needing a rolling road to determine needle profiles then you shouldnt have any problem. Stuart. Quote Link to post Share on other sites
TriumphV8 Posted October 4, 2015 Report Share Posted October 4, 2015 The 4-cylinder TR is one of the rare engines that benefit from a larger exhaust valve. I choose 35mm from a Subaru but even bigger ones are in use elsewhere. The undercut between the cylinders to minimize wall shrouding should be smoothed at these engines. We had trouble to find a suitable needle for SU HS6 and at the end started with a stock needle that we grinded down on the top end. Wideband AFR is required to do that job, recommended needles did not work properly on those engines employing cams similar to the envisaged 280 degree cam. Quote Link to post Share on other sites
Peter Cobbold Posted October 4, 2015 Report Share Posted October 4, 2015 Wideband UEGO and a camera to measure piston lift while driving, to find the region of the needle to richen. Quote Link to post Share on other sites
roy53 Posted October 5, 2015 Report Share Posted October 5, 2015 Mike i ran an 89mm full race engine for 15 years on 1 3/4 su. in excess of 190 bhp ,no problem. Did change to 2" later then webers . webers better at top end , su lower down. Over all not much to chose between them.All coped fine. ROY Quote Link to post Share on other sites
mike3md Posted October 5, 2015 Author Report Share Posted October 5, 2015 Thanks all for the thoughts, which give me encouragement! Peter - playing with needle profiles sounds like fun, but I thought watching TV while driving was a no no? I do have Des Hammill's book, though. Mike Quote Link to post Share on other sites
Peter Cobbold Posted October 5, 2015 Report Share Posted October 5, 2015 (edited) Thanks all for the thoughts, which give me encouragement! Peter - playing with needle profiles sounds like fun, but I thought watching TV while driving was a no no? I do have Des Hammill's book, though. Mike MIke, Passenger's job to run the video recording ! You shout out rpm ( at wot) into the mike, passenger shouts AFR from gauge. Who needs digital logging By all accounts Hammil's trick of removing the SU piston spring and damper will make the engine run like a bag of nails at much under wot and high rpm. Lightening the spring and damper oil should be beneficial, once you can measure AFR and lift. Peter Edited October 5, 2015 by Peter Cobbold Quote Link to post Share on other sites
hoffman900 Posted October 6, 2015 Report Share Posted October 6, 2015 (edited) 1 3/4 SUs will support about a TRUE 100hp/carb without modifications to the bridge. There are modifications you can do (through bore (increased 40cfm @ 28" H20) and some mods you can make to the slide that actually improves the jet signal vs. what it is with the bridge in place. See here: http://www.vintageracerules.com/forums/ubbthreads.php/topics/11724/SU_Carburators#Post11724 Webers make more power everywhere over SUs when done correctly. You can do a lot better than Peter for the cylinder head, but you'll have to ship it to the States. The TRs want and need more intake valve... but you are limited due to the zero included valve angle (fouls on the cylinder bore). So you're really stuck just fitting as much as you can without hitting or notching the bore and lifting it as far as the valvetrain can take it. It's all about CFM demand / velocity for your driving range and jet signal. Jet/needle will depend on a lot of things, but will ultimately be dictated by the pressure drop over the jet. How well the head is modified, revs, intake tract length for the rpm range, cam timing events, to the exhaust system will dictate that. Ideally, the less jet/needle you need for a given power range, usually the better. Figure out piston cfm demand at your peak (5500rpm) and work backwards from there. PipeMax is exactly what you need to figure this stuff out: http://www.maxracesoftwares.com/forum/viewtopic.php?f=13&t=242 One thing you'll likely find is that the intake ports are already too big in areas to maintain a target fps of 290-300 at 5500rpm peak. Edited October 6, 2015 by hoffman900 Quote Link to post Share on other sites
Peter Cobbold Posted October 6, 2015 Report Share Posted October 6, 2015 (edited) @Hoffman900. re: http://www.vintageracerules.com/forums/ubbthreads.php/topics/11724/SU_Carburators#Post11724 By removing the bridge he has converted an SU to a Keihin : http://www.vulcangadgets.com/files/keihin_carb.html The air flow for a given depression will not be faster** but the carbs take on an "on or off" delivery, OK for Bikes and circuit racers but terrible for a road car. The SU bridge is designed to deliver the best possible atomisation of the fuel flow and spread across the choke tube and prevent fuel flowing as a film along the floor of the carb. That is critical for a road car and ensuring the mixture has droplets fine engough to distribute eqaully between cylinders. The last can be very trciky with one carb feeding three cylinders. Peter ** But we must not forget where that depression arises. The SU is a 'constant depression device'. His claimed gain in air flow from opening up the area can be misleading. It is not an across-all-rpm gain, it wont happen at below near max horspeowers. It will only happen at the very highest horsepowers. At road loads there will be no gain. This is because of the 'constant depression' operation. . That pressure determines how much air the engine will take in. At airflows ( ie rpm ) that do not fully lift the piston, the absence of the bridge makes no difference to depression and hence to air flow. The bigger flow area will only come into use at the very highest air flows. Removing the bridge could keep the depression constant up to say 120 hp when with the bridge in place it maxes at 100hp. But at 80hp there will be no gain in flow: depressions will be the same. The key to understanding the SU is that feedback between air flow and piston lift: that is the source of the 'constant depression', and that determines air flow past the intake valve ( ignoring the butterfly loss). And that air mass flow rate determines piston lift by restricting the flwo area just enough to give that constant depression. The tweak that matters across the pm range is the removal of the spring, which allows less depression to lift the piston and so better intake at the iv. Piston lift will then max out at lower horsepowers, so opening up the flow area will help then. But for road car he mixture quailty will be horrible with no spring and no bridge: I would not go there. To be fair on the poster does say he doesn't do SU tuning for road use, racing only. Edited October 6, 2015 by Peter Cobbold Quote Link to post Share on other sites
hoffman900 Posted October 6, 2015 Report Share Posted October 6, 2015 Hi Peter, I disagree. My background is in motorcycles and the slide carburetors work very well at all throttle openings. The needle is ultimately what meters the fuel and its position is governed by the pressure drop. In a motorcycle carb, the needle is adjusted by throttle opening. Furthermore, flowbench work and clever and careful modifications actually increase the jet signal vs. with a bridge in place. The thru-bore motorcycle carburetors use a shield around the jet for this. Jet signal is ultimately the key factor in any carburetor modification and can be measured on the flowbench (even when flowing heads, you want a minimum of 25-28" of H2O depression). You can see how in that particular instance the tuner data logs the piston height so he knows exactly where the needle is running at all points in the powerband and at varying throttle openings. This lets him REALLY dial in the needle tapers. Quote Link to post Share on other sites
Peter Cobbold Posted October 6, 2015 Report Share Posted October 6, 2015 (edited) Hi Peter, I disagree. My background is in motorcycles and the slide carburetors work very well at all throttle openings. The needle is ultimately what meters the fuel and its position is governed by the pressure drop. In a motorcycle carb, the needle is adjusted by throttle opening. Furthermore, flowbench work and clever and careful modifications actually increase the jet signal vs. with a bridge in place. The thru-bore motorcycle carburetors use a shield around the jet for this. Jet signal is ultimately the key factor in any carburetor modification and can be measured on the flowbench (even when flowing heads, you want a minimum of 25-28" of H2O depression). You can see how in that particular instance the tuner data logs the piston height so he knows exactly where the needle is running at all points in the powerband and at varying throttle openings. This lets him REALLY dial in the needle tapers. Hoffmann, A Keihin is a constant depression device like an SU, not a slide carb like Mikuni. I quoted the Keihin as the sloped front to the piston resembles the cutaway in the thread you posted...In my view they are both needed to atomise the fuel not to pull it up the jet.Here's why: The pressure drop needed to lift fuel flow sufficient for 150hp through the jet ( of a 2" SU) is about 0.12psi or 3inches of water.** This is closely comparable to the constant depression set by the spring+piston. ( Of course - thats how SU desiged it, within the constraints of fluid mechanics.) The pressure at the jet is only 0.8% down on atmospheric. So where does your 25-28inch water comes from? That is a depression of 0.9psi, a loss of 6% off atmospheric. That 6% dropped is a lot worse than my 0.8%. I am puzzled what you are doing at those depressions.... Peter **page 8: https://supertrarged.files.wordpress.com/2015/05/the-operating-limits-of-an-su-carburettor12.pdf Edited October 6, 2015 by Peter Cobbold Quote Link to post Share on other sites
roy53 Posted October 6, 2015 Report Share Posted October 6, 2015 Read Mikes first post and answer his question. And mr Hoffman why do you assume that we don't do the type of mods you mention ? ROY Quote Link to post Share on other sites
Peter Cobbold Posted October 6, 2015 Report Share Posted October 6, 2015 (edited) And mr Hoffman why do you assume that we don't do the type of mods you mention ? ROY Because you never post anything describing them. Mike has your answer: twin 1:3/4SU are fine for 190hp. He's going to 5500rpm so wont see anywhere near 190... Peter Edited October 6, 2015 by Peter Cobbold Quote Link to post Share on other sites
Alec Pringle Posted October 6, 2015 Report Share Posted October 6, 2015 " Because you never post anything describing them. " Well no, why would any active competitor wish to reveal the detail of his personal tweaks on a Forum for all to read ? That really would be a case of opening mouth only to swap feet . . . . enough of that on here already . . . . Discussing generalities, or theory, or detail of what was done years ago . . . . that's a different matter. Cheers Alec Quote Link to post Share on other sites
TriumphV8 Posted October 6, 2015 Report Share Posted October 6, 2015 (edited) Well no, why would any active competitor wish to reveal the detail of his personal tweaks on a Forum for all to read ? I had the chance to investigate on a TR3 head lifted from one of the fastest cars in TC Challenge at Spa, Belgium. It was dropped on the side to change head gasket and if you are familiar with the details you can see what has been done to get it fast. Also I checked a TR4 head made by a well noted British tuner and I am happy to share what I learned and hopefully get some informations back for my TRs that I do not know today....... Last but not least I could check GTs heads brought to Stoneleigh some years ago while he was busy with a customer. Edited October 6, 2015 by TriumphV8 Quote Link to post Share on other sites
hoffman900 Posted October 7, 2015 Report Share Posted October 7, 2015 (edited) Peter, Attach a SU carburetor to a flow bench. At minimum use 25" depression, ideally you would go higher. Then with a second manometer, attach it to the jet well. Measure the pressure drop in the jet well at varying piston positions. Measure the pressure drop when performing any mods. Lots of flow gaining mods will cause the pressure drop (signal) to decrease. However, not all do and it's possible to increase it over the baseline. Furthermore, datalog the piston position on a running engine as well. That report is very good, however, one must take care to calibrate their models* to accurately capture reality. Ornery Roy, Not sure where the attitude came from but it seems to be a trend in all your posts. Never said you didn't. However, my experience has been Webers are typically used "across the pond" due to rules being historically more liberal in that regard. For 60+ years, SCCA has sought to equalize performance through carburetor size. This has resulted in a lot of testing and r&d work to maximize the performance and flow potential of he SU carburetor. * natural scientist with some background in modeling. Models (which are composed of equations) typically need to be stacked up against measured data. Then you tweak the model to match observed conditions. Then you apply said model to scenarios which you don't have data for, as long as certain conditions remained unchanged. A real life example: The black car in this video is a 1275cc MG Midget. .040" (=1310cc), stock valve sizes, stock connecting rid length, 2x1.5 SU carburetors with modifications I described. Pretty much everything else is open (can't relocate the port or valves or add material in the flow tract). These engines typically make 143-148bhp @. ~ 7600rpm. VE is over 100%. Top speed at this track is 138mph without a draft .., with a draft it was we over 140mph. https://m.youtube.com/watch?v=agEDGBcrPrM Edited October 7, 2015 by hoffman900 Quote Link to post Share on other sites
Peter Cobbold Posted October 7, 2015 Report Share Posted October 7, 2015 Hoffman, OK I get it, the 25" depression is a flowbench operational measure, an indirect measure of mass air flow. I'm interested in the depression you measure at the jet. And how it changes with the imposed lift from the lifter device. Where I'm puzzled is that without the lifter/potentiometer the piston will find it own level and the depression at the jet remain near constant ( except perhaps at very low lifts). Does it ? And is your emphasis on maximising jet depression because jet bore is restricted by the regulations? you need to get the max fuel flow faster? Looks like you have lots of data that could help elucidate the fluid mechanics of the SU. My blog post was focused on mixtures at mass air flows two to three-fold higher than needed to lift the piston fully. I think - hope - I have that nailed. But there is doubt whether the 'constant depression' at low lifts really is constant - it might be augmented by high air velocities and a crude venturi effect. I have not seen any data published anywhere. Peter Quote Link to post Share on other sites
hoffman900 Posted October 7, 2015 Report Share Posted October 7, 2015 Peter, It isn't my data, just sharing. I can put you in contact with the fellow though. Very nice guy and builds what are probably the most powerful Spitfire engines around. Often people do "mods" are carburetors and hurt things. When the jet signal (pressure drop) is low, you have to use a larger gas jet which causes drive ability issues, poor atomization, etc. The game is to maximize airflow while preserving the strongest jet signal. The Holley carburetor guys figured out this was a big part of carburetor modifications a long time ago. Here it is applied to modifiying a DCOE Weber: http://www.landracing.com/forum/index.php/topic,4087.msg223502.html#msg223502 Let's get back to the OP: Quote Link to post Share on other sites
hoffman900 Posted October 7, 2015 Report Share Posted October 7, 2015 (edited) Getting back to the OP: Let's assume you want to build an engine with a 5200rpm peak, Make say ~135 corrected bhp and ~148.7lb-ft. To reach that target, you'll need: 89mm bore 10:1 compression Obviously go with a good ring pack, preferably a modern thinner "metric" design. Total Seal sells a ring package that is 1mm, 1.2mm, 2.8mm ring pack in that bore size. Go with a napier (hook style) 2nd ring. Head: Hard to say as the ports are much too big for rpm this low. The TR4A head has an inlet entrance diameter of 1.5" or 1.77inch^2... this will produce a FPS @ 28" well below 200fps. Target is 225 or so at this point and you only need 1.390" , or about 1.340" in diameter. This should be telling you to NOT make this area any larger. Valve sizes will likely need to be worked out on the bench, but you really can't go wrong with something around 1.61-1.63" I and around 1.41-1.42 exhaust. The gains in this head will be the valve shapes, valve job, and working the throat, short side, and bowl. Valve jobs and the short side radius are typically the key to making power on any engine. Target cfm with the intake manifold and carburetor on will be: 130-138cfm at .486" valve lift and exhaust will need to be around 106-116cfm at .424" valve lift. Figures at 28" of H20. You want to be shooting for 260-280fps average, which you're not going to get with a TR4A head (too big). Likely, you'll find your engine is going to peak higher than you want. The head will be the trickiest part of your plan. Camshaft will only need to be around 260* seat to seat, around 215* @ .050", and will need at minimum .485 and .436 valve lift. Smaller valves will require more lift. That should give you some targets. I think you'll find you'll have an engine that will likely peak around 5500-5800rpm though and even then, the intake ports are too big. The name of the game of a TR4A head: Too big inlet diameter, too small valves. Edited October 7, 2015 by hoffman900 Quote Link to post Share on other sites
mike3md Posted October 11, 2015 Author Report Share Posted October 11, 2015 Again, thanks all for the thoughts, I shall ponder on..... Mike Quote Link to post Share on other sites
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