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MG TD TF 1500 - Now at .120 over what carb?
|My XPAG engine is now bored to .120 over. Would the larger 1 1/2 " carbs be appropriate or are the stock 1 1/4" carbs sufficient?|
|Tim if you could find a set of original 1 1/2" as fitted to the TDC I'm sure you wouldn't regret it. You shouldn't be punished if planning to show the car as the bigger carbs were a factory standard on the TDC. Cheers|
Peter TD 5801
|There is a set on EBAY right now if you want to spend $600.00. |
Dan, Thanks for the heads up.
|Save your money, or put it towards headers, a roller cam, supercharger.......|
The Mark 2 went with the bigger carbs, new cam, oversized valves and larger air intake and wound up with a whole 3 horepower.
The Marshall-Nordec was equipped with just a single
1 1/4" SU, and they certainly don't starve for air/fuel.
|A standard TD has 54.5 bhp at 5200rpm, a TDC has 65 bhp at 5500rpm, a standard TF 1250 has 57 bhp at 5500 rpm and a TF1500 has 63 bhp at 5000 rpm.|
(All on paper, in practice there will be differences)
The difference in BHP's withe standard cars is small, and real progress is made when a supercharger (side or front mounted) and/or a crossflow cylinderhead is used.
I agree that only 1 1/2 carburetters won't do anything; at least you will need a different inlet manifold.
Erik van Hardeveld
-TD 1952 with spaghetti exhaust manifold
-Xpagengined TA/Q with crossflowcylinderhead and frontmounted Volumex supercharger
|Erik van Hardeveld|
|A couple of quick comments:|
1) The Mark II or TD/C engine uses the same cam as the regular 1250 engine.
2) The Mark II increase in HP may not sound like a lot, but it's almost 5%, which is noteworthy
3) We use a single 1-1/2" H4 carb with our new supercharger, and I believe the engine would be strangled with a single 1-1/4" carb.
4) It's all a question of breathing, and 1-1/2" carbs will breathe better than 1-1/4" (H2)carbs. The difference may be small, but my seat-of-the-pants experience is that high end power and acceleration will be improved with the larger carbs, particularly in conjunction with other engine mods.
5) I don't think that H4 carbs on a stock 1250 will feel much different than one with H2 carbs, and might even cause the engine to bog down a bit at certain speeds and conditions.
|For reference only, I did an A/B test of 1.25" and 1.5" carbs on my TC. Departure from stock is 9.1:1 compression, street performance roller cam, +30 bore with high top pistons.|
The 1.5 carbs made little to no difference. The only thing that actually gained me power was the overbore and compression bump. Cam and carbs did nothing compared to the old cam and carbs. Possibly a tiny bit of extra torque down low, but not much. I plan to do another 1.5" carb test after some head work.
|It will be interesting to hear what difference the head work makes. I already convinced myself that the ports need to be enlarged and matched to the manifolds, especially on the exhaust side.|
|I was very surprised to see so many feel that changing to 1 1/2" carbies made no real difference. I have the 210 210 08 roller cam & 10:1 CR unblown so maybe I wont bother with fitting the TDC 1 1/2" carbs.. Cheers|
Peter TD 5801
|Peter, I can see no reason to change to 1.5" (HD4) carbs when you consider that Jaguar in their 3.5 liter MK5 and Riley in their 2.5 liter RMB used essentially the same carb as TD/C. Obviously both these cars had a much bigger displacement than a bored out TD. |
|Thanks Hugh. When I first bought the car in the late 60's many of the owners here had already got rid of the oil bath air filter & upgraded to the 1 1/2". The TD had the 1 1/4" (H2) as standard & the TDC, the competition model, had the 1 1/2" (H4) which I guess is why everyone assumed, me included, that the upgrade improved performance. Cheers|
Peter TD 5801
|Peter, further to my comment - my 1934 Bentley Derby sports saloon has a 3.66 liter engine and 1 3/8" twin SU's. All three cars, the Jaguar, the Riley and the Bentley were no slouches but used small carbs compared to what is used today. My guess that this trend to multiple large carbs (ie triple 2" SU's on Series 1 E-types) has more to do with "swagger" than performance. That being said the triple SU's do look very nice!|
|Air/fuel mixture relies on both volume and velocity. If you open the size of the venturi (carb's throat), the air WILL move more slowly. At some point it will move so slowly that failure to efficiently atomize the fuel for proper combustion will result.|
|Isn't the use of triple carbs on a 6 cylinder more to do with optimising and balancing inlet tract length - impossible with twin carbs? |
|M Andrus, if nothing else is done but to install larger carbs then your statement is true. But if the volume of the engine is increased (increased piston and bore size) then the original (smaller) carbs could cause a restriction to air flow. The larger pistons and bore will increase the volume of air drawn into the engine thus the possible need for a larger venturi. Figuring out at what a size of a rebore an engine could benefit from a larger carb(s) is, of course, the problem. Obviously the correct cam and larger exhaust could also benefit the performance but I'm only talking about the increase in engine volume vs venturi increase in this instance.|
|Tim -any good carburetor person (Curto and Braun come to mind) can bush the shaft holes in the carb body. Burlen Fuel Systems used to carry teflon lined bushes that will out last just about anything. Cheers - Dave|
|Thanks Dave. Not sure why you are giving me that info but as always I appreciate your input. :-) I actually re bushed my own carbs an MGB's HIF4s and the carbs on my TD. There is a kit available that comes with a reamer and the bushes.|
This thread was discussed between 28/03/2016 and 31/03/2016
MG TD TF 1500 index
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