Re: Cylinder Head Recommendations, long post
I don't buy into the claim that modified cylinder head is capable of making 25-30 % HP gain, there not many single things that can boost power to that level, forced induction being one of them. Ok getting back to naturally asperated engines, say you got a nicly ported head, didn't raise your CR and did nothing else, what could expect, I think well below 10% increase. A combination of improvements in the end result in a overall increase. Not long ago a thread started on another forum about the single best thing to do a normally asperated engine, you got alot of folks claiming a ported head, but I know better than that from personal experience. I know compression ratio increase is the number 1 power increaser and we are limited to how much by the gas we have avaialble to use on the street, now I give you the proof that help convince me.
There was on going article recently in a magazine we mostly all read, going thru the build of a very serious A series 1275 build, the builder is very much considered a expert in his field, and that is exactly what I think of him as well, very good builder, and friend of mine. He built a no-holes barred 1275 Midget engine, 1380ccs at 10 to 1 CR. fully ported head, big Rimflow valves, agressive camshaft, 1.5 roller rocker arms, you name it everything that it takes to make power, no expense spared. This engine developed 105 HP at the flywheel on a engine dyno, which is alot for street engine, a very impressive and realistic number, something to be proud of no doubt.
Ok, about 5 years ago, we SCCA guys were handed our first new engine formula in over 30 years, limited prep, the whole idea was the put larger displacement engine with less mods in classes where smaller engines with more mods now only existed. Here's run down of the formula as for items that could make or limited HP, no head portig other than 1" gasket matching, stock carbs other than velocity stacks, .450" limited cam lift measure at the valve with zero lash, stock rocker arms, ;040" over max overbore, stock valve sizes, and a max of 11.0 CR. Mike Miller of Comptune racing and myself did alot of stock head core flow testing on my flowbench to see if some stock heads flowed better than others and no doubt after testing maybe 40-50 heads over the last few years,you better believe there is a difference the better heads flowing a good 5-6 cfm per port than the bad examples. These engines under this formula made 100 hp at the rear wheels, and as much as 115+ at the flywheel.
Ok you've seen a quick run down of the two engines and what hp they developed. So the big question is how can a engine that has less cam lift, no head porting, stock carbs, smaller valves, a good bit less displacement possibly develop a good 10% more than the other engine, what is the only thing the SCCA LP engine has over the street hot rod engine to make more power, the answer is one point more compression ratio. The street engine has the LP engine totally dominated in all aspects other than CR. So for me, this clearly told me the importance of raisng the CR on a given engine as being the #1 hp improvement you could ever do.
Now of course a overall plan of a better camsahft , more CR, (I don't recomend anything over 10.0 to 1 on the street for obvious reasons), a nicely ported head, bigger valves, big bore, all will pay off in the long run, but if you're looking for the smoking gun as the very best way to increase power, raising the CR is the way.
Now I port cylinder heads for customers, so I like to sit here and tell you it is the big punch you are looking for, but IMHO, it only one ingredient in the recipe needed to bake the cake.
Darren what pistons do you have that alone raise the CR to 9.5, most MGB shelf piston only claim 8.8 to 1.
FWIW, I'll be starting on my new personal engine in the near weeks to come, here's my recipe fo what I'm building for myself.
Early 18GB engine
1900 pistons
later straight rods 18v rods,needed for the 1900 piston, ligtened , polished, balanced and ARP bolts
Flat side steel crank, wedge lightened, balanced
Ported cylinder head with bigger 1.7 gas flowed intake valve and larger exahust valves, uprated valve springs I don't have to worry with the exhaust vlave reliefs with the .080" over bore. I'm not bothering with roller rockera rms, I'm convince 1.5 offer little to no gain over stock, and only the 1.625 increased ratio offer an increase and with the cam I'm using I'm satisfied with the cam lift I'll have.
APT VP 12 cam, I could go a little more on cam, but this will make for pretty serious street cam with having a crazy high idle, it should cleanly idle at say 1200 rpms, and be plenty streetable.
10.0 to CR
Aluminum flywheel
ARP hardware thru out.
The idea is to use lightened internal comnenets and aluminu flywheel to hope to induce quicker reveing engine from the B, compared to the 1275, it's kind of dog at reving quickly. The 1900 over bore should add torque, and the ported head with bigger valves should help it breath good, and the cam should shoulf give a nice powerband.
Ok now on to carbs, I'm a frim believer tha bigger isn't always better, every HS6 carb MGB I ever seen has a slight bog off throttle, even in race trim where it doesn't really matter, asn they spend most of their time wound uo anyway. DCOE Weber increase top end powe but at a sacrafice to low and middle power, a downdraft Weber is not even a consideration. So I'm using modified HS4s, port intake to match the head, thinned shafts modified stock velocity stacks open up in the cneter and the throat of the HS2 ported to math the ID of the modded stock v stacks. The HS4 will give me the quickest throttle response and since I do alot of very tight mountain running, this clearly the choice for me, I need the carbs to snap when I ask them to perform in the twistys.
I already have a Peco headers and complete exhast that will stay the same, outside coating the header. Also I've got a new T( 5 seed kit, but this more about highway cruising when I need to do that, and making the car more liveable for travel.
In ever rwally considered the SC, I want a easy to work on car, and the SC takes up alot room, I want simple clean, uncluttered and easy to work on.
I never even considered the x-flow head, they are cast here in the US Darren in case you wondered, not in the UK, no reason to make one travel across the Atlantic twice
The x flow to me is a add on not necessarly meant to be, it extremely clutters up the right side of the motor, makes oil filter changes , ignition tuning and other work a nighmare, no thank you. Then add to that most pro head porters done't want to port these heads due to cast flow issues, same goes for the normal 5 port aluminum head, I don't want any aluminum aftermarket head on my engine, again no thank you.
Ok, there's my personal recipe of what I want to do, not for everyone, but what I want for my MGB.
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