Straight Port v B head

I have just replaced my B type head with a straight port head on my 3.4.
I am trying to find what the difference in performance between a Straight port head verses a regular B head is.
I have 8:1 comp with plus 30 on the bores, I probably will have to change my timing from 6 to 9deg.
Engine will not be installed until next year, so I am wondering if any of our members has done this before.

It’s a bit hard to gauge exactly what difference an SPH makes, as there was no instance of a straight swap without other changes like carburation or displacement increase. The 3.4 XK150 went from 210 to 250, but also 3 carbs. 2.4 went to SPH with 240 and power from 120 to 133, again, carb change , Solex to SU. My best guess is this: 3.8 and 4.2 triple carb motors produce the same (claimed ) power (265), the 4.2 with twin carbs (420) 245 hp, so an extra carb is worth 20 hp. If a 3.8 with SPH and 2" SUs also produced 245 and 220 with B Type and 1 3\4" carbs, I’d say the head was worth 15 hp and the carb 10hp, but that’s pure speculation.
Be aware that the SPH produces its extra torque only at higher revs than the B Type, ( 4,500 rpm for 3.4 XK 150S vs 3000rpm for B Type ) and top gear acceleration times are slightly in favour of the B Type until 60 to 80mph. A straight port head swap alone may not actually result in improved performance in ordinary driving, IMO.


Thanks mate, lots of info there to mull over.
Without a before and after on a dyno its impossible to gauge.
At this time l have a 4.2 with twin HD8, alloy Fly/whl. With big bore exh man.
My original 3.4 will get installed next year with the Twin HD8’s, alloy Fly. And stage 1 cams, and big bore exh, also a polished inlet man.
So l am hoping all that should be about equal to a 3.8 S motor.
Time will tell, but the 4.2 is a beauty of a powerhouse.
Much appreciate your knowledge.

It will be interesting to see how the 3.4 compares to the 4.2. Sounds like a nice project. Good luck.

3.4 B head and 340 s/p have the same bhp in my book , both with 210 , both with SU carbs !
4.2 XJ6 has about 245 bhp with 2 2" SU carbs .
Not sure why anyone would replace a good running 4.2 for a 3.4 that needs a rebuild , but each to there own :laughing:
I would spend the cash on a 3.54-1 LSD , that gives the old girl far longer legs in every gear !
On saying that , im a boob man :rofl:

Not impossible !

Do as Hammill suggests and perform a simple stopwatch over known distance test, before and after modifications

1 Like

Car has a 3.54 with a 5 speed, 3.4 is original engine and fully rebuilt, eventually has to go back in for resale value to keep as matching numbers.
Thanks for your notes. boobs eh !

Well I would tend to disagree , as the 3.54 and 5 speed are not standard , having engine in the car would make no diffrence , the 4.2 and the rest of the running gear is a great power increase , if you sold car , and the engine was on a pallet , no value would be lost , not forgetting the 3.4 engine is not standard now , with the s/p head .
Cars are only original once , as they left the factory ! :+1:

1 Like

I get your point Ian.

Do the stop watch tests, four times, in each direction, to determine performance more accurately.

1 Like

Will do the 4.2 this year and 3.4 with mods next year… but l hate to remove the 4.2 as it was meant for the 150 ots.
Watch this space.:racing_car::oncoming_police_car::oncoming_police_car::oncoming_police_car:

I replaced my engine with a XJ6 one , made it look like a 340 , only giveaway is the HS8 Carbs , kept the old engine for Originality :rofl:

Don’t think that a pine crankshaft was ever specified.

1 Like

That’s not a Crank , was going to make Cylinder head Gasket :rofl:

1 Like

Might lower the compression too much. Is that a 2.4 or 2.8 block? Looks lower than a 4.2.

Eves says 2.4 if you zoom in far enough :smirk: