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:

Factory list 340 with SPH on a 3.4 at 225 HP. Standard 3.4 is 210HP

225 hp. That’s interesting and something I’ve never seen before. I’ve also heard it suggested that the 340 or some 340s might have had different cams as well as the SPH; however the parts manual says they’re the same cams as the Mk2. The 340 certainly seemed to be fast compared with the 3.4 and 3.8 Mk2.

Not all 340s had SPH. I had an early 340 numbers matching and had a B curved port head

Yes, I think the 340 might have served to use up available parts. In addition, a few early ones, especially LHD 340s sold in the US, were basically re-badged 3.4 Mk2’s. Some even had the Mk2’s big fluted bumpers.