Cnc ported Pre HE heads

This is what I did and why.
So, deciding witch level you should go for when modifying an engine is important. Here I wanted to be able to make low rpm torque and also use original inlet manifolds, but also more flow than my 493 hp Gr.A engine, because I could use bigger valves on this job.
It is important to not open the upper part of the port to much as it is not there the flow restriction are, the biggest restriction are around the valveseat and up into the bowl.
And with to much cam and large ports you only loose airspeed and the possibility to take advantage of pulse tuning. You will never get above 100% volumetric efficiency. And also the smallest part of the port should not be bigger than the inlet manifold for a harmonic intake.

So, with this port I actually got more flow than my Gr.A heads and they are 32mm at the tightest throat. This is pretty good and will open for a broad use and possibility to make many different setups.

Technical Data:
Inlet valve diameter 43mm
Exhaust valve diameter 35.5mm
Inlet port volume 90cc
Inlet port diameter at manifold face 36mm
Smallest diameter of inlet port 32mm
13mm lift capacity with seals installed
Valve guide material Colisbro
Valve material is manufactured from a one piece electrically upset 214N austenitic stainless steel forging, machined all over, with a Stellite 12 hard tip and a pulse plasma nitrided treatment for low friction and wear. Exhaust valve has additional Stellite 6 valveseat.
Compatible with std inlet manifolds (ports are not oversized).
Flow supports 600HP
Compatible with STD springs and retainers
Available with Mobeck springs and retainers to handle race cams up to 13mm lift and 40 kilo seat pressure.
Inlet flow max 196.5 CFM @10mm lift
Exhaust flow max 141 CFM at 12.5mm lift

Ole Martin Mobeck


Holy cow! Did you start with a stock pre-H.E. head, or are you fabbing new heads from scratch? Will this mod cost more than $50 when someone ships their heads to you?

Yep Kirbert, you are absolutley right, the mod cost more than 50bucks :+1:

I have been hoarding heads and engines for some time and now I have 3 pairs finished for sale in stock and two more pairs cnc ported, but missing guides and valves. Have to sell the three pairs to be able to order the rest of smallparts and finish all.

I don’t expect many heads in return, that works pretty bad actually, but I am very happy to get some in return for shure.

And yes, this is old heads, boiled, cleaned, shootpeened, cnc ported, new guides, cut seat, repair old freezeplugs, fix sparkplug threads if needed, mill the head until gasket face is clean, install valves with teflonseals and lower spring retainer.

I really hope someone want a pair, they flow better than my Gr.A heads

Ole Mobeck


I’m one lottery win away from having ol’Mobucks do my XJS :slight_smile:

Not sure if that joke translates well, but Americans will appreciate the pun. Those who appreciate puns.


Ole. is there any reason for the small 32mm intake port dimension compared to the 36mm manifold face dia.?

Well, @Craig_Balzer, a set of these might light up your car a notch!


I have two sets of Pre-HE heads sitting on a shelf just waiting to get fairy dust sprinkled all over them. Wanna swap two for one :smiley:

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Sure, we can swap, but pairs, and a bucket of cash between :+1:
No doubt I will need new cores to work on

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Norman. Well, there is something about where you want the smallest cross section in a intake runner path. And I dont want it up in the manifold. Also, the flow restriction are not in a strait section of open runner, it is most likely in the complex route between the valveguide and valveseat, and between the valve head and seat. So a 32mm hole does not restrict flow at this level, but it makes it possible to use the std intake manifold without any problems. And I think most of my customers have std intake manifolds, so this was a priority. Anyway, my gr.A heads has smaller runners and valves and developed 493 hp so it should be a good balance between use, customer needs and power.

Many many people increase the runners too much and loose air velocity, than to add up on thet they mount cams with longer openimng periond and loose even more velocity. This make the engine performe pretty good only at high rpm, where the air velocity starts to be good for tuning agan, and the feel is like wow, you can really feel the engine avake at 6000rpm. In reality you could have just the same power with smaller runners and more power further down witch would make a better engine, but you would not feel the kick… I have developed some special cams to work with this also, only 234 degree at 1mm and 12.7mm lift. it will be just great.

The 36mm entrance has something to with Don’s Gr.A manifold can be delivered with that and also it works with std manifold gaskets and the mechined bowl in the end of the std manifold. Just use your grinder around the injector protrucion in the std manifold and it will be much better than std.

Ole Mobeck

This is well above my pay grade level, but I have a set of altered intake manifolds for the Pre-He heads from Roger Bywater and I am wondering how using them might effect your findings? Or, do you think it would be of no consequence?

That will be just fine. I guess you have sligtly longer runnes then and with bellmouths. This promotes low end grunt and you can use it with std or mild tuning cams. No point of using Baywater manifolds for top end power at high levels, but good for street and auto cars.

Ole M

Thankyou Ole for your reply. Matching heads to manifolds is my normal practice when porting heads. Definitely improves Dyno results.