To port-match or not to port-match? ...

port-match the intake and the cylinder head on a street engine, what is your advice?

  • match intake ports and manifold to gasket size

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Dear community.

I actually rebuild my engine.
The heads are off and also the intake manifold. After test fit of the intake, the heads and the new gasket to both i realized that all those openings do not perfectly match each other.

I asked one of the forum members in here, who already rebuild his 4.6, if he port-matched the intake manifold and he gave me the hint to use the gasket as a guidance.
Gasket to head and intake --> mark position --> port head inlets to match gasket --> mount gasket on marked position to intake --> port intake to Gasket.

Sounds understandable. Seems logic.

But then, ...
I found an older thread on the V8owners-forum started from 3xpendable with the title Rover V8 cylinder head porting advice.
One of the experienced users (kiwicar, Moderator, 5461 Posts) argued against it :

"Match the inlet port sizes to the manifold gaskets "
this is just what you do not want to do on the rover, the port at the head face is already too large to get proper taper into the bowel area.
I am going to list a summary of his main arguments and statements against matching the inlet ports and i would really like to get your opinion on this:
  • The port at the head face is already too large to get proper taper into the bowel area
  • If you take the head face to the gasket then the port at this point will be vastly too large to flow properly and you will get less flow than unmodified heads
  • I am saying is that on rover heads it is counter productive to getting the best flow out of them.
  • by all means match the port to the manifold if you feal you must but do not match either to the gasket do it with templates and take the absolute minimum of metal off either. You are aiming at a constant taper of around 7% area from the end of the bowel area to the open air (or plenum) the key word here being constant
  • you want a constant narrowing of the port from the entry of the manifold runner to the valve area. This builds up the velocity of the air and helps cylinder VE
  • If you open up the manifold and the port face to match the gasket, you will end up with a taper in the manifold runner and then an expansion where the manifold meets the head and then a taper again from the port face to the valve. Not good for VE as the velocity slows down when it enters the port.
  • There are a number of reasons for opening the ports up to the gasket,
    1/ "It has always been done that way"
    2/ "It is what the customer expects"
    3/ "It Obviously is correct"
    You don't do it for the reason below.
    4/ "It is what produces best results"
    Because 95% of the time it doesn't
    If you are buying "stage three" and had 2 sets in front of you, one opened up as far as they would go at the gasket face and one opened up very little, which would you buy? Come on honestly you would expect to see "really big ports"? I bet you do because you will be judging them on criteria 1 and 3, the head porter will be operating on 1 and 2 so when you by "stage 3 heads" that is what you get, in this case number 4 does not get a look in because your average head porter wants to sell heads.
    If you don't want to accept this as an argument then fine, but think for a minute who decides which tyre tread patterns you get to buy, it is not the R&D department it is the Marketing department!
  • The original post was about matching the inlet manifold to the heads by matching both to the gasket This causes a reverse taper in the port and is generally bad for overall flow and is to be avoided where ever possable.
    On 95% of heads matching the inlet to the ports can be done, and if done carefully removing the minimum of material can show some small gains, however it is often the case that under 80% of circumstances it results in no measurable power gain as there are other things going on here than absolute flow numbers. Power comes from an engine not a flow bench bigger flow numbers do not always result in more power. Often it is not worth the effort on anything other than full race engine and it most certainly does not always result in more power.



Well-Known Member
Yes I’ve read his post on that subject many a time and have to agree entirely.
There’s not much I want to add to that as it’s written by someone much more experienced than I, but here’s a few thoughts:
It is bad practice to open ports up to gaskets as it’s essentially a complete guess and unnecessary material is removed.
I’ve read quite a lot on porting from David Vizard’s main book on the subject. It’s a very complex matter and whole engine dynamics need to be taken into account.
Intake manifold ports for example usually work better a little smaller than the head port they attach to, when compared to the exact same profile.
I would leave the intake manifold well alone unless there are serious irregularities in the casting. The P6 SU manifold is restrictive compared to other options but you’ve planned on using the SUs so you’ll have work with that I think. I have an Edelbrock Performer on mine which is a better design, the later Rover ones are quite a bit better too..I plan to space my 500 carb higher to increase intake port runner length, but that’s not really something that could be done with the SUs.
Height of port can be very important (as in the positioning of the floor /ceiling and how that aligns with the attaching port..) This is something gasket matching could potentially ruin.

Oh, I wrote more than I meant to!

Thanks Jim for your answer.

In my specific case i don't built up an engine that is thought trough thoroughly, it is more like working with the parts that are available.
  • The short block is a 4.8 (i measured with a calliper)
  • The heads are standard 4.6 with piper springs
  • Pistons seem to have a high compression. Usually the HC pistons for the 4.6 have a 9.35:1 CR but i guess mine could be a little more because of its valve notches cut in. Anyway i use a composite gasket
  • Camshaft is a Piper 270
  • manifold is the standard rover one for 2x SU, only the ports where the carbs sit are already ported to 2"
  • Carbs are 2" HS8. (Usually i thought the engine is a 4.9 and i do not wan't it to run lean on higher revs)
  • jets are 0.100 and needles BAW (new) to have a point to start from
  • headers are standard 3500S, exhaust is stock too (stainless), there is a chance i will remove one of the silencers this year
  • Final setup should use the standard ellbows with the HS8, and the SD1 Airbox with the bigger intake snorkel and maybe less restrictive airfilters hidden in the airbox
With the piper 270 and the standard gear the engines
useable powerband should be between 2000 - 4500. I don't think it will become a revvy engine from its characteristics.

As the engine is in parts now i want to work on improvements that are useful, like the uprated oilpump.
If a port match is useful, i will do it, if it get's 5 more bhp at >5000 rpm i will spend my time on other mods.

I know that this setup does not use the full potential this particular short block could deliver but this was never intended. My first plan was a rebuild of the 3.5 with ported heads, tubular headers, other cam and those american carbs... but the offer for the block was good and therefor there is no need for me to do more. (for now :p)
What do i expect?
  • i expect the engine to deliver around 220-240 HP as most of you guys running a 4.6 on SU carbs also do
  • i expect the engine to deliver a midrange torque of ~300 ftlb
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Well-Known Member
You'll not know if you've lost, gained, or unaffected power unless you dyno test before and after. If there's a tried and tested combination that somebody has proved in real terms that is a good option or starting point. But as you'll know there's so many variables in the equation that things won't always turn out the same!
Since you already have the cubic capacity increase, I'd say it's probably not worth the small potential gain at the risk of any potential loss of power if that makes sense?

Yeah not many of us have the luxury of building up exactly the engine we want. Mine has been through many stages of evolution from when I first had the idea to buy a 4.6. I've compromised and let go of certain ideas I had, but it is all working out for the best I think.

It sounds like you have a nice collection of puzzle pieces now and I'm sure you'll end up with a smooth, solid engine that does what you want :)

I bolted the manifold down with thin card (an old cereal box) instead of the gasket and poked holes with a pin around where the ports left an impression so I could see how well the manifold & head ports lines up. I then just removed any projecting edges, I've no idea if it made any noticeable difference but it only cost time so I thought why not?
What's the bore and stroke of your engine? I'm just curious because 4.8 is an odd size.
@ratwing @corazon
Thank you both for your reply. This weekend i intend to fit the studkit and the heads, so i will have a look with the manifold.

The bore is still 94mm, but as the pistons are marked with 0.020 i think they are oversized.
They are definitely not 96mm. The stroke is - measured with a caliper - 68,4mm.
Result: something around 4795ccm, almost 4.8

I still don't know anything about the pistons. A few years ago a guy in the v8forum posted information on this pistons and wrote he bought them from "wildcat". The company, not buick wildcat pistons!
The conrods are also not original, they look like forged h-beam, defintely not standard. I forgot to make a photo, and the oilsump is already on.
The crank is a Rover 4.6 one, clearly marked as one, but the journals for the conrods have been machined to the 50.8mm (3.5 specification). The interesting thing is, when machining to the smaller size it the center of the 50.8 was moved outwards resulting in a longer stroke.

carbs keep me a little busy.
I bought a kit to overhaul them and they are almost done now. They are from a 4-pot P6, so they have all their levers for operating butterfly and choke on the same side of the carb.
When using on a V8 manifold, i need all levers on oposite sides to match. I thought to just swap them, but i thought wrong, this can't be done.
I am still thinking how to solve that.


edit: cc typo changed to 4795
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Well-Known Member
@ratwing @corazon

The bore is still 94mm, but as the pistons are marked with 0.020 i think they are oversized.
They are definitely not 96mm. The stroke is - measured with a caliper - 68,4mm.
Result: something around 3795ccm, almost 4.8

I still don't know anything about the pistons. A few years ago a guy in the v8forum posted information on this pistons and wrote he bought them from "wildcat". The company, not buick wildcat pistons!
The conrods are also not original, they look like forged h-beam, defintely not standard.
I'm guessing that was a typo and should say 4795ccm. That would be a 292 in inches as opposed to a 281 for a 4.6, both have a nice ring to it!
Forged conrods could well be machined Chevy items
I wonder if the way your crankshaft has been machined is to wildcat spec?..

You are right. I corrected to 4795.
The buick specification could explain it...

According to wikipedia Buick V8 engine - Wikipedia
the buick 300, had a bore/stroke ratio of 3.750 in × 3.40 in (95.3 mm × 86.4 mm)
So the 86.4mm is the same as i have, but at 94mm bore.

But this engine was a iron block "based on the 215" ... whatever this mean
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much ado about gas flow and porting. when I was young and had an escort mexico Mk1 ( 16oogt kent lump) I thought about that and got advise that as long as outer gasket and exhaust flange was slightly larger than exhaust and inlet ports then gas flow would be good. any further modifications such as polishing ports can actually reduce good air.fuel mix as some turbulence and ideally high vortex for inlet and exhaust gas's is good.
a better exhaust manifold make a greater difference . eg banana bunch (4-2-1) rather than standard adds better 'scavenging' and with good matched system we can increase power a bit and get better mpg. however this is a known are for people with equipment and know how to get best results. much like changing cam lift and over lap.valve sizes .carbs . etc all have an impact and all can get very expensive fairly quickly.
The carbs are fine Jim (when I took it to the rolling road they used parts from yours and mine to build the best pair), just got to replace a recently perished choke O ring.
It sounds like you'll have quite an engine when its done Simon, have you opened out the inlet manifold to match the bigger carbs?
This doesn't really show much detail but it does look like the throttle linkages are on the same side, have the float bowls been altered?

Please excuse my late answer.
Thanks for this video of alex's car. I never watched it in detail, and it shows some interesting details.
@cobraboy made a good picture of his linkage.
I did not realise that the HS8 open counterclockwise, while the HIF open clockwise. So i have to make a complete new linkage. I found a way to manage this without changing the carbs levers. So they will have their float bowls vice versa.
I opened the inlet to 2".


The float bowls will be leveled of course.
My fault, I wasn't clear - I was wondering if the float bowls on Alex's carbs had been altered or swapped because one is on the right and one is on the left which makes the throttle linkage a lot easier to arrange.
I wonder why HS and HIF open in opposite directions?
Now i understand.

I had a detailed look on them the last days. You could change one from left to right, but it is not easy.
  1. Most of the holes for the bolts are already there, but i would need to cut new threads (and i only have metric cutters.)
  2. You can't flip the levers. I would need to buy new ones, flipping doesn't work
  3. If i could manage point 1 + 2, the vaccum connections on one carb would interfere with some srews... so forget it.
Oh yes, as you mentioned they open in oposite directions to the HIF6 :mad:
So a complete change of the linkage is needed.
On this vid you can clearly see how the HS6 linkage on a rover V8 works!

I managed a way to use my existing carbs without changing sides, as a result they still have their levers and the floatbowls on different sides when facing each other on the V8 manifold.
If you want to keep your choke mechanism working levers and float bowl have to be on different sides.
But it works, the space is not a problem, the only thing is, it is not looking as good. I could live with that.
If all is finished, i place some photos in here.
Hopefully after this weekend...

BTW: @RoverAlex mentioned in his vid that the HS8 came out of a jag or daimler, mine are from a P6.
To proove using P6 HS8 work, i found a video on youtube from a user who managed to do it, unfortunatly quality is bad and you can't see the linkage

The next problem to solve is the choke mechanism, the jets move downwards quite a few centimeters. @cobraboy modified the valve covers to fit that.
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Well-Known Member
I had to mod the P6 rocker covers just to get the jets to clear in the up position, I was trying to get a bit more room to allow the jet to drop, I did but not much. In the end I could not get the jets to return properly so gave up on the idea of dropping the jets to choke the car for starting.
I now inject fuel through the old AED stubs and keep the car running on the throttle until warmed a bit.

It is worth the hassle to fit HS8, although the HIF6 can be jetted to suit a 4.6 I wanted to get more air in, I am very pleased with how the car runs. I have not had the chance to try a full throttle run on the road yet due to only just fitting a tach that works with electronic ign.
I have an Autolite 4100 four barrel carb that I intend to fit to an Offy 360 in order to do a back to back test with the HS8, but TBH the SU's are pulling so strong I doubt there will be much gain, it might be a bit more responsive, we will see.


Well-Known Member
Nice tidy solution. What are the shiny 'S' shaped things at the top of the first two photos in your 'zusammen' montage of three photos? Something non car related in the background? How much more air and fuel will these larger carbs flow in comparison to the standard HS6 set-up? Are you going to have a go at port matching your intake to the heads, and the heads to the exhaust? I'm watching and learning.
"What are the shiny 'S' shaped things"

;) Nothing car related. It's a chrome candle holder. I have a very very understanding wife, and she allowed me to rebuild the carbs and the intake manifold on the table of our dining room :p