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PeteStupps

Mk3 Spit cylinder head quandry

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Hello all, I introduced myself a few months ago on the 'welcome' thread, but work / kids etc have distracted me all summer.

Can you give me opinions on my plans please, I am indecisive and could spend another 6 months weighing up options. Apologies if this is long.

I’ve got a mk3 Spitfire with standard FD engine and I’d like to squeeze a bit more power out of it, without going mad or bankrupt.  I really want to avoid is losing all the mid-range power - I have to pootle through town for an hour any time I want to go anywhere.

The head is off and my current plan is:

  • Open the short-side radius up a little, using dremel & guidance from the DIY threads on here
  • Have my local machinist re-cut the inlet seats to take the larger MkIV valves (including blend the throat a bit if necessary)
  • Skim head to 9.5:1 (?) CR. Not sure how high to go with 25-65 cam and 98 octane.
  • Maybe back-cut new inlet & ex valves, using a pillar drill as a lathe
  • New unleaded exhaust seats, new guides, new standard single springs, re-use the old spring cups and collets.
  • Properly align the exhaust manifold somehow or other.

For induction it’s just got the standard HS2’s, plus radius stub stacks and K&N’s. When the head goes back on it’ll have a Phoenix 4-2-1 manifold which has been hanging on the garage wall for about 2 years. Donning my tin hat for criticism, I am still using points & condenser ignition.

My main uncertainty is the valves. I’ve read that bigger valves will hurt the mid-range, because your gas velocity is lower. However I’ve also read those larger 1.44” inlets are best!

Should I just stick with standard 1.31” valves and SSR mods? Should I use my spare 1500 head with its 1.38” inlets (will require a big skim)? Should I buy a big-valve head?

My longer term plan (2021 probably) is to go for a slightly hotter cam, maybe Newman PH2 or TR5 profile. That will come with a full engine-out rebuild though, which I can’t stomach at the moment.

Thanks for reading!

Pete

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Hi Pete

Others out there with a lot more experience than me I'm sure but I can give you some of my thoughts.

In my experience as long as you don't go daft the 1300 has pretty decent mid range almost whatever you do (more on that later).

So, unless you are going to go for a significalty more radical cam/carb arrangement I would be careful about doing too much on the inlet side or you can make things worse, or at the very least waste your hard earned time/money.  I would suggest matching the ports to the inlet manifold/gasket and tidying them up along the length to the valves but don't remove much more metal than needed to keep things parallel along the length.

The larger valves probably won't make a lot of difference either way at this stage but if you are getting them done then go for the larger size and get them flowed a bit too. I would tend to stick with the 1300 head rather than faffing about with the 1500 but maybe talk to whoever is doing your head to see which works out cheaper. I would suggest that until you've decided on your cam there is no point getting major head work done or chances are you'll have to do it again later.

Again at this point until you decide on what cam/carb you plan on running I wouldn't go much beyond the stock CR of 9.0 unless you spend some time and effort on the ignition side. If you are still running the Delco destributor with points it's very hard to keep the thing on song.

With what you're doing, new standard springs are probably fine but you'll want slightly stronger/double springs when you go for a more radical cam.

Exhaust ports and exhaust manifold should also be matched or at least and step between the two should have the exhaust side larger than the head side. The mounting studs should be good enough to keep this matched. The inlet is more critical but the pins in the head do a good job for this.

I assume you know how to match the ports using a gasket? If not I'm sure there are guides on you tube. I learned how to do this sort of thing when all we had was books!

If you are rebuilding the head and replacing the valves then you need to replace the rocker gear as well or worn rocker tips will wreck the top of your nice new valve stems. If you haven't done so already I also recommend that you fit the external oil feed to you valve gear for the same reason. Remember to block off the oilway on either side of the head gasket at the same time.

The best off the shelf exhaust manifold for the Spitfire is im my opinion the Bell one from the TSSC. This is because it is the only one I've seen where the arrangement of the secondaries allows the primaries from cylinders 1 and 4 to be approximately the same length. But if you got it then what the heck eh!

Points and condenser are OK is but as the biggest improvement you can make is keeping the ignition timing constant then I would strongly recommend that at the very least you go for one of the electronic systems that fit in the existing distributor. I've used the Aldon Ignitor on various cars for years and never had a problem with them. I would also suggest that you replace the timing chain and tensioner to help reduce distributor scatter.

My Mk1 Spitfire currently runs a modified small bearing 1300 engine that I built myself for historic competition. Balanced and polished crank and rods, standard pistons (yep that's right). Larger inlet valves, head and valves lightly flowed. Bronze valve guides. CR is 9.75:1. Runs a Newman race cam 40-80 80-40, 300 degree duration with standard ratio roller rockers and duplex chain. Super lightweight alloy flywheel and 6.5" paddle clutch. Carburation is by twin 40 Webers. Ignition is by a Lucas 25D6 distributor (for the mechanical tacho drive) into which I have fitted the topworks of a 25D4 so I've got the right number of sparks! Aldon Ignitor ignition and 12V coil.

I realise that all rolling roads are different and with varying degrees of exageration built in but with this setup I got an estimated 126hp at the flywheel at 7000 rpm. Although the engine just keeps revving, with the build I have I've set 7200 rpm on the rev limiter. Engine will idle reasonably happily at 900-1000 rpm.

I previously ran the car on a Kent Cams TH6 which has a 290 duration and found the car very 'cammy' not really wanting to do much until about 3800-4000 rpm. With the Newman cam there is vastly more mid range and it will start pulling hard from about 3000 whilst still being pretty docile as long as the revs are above about 1500 rpm. Top end was much the same with either cam but the Kent seemed much more sensitive to fueling/ignition in the mid range. Pootling around in traffic with the Newman cam isn't really a problem (except the paddle clutch being a bit sharp) although after a while the engine will go excessively rich because of the Webers overfueling at low rpm. On a 4.55 diff and stripped out interior the performance is 'vivid' but maybe a bit over the top for what you want!!

I guess the point I'm trying to make is that even with what is not far off a full race spec the 1300 engine still has decent mid range torque and is a very driveable little engine given it's almost 70 years old as a design.

 

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Early MkIV 1300s have the bigger inlet valves. Ours was one such and it got plonked on an FD bottom end with the standard mk3 cam. I did tidy the short-side radius and valve throats up a bit but that was all.

It went ok with the std HS2 carbs with decent mid-range but was never very willing to rev. Not sure we ever got the needles right. Recent addition of EFI and coil pack ignition seems to have woken it up a bit.

Years ago now I built a small crank 1300 for my Herald. That had a professionally done (Silverstone Eng IIRC) stage 3 head with big valves (not much bigger than the biggest OE ones in fact) and some very nice port work. Bottom end was lightened, balanced, tuftrided etc and a Kent TH3 fitted. I tried various SU combinations and many different needles, none of which was wholly satisfactory. I did have the loan of a pair of 40DCOEs from a successful racer for about a week. They worked, but damn it was thirsty! With the SUs it was always pretty crabby below 1500rpm (traffic no fun) but took off about 3000 and revved to Smiths. Would outdrag XR3s, Volvo 360GLTs and give a Rover 216GTI a big fright. Did get wearing a a daily driver though. Eventually the crank broke (too much bouncing off Smiths no doubt) and when I rebuilt it I returned to a std mk3 cam which was a very noticeable come-down, though a lot friendlier in traffic.

Bottom line.... the power is in the head and cam. Yes to the short-side radius work and yes to to the bigger inlet valves though you’ll not see the full benefit without a cam change. If you are having unleaded seats on the exhausts and bigger valves on the inlets, have the machine finish the seats with a 3 angle profile.

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17 hours ago, Escadrille Ecosse said:

Hi Pete

Others out there with a lot more experience than me I'm sure but I can give you some of my thoughts...

 

Thanks very much for this comprehensive reply! I will be referring back to it fairly often I reckon.

The machinist ummed and ahhed about my options but ended up recommending modifying the Mk3 head to take the larger valves, rather than buying another head or heavily skimming the 1500 head.  I will also be replacing the rocker gear, it's all quite worn. Interesting that you suggest the external rocker oil feed - I've read mixed opinions about them! But maybe my worn rocker shaft is a clue.. Likewise port matching - I was only going to do something about alignment of the exhaust manifold, as it has slots rather than stud holes and there is a lot of free movement if just aligning to the studs.

New timing chain and sprockets are on the shopping list too - am not certain whether to go duplex or not. Have heard the available simplex chains stretch quite quickly.

Glad to hear your positive view on the Newman cam. I am leaning towards their 280-degree 30/70 profile.

 

Thanks Nick as well for your experience, it seems encouraging so I'll take the plunge and get some valves / guides / etc ordered! 

Is it worth getting 3-angle cuts on inlet and exhaust seats? Will longevity of exhausts be an issue? 

 

Part of the reason for my hesitance over exactly what to do is that I did the Coast 2 Coast in a MkIV spit which had a Piper fast road (I think) cam, and really didn't like the gutless performance below 3k rpm. Top end was good but not worth the trade-off in my opinion, if you have to scream up the gentlest gradient in 2nd gear...

But after last year's RBRR I had been thinking about getting a bit more performance (and reducing the oil consumption). 

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