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Combustion Chamber Shape - Vizard


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#1 grizzler73

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Posted 07 June 2018 - 01:08 PM

Hi All 

I am looking at setting out the chambers on my 12G940 1275 race head using Vizards bible.

I note on page 192 that the diagram shows an off set of 0.050" to the exhaust valve for valves of 1.440" and 1.480"

is this still the case? I was planning on using 1.440 (35.6mm) inlets on mine.

Also, what is the dimension between the valve centres supposed to be? I have 1.5" in my head for some reason. Finally, are the valves centred on the bore or slightly offset?

Can anyone advise? 

Ta



#2 Moke Spider

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Posted 07 June 2018 - 06:11 PM

The Centre Line of the Valves is across the Bore Centre - that's to minimise any curtain from the Bore.

 

Just before we go too far down any road here, what's the intended use for the car that this head is going on to?



#3 grizzler73

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Posted 08 June 2018 - 12:10 PM

Mokespider

It is a race car engine for my GTM, it has to be MOT'd for the regs but it will not be used on the road really (trailered to meetings etc)

So far I am planning torque over top end as I am restricted by Twin 1.25 SU's (with Vizard tricks), so a good mid range cam (Swiftune SW8 or similar) 1.5 rockers and a fairly standard bottom end. I will probably go with MS evo pistons. Can't see me using over 7000rpm. as for the centreline, sorry I should have been clearer, I meant the other direction, so if the spacing between the valves is 1.5" is the centerline between the valves (0.75") on the centre of the bore?

Cheers



#4 Moke Spider

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Posted 08 June 2018 - 07:09 PM

Turbo Phil or one of the other guys would probably be able to better advise here, however, given that you're limited to the twin 1-1/4" SUs, that really

limits you to circa 100 HP unless you can modify the carbs, but even then, it won't add much more to the potential of these Carbs, maybe add another 5 HP at best.

 

Given your criteria here, being that you are looking for overall improvement, but a better bottom end and an upper rev limit of 7000 RPM, I don't feel that going to such big valves will be helpful here, in fact, likely to opposite, however, your choice of 1.44" Inlets would likely be the way forward. I don't think going any bigger would be helpful. It would be a good idea to off-set the guide(s) if for no other reason than to improve the reliability of the casting (ie, less likely to crack).

 

Gotya on what you are meaning with the valve centres vs the bore centre. They are not equally divided from the bore centre, they are off-set towards the exhaust as that is the smaller valve, just exactly what they are, I don't have to hand, but in any case, do vary from engine and head combinations. On that' if you are going to these bigger valves, I'd suggest a dry assembly of the head to the block and checking, as they may also not be over the bore centre line either. If they are out, this could (and should) be corrected to take better advantage of the head you are preparing.



#5 Turbo Phil

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Posted 08 June 2018 - 07:57 PM

If you're retaining the 35.6mm inlet you can use a 31mm exhaust without offset, you only need to offset when using a 32mm exhaust. That said I offset all my turbo 36/31 valved heads mainly to increase reliability, though there is a small increase in flow to.
Beware a 0.050" offset 32mm exhaust will require a 73mm+ bore size to clear the block.

 

Phil.



#6 grizzler73

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Posted 09 June 2018 - 07:09 AM

Thanks for the answers guys, so I can use the 35.6/31 combination without having to offset, but can I fit a hardened seat in for unleaded? Not a big problem if not as it is a race engine but would make life a bit easier if I could. I plan the draw out the shape of the chamber at the Base and at the face of the head in Cad then cut 2 templates in Ali so I can get a fairly uniform profile before checking volume more accurately and tweaking.

#7 carbon

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Posted 09 June 2018 - 09:57 AM

Grizzler, as a rough rule of thumb for 940 head valve sizes = for standard guide spacing no offset the sum of inlet valve head + exhaust valve head must be less than 67mm. The original S valves were 35.6mm inlet + 31mm exhaust = 66.6mm and gives enough clearance.

 

You can get 36.5mm inlets, and the max exhaust size to use with this is 30mm (Longman used to make heads with 38.2mm inlet and 29mm exhaust, you can just about get a fag paper between these two...).

 

Some of the quickest classic hillclimb Midgets have been running twin HS2s on Maniflow inlet. With a well built Oselli 1380 and 286 cam head these have reportedly been putting out 100bhp at the wheels using well ported head but not excessive valve sizes.



#8 Turbo Phil

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Posted 09 June 2018 - 10:10 AM

Just something to consider but a big valve doesn't always mean it will flow better. I flowbench tested a head last year with a 37mm inlet valve from a well known company that flowed less than a decent 35.6mm valved head.

Phil.

#9 grizzler73

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Posted 09 June 2018 - 10:23 AM

That's a great bit of info, I was thinking to use Swiftunes 35.7/31 valve combination. The guy that runs in the class I want to enter in an Ardley reckons he gets over 100 bhp from his in line A series, that also has to run the same carbs.

#10 carbon

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Posted 09 June 2018 - 11:18 AM

That's a great bit of info, I was thinking to use Swiftunes 35.7/31 valve combination. The guy that runs in the class I want to enter in an Ardley reckons he gets over 100 bhp from his in line A series, that also has to run the same carbs.

It could be worth finding out which cam and rockers he is using. The SW8 may be a bit on the tame side for competition.



#11 grizzler73

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Posted 09 June 2018 - 07:52 PM

I did ask him but he was reluctant to let me know! Anyway I think I will look at several different cams and pick a few brains, I know a guy who worked at Swiftune so I will ask his advice too. What about Unleaded seats though, what do people think?

#12 Turbo Phil

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Posted 10 June 2018 - 01:13 PM

They can be fitted for a 31mm valve, though I personally don't like it as it leaves very little in the insert.

Phil.




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