Cylinder head deliberations.

[dougal9887]

Hi all,
I'm deliberating on the options for some, if any, modifications to my big valve cylinder head which requires new valves (I plan to stay with standard big valve sizes) and guides.
The car is a Plus 2S130/5. Road use only, some spirited driving and some longer cruising trips (ie when the Mrs is in the car!). And not an every day driver. Torque is more important to me than the last few bhp at 6500 rpm. Also I'm not thinking of big bucks for a few more hp. I prefer to do as much as possible myself. Originality is not a concern. (I have already converted to cv drive shafts, MX5 headlamp motor and X1/9 seats, all for personal preference).
Opinions on these possible modifications, subjective or otherwise, or suggestions, much appreciated.
I think! I have decided to use the 1692cc tall block conversion I have ready to go. (Once head work complete I would need to have the pistons machined for a 10.5:1 compression ratio). I also have a 1558 block which may need a rebore, the pistons having siezed in storage.
1) I need a new exhaust system anyway so I understand that a big bore system is money well spent and I'm thinking of TTR for this. Any other recommendations? I would also match the exhaust ports to the manifold, which appears to be a relatively risk free operation, with care. The David Vizard book recommends boring the exhaust valve throats to 1/8" less than the valve diameter to give a seat width of 1/16"? If desirable for my application, this would need to be done by a machine shop. Replace valve guide in cast iron and ream it myself? or replace in colisbro and have it reamed by machine shop?
2) Inlet port. Check/match to spacers and smooth any casting marks etc. Replace valve guide with new one in cast iron, ream it to size and re-cut 45deg seat myself?
OR
Replace guide with new one in colisbro and have machine shop ream it to size and re-cut seat to 3 angles?
POSSIBLY
Form inlet port venturi shape and further port work as shown in the David Vizard book? This appears to carry some risk!
3) Combustion chambers. These vary from 36.6 cc to 37.5cc. Remove some material to even out.
OR
As above but further relieve around valves to improve flow and polish? Any additional volume will help reduce compression ratio.
4) Cams.
Use existing sprint cams
OR
Have existing cams re-ground to QED 420? and install with Q55 springs etc and shim to correct installed spring length. Make QED suggested carb changes. Piston cut-outs are large anyway but would be checked for clearance.
5)Anything else I've forgotten about?

Quite a lot to think about! Help!

[rgh0]

The starting point for cylinder head work is really the selection of cam and valve sizes based on desired HP and planned use (and how much money you want to spend !).

For road use and with a standard or tall block bottom end and a 6500 rpm rev limit the standard big valve, valves sizes are fine I believe.

From a cam perspective the meaningful next step beyond the standard sprint cam is I believe to go to a short duration ( less than 290 degrees seat to seat) and high lift (0.410 and above) cam. i.e. something like the QED 420 or similar cam. The downside is you need to spend more money on new springs to suit the cam and spend some time making sure the rest of the valve train set up is OK and set up to suit the smaller base circle that the high lift cam will have.

The so called fast road cams with longer duration but about the same lift as the sprint cam are cheap and easy to install as the use the same standard springs and give a little more top end compared to the sprint cam at the expense of mid range torque.

If you stick with standard valves and standard sprint cam then I would not do much else to the head apart from cleaning up any obvious large casting issues in the ports and matching the carbs and exhaust to the ports. The standard engine responds well to a bigger exhaust system but does not need the exhaust port opened up as it is plenty large enough. A step up in size at the exhaust port to manifold is fine ( in fact some people say it is a good thing to have).

Spending the time to match the combustion chamber sizes is easy to do and worthwhile

If you go with the QED 420 or similar then porting work on the head in line with what Vizard says will add a little more power but it is expensive to pay for or time consuming to do it yourself. Also like you say there are some risks involved! Whether the extra effort is worth the 5+ HP extra you may get is a personal decision. personally if I was going to go to the effort of porting the head I would also fit bigger 1.625 and 1.4 inch valves I think to get the full benefit.

Fitting a tall block engine into a Plus 2 can be a squeeze for bonnet clearance and you may need to drop the engine on its mounts a little. A 1700 cc engine with a QED 420 cam, 10.5:1 compression ratio but otherwise standard head and a big bore exhaust is a good high torque engine for a Plus 2 that should give around 145+HP.
Doing a lot of porting work and bigger valves will give over 150HP but not change the mid range torque much

From a reliability and life view point I would stick to the standard width valve seat faces and use colsibro valve guides. If you are having a machine shop do that work then you may as well get them to cut new 3 angle seats at the same time.

cheers
Rohan

[dougal9887]

Rohan, thanks for the detailed reply, it is much appreciated.
QED 420 cams it is then with larger bore exhaust and matched ports. You mention a step up in size at exhaust port to manifold, does that mean that the exhaust port diameter should be marginally larger than the manifold diameter? Quite happy to tackle the valve train work, I suspect QED have it well sorted with their Q55 springs etc so that shimming to the correct installed spring length after valve work should be all that's required then a valve to piston check. This should be ok since the pistons are QED with the large cut outs. I shan't get the pistons machined for compression until then just in case the cut outs need adjusting.
On the subject of machining the pistons, I asked QED for some advice. They were quite happy with machining off the raised part but suggested not to go too much further. This would give me 1cc of the 2cc's I'm looking for. I would probably chance going another .22mm for the other cc as that sounds not too much further to me? They couldn't give me a crown thickness as it's the older type cast piston from ASSO. I might gain a little volume also after work on the combustion chamber, but do you imagine I could gain 1cc? Presumable I would aim to smooth away the slight well that the valves sit in, blending into the rest of the chamber?

[rgh0]

The exhaust manifold can be slightly larger than the ports in the head so there is a step up in diameter as the exhaust exits the head


You have 1 cc variation in combustions chambers so you will gain a little when you work on them of around 1 cc in the smallest at least. When I open out the combustion chambers I focus on the area around the inlet valve to minimise the shrouding effect of the surround head. If I still need more volume I do the same on the exhaust valve. I also round of any sharp edges to minimise opportunities for hot spots.

0.22 mm does not sound excessive off the tops of the piston. I would measure the total crown thickness and if 0.22mm is only a small percentage of the total removing it should not be a problem

Quite a few variables in setting up a high lift cam and valve gear right in the twin cam and no single "ideal" way but QED should have the formula sorted by now for their QED 420 cam that they recommend for a road engine and can supply all the parts for.

cheers
Rohan

[dougal9887]

Thanks again Rohan. Time to get down to work then!

[john.p.clegg]

Are you thinking of a pair of 420 cams...I don't think you will need 420 on the exhaust...I'm about to take my 420 out as it's moved the power up the rev range...never have like high revving engines ( ever since a 250 yamaha burned off my 650 BSA ) and that's why many people fit five speed gearboxes...

John

[BullAndrew]

Hi Dougal

Apologies for being off topic but could you post some pics and details of your MX5 headlamp motor conversion, probably a different topic but it would be good to see your solution,

Thanks
Andrew

Ps the tread is good I am in a similar position Rohan?s comments are very useful

[dougal9887]

John,
I never even considered the possibility of different lifts and periods on the 2 camshafts. I assumed they would always match. Has anyone experience with this solution.
Dougal.

[john.p.clegg]

Dougal
There is a formula somewhere ( Vizard book? ) that gives the optimum lift for valve head diameter,I think .420 is good for 1.625" and smaller valves need less lift..

John

[dougal9887]

Andrew,
I agree. Rohan's advice is indispensible. How can you make these decisions when it's a once in a lifetime, for me at least, operation? ( Might have been different if the car hadn't sat in the garage for the last 35 years )
The MX5 conversion does indeed deserve a new topic. The mechanical side is straightforward, the electrics less so if you wish to flash. I incorporated the MX5 retractor unit but some additional relays and diodes are required due to the wiring layout in the Elan. I'll do my best to start a new topic shortly under Electrics/Instruments. I have the pics but need to do a proper wiring diagram.
Dougal.

[mbell]

(The MX5 lift motor has been done and covered many times on here. Enough for me to manage to do it to my car. Searching is always tricky to find info but there is plenty on here if you can find it.

Dougal, please still do post information on your conversion its always useful to see how different people have addressed the problem to help other solve the same problem. Even though I've done my car I am still interested to see how other people do this.)

[vernon.taylor]

Salut Doug

Another off-topic request - a photo of your X1/9 seats in your car, please.

Merci

Vernon

[msd1107]

The guideline of valve lift is 1/4 valve size and is just that, a guideline. You get improvements in flow by going with higher lift.

However, the TC head is not well suited for higher lifts. I think Rohan has covered this topic before. But you get into more or less ugly constraints the higher the valve lift.

David
1968 36/7988

[oldchieft]

D/4 was alway the guide for valve lift, as that is the annular area that is equal to the bore D.

This is used in all screw lift valves in water lines on ships.

More then D/4 does not give better flow and can give a loss of gas velocity, this can give poor charging of the cylinder and less turbulence for combustion.

This is all from memory so I could stand to be corrected.

Jon the Chief

[rgh0]

D/4 is a good guide for maximum valve lift as it typically means that the valve throat rather than the valve curtain area becomes the dominant flow constraint. However it is only an approximate rule of thumb and a not a hard fixed limit. Twin cams in practice benefit with up to around 0.5 inch lift with 1.625 inch inlet valves in practical experience.

Using a lower lift / lower duration exhaust cam can benefit in certain situations as it does tend to move the torque peak down the rev range a little and broaden the torque band with only a little loss of power. This seems to be most significant and useful when using something like a 0.5 lift 310 degree inlet cam and where using the same cam in the exhaust results in a powerful but very narrow and high rpm torque band good for a light open wheeler where you can swap gear rations for every track easily but no so good for a heavier sports car like an Elan where swapping gear and diff ratios I not an easy option. The benefits of doing the same thing in less extreme cam profiles is less. I have not tried it with a combination of something like a QED 420 on the inlet and a standard sprint cam on the exhaust but it would be an interesting experiment. I will have a look at it in my engine simulation program when I get some time to see if its worthwhile trying in real !!!

cheers
Rohan

cheers
Rohan