Burton BLF14 cams; engine mods

[RemcoB]

I recently bought a Twin Cam to install in my Marcos 1600GT, instead of the standard Crossflow (see http://marcossportscarenthusiasts.yuku. ... Tya_o6GbS8). At the recommendation of my mechanic I bought a pair of BLF14 cams (.410, 280 deg.) as I needed new cams anyway. The inlet side cam fits fine, but the exhaust cam doesn't, as the valves seem to be too short. Which modifications are necessary to make the exhaust cam work properly?

My mechanic suggested mounting the standard sprint exhaust cam, while retaining the BLF14 for the inlet. What are your thoughts about this suggestion?

[types26/36]

Burtons do sell longer valves, these are 1.5mm longer but if that is not enough I think you may need ?top hat? type shims as I believe they come in thick sizes although I have no experience of them.

[RemcoB]

1.5 mm works for the inlet side, however the outlet valves need about 5-6 mm which is too much for the biggest shim + longer valve stem. The top hats might work, but I'd need some more information on durability etc.

[Esprit2]

1.5 mm works for the inlet side, however the outlet valves need about 5-6 mm which is too much for the biggest shim + longer valve stem. The top hats might work, but I'd need some more information on durability etc.

5-6mm (0.197-0.236 inch) ?? That's a lot! Are you missing a decimal? Did your mechanic install new valves, and get the stem lengths wrong? Personally, I don't see any cam you install making up that difference. If the valves are wrong, replace them.

Are the cams reground with a smaller than standard base circle diameter? If they're regrinds, ask if the BLF14 profile is available ground on new blanks with a full spec base circle diameter. I still don't see that compensating for 5-6mm short valves, but it's something to look into.

The range of Lotus OEM shims only goes up to 0.150" thick, but some independent Lotus specialists offer thicker shims specifically for use with reground cams. I've purchased shims from JAE up to around 0.180"-plus thick. Call around and ask.

Look inside the tappet and you will find a raised pad in the center. Use a micrometer to measure the overall thickness of the crown including the pad. I don't know the thickness of the stock Twin Cam tappet (026E0024Z, chilled cast iron), but I think it's around 0.180". I know JAE sells an aftermarket tappet for Twin cams that is 0.180" thick.

The upgraded tappet for the Lotus 910 Turbo engine (B920E0017Z, Steel) is 0.224" thick measured over a taller pad, and will fit in the Twin Cam head (early 907-910 engines used Twin Cam tappets). If your tappet is closer to the 0.180" number, then installing a set of later 910 steet tappets will give you another 0.044" of overall thickness.

The 910 steel tappets also have a slightly longer side wall (+0.031 ?) to prevent cocking in the bore when used with aggressive, high lift cams. In their 9XX engines, Lotus recommended against using the short skirt, cast iron tappets with high lift cams.

*~*~*
The inlet cam has more impact on the engine's personality and performance than the exhaust cam does. If you're going to mix profiles, then putting the milder cam on the exhaust side will have less of a negative impact on performance potential than it would if installed on the intake. In this case, there's not a huge difference between the BLF14 and the Lotus Sprint cam (D-cam).

280 deg / 0.410" Lift -- BLF14
272 deg / 0.360" Lift -- Sprint (D-cam)

Lotus did something similar with their 912HC engine in the Excel SE. They used their hotter 104 cam (272 deg / 0.420") on the intake side, and their milder 107 cam (252 deg / 0.378") on the exhaust. The result performed well on the top end, but without suffering quite as much from a loss of low end torque and off-idle performance. Two 104 cams would have made for a hotter engine in the upper end of the rev range, but the combination was a good street compromise for a 4-seater that was expected to have good manners.

Will the Sprint cam work for you on the exhaust? It depends upon what your goals are for the engine. If you're after performance, then no, you'll be giving up some performance potential. But if you're just after good street performance with some manners, then yeah, it could work quite well. Performance should still be a step up from a pair of Sprint cams.

My only (minor) concern is that the Sprint cam (D-cam) has only 0.360" lift, compared to the L14's 0.410" lift. With stock valves, it should be okay, since once the annular area around the open valve equals the port area, additional lift has less significant incremental benefit. But, if (??) you installed larger valves to go with the L14 cam, then the 0.360" lift might be a bit of a limiter in the combination... ie, you may not be getting the benefit you paid for with the larger valves. On the exhaust side, with lots of cylinder pressure available ("Critical Flow"), the exhaust "will" get out; so for the street, no problem.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[rgh0]

There is something fundamentally wrong with the head setup if the BLF14 cam fits on the inlet with a 1.5mm shim which is the absolute minimum but requires a 5 to 6mm shim for the exhaust.

It sounds like the valves are not the right length or not seated in the head in the right location or both. The springs will also most likely not have the correct installed length. I would not run this head until i understood properly what was wrong as you could have a catastrophic failure due to clearance problems such as spring binding. Given the problems observed the whole head assembly is doubtful and everything must be checked in detail

The valves need removal and the right setup dimensions established for use with the base circle on the cams you have. If you cant do it yourself take it to a engine shop who knows how to do twin cam competition heads.

With a .410 lift cam you need longer valves and special valves springs and retainers as the standard items will not work with that lift. You will also probably need thin pad steel followers depending on the exact valve location versus the cam and the cam base circle. Lots of details needed to get it right but worth the effort as the engine loves the high lift cam.

cheers
Rohan

[Elan45]

Sounds almost like the PO had the exhaust seats replaced, but then didn't have them cut to the proper seat angle.

Roger

[RemcoB]

Thank you for your elaborate replies!

5-6mm is indeed the correct difference. The BLF14 does have a smaller base circle diameter than the Sprint cam (by some 3mm). But that still leaves some 3-4 mm of tolerance unaccounted for.

On the inlet side, larger diameter valves were used with a longer stem. However, the longer valves arent available for the exhaust side (which still uses standard diameter valves). We already use the 910 steel tappets. Top hats might do the trick, unless there is something seriously wrong with the head. I'll ask my mechanic what type of valve springs and retainers he installed.

As the Marcos is slightly heavier than an Elan, performance is a factor, so I would like to see the engine working with both BLF14s. However, I need to keep in mind the Twin Cam is a step above the standard Crossflow anyway.

Thanks again for your insightful replies; I will keep you posted on the rebuild.

Remco Bruins

[rgh0]

Hi Remco

With the exhuast you would normally go to a longer stem valve with a 1.4" head. These can be seated on the standard exhaust seat and it is rebored and ported to suit rthe bigger valve. Not absolutely necessay to do the exhaust on a cam like the BLF 14 as the difference is small but like you say getting the longer stem in the standard valves is hard to do and normally requires a special to be made which is not that hard to do and not that much more expensive.

If you have a standard length exhaust valve seated to low in the head you probably have a standard spring that has even less lift than standard which is a max of around 0.37 inch and it will not work with a .410 lift cam. There are a few springs that could potentially be suitable depending on where you end up seating the valve for shimming - have a look at a post I have done on the various springs and design instal;led height and lift ranges, it in the mods section - search my name and springs

Incidently I raced against a guy here with a 2 Litre volvo engined Marcos for a lot of years, it was are very quick car and much faster than me. He used to beat Tony Tompson in his 26R when he raced in the UK before he migrated with the car out to Australia. His car was faster and I supect he was a better driver also

cheers
Rohan

[RemcoB]

Hi Rohan,

For the inlet the thickest shim was used + a longer valve stem; as for the exhaust side we used standard valves, this might explain that we only encountered these issues on the exhaust valves. Currently searching the forum about more information on springs.

Also, I wonder whether it would make any sense to use a big bore exhaust manifold. Would it bring any advantages on a road car? There's ample space under the Marcos' bonnet.

The guy racing a Marcos wasn't Roger Ealand by any chance?

Greetings,

Remco

[rgh0]

Hi Remco

Here is the link to my valve spring selection spreadsheet

elan-mods-f31/lotus-twin-cam-alternative-valve-springs-t25111-15.html

Yes it was Roger, he was very very quick in his Marcos. The only time he did not win his 2 litre class and be near the front even against the 6 lire Panteras 7 Litre Corvettes and 3 litre Porches on the long power circuits was when he blew a head gasket which happened sometimes with what was a very highly developed Vovlo engine.

A big bore exhaust for a road car with BLF14 cams is well worhtwhile.

You will struggle to find a spring that will give the BLF14 lift in a head with the standard valve length. There are lots of possible combinations of dimensions to achieve a workable setup and some are better ways to do it than others. The starting point for the design is the valve position in the head and the dimension from the cam centre line to bottom of the spring centre line which varies signficantly between heads and between individual pockets in a head. You do not need to deepen the spring pockets as you can generally find a spring / valve / retaner combination so that this is not needed as it once was in the past.

regards
Rohan

[RemcoB]

Hi Rohan,

We're currently looking into the valve springs. Spreadsheet is very helpful - thanks! Still undecided whether to opt for the top hats or specially made exhaust valves. The latter are probably the better solution, but would involve taking apart the engine again.

Roger Ealand was highly succesful in the Marcos. I tried to persuade my father to buy a Mantula in the early nineties, pointing him to the great racing heritage of the marque as displayed by Roger's many wins. To no avail - he bought a Fiat Strada.

Kind regards,
Remco

[rgh0]

HI Remco
If you use top hats - they typically come 0.200 inch thick unground and you grind to thickness you want you will still struggle to get the higher valve lift needed on the spring that been installed. You will really need to disaasemble the engine to check the spring installation is OK for the planned cams lift and valve stem lenght if it was not done on assembly and that gives you the option to change the exhaust valves to a longer stem.

cheers
Rohan

[RemcoB]

I got it working with a standard camshaft on the exhaust side (Burton BLF 14 cam 280 deg / .410 lift on the inlet), but it kept bothering me that the BLF14 cam didn't fit. As I'm cleaning and tidiying the engine bay this winter, I pulled out the engine and took it apart. As Rohan suggested above, could there be something wrong with the way the valves have been installed, i.e. angle and depth? Valve seats are stellite iirc. I also wondered why the valve recess edges hadn't been smoothed out. Does this look correct, especially the exhaust valve?

[rgh0]

Long time since last post. The inlet valve looks like its high in the head chamber and the exhaust valve looks like its low in the head chamber which could be an outcome of the high lift inlet that's been installed. To fully understand you need a lot more measurements to see how its been machined to seat the valves and setup the valves relationship to the cams and to the spring pockets and the springs used. Happy to advise on what to measure and how but its not simple and you need to be patient

cheers
Rohan

[RemcoB]

Hi Rohan,

Time flies, no matter whether you're having fun or not This Friday I'll take the head apart. What measurements should I start with?

Cheers,

Remco