Sprint Cams?

[Stuart+2]

My '68 Plus 2 runs 190 p.s.i on all cylinders and is solid.
I'm thinking of switching to Sprint camshafts and bigger jets, etc for the Webers but aren't sure what performance gain to expect given the benefit of the Sprint's higher compression is already there (no big valves obviously, but not too material).
I'm trying to decide whether it's worth it - I'm pulling the engine out soon anyway to do the timing chain and clutch.

Cheers

[andyelan]

Hi Stuart

I would have thought this should be a worthwhile mod. As I understood things, most of the performance increase of the "Big Valve" over the SE engine came from the increase compression ratio (10.3:1) and the D Type/sprint cams. The larger inlet valves actually only accounted for about 1 or 2 hp so. Rumour has it, I believe, that such an engie was built (ie high comp and D-type cams and called a Super SE) and this was fitted to early road test Plus 2s which is why they went so fast.

Andy

[Stuart+2]

Thanks Andy - I had the Super SE in mind.
I'd expect maybe 5-6 hp extra from the cams and carbs.
I noticed that QED will re-profile cams for GBP70 each which seems reasonable.
Cheers

[andyelan]

Hi Stuart

I'd just be a little careful re-profiling existing cams. My friend had some done for his E-Type and it pushed the shims requirements for setting the valve clearances outside the range of what's normally available. He ended up having to use special extra thick ones which was both inconvienent and expensive. That said however, QED are very good so should know what they're doing. I would just quiz them on this issue though if I was you.

Andy

[dusty]

I'm just waiting for my S/E cams to come back from QED after requesting a reprofile to their Q360 spec. I will be trial fitting them in the next week or so and will let you know the shim requirements I come across, I also have new valve seats fitted so it should be the worst case scenario re thick shims.

Cheers
Jon

[Stuart+2]

Jon

That's the grind I'm looking for - I would really appreciate hearing how it goes so thank you.

Unfortunately I was outbid on a new set of Kent cams on Ebay a few hours ago

Cheers

[msd1107]

If Lotus had kept the standard intake valve size rather than increasing it to 1.56, in theory, peak power rpm would have decreased from 6,500 to 6,250 and peak hp from 125 to 120.

I know it is common to say the increase in valve size had only a minor effect. But if we look at the SE engine rated at 115 hp, half the power increase was due to the increase in valve size, and half to the cam and carburation changes.

And if Lotus had been really brave and increased the intake valve size to 1.625, in theory, peak power rpm would have increased to 7,000+ rpm and peak hp to 135. This would have been much too risky with the standard bottom end, so the big valve as introduced was a sophisticated compromise between performance and reliability.

If you are spending money on modifying your cylinder head for better power, it makes sense to install the larger valves together with appropriate porting and use a cam with slightly less duration. The shorter duration cams, by a small increment, increases the effective cr, increases the thermal efficiency, and maintains the torque curve at lower rpm.

David
1968 36/7988

[Stuart+2]

Thanks for your perspective David.
I must admit I was encouraged by reports in the Miles Wilkins book of Super S/Es producing Sprint-like h.p. with standard valves.
I wasn't planning to remove the head at all - just change and re-shim the cams along with a new timing chain and sprockets while I have the engine out to do the clutch (plus a few smaller things).
Cheers

[dusty]

Jon

That's the grind I'm looking for - I would really appreciate hearing how it goes so thank you.

Unfortunately I was outbid on a new set of Kent cams on Ebay a few hours ago

Cheers

I will let you know as soon as I can, I'll only be able to tell you how they shim up though as I won't be running the engine for a while (the rest of the car is still in lots of pieces!

Cheers
Jon

[Stuart+2]

That's all I need thanks Jon.
Sounds like a thorough rebuild so hope it's all going well.
Apart from the cams are you doing any other head work/replacements?
I'm tossing up whether to do the cam followers when I pull the cams.
Cheers

[rgh0]

Stuart
When putting in a new cam or a newly reground cam you really should put in new followers or at least grind the surface of your esiting followers back flat again if they are not to badly worn. Otherwise you can potentially have problems with rapid failure of the cam and or follower as you try to get them to lap themselves to match up and bed in properly during the first few hours of running.

regards
Rohan

[rgh0]

Stuart
When putting in a new cam or a newly reground cam you really should put in new followers or at least grind the surface of your esiting followers back flat again if they are not to badly worn. Otherwise you can potentially have problems with rapid failure of the cam and or follower as you try to get them to lap themselves to match up and bed in properly during the first few hours of running.

regards
Rohan

[Stuart+2]

Thanks Rohan
I was going to do them with kerosene and emery but I think I'll just fit new ones.
Cheers

[steveww]

While the head is off I would check for any wear in the valves and guides, it is also worth check the water pump as well.

As Rohan has already mentioned a new or reground cam will need new or reground followers. Remember to run the engine at 2000 rpm for about 20 mins when you finally restart it to bed in the cams.

I had some old L1 cams reground to Sprint specification, I now have to use some thicker shims but still well with in the range that QED supply. With reground cams you will need adjustable sprockets as the chance of them being exactly in time is remote.

[Stuart+2]

Thanks Steve.
I'll have a range of offset dowels at the ready.
Cheers