Lotus Elan

Proposing to fit L1 Cams into Big Valve

PostPost by: fastcat7 » Thu Jun 25, 2020 5:39 am

Hi;

I'm proposing the fit higher lift L1profile cams into my Elan Sprint. What I need to know Is what the existing clearance is between the Inlet Valve and Piston when at TDC.

The required clearance for the L1 camshaft is 5mm from top of piston, Could anybody please tell me that the distance is greater than 5mm. This would then mean that L1 cams can be installed without further machining being required to the top of the pistons.

The Specification has been copied from Burton Performance website and is posted below:-

Very popular rally/race camshaft. Best power band between 4500-7500rpm. To check for piston to valve contact, ensure there is a minimum clearance of .200” (5mm) between the valve head and piston at TDC.
Power Band: 3500-7500rpm

Valve Lift: 0.398"

Duration: 306° inlet

Timing Figures: 47/79/71/55

Inlet/Exhaust Full Lift Position: 106° ATDC

Inlet Valve Lift at TDC: 0.130"

Exhaust Valve Lift at TDC: 0.127"

Valve Clearances: .010" inlet/.010" exhaust

Durations quoted at .010” / .25mm checking height.

Thanks in Advance

fastcat7
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PostPost by: rgh0 » Thu Jun 25, 2020 7:29 am

Given the large number of modifications made to these engines over the years you really need to check your individual engine as a huge number of factors affect piston to valve clearance. Do a trial assembly and do a "clay" check of the piston to valve clearance around TDC. Lots of videos on U-Tube on doing it.

Generally modern high lift short duration cams to around 0.42 cam lobe lift will clear standard pistons assuming the head and block have not been machined to much over the years and the valves are set correctly in the head but it also depends on the pockets machined in the pistons which can vary a lot depending on who made them. These modern cams have shorter duration than the L1 so less overlap and less lift at TDC so generally achieve acceptable piston to valve clearance.

If going to the trouble of fitting high lift cams there are much better ones than the L1 profile available now for either race or road use.

cheers
Rohan
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PostPost by: fastcat7 » Sat Jun 27, 2020 9:45 am

rgh0 wrote:Given the large number of modifications made to these engines over the years you really need to check your individual engine as a huge number of factors affect piston to valve clearance. Do a trial assembly and do a "clay" check of the piston to valve clearance around TDC. Lots of videos on U-Tube on doing it.

Generally modern high lift short duration cams to around 0.42 cam lobe lift will clear standard pistons assuming the head and block have not been machined to much over the years and the valves are set correctly in the head but it also depends on the pockets machined in the pistons which can vary a lot depending on who made them. These modern cams have shorter duration than the L1 so less overlap and less lift at TDC so generally achieve acceptable piston to valve clearance.

If going to the trouble of fitting high lift cams there are much better ones than the L1 profile available now for either race or road use.

cheers
Rohan


Thank You for your response Rohan Which cams would you recommend for fast road use and occasional trackdays

Thanks in Advance
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PostPost by: rgh0 » Sun Jun 28, 2020 1:54 am

fastcat7 wrote:
rgh0 wrote:Given the large number of modifications made to these engines over the years you really need to check your individual engine as a huge number of factors affect piston to valve clearance. Do a trial assembly and do a "clay" check of the piston to valve clearance around TDC. Lots of videos on U-Tube on doing it.

Generally modern high lift short duration cams to around 0.42 cam lobe lift will clear standard pistons assuming the head and block have not been machined to much over the years and the valves are set correctly in the head but it also depends on the pockets machined in the pistons which can vary a lot depending on who made them. These modern cams have shorter duration than the L1 so less overlap and less lift at TDC so generally achieve acceptable piston to valve clearance.

If going to the trouble of fitting high lift cams there are much better ones than the L1 profile available now for either race or road use.

cheers
Rohan


Thank You for your response Rohan Which cams would you recommend for fast road use and occasional trackdays

Thanks in Advance


You want a cam with a seat to seat duration of less than 290 degrees and the maximum lift possible. This lift and timing gives a very strong and broad torque band and great power to 6500+ rpm. If you have a steel bottom end the McCoy cam below pulls strongly to 7500 rpm.

My favourite is the McCoy / Omnitech cam with 0.440 lobe height and 285 degrees duration at 10 thou lobe height ( equivalent to seat to seat duration with 10 thou clearance. ) This is the highest lift cam under 290 degrees I have found. Depending on your specific head's exact cam to spring pocket base dimensions and valve springs selected you may need longer valve stems and thin followers to fit it in.

The QED 420 cam is another popular choice with a 285 duration and 0.420 lobe height. With their Q55 spring pack this fits most normal heads and standard valves assuming nothing major has been done in terms of head thickness reduction or strange valve height setting in the head.



Other cam suppliers do similar cams. Tony Ingram at Lotus7.com is also another good source of cams and springs

With all these high lift cams you need suitable valve springs and need to check clearances carefully. In particular check piston to valve clearance and check the bottom of the retainer to the top of the valve guide. On some heads you may also find the cam lobe can hit the top of the follower guide if the semi circular clearance cut out has not been machined quite right or if the lobe on the cam casting is not precisely centred on the cut out.

cheers
Rohan
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PostPost by: fastcat7 » Sun Jun 28, 2020 7:29 am

Rohan,
Thank you for the valued information, which gives me many things to mull over, I'm not in a hurry to get it done immediately as I really want to get it correct.


Many Thanks

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