Lotus Elan

CR and valve timing

PostPost by: cabc26b » Wed Feb 17, 2010 1:37 pm

I was on the phone recently with a trusted advisor buying parts for the elan and he asked some questions about the engine build I have going on. Anyways - the discussion turned to valves, cam timing and compression ratio. He mentioned some constraints that I had not heard before.

So, here's the question - if I am running 10.5/1 compression with small valves and race porting is there a diminishing return on valve lift ( short duration) ?

Thanks

George
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PostPost by: rgh0 » Wed Feb 17, 2010 6:10 pm

Yes there is diminishing return not driven so much by higher lift directly but by the fact that you need longer durations to get the very high lift possible and the longer duration starts to cost you mid range torque. This loss of torque is more noticeable in a 10.5 comp engine than in a 13;1 comp engine as the higher compression compensates to some degree for the loss of compression caused by the longer duration of valve overlap. Also the lift to get maximum flow out of a small standard valve FIA head is less than a big 1.7 inch inlet valve hart style head.

In practical terms going beyond around .45 lift and around 290 to 300 duration in a FIA style head to .5 lift which requires around 320 degrees duration gets into the area of dimishing returns where a potential small increase of power is not warranted due to the mid range torque loss.

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PostPost by: elansprint71 » Fri Feb 26, 2010 1:03 am

That's pretty much what I thought, apart from the "hart" bit, which, I have to say, has got me somewhat baffled. :twisted:
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PostPost by: Dag-Henning » Fri Feb 26, 2010 8:39 am

rgh0 wrote:Yes there is diminishing return not driven so much by higher lift directly but by the fact that you need longer durations to get the very high lift possible and the longer duration starts to cost you mid range torque. This loss of torque is more noticeable in a 10.5 comp engine than in a 13;1 comp engine as the higher compression compensates to some degree for the loss of compression caused by the longer duration of valve overlap. Also the lift to get maximum flow out of a small standard valve FIA head is less than a big 1.7 inch inlet valve hart style head.

In practical terms going beyond around .45 lift and around 290 to 300 duration in a FIA style head to .5 lift which requires around 320 degrees duration gets into the area of dimishing returns where a potential small increase of power is not warranted due to the mid range torque loss.

cheers
Rohan



- like the QED 450 profile ; - .44 lift / 298 deg. above 170 Nm at 5.600 rpm with FIA valves and 11,6 CR. Driveable ! :D


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PostPost by: rgh0 » Fri Feb 26, 2010 11:16 pm

Hi Dag

I actually sent to George a pm with the details and dyno results you sent me on the QED 450 cam as an alternate recommendation a couple of days ago.


The hart style heads i referred to were the heads developed by Hart for their 416B Lotus Twin Cam engine for Formula B racing in the USA. Basically they did everything possible to make the head breath - biggest possible ports of the best possible shape, biggest possible 1.7 inch inlet valves, biggest possible lift cam around 0.5 inch. Lots of power but all top end which really only suits a lightweight formula car with gear ratios being selected for each circuit.

The downside of these heads was that they were fragile and really a throwaway item after a seasons racing as so little metal left in them

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PostPost by: jkolb » Sat Feb 27, 2010 8:08 pm

My Elan came equipped with a Brian Hart head and I can vouch for Rohan's comments. The 1.7 intakes cause metal erosion around the plugs. It would have required a lot of welding to bring back, with no guaranty that I would ever have a usable head. Now a big expensive door stop.

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PostPost by: cabc26b » Sun Feb 28, 2010 2:42 pm

As Rohan has mentioned I have some alternatives being suggested , Dag's being but one of them along with what my builder knows plus some other reputable builders ( may thanks to Rohan for the back and forth over the last week) . No one seems to know why the configuration is the way it is, but on the other hand no has taken the time to work up a cnc porting configuration/program like this either. Out of respect for John we are moving forward to test his head as supplied ( unless my builder is not comfortable with mechanics of the head)

At this point , I think it comes down to spring pressures and what the dyno says - I have more than one source saying that my HP limit is driven by the exhaust manifold at 175hp - big bore TTR - ( I bailed on the stepped header we were cooking up) which is fine b/c I'm looking for torque anyways. ( Hmmm we have a couple of different 4into1 twin-cam header sets there.... have to see how dyno time I can afford)

I am also going to bump the CR up a little ( I guess I get to dose the gas like my last street elan) The target is 170 drive-able hp -

George
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