Lotus Elan

Large port cylinder head - cam choice

PostPost by: promotor » Thu Nov 16, 2023 9:24 am

I've got an engine to assemble and the cylinder head has been ported fully (not just cleaned up in the bowls and manifold faces) and is for 45 dcoes at the manifold face. The ports are quite large the full length (I will get some measurements today at some point).
Engine is a 1558 bottom end with some Vegantune cams around a modern 420 spec. Currently at approx 9.8:1 compression ratio with a view to changing this to 10.5:1 when new pistons are fitted.
Valves are Sprint size.

I'm assembling the engine for someone who has concerns about the cams being too much and he is thinking of fitting S/E or Sprint cams to dial the engine back and make it easy to live with. One of the cams fitted is very worn on one of the bearing journals (0.003" wear!) so a replacement should be fitted. This has prompted the re-think on cams.

My concern is that due to the ports being so much bigger than a stock head that fitting S/E or Sprint cams together with 45's (even with very small chokes fitted) will make this engine very tame and likely worse to live with than the 420's. I suspect it's going to be lacking torque low down.
I think the porting would probably support much more than the 420's, possibly 440/450's (although I'm not recommending going beyond the 420's) which gives an idea of where this head was originally meant to be.

One "halfway house" option is to run the 420 on the inlet and fit a sprint / S/E cam on the exhaust (in part, due to cost and availability of standard used parts, also due to not needing as much cam on the exhaust due to its ability to breathe) to lower the usable rev range.

This cylinder head in my mind is not best placed to give the desired results (ie as pleasant as a S/E / Sprint engined car) but it looks like the head is here to stay, so making what we can of it is the current route.

Has anybody run a large port head with tame cams, and what was the result?
Otherwise, experience and opinions (regarded in that order!) are more than welcome.

Thanks for reading.
Last edited by promotor on Thu Nov 16, 2023 9:58 am, edited 1 time in total.
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PostPost by: rgh0 » Thu Nov 16, 2023 9:49 am

As long as the cams are less than 285 degrees duration seat to seat and the carb choke is around 34 mm You should be able to set it up on the dyno so its even more tractable with better mid range torque and better top end power at 6500 rpm than a standard sprint cam head. Using a sprint exhaust cam will loose a little top end and maybe increase the midrange torque a little more still.

Many years ago I used a full race ported head with sprint cams while my Elan was still more of a road car and it was good on the road and better than the standard sprint head setup it replaced

More recently I have built a McCoy full race head engine with McCoy 285 degree 0.440 lift cams and 36 mm chokes this again was a great road engine.

cheers
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PostPost by: nmauduit » Thu Nov 16, 2023 10:16 am

This is a somewhat puzzling situation (what are the goals of the customer, precisely?), yet here are my 2-3 cents

- Q420 cams are very road compatible in my experience : if this head has to be retained I would fit news Q420 cams and try it like that (cams could easily be swapped further down if somehow need be) - limited overlap would provide at least stock level torque over the rpm range (proper carbs tuning is required, even with stock cams and carbs). Q420 can be used with "stock" valves/head provided that the matching "Q55" springs are installed (I've tried it myself, it works - fitted length 1.150") theses springs are good to 6500rpm and would be light enough not to reduce much lifetime due to contact pressure (while avoiding binding at full lift).

- tuning an engine is like a chain: it's limited by the weakest link. Here a restriction would be cams
duration/opening I suppose within a head "fully" ported (not a new SAS I guess). The rationale to underuse the potential of such a head is a bit unclear to an external reader (retain the possibility to increase power later ?), as if anything very large valves would be a bit less reliable than standard ones in the long run: if absolutely set to stock power and specs, why not using a standard head rather? In any case the full power of such a head would only be apparent at higher revs. Likewise using 45 (even with minimal venturies) rather than 40 is somehow puzzling and is not the easiest to downtune an engine (carbs tuning will require more work than with 40s I bet), but should work in the end. If only downtuning the engine is the goal, the fastest/simplest/cheapest would be to take out some spark timing imho...
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PostPost by: 2cams70 » Fri Nov 17, 2023 6:52 am

It might be worth your while talking to SAS about their standard versus race port heads and the pros and cons of each and recommended set ups. The information gleaned may be useful for your situation
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PostPost by: promotor » Fri Nov 17, 2023 7:45 am

rgh0 wrote:As long as the cams are less than 285 degrees duration seat to seat and the carb choke is around 34 mm You should be able to set it up on the dyno so its even more tractable with better mid range torque and better top end power at 6500 rpm than a standard sprint cam head. Using a sprint exhaust cam will loose a little top end and maybe increase the midrange torque a little more still.

Many years ago I used a full race ported head with sprint cams while my Elan was still more of a road car and it was good on the road and better than the standard sprint head setup it replaced

More recently I have built a McCoy full race head engine with McCoy 285 degree 0.440 lift cams and 36 mm chokes this again was a great road engine.

cheers
Rohan


Thanks Rohan. My feeling is camming down the exhaust would be the best option.
The McCoy head spec sounds nice. What were the BHP & torque figures and at what revs?
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PostPost by: promotor » Fri Nov 17, 2023 7:58 am

nmauduit wrote:This is a somewhat puzzling situation (what are the goals of the customer, precisely?), yet here are my 2-3 cents

- Q420 cams are very road compatible in my experience : if this head has to be retained I would fit news Q420 cams and try it like that (cams could easily be swapped further down if somehow need be) - limited overlap would provide at least stock level torque over the rpm range (proper carbs tuning is required, even with stock cams and carbs). Q420 can be used with "stock" valves/head provided that the matching "Q55" springs are installed (I've tried it myself, it works - fitted length 1.150") theses springs are good to 6500rpm and would be light enough not to reduce much lifetime due to contact pressure (while avoiding binding at full lift).

- tuning an engine is like a chain: it's limited by the weakest link. Here a restriction would be cams
duration/opening I suppose within a head "fully" ported (not a new SAS I guess). The rationale to underuse the potential of such a head is a bit unclear to an external reader (retain the possibility to increase power later ?), as if anything very large valves would be a bit less reliable than standard ones in the long run: if absolutely set to stock power and specs, why not using a standard head rather? In any case the full power of such a head would only be apparent at higher revs. Likewise using 45 (even with minimal venturies) rather than 40 is somehow puzzling and is not the easiest to downtune an engine (carbs tuning will require more work than with 40s I bet), but should work in the end. If only downtuning the engine is the goal, the fastest/simplest/cheapest would be to take out some spark timing imho...


Thanks Nic.
The requirements for the engine are to make it as usable as possible but the main components ie cylinder head etc are here to stay so it's make do with as much as possible within the realms of what is usable / not worn out.
It is not the ideal starting point for an everyday car but I feel the porting has been done nicely. The head already has Q55 springs and retainers.
Cams are Vegantune 420 spec which I feel is around where the head is designed to work, if not a little more.

I don't like the idea of fitting 40's to a head that is at 45mm at the manifold face, it'll work but fitting 45's will eliminate the step. I appreciate it isn't the same as fitting 45's to a head that is ported at 40mm (ie a big step up) but still, basic port matching should be better.

I think new carbs are being bought regardless so 45's aren't too much of an issue.
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PostPost by: promotor » Fri Nov 17, 2023 8:00 am

2cams70 wrote:It might be worth your while talking to SAS about their standard versus race port heads and the pros and cons of each and recommended set ups. The information gleaned may be useful for your situation


Thanks, certainly may be worth a call.
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PostPost by: promotor » Fri Nov 17, 2023 8:08 am

Just to add info even if it's not too helpful other than for reference for other peoples projects I have obtained the following info :

The inlet ports on the head in question go from 45mm at the carb face down to 33mm at the tightest point and then back out to 36.5mm behind the Sprint size valve.
Exhaust port goes from 31mm behind the valve out to 33mm at the exhaust manifold face.

A standard Sprint head I've measured goes from 40mm at the carb face, down to 28mm at the tightest point and then back out to 34.75mm behind the valve.
Exhaust port goes from 30mm approximately the whole way to the exhaust manifold face.

That's a fair increase in cross-sectional area at the tightest point on the inlet ports. It appears to be consistent with 420 spec - and perhaps a little more - from what I've seen except for the 45mm carb face.
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PostPost by: nmauduit » Fri Nov 17, 2023 8:28 am

promotor wrote:Just to add info even if it's not too helpful other than for reference for other peoples projects I have obtained the following info :

The inlet ports on the head in question go from 45mm at the carb face down to 33mm at the tightest point and then back out to 36.5mm behind the Sprint size valve.
Exhaust port goes from 31mm behind the valve out to 33mm at the exhaust manifold face.

A standard Sprint head I've measured goes from 40mm at the carb face, down to 28mm at the tightest point and then back out to 34.75mm behind the valve.
Exhaust port goes from 30mm approximately the whole way to the exhaust manifold face.

That's a fair increase in cross-sectional area at the tightest point on the inlet ports. It appears to be consistent with 420 spec - and perhaps a little more - from what I've seen except for the 45mm carb face.


interesting, thank you, and even more if you eventually get the build dynoed and can post the results...
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PostPost by: promotor » Fri Nov 17, 2023 9:04 am

nmauduit wrote: interesting, thank you, and even more if you eventually get the build dynoed and can post the results...


No problem. If it does get dyno'd and I get the results I will let you know - that'll be the responsibility of the owner.
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