CR block decking

[nmauduit]

I'm slowly making progress in building an engine bottom, and will shortly be facing the choice of compression ratio via deck height, and thought I should seek comments.

The intended use would be targeted at rally/track days but a road use should still be possible and reasonably enjoyable.
The engine will be running an 123 distributor, which offers good control of the ignition (and rev limiter) but is not a fully mapped ignition system with ping detection. 2 curves are stored in the system, one would be more conservative than the other to help stay away from pinging on the road (which also depends on engine temperature).
pistons (forged, with intruders) and valves will be ceramic coated for heat insulation (but not the chambers, only polished for hot point removal)
camshafts is to be of high lift/short duration type (typ 0.44 duration 280 degrees)
head gasket will be standard cometic at first, leaving room to increase thickness (decrease CR) if the need arised
pump gas here is 98 unleaded - the option of using an additive would not be a problem (esp. for track days);

I'm tempted to go as close to 12:1 as possible : how does that sound (engine will be tuned on a rolling road before being pushed) ?

side question : is in case of momentary unavailability of additive or adequate high octane gas, can an adequate driving style help avoid trouble till the next pump (e.g. limiting revs and temperature, limiting load...) ?

[rgh0]

I would stay away from decking the block any significant amount. I would only deck if required to get a flat parallel top surface the minimum amount possible. I would use the piston compression height and intruder size to get the compression ratio wanted.

With the cam proposed I would personally aim at around 11:1 above that you may run into pinging problems at full throttle medium revs situations using premium unleaded 98 octane. You may be able to go a bit above 11:1 but I have not tried it. You need 105 octane racing fuel or Avgas to go above 12:1 in my experience with that type of cam.

Just backing off on the throttle can stop pinging happening as closing the throttle reduces the actual compression pressure in the engine which is the cause of pinging, if you are forced to run on lower octane fuel for a short while. However you can do damage without being able to hear it so need be very careful.

cheers
Rohan

[nmauduit]

thank you for your input Rohan, food for thoughts...

I should have added that this is not a lotus block but a 711M block, which will be decked some anyway with the change of pistons. I'm planning to test the head with that, and if satisfactory will build a suitable lotus bottom at a later date.

for 75 ABDC (Neuman ph5 280/443) dynamic compression evaluation look like :
static 12:1 > dynamic 8.7:1
static 11.5:1 > dynamic 8.4:1

Has anyone used an after market detonation/knock sensor kit for tuning a lotus twincam?

[rgh0]

While many people deck 711 blocks you dont have to depending on the rods and pistons you use.Various combinations can give you none, a little or a lot of decking. The lot of decking combinations I would not use as it gives you little meat left in the top deck of the block

cheers
Rohan

[nmauduit]

The block decking is in the hands of a UK builder, I understand 711 rods (4.928) and lotus pistons call for about 5mm decking (to be adjusted to target compression ratio) - which would be useful if not necessary to close the hood... I believe this is a common option that has been put to use since the 70s.

regarding tuning and the exploration of detonation conditions, I was tempted to try fitting a standalone Bosch sensor and ping counter to see how it goes : if it's too bad I can fit a thicker head gasket and reduce CR by 1.4 or even more, while an intermediate option would be to use octane booster for the tuned ignition curve and reduce advance for the road curve (knock detection can also be used to switch automatically back to the more conservative road advance and rev limit). I understand detonation is not fully controlled by timing advance though.

[rgh0]

If your buying custom pistons to achieve the target compression ratio there is no need to deck the block as you can get the compression height you need.

The 5mm decking has proved OK in practice as you say - its the "little" decking amount I alluded to but you can get 5mm for bonnet clearance in other ways if you need it

If you setup for 12:1 for a thin head gasket you can always reduce it like you say with a thicker one. I build my race engines that way as they keep threatening to ban use of Avgas and force use of 98 Octane pump fuel in my racing category - so far they never have

If the combustion chamber shape is good with no hot spots then ignition retard will eliminate ping / knock in most circumstance if you have a too high compression ratio so fitting an after market ignition retard with Knock kit will probably help if 12:1 is to high in some circumstances

cheers
Rohan

[nmauduit]

this is a budget exercise, and I already had a nice set of pistons from Tony Ingram which triggered it... Regarding bonnet closing, I presently have a similar engine in my car while the matching number engine is stashed for TLC, and I was running short of tricks to get a few mm clearance (optimized engine spacers and filed up engine mounts), so 5mm extra would be a new headache I'd rather avoid.

I'm planning to put some work to the head smoothing whatever sharp edge that could turn into hot spot, then ceramic coating the removable parts (don't want to ship the head, though the chamber itself would also benefit from it).

I have to admit that the tinkerer in me is eager to give a try to 12:1

[1owner69Elan]

At least you have access to SP98 gas. I think this would be equivalent to 93 (R+M)/2 here in the US.

In California the best we have standardly is 91. The 93 seems to have disappeared but maybe you can still find it somewhere? I also have access to 100 if I drive over to the nearby raceway but the cost is rather high (4x).

My newly built street engine when trial assembled had a CR of 11.5. The builder, Dave Vegher, then machined the piston crowns to achieve 10.25. On the dyno on our 91 pump gas we did not have any pre-ignition and the performance was quite good (181 hp, 143 torque) with an imposed 7000 red line. I believe Dave also took great care to carefully modify the combustion chambers (around spark plug holes, ..) to alleviate any hot spots.

I intend to run pump gas most of the time with an occasional splurge on race gas. One thing that Dave did do was to run race gas through the engine after the dyno tests on pump gas as the race gas would be better for storing the engine before I got it back into the car sometime later. So it sits with race gas at the moment.

[nmauduit]

My newly built street engine when trial assembled had a CR of 11.5. The builder, Dave Vegher, then machined the piston crowns to achieve 10.25. On the dyno on our 91 pump gas we did not have any pre-ignition and the performance was quite good (181 hp, 143 torque) with an imposed 7000 red line.

indeed that sounds very good to me, esp. nice torque... is that a 1600cc or an over capacity engine?

[1owner69Elan]

It's a stroked engine at 1700cc.

Dyno shows torque curve quite flat from 4000 to 7000.

Haven't yet been able to experience it as the car is undergoing a complete renovation. Right now I am deep in the rewiring weeds.

[nmauduit]

this is very promising, my test block will be 1700 also...

what kind of camshafts did you use?

[1owner69Elan]

Still waiting on camshaft specs from builder.

But I believe similar to what you are intending.

I'll post them when I get them. Should be soon.

[vstibbard]

Hi,

I've just finished an S1 Elan which has 11.3:1 comp with high intruders, running QED 420 cams, 45mm Webers with 32mm chokes on 98 octane pump fuel. We spent a lot of time setting up cams and timing, advance curve etc and have no pinging. Its producing 132BHP at the wheels at 6400 and 122 ft lb torque, it has lovely flat curve, with no dips and is still climbing, it idles at 800rpm from cold, does not get hot in traffic and is an absolute blast to drive.

We were worried about the static compression with small duration cams, and were prepared to go to the QED450 if required.

Cheers

V

[nmauduit]

I've just finished an S1 Elan which has 11.3:1 comp with high intruders, running QED 420 cams, 45mm Webers with 32mm chokes on 98 octane pump fuel.
V

thank you for sharing, an other interesting data point. Is the use of weber 45 with 32 chokes the result of an optimization or somewhat circumstantial?

ps: for those interested in knock , I've come across a detailed description of knock sensing

http://www.efi101.com/forum/viewtopic.php?t=7972

which cites an interesting systematic study (esp. regarding ethanol effect and knock prediction models)

https://dspace.mit.edu/bitstream/handle ... 45-MIT.pdf

[1owner69Elan]

As promised the cam specs on my stroked engine (1692 cc).

From Kent cams.

Lift: .440"
Duration: 250 @ .050