Lotus Elan

(v) Fast Road Car

PostPost by: 1owner69Elan » Fri May 04, 2018 7:19 pm

Out of curiosity what is the rod length that you end up using in the Frankentwincam?

How much weight savings have you done? I did some obvious things (several of these for other reasons than weight) that shave off about 60 pounds net (mag bell housing, alloy flywheel, aluminum radiator, no spare tire (don't have a spare 26r wheel that would work anyway), alternator, starter, ...).

BTW, I was little concerned about the light flywheel but have noticed no lack of drivability and the engine spins up more readily I suppose (but I don't remember enough to compare with stock). Also, no lack of smoothness at idle.


Frankentwincam looks like a nice build. Should give you plenty of power/torque for a road car.
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PostPost by: knockoffnut » Sat May 05, 2018 11:09 am

Thanks 1owner. I just checked with my buddy on his jetting, and the 45DCOEs on his 2.0L LTC are set up on 38mm chockes, which surprised me as that's quite large for a street engine. Would you by any chance know the jetting specs for your engine? It would be very helpful for a starting point on my engine when I get to that stage. On a properly balanced engine the flywheel acts purely as a flywheel, storing energy for moving from a stop without the engine bogging. These big(gish) engines in tiny cars don't need much of a flywheel effect, so everyone I have spoken with says alloy flywheels are fine for that. It is the high load on fasteners from the torque, that makes racers recommend steel flywheels, but severely lightened steel works very well. I have one of each to choose from. (next in line is a very light LTC Seven project) I expect to use 5.2" steel H beam rods, but I haven't finalized those calculations yet. I have all the other bits and I am just doing the rod calculation now. I need to check my figures against actual chamber volume, make sure I have enough meat in the pistons to cut the required dish to bring the CR down to 11:1 for 94 octane fuel, and then work backwards to rod length. I am told by the folks who have done it before that 5.2" is the right setup. I will post an update when I know the final setup.
The car has an alloy bellhousing, trans case, diff case (nose), wheels, brakes, radiator, lightweight body. I will likely add alloy door handles, and maybe the fueltank too. Some of this "added lightness" will be offset by the weight of the rollbar.
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1967 S3 SE DHC
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Seven S2 F
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PostPost by: 1owner69Elan » Sat May 05, 2018 4:59 pm

Here are my jetting specs along with some others for comparison:

lotus-twincam-weber-jetting.jpg and
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PostPost by: knockoffnut » Sat May 05, 2018 11:38 pm

Thank you very much for posting this 1owner. It is very much appreciated. Your curves start at 600RPM. Does your engine idle at 600? If so that is quite an accomplishment for a 181HP 1700cc engine. I expect that could be one benefit of the 40DCOEs and the 32 chokes. Must be great fun to drive!
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PostPost by: 1owner69Elan » Sun May 06, 2018 12:35 am

The 600 rpm data point is from Pertronix, not an idle point measured by me.

My car idles around 800 rpm, quite smoothly and quietly, cold or warm. In fact, I am a bit startled that the idle is so good.

It also starts easily from cold (not that cold here), no choke hooked up - not needed.

In the car's previous incarnation (Stromberg head, stock SE cams etc., but with emissions removed) it was about the same idle rpm but I don't recall it as quite this smooth and balanced. Perhaps the Stromberg head siamese intakes had something to do with that.

The car while cosmetically in stock form (except for 26r wheels and wide tires) is a very different experience now with all of the uprating throughout. Not that I didn't thoroughly enjoy my car before.
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PostPost by: bill308 » Sun Jun 03, 2018 10:17 pm

Hi guys. It looks like I’m late to this party, but let me add my thoughts.

1owner69Elan, it looks like you have a really nice engine. Congratulations.

As another data point, my ignition timing (crank triggered-programmable) (total advance verses engine rpm)

rpm……..degrees
600……….14
1200……..14
3500…..…32
8000……..32
9500……..20

My ignition curve was somewhat arbitrary as we needed something to break in the engine and get a representative dyno pull. It is not very different than your Pertronix Chart from Sheet plot. At some point, I’d like to add in vacuum advance, for economical cruising, as this is a street engine. Light throttle at cruise should enable additional advance for improved mileage.

The head Dave Vegher showed you was likely a John Stowe designed head. This head has been tweaked in a lot of areas for better performance and structural robustness. It is legal for most race classes. Defining characteristics are improved ports, compact spark plug, revised water jackets, internal compression columns, and improved metallurgy, but retains standard cam bearing shells. This needs minor porting to match the owner’s selected valves. This is the head Steve Jennings supplied to Jay Leno when John Stowe was casting and machining them. Later, John sold the tooling to Dave Bean, who has them cast and machined by someone else. Incidentally, it was Steve Jennings that introduced me to John Stowe. I was talking to John last week and he said he’d like to do a redesign of the twin cam head but with a revised combustion chamber, that improved squish and therefore flame front propagation. John is a fan of David Vizard, who has probably more dyno time than any other developer, puts on technical seminars, and is a degreed engineer. John comes from the aerospace (Pratt & Whitney Aircraft Engines) world and is currently an aluminum foundry/machine shop owner (Sycast Inc.) in Hartford, CT, USA
.
I note some twincam head manufacturers run the cams directly in the aluminum head. I believe most modern production heads are designed this way, probably because it is cheaper. Aluminum is a good bearing material. If the bearings wear, presumably the head can be machined for traditional bearing shells.
Port polishing makes sense on the exhaust ports and combustion chambers, but not on the intake ports. Intakes really want a rougher surface to help transport the mixed in fuel to the combustion chamber.
Bigger valves are not always better. They take up more room and are heavier. I believe the law of diminishing returns plays a big role here
.
I think once you go above about 160 bhp, you probably should switch from 40 to 45 mm carbs. I believe the 45’s will be easier to dial in. As the venturi size increases, it approaches the carb bore size. The whole idea of a carb is that air speed increases as it flows through the venturi (smaller flow area).The increase in flow speed results in lower pressure. It is this lower pressure, amplified by the auxiliary venturi, that draws fuel into the air stream. As the venturi diameter approaches the bore size, the air velocity does not increase as much, so the vacuum signal will be diminished. One really wants to amplify this vacuum signal for best tuning results.

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PostPost by: 1owner69Elan » Sun Jun 03, 2018 11:11 pm

Bill, thanks for the additional info and comments.

When I was researching getting a new Weber head I also spoke to DBE (Ken at Bean). Ken basically told me that their heads were designed for racing, not for the street, with large ports. I also got similar advice from Tony Ingram that for a more tractable street engine I would be better off with a "small port configuration". Thus, I special ordered a street ported head from SAS Engineering in the UK. While I may have sacrificed some high rpm horsepower, the build still yielded 181 hp. If you look at Tony Ingram's dyno results (Lotus7.com) for a SAS competition (large port version) he does exceed my outputs (for similar displacement) albeit at higher RPM (196 peak hp @ 7700 vs [email protected] 6900, ~ same peak torque but at 5600 rpm vs 5100). Ingram's results are with Weber 45's vs my 40's

ingram-dyno.jpg and


I think one of the most puzzling things is that my outputs are achieved with 40 DCOE's and only 32mm chokes. Great street tractability but still elevated outputs. If I had independently made the decision I would have probably used 45 DCOE's but Dave Vegher made that decision for me (and it clearly worked out).
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