Advance Curve

[jkolb]

I am building a TC for my S1 Elan that will have the following: 10.5:1 CR, 1.625/1.375 valves, 300 degree cam w/.440 lift. Forged pistons, Cosworth rods, steel bottom end with Leystall fully counterweighted crank, dry sump.

My 23D distributor needs rebuilding and I want to give the shop an advance curve. Any recommendations? I assume 10 degrees static, but what should I use for total advance in the distributor and how fast should it reach full advance? Spirited road driving with track days and autocross.

Thanx

Jerry

[type26owner]

Don't know exactly for your application but what you want to do is feed in all advance as quickly as you can without causing it to detonate. Best guess on a road car that would be between 3000 and 4000 rpms. The curve in the Lotus Manual is extremely conservative. If you take the time to tweak this parameter the hp gain can be a little neck wrenching. It really is dependent on the fuel you can get. Here in California it's a real challenge to get this stuff to perform in a carbie particularly on the summer stuff. There are rumors they are trying to formulate the new fuel so it will not work in a carbie and thereby remove the older cars off the roads. :angry:

Just remapped my son's dizzy on his 383 so it's at total advance at 2000 rpms. It nearly doubled the hp. He says it's scary to drive now cause the steering is extremely slow and heavy. It gets totally out of shape now at half throttle and he can't recover it and still stay in his own lane and that's with a locking posi. Bet I lowered the ET by at least 2 seconds. With sticky tires I'll bet it'll lift the front wheels off the ground now. He needs to enter into the dark world of twincams and stop screwing around with the heavy iron. Where did I go wrong?:D

[mikefromengland]

on a spint engine 24 degrees full advance is plenty.this is normally at about 6000 revs then it drops off sharply after this.the advance starts at about 1500 revs.if the engine is balanced etc and revs higher i would set static at about 14 degrees btdc.if you can get away with it with the type of fuel

[rgh0]

Jerry

Fron the sound of it you are building an engine capable of running to 8000 rpm to get the full benefit of the cam.

For an 8000 rpm 10.5 comp ratio engine running premiun unleaded I would aim for about 30 crank degrees total advance. It should come in a straight line from 10 degrees at idle to 30 degrees at 3500 rpm to 4000 rpm. So with 10 crank degrees static you need the distributor to give you 20 crank degrees centrifugal advance.

Best then to setup your engine on a dyno to get the carb jets right and finally set the total advance for best power. This could probably vary plus or minus a couple of degrees from the nominal 30 crank degrees depending on your fuel and engine details such as piston intruder shape. This will vary the static setting between 8 to 12 crank degrees which will not be a problem. If you find best power comes in with total advance less than 28 crank degrees (possible) or above 32 crank degrees ( unikely) then best to recurve the distributor again to hold static advance in the 8 to 12 crank degree range.

What carbs and chokes are you running? You really need to go to 36mm chokes and do a proper porting job to get the best out of your engine with the specs you quoted.

Rohan

[jkolb]

Thanx. I am running 45s with 36mm chokes. While I've got you, what plugs do you recommend for this engine?

Jerry

[jkolb]

Forgot the porting. The head has been ported as well to take advantage of the valves and cam.

Jerry

[rgh0]

Jerry

I would use as follows

NGK BP6ES for every day road use in normal traffic
NGK BP7ES for long duration high speed freeway cruising
NGK BP8ES for full throttle continuous 8000 rpm rev limit track work

You could probably get away with BP7ES in all 3 circumstances provided the carbs are jetted right and not to rich so that it fouls in road use or to lean so it burns the plugs in track use. Generally being one heat step to hot or to cold will not hurt. However BP8ES will almost certainly foul in normal traffic and BP6ES will overheat in most race situations. All three will work in freeway cruising.

Do you have starting jetting for your carbs ? if not I can give you what I use which should not be to far out.

Rohan

[type26owner]

Hey Mike,
Like I said the advance curves in the manual are very conservative. If you ignore that nonsense and quicken up the advance there is about another 10 hp to be gained out of the stock twincam. If you've got it way off then the horsepower gains can be quite startling.

The guy that my son bought his car from claimed it was tuned to perfection. The guy was a hack and he was the president of the local Mopar Club too. What an imbecile. :angry:

[jkolb]

Rohan

I am starting out with the following set-up for my DCOE 45's:

Venturis ? 36mm
Auxiliary venturies ? 4.5
Main Jets ? 155
Emulsion Tubes ? F16
Idle Jets ? 60 F8
Pump Jets ? 45
Needle & Seat ? 300mm
Pump Exhaust ? 0

Short air horns (20 mm) are being used inside the elan airbox with a K&N filter upstream in cool air. I need to use good air filtration because it is quite dusty where I live.

Thanks for all your help.

Jerry

[stelz]

Fellas,

If there is so much to be gained by setting a good accurate advance curve, wouldn't there be a fiar bit to be gained by using a digital controller like Deltamax

<a href='http://www.delta-digital.com.au/Deltamax%20Info.htm' target='_blank'>http://www.delta-digital.com.au/Deltamax%20Info.htm</a>

I haven't tried this as my car is still in a thousand or so bits, but this modernisation looks like the way to go for good realiable power. It can program the advance curve in 800rpm increments via a PC serial cable. Also includes rev limiter.

Ben
S1 unfinished adventure

[tmr]

Jerry:

With an engine that has been modified such as yours, I agree that some sort of programmable system would be best. Better yet, have the engine put on a dyno to get the best settings for jets, advance curve, etc.

Otherwise, Miles Wilkins book Lotus Twin-Cam Engine has curves listed for different variants of the Lotus T/C. I think that I scanned and posted it a few years ago, but if it's not still there, I'll email it to you.

Tom Roberds
69 S4

[rgh0]

Jerry



My current jetting for 45COE -54?s as set on the dyno for Avgas with 12:1 comp ratio, 300 degree .41 lift cam and 8300 max rev limit. 31 to 32 degrees max ignition advance and large diameter 4 to 2 to 1 extractors

Choke 36mm
Main jets 150
Air corrector jets 170
Emulsion tubes F16
Idle jets 50F8
Pump jets 40
Aux venturi 4.5
Pump spring No data
Pump stroke No data
Pump vent No data
Float valve and needle No data
Intake trumpet length 40mm with oil treated foam socks

Notes: First progression hole peened partially closed to get correct progression on dyno


A couple of other alternatives as starting points

Original S2 26R 45DCOE -13

Choke 36mm
Main jets 140
Air corrector jets 160
Emulsion tubes F16
Idle jets 45F9
Pump jets No data
Aux venturi No data
Pump spring No data
Pump stroke No data
Pump vent No data
Float valve and needle No data
Intake trumpet length No data

Notes: This is a prety standard spec for a 1600 with 36 mm chokes I dont think lotus did much work on its development. They expected buyers of 26R's to do their own dyno work to setup the cars I think.


Brian Hart Big Valve 45DCOE ( light open wheeler 9000 rpm 190 hp)

Choke 40mm
Main jets 160
Air corrector jets 160
Emulsion tubes F16
Idle jets 45F9
Pump jets 45
Aux venturi No data
Pump spring No data
Pump stroke No data
Pump vent No data
Float valve and needle No data
Intake trumpet length No data


Notes : The 40mm choke Brian Hart spec is probably to large for the heavier Elan and milder cams that most people run. I have a friend who just started running 38mm chokes in his S4 seven with the same basic engine spec as mine, this may be a more doable step in an Elan


Does not look as if your proposed jetting is to far out, you did not give an air corrector. Your jetting is going to produce a richer setting across most of the range compared to mine but that variation could come out of fuel density and viscosity variation.


Modern Fully Mapped engine managment systems with crank position sensors, throttle bodies and knock sensors etc will enable you to get more power more flexibly by running closer to the limits for any specific fuel and engine design than is possible with carbs and mechanical distributor. My historic racing class does not allow this sort of stuff so I have never gone down that route. Plus I like playing with the old mechanical stuff. If I did not then I guess I would have to buy an Elise


regards
Rohan

[jkolb]

Sorry about the omission - my air correction jets are 180s, which should tend to lean out the otherwise rich 155 mains. You are right, of course, on sorting the engine on a dyno. Unfortunately, the nearest dyno is at Jay Ivey's, 150 miles away. Ivey has blueprinted the bottom end in this engine, and sleeved it, however, and it would probably be a good investment to haul the engine back over to him for proper set-up.

Thanks for all of your help.

Jerry

[SJ Lambert]

Any more thoughts on jetting settings for race motors that are maximized for mid range torque?

I'm thinking about engines for open wheelers with capacities in the 1500 to 1600 cc range.

[SJ Lambert]

Any tips on the type and set up for a distributor to run in a Stage 4 McCoy topped engine?

Wouldn't mind it looking suitably historic (though am happy enough for appearances to be a ruse!!).

Will take advice from John of course.

James