Lotus Elan

crank rods and pistons

PostPost by: pauljones » Mon Jun 22, 2009 6:28 pm

im very soon about to build the bottom end of a 711m block to go in my elan.but first i have a simple question,i intend to use the standard 1600 x flow crank,for a small amount of extra cc and torque.so what are the rev and horse power limits for this crank.to go with this if the block is good and can take them,then 85mm pistons.but does anyone have experience with this crank and rod length/piston size's.
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PostPost by: rgh0 » Tue Jun 23, 2009 11:49 am

I have never built a long strok 85mm bore 711m block engine so cant talk from personal experience.

However I can make a few comments based on other engines I have built and analysis work done

1. Getting a block good for an 85mm bore is easier said that done. Most 85mm bore engines only last a couple of of year in competition use before you split a bore as the wall thickness is to thin in some areas. You need a really good block and very precise and detailed measuremeant and offset boring to get the greater than 0.100 inch wall thickness required all round. Ideally you need greater than 0.120 inch to with stand compettion stresses but this can rarely be done - you may need to test 20, 711m blocks to find 1 that you can achieve this on. Personally I would aim for a 83.5 mm bore as most blocks will do this safely.

2. The Dave Bean catalogue talks about the options for rod length and pistons they have available for building big bore long stroke blocks. Wlicox in the the Uk would be another source of information for out to 2.2 litre money no concern alloy block options. In general as you go for longer stroke you need to go for longer rods to match. In going from a road to comptition design of the same engine and stroke people tend to go to longer rods again as this achieves some further reduction in engine stresses and friction losses at the cost of more complex piston designs to accomodate the resultant lower compression height but this is generally easily accomodated in the higher quality forged pistons used for compettion use

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PostPost by: types26/36 » Tue Jun 23, 2009 12:48 pm

I've had two engines bored to 85mm, one was a 711 and gave no problems at all, it was randomly picked from various 711 engines and was not checked or tested for wall thickness or anything else, it was just bored to 85mm, used standard Ford 1600 rods and crank. It was fitted in my Seven and used in mild club competition for several years before I sold it two years ago and is still running today although not in competition.
I also had a 120E (originally Lotus engine) bored to 85mm but it was sleeved, it was also used in classic rallies but over heated and was eventually consigned to the scrap heap. :cry:
I once had a scrap 711 engine bored to destruction just to see how far it would go and apparently it went to 90mm before it broke through on one side.
An other engine I tried was a 681F, this was bored to 86mm and fitted with VW pistons, it lasted about a year and also had slight competition use until it cracked the bore and looked like a steam engine :lol:
I would have no problems boring a 711 to 85mm and taking a chance but if you wanted to be sure then have it tested for wall thickness and position of the bores.
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PostPost by: rgh0 » Tue Jun 23, 2009 1:43 pm

If you ultrasonically test blocks and I have done lots you find all the outside of the bore castings between around 89.5 and 91.5 mm, this is the same for all block types , 116E, 120E, 681F 711M , L blocks etc there are no thicker wall block types and the t numbers are meaningless in terms of wall thickness, all the stuff you read is myths by people trying to sell blocks if you dont believe me buy an ultrasonic gauge , buy yourself 30 blocks and do the measurements yourself and then come back and discuss it with me - I may be wrong and always willing to be proved so with real data

I am sure some special thick wall blocks were made may be the South African AX blocks had thicker walls never measured a repesentative sample of those (anyone got any real data on bore OD's for them) but you dont find them at your local wrecker that easily. The limit on OD of the bore is based on the sand casting methods used and bore spacing which was constant for all the block types and the range reflects the variation in accuracy of Fords casting methods.

At the maximum 91.5 mm OD you find you have 3.25mm wall thickness for an 85mm bore if the bore exactly centred in the casting, its circular and it has no signficant corrosion or thin spots and the cross drill for the oil feed between cylinders 2 and 3 is centred so you miss it. There are blocks out there that meet this but it not "common". Seen lots of 85mm bore blocks fail including my own and other people, not seen to many last long in competition usebut not denying they do exist, they also last longer if gently used in relative terms on the road.

Get a block, bore it out and have a go - you may be lucky I have 12 blocks in my workshop currently and about another 8 I have tested recently - only one got close to being OK for 85 mm bores, may be I am just unlucky who knows :D

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PostPost by: msd1107 » Wed Jun 24, 2009 12:07 am

Rohan,

The concept that a long rod reduces engine stresses and friction losses falls under the category of "old wives tales" or "hoary legends"

The variation of length of TC connecting rods has an almost infinitesimal effect on peak G loads at TDC or peak side thrust. What effect you will see is due to the decrease/increase in weight/stiffness with the shorter connecting rod vs the longer rod.

Race Engine Technology magazine had an interesting article comparing the latest F1 engines with cup (NASCAR) engines. Despite the huge advantage in technology, the F1 engine developed only a few % more BMEP over the dinosaur era Cup technology. The authors puzzled over this (maybe American engine builders are more clever than F1 designers???) but missed the (to me) obvious answer. F1 engines use a rod/stroke ratio of around 2.5, very, very long. Cup engines are down around 1.5/1.6. Mathematically, this translates to cup engines having around a 3% greater volumetric efficiency compared with the F1 engine, a huge difference when it comes to race tuned engines.

There are other secondary effects. A shorter rod causes the peak airflow speed to increase, and occur earlier/later in the intake/exhaust stroke. Depending on the details of the porting, this can lead to an increased pressure depression, and higher pressures at intake closing.

I have prepared an article "The influence of rod/stroke ratio on the theoretical volumetric efficiency of the internal combustion engine". This has not yet been published, but I am including an extract plus a supporting spreadsheet (gee, a spreadsheet less than 1MB!) for those whose curiosity has no bounds. Have tons of fun! I know of no controlled studies to validate any of these theories, so believe that what you wish.

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PostPost by: europatek » Wed Jun 24, 2009 2:06 am

I've had a number of larger capacity engines built on the 1600 x-flow block with bores from 83.0 to 85.0 and have never experienced any problems - lucky maybe. They were all street engines but pretty well tuned with high compression, cams etc... and were used well!
As far as pistons and rods are concerned I used Datsun components. What I have discovered is that Ford and Datsun engines are very similar dimensionally and Datsun components can be made to fit quite easily. Datsun parts are also strong, reliable, plentiful and relatively cheap. A number of mates have owned tuned (160-180bhp) Datsuns over the years and I've been most impressed with the engines. That said, I use Datsun pistons and rods connected to either a Ford or Datsun crankshaft. Pistons are available in 83.0 thru 86.0 (Datsun L16, L18, L20, L24) and only need slight milling to lower to the deck height you want. The rods I have used are the L24, 6cyl rods which have larger bolts than the equivalent L16 rod which is the same length. Some clearancing in the crankcase and around the bottom of the cylinders is required as the big end is physically larger than the Ford rod. If using the Ford crank then it needs to be no smaller than .010" under on the big ends. If using the Datsun crank this is not so critical. The twincam in my Cortina is running with its L clock, 1600 x-flow crank and 84.0 Datsun pistons. Capacity around 1720cc. With big valves, mild porting and cams, 42 DCOE's this engine is very torquey. It keeps up with modern traffic no problem. I'm thinking of something similar for my Elan which should really get it moving.
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PostPost by: types26/36 » Wed Jun 24, 2009 7:46 am

[quote="europatek"]
As far as pistons and rods are concerned I used Datsun components. What I have discovered is that Ford and Datsun engines are very similar dimensionally and Datsun components can be made to fit quite easily. I use Datsun pistons and rods connected to either a Ford or Datsun crankshaft. quote]

...........and with that said I just happen to have some Datsun rods for sale on ebay
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 0438629801
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PostPost by: rgh0 » Thu Jun 25, 2009 10:38 am

David

I agree the dynamic and force effect of longer rods versus shorter rods over the range used in racing twinks is small. But it isn a myth or wives tale - the geometry change of a longer rod does change side thrust and acceleration by a small amount which is why i guess people do it - personally I dont bother as the effects are too small to worry about. Similarly the changes in breathing for long or short rods in an otherwise identical engine is also not really signficant.

The real difference between max BMEP between a F1 and a NASCAR is not the amount it is but the revs it occurs at. Getting a high BMEP at 6000 rpm in a 2 valve push rod wedgehead V8 is very different to getting the same BMEP at 15000 rpm in a F1. The resultant HP difference is what is important as HP = k x BMEP x revs.

The effect of the rod length on the volumetric efficiency is trivial compared to the effect the very short stroke and big bore has on BMEP and the ability to achieve it at high revs.

I think the principle reason F1 goes to extreme rod ratios is to enable relatively narrow V angles in their big bore engines for aerodynamic reasons to give better air flow around the rear of the car and thus better downforce. Nothing to do with engine dynamics or engine breathing at all

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PostPost by: msd1107 » Sat Jun 27, 2009 7:08 pm

Rohan,

Good observation about the effect of the engine V angle on car aerodynamics. In older formulas, you wouldn?t see 120 or 180 degree V-12s or 144 degree V-10s for that reason, despite the obvious advantages in lower CG height. Also, F1 goes to extreme lengths to minimize the width of the exhaust system for the same reason. Some of the exhaust bends are really extreme!

As far as shorter rods generating more side thrust, this can be minimized with an offset crankshaft or piston pin. The spreadsheet has a field in which to specify the crankshaft offset. Unfortunately, I didn?t supply a column so that the side thrust could be calculated, but the user can supply different values of offset and observe the change in acceleration and velocity at different crank angles. If you want to see some interesting curves, specify a very short rod length and a large offset. There are geometric limits, so the program limits values to that possible.

Also, materials and processing improvements in piston ring shape, machining, and coatings, piston shape, machining, and coatings, and cylinder wall material, finishing, and coatings have led to a measurable decrease in friction losses. Some of these techniques are becoming available to non-F1 teams if you know where to look and are willing to pay a premium over normal commercial parts.

F1 engine designers took a small hit on BMEP to gain percentage more in RPM. Unfortunately, the rules writers have negated much of the promise of these advances by outlawing beryllium and MMC (we probably would be in the mid-20K RPM region by now), specifying minimum weights for pistons, pins, and rods, and now mandating lower RPM. With F1 engine designs frozen, the designers cannot go back to optimize the bore/stroke ratio around the lower RPM limits. It used to be that F1 and aviation pioneered new technology that years, decades, or generations later made their way into the commercial arena. No more.

200+ BMEP with a wedge head is a real achievement, no matter what the RPM.

A long stroke twinc with 4.928? rod has a rod/stroke ratio of 1.613, getting toward the short rod side of the continuum.

As Rohan pointed out, the extremes of possible con rod length in a twinc do not produce enough of a change in rod/stroke ratio to be meaningful. Actually, the interesting effects of shorter con rods only begin to manifest themselves at rod/stroke ratios substantially less that the current short rod ratio of 1.5. The implications are sufficient enough to require a research effort to quantify the potential changes.

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PostPost by: CBUEB1771 » Sun Jun 28, 2009 4:09 am

msd1107 wrote:Unfortunately, I didn?t supply a column so that the side thrust could be calculated, but the user can supply different values of offset and observe the change in acceleration and velocity at different crank angles.


While planning a long stoke TC for my +2 I had been advised to use the 1600 crank in a TC block with 125E dimension rods. I was concerned about the rod length to stroke/2 ratio. A simple free body diagram shows that the 1600 crank/125E arrangement causes about 3% greater maximum piston side thrust when compared to the 1600 crank/1600 rod arrangement. Significant? I don't know, I am just a butcher, but I bet the F1 lads spend a lot for 3% improvements here and there. In the end I am working with a 1600 crank, AX block at close to full deck height and BD (5.23" CL to CL) rods.
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PostPost by: pauljones » Wed Jul 01, 2009 6:53 pm

i think ill go with the 5.23inch rods,and pistons of the right compression height.i seen some in burtons on line.a bit costly but steel rods,forged pistons,never a cheep option. as far as all the rod length to stroke ratio and the other pros and cons go,i think it will achieve a lighter over all rotating mass,with a lighter piston giving a faster acelerating engine.i think the 12 to 1 compression is a bit high so will need sorting.
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PostPost by: Davidb » Thu Jul 09, 2009 9:30 pm

What are the opinions here re the BEST crank for a 1600 TC for out and out racing?

What is the best deal?

Does anybody do a forged, narrow throw crank for the TC?

I have been out of it long enough that I have no current knowledge on these engines.
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PostPost by: Dag-Henning » Fri Jul 10, 2009 6:05 am

- under what rules will you be racing ? There are all patterns available in 4 or 6 bolts and different numbers of dowels acc to your wish. Narrow and wide journal cranks with 72 or 77 throw..... The rules and your ethics will descide.... :wink:


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PostPost by: Davidb » Fri Jul 10, 2009 3:25 pm

Ethics and cranks! The world is getting too complicated :roll:

It's for racing in N.America--ethics (FIA) take second place :D
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PostPost by: msd1107 » Fri Jul 10, 2009 3:39 pm

Davidb

Dave Bean and others here in the states have billet steel eight counterweight crankshafts in both wide and narrow journal, 6 bolt or 12 bolt, standard and long stroke.

Also strong rods, aluminum blocks, etc.

Just open your pocketbook!

For many of these people, if they do not have it in their catalog, they can still get it for you.

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