Conrod length on 1995cc motors

Question and answer direct with GC. Competition engines and 'live' projects only. Good photos to illustrate your post are expected.
FiatRally
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Conrod length on 1995cc motors

Post by FiatRally » Thu Jan 28, 2016 1:16 pm

I am looking at spending some money to test my theories and abilities.

This will need a lot of input and help from the clever people.

I want to make longer conrods and pistons with lower compression height.

This said I also want to move to thinner small end pins. Maybe 19mm or 18mm.

Looking at the 1800 engine the conrod ratio is about 1.71

The 1995cc motor is 1.61

To get a better ratio the 1995cc rod should be 154mm to get to 1.71

The question now is can a piston be made with a thinner small end pin to work with the 9mm longer rods?

Will there be enough space for the rings without jeopardising integrity or exposing the top ring to the chamber under pressure and ignition?

Any and all input will be appreciated.

Or should I just make the payment to GCRE and sit back for the master to teach me what is possible :-))
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Guy Croft
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Re: Conrod length on 1995cc motors

Post by Guy Croft » Thu Jan 28, 2016 7:07 pm

Long rod makes sense for one particular reason though I shall let you (first) explain why..!

the longer rod may strike the crankcase at the bottom of the cylinder, that said you could grind some metal out but to be honest I would make a dummy rod first to find out.

The shorter piston may not suit a conventional design, I'd be thinking of using an X Type forging for that design (see attached). I would not, for instance - use a smaller dia pin.

G
Attachments
X Piston 3D.pdf
(1.33 MiB) Downloaded 69 times
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cormac
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Re: Conrod length on 1995cc motors

Post by cormac » Thu Jan 28, 2016 8:03 pm

Guy

I am no engineer nor even an accomplished auto mechanic though I can follow directions/instructions and have a great thirst for info/knowledge/advice and have an innate ability to separate the chaff from the wheat (at least I believe I can)

I very much enjoyed looking at the 3D image of piston, just wish I could view from an engineers perspective, but what really struck me as I viewed it was your expertise, commitment, and devotion to your craft and trade

Thanks, and I just hope this resource (you and your forum) remains open to experts and neophytes alike, long enough for me to work through my project or at least until I start it

personal note here (I include these as I have decided that beneath your sometimes gruff exterior is a softy, and at this point I have no project pictures or progress to post...so want to share something of note, UNACCEPTABLE)

some responses to my daughters's communication (soccer recruitment) with coaches

Wow…this may be the most in depth and well written response I have had to those questions, I am impressed.

What a thoughtful and incredible e-mail to receive. I enjoyed your highlight video and would like to speak with you very soon and hopefully meet you

there in no doubt in my mind that you have the ability to help our team reach new heights. I truly believe that you would fit in very well as we continue our forward momentum

have a great late evening
GC_85

FiatRally
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Re: Conrod length on 1995cc motors

Post by FiatRally » Tue Feb 02, 2016 9:04 pm

Mr Croft,

Thank you very much for the 3D image that you posted here on your forum.
I have done some work and busy with drawings of the conrod and piston position.

This is very interesting and I am learning so much from something that looks so simple.

The piston position at different crank angle positions is a whole story in its own.
Making drawings is not easy for me as I do not have CAD or the resources for it.

I use MS Visio and it helps me to understand what I am busy with.
It is so much easier when you can see it than to try and work it out.

Thank you once again for the great work and help that you give us all.

Ernst
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FiatRally
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Re: Conrod length on 1995cc motors

Post by FiatRally » Tue Feb 02, 2016 9:06 pm

This is the important part for me.

Understanding the piston position in relation to the crank position.
Also the angle of the conrod at that point!
This is the standard 145mm Fiat conrod.
Attachments
Fiat 1995 Conrod Angles for 145mm Rod.jpg
Fiat 1995 Conrod Angles for 145mm Rod.jpg (94.85 KiB) Viewed 1357 times
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FiatRally
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Re: Conrod length on 1995cc motors

Post by FiatRally » Tue Feb 02, 2016 9:38 pm

I have taken note of your expert advice and warnings regarding the piston pin diameter.
I am looking for lighter weight pins. I will have to look for the correct material to make pins from them.

Is it advisable to have offset small end bushes if available rod lengths is a problem?

The question is - Why all this changing to a brilliant engine?

The 1995cc engine was a change made to the brilliant 1600 and 1800cc engines for capacity reasons.

The cylinder spacing was tight to make sure the blocks were as small and light weight as possible.

No chance of increasing bore size to reach the required capacity.

Lampredi understood the cost implications to design everything from scratch.

The stroke required to get to the requirement meant that the block had to be able to accommodate a 90mm stroke.

The block had to be taller. This was the cheaper method to achieve the goal set for him.

Remember the engine developed from the original 1608cc square block motor. 80mm x 80mm

The design was already stretched to its limits!

The longer stroke made it possible to save millions on engineering and design and use most of the existing parts, except for the crank and conrods.

To fit this configuration a compromise had to be made on conrod length!

The forces exerted on these conrods are extreme at the top and bottom ends of the crank cycle. The piston basically comes to a standstill and changes direction of travel with enormous acceleration of the piston assembly.

A longer conrod will reduce the angle of the conrod as it moves towards the 90 deg position and therefore lessen the stresses on the big end bearings and bolts.

This is my understanding of Conrod Piston Ratio.

Please help and advise where I am wrong.

Ciao,

Ernst
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Guy Croft
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Re: Conrod length on 1995cc motors

Post by Guy Croft » Wed Feb 03, 2016 1:10 pm

Using a longer rod for given stroke has very little influence on acceleration per-se. Very little. You'd have to use a rod 200mm long to see a noticeble difference.

However making the rod longer makes it heavier and that really does impact on the rod stresses. So the rpm limit for the engine might have to be reduced,it all depends on the stress ananlysis. I've done all of all that so - trust me!

The only advantages of a longer rod are that 1) the piston dwells longer at TDC giving the flame front more time to devlop where the pressure is most needed - so the ignition timing can be started later and 2) the thrust load of piston to bore is reduced because of the shallower angle of the rod to the bore approaching TDC. These things do give more power.

You should note that whilst a production rod may offer generous clearance to the lower part of the cylinder I've had production length rods (from Carillo) that actually touch. Now whilst GC_Cunningham ones do clear, I cannot say they would if made longer.

GC
Attachments
rod stress std 2 liter rod_varying rod length.xls
here's the maths if you want to see what happens..
(26.5 KiB) Downloaded 54 times
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Piotrek125p
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Re: Conrod length on 1995cc motors

Post by Piotrek125p » Wed Feb 03, 2016 9:58 pm

Very strange why almost no one downloaded Your amazing .xls I think this is necessary if we want to analyze a piston-rod matching.

BTW. GC, I am making now 2-3 new threads in Q&A, You wrote about closure this forum, if I can before this fact make it and could we chat about some Fiat TC performance engines nuances ?
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Guy Croft
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Re: Conrod length on 1995cc motors

Post by Guy Croft » Wed Feb 03, 2016 11:18 pm

If I close the forum, it will be in June 2016.

GC
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FiatRally
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Re: Conrod length on 1995cc motors

Post by FiatRally » Thu Feb 04, 2016 8:34 am

THIS IS A TOTAL DESASTER IF THIS SITE WILL CLOSE!!

I will get rid of all my Fiat and Abarth parts and sell it all!

This is probably the best forum for any person trying to learn and understand what a racing engine and the development of such takes.

I thank you for all the knowledge that I have gained over the past few years and will be extremely sad when that day comes.

Guy thank you for the help and information.

It is clear now why it is not worth investigating this matter further.

I just see that the old 2000 Opel motors make so much power and torque.

Therefore trying to see what is holding the Fiat TC engine from making the same type of figures!
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Guy Croft
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Re: Conrod length on 1995cc motors

Post by Guy Croft » Thu Feb 04, 2016 1:12 pm

well, I have made 200bhp on the 2 liter on 45 DCOE at 2050cc...!

Not saying that is for road use,

full race..!


G
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Piotrek125p
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Re: Conrod length on 1995cc motors

Post by Piotrek125p » Thu Feb 04, 2016 2:59 pm

It is a good place to ask You GC for one thing.

What is the most powerful Fiat TC that You ever built ?

Returning to .xls files, GC, it is look that You often use this type of files when You designing Your parts, do You use something like it to design a pistons ? Eg. for crown, deck thickness, top land height etc. ?
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Guy Croft
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Re: Conrod length on 1995cc motors

Post by Guy Croft » Sat Feb 06, 2016 6:43 pm

Excel is vital to get the compression height and dome/dish correct for the CR. It is also a very useful record and I have hundreds of configurations for many kinds of engines.

I have cad for the designs..

most powerful? No comment, does nothing for me quoting power, I have to work with (usually rather limited) client budgets and derive the best solution for them, all that matters to me..

I never get 'let loose' to play..

G
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tmvolumex
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Re: Conrod length on 1995cc motors

Post by tmvolumex » Sun Feb 07, 2016 6:14 pm

I am a Retro, VX supercharged, Fiat Lancia, engine fanatic, so my interest is torque and throttle response in my cars.
Honda made rod length changes etc on their 2 liter engine for the S2000. The original 1999 vintage, 2000 cc Honda S 2000, revved to 9,000 RPM
had long rods, very light weight pistons, 16 valves etc as a street legal car, but did not have enough torque, to satisfy the production sports car market. Thus Honda increased the displacement from 2000 cc to 2157cc but still called it a S2000. Road and Track magazine (USA) did an in depth article on the engine that was very interesting, you should look it up. The article detailed the various design decisions made and the pro's and con's of the decisions. Note the minimal increase in torque and the decrease in rev limit.

Original 16 valve, 2000 cc, Honda S200 1999 street version.
Displacement 1,997 cc (122 cu in)
Power 177 kW (237 hp) @ 8,300 rpm (US & EU)
184 kW (247 hp) @ 8,300 rpm (JP)
Torque 208 N•m (153 lbf•ft) @ 7,500 rpm (US & EU)
218 N•m (161 lbf•ft) @ 7,500 rpm (JP)


2004 and on "S2000" engine, increased to 2157cc street engine
2,157 cc (132 cu in)
177 kW (237 hp) @ 7,800 rpm (US)
178 kW (239 hp) @ 7,800 rpm (JP)
220 N•m (162 lbf•ft) @ 6,800 rpm (US)
221 N•m (163 lbf•ft) @ 6,500–7,500 rpm (JP)
8,000 rpm / 8,200 rpm
87.0 mm (3.425 in) x 90.7 mm (3.571 in)
11.1:1

You didn't say if this engine is for racing or play or street use.
If not constrained by regulations I would supercharge it.
tmvolumex
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FiatRally
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Re: Conrod length on 1995cc motors

Post by FiatRally » Mon Feb 08, 2016 8:51 am

Mr Croft and TMVolumex,

Thank you so much for all the information and advise.

It is clear that the cost does not warrant any further investigation on the Fiat TC engine.

Money would be much better spent on the correct pistons and conrods.

Then - to build it to the GC spec and fit the correct accessories to ensure it is a rugged and a long lasting engine.

I am satisfied that the experience gained from my thoughts would be futile and a total waste of time and money.

I will work on what we have and make sure it is put together properly with the correct preparation as per your instructions.

Thank you very much for the help.
GC_75

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