arithmetic, maths & sums for engines
Posted by danster
Haggis Muncher Professional Moderator Location: Haggisland UK Join Date: 01/04/2013 Age: Possibly Wise Posts: 409 Rally Car: VWs (for my sins) |
Noticed a lot of science and numbers being banded about in another thread regarding suspension springs and damper rates.
So not wanting to feel left out thought I'd chuck this topic out for general assesment and see how far off track it wonders... 
Ok, playing around with engines as one does it became apparent to me that there are many differences in the components used over the years as the manufacturer developed and finessed their designs. This being the case a bit of time spent weighing and measuring the parts used with a view to "optimising" may be beneficial. I wish to use a piston assembly with rings, pin and circips that currently weighs 454g. The pin is heavier than the ones used in other engines of the type. Unfortunately it is also a few mm longer so I cannot just swap it out for a lighter one and save 15g. Also the piston skirt (cast piston) is slighly longer than the piston originally fitted to the engine. I ran some calcs regarding inertia and mass. If I could reduce the weight of the assembly by approx 50g which is a bit of an ask I found that the upward force at 8000rpm could be reduced from 3,485lbs down to 3,079lbs. That is a substantial drop. So questions are: 1: Thoughts on whether I can bore the gudgeon / wrist pin to reduce it's weight. The pin has an OD of 20mm and an ID of 11mm, I have other shorter 20mm pins that have IDs of 12mm and 13mm suggesting it would be ok to lighten the pin. Case hardening an issue? 2: I can also take a few mm of the piston skirt to make it more like the oem piston I removed which is erring on the slipper design with it's short skirt. The pistons actually look like they are the same casting and just machined differently for each application. So, benefit or grenade in the making? Disappointingly not yet a Jackass Edited 1 time(s). Last edit at 01/22/2013 07:43AM by danster. |
Robert Gobright Super Moderator Location: White Center Seattle Join Date: 04/27/2008 Age: Possibly Wise Posts: 1,292 Rally Car: 91 VW GTI 8V |
Haggis Muncher Professional Moderator Location: Haggisland UK Join Date: 01/04/2013 Age: Possibly Wise Posts: 409 Rally Car: VWs (for my sins) |
Everyone likes a picture so here are the pins in question.
The pin on the left is 100g, the pin on the right is 75g. The pin on the right is 5mm shorter so I reckon there could still be a good 20g saved if the heavier pin could be bored safely. If not a lighter pin of the correct length may have to be sourced. Had another look at the piston and I reckon 20g could be taken off it. So 40g overall or a 10% weight saving for just a bit of time and effort. I am not going to drop a grand on forged pistons and rods. This is just a cheap ghetto build. Disappointingly not yet a Jackass Edited 2 time(s). Last edit at 01/22/2013 10:02AM by danster. |
John Vanlandingham Super Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Haggis Muncher Professional Moderator Location: Haggisland UK Join Date: 01/04/2013 Age: Possibly Wise Posts: 409 Rally Car: VWs (for my sins) |
It was not so much will the engine still make power at 8k. I very much doubt that.
It would need higher CR and longer duration cam requiring forged internals for that to safely happen. And the powerband would be narrow and unuseable without a pukka gearbox. yadda yadda..... But even on a budget build having the over-rev ability for the odd occasion between corners to avoid a momentary upshift then downshift, or a missed gear may be useful, it seems silly not to lighten components to a safe degree if it could help prevent a failure. But likewise there is no point engineering in a weak point either. VW rods are pretty strong, the turbo cars put way more compressive force on them than a NA application like mine will. However I understand they are not so good in tension and it is the studs / nuts and big end cap that give the issue. So it is high revs on the overlap of the exhaust / intake stroke at TDC where there is no pressure acting down on the piston that presumably causes the most strain. By keeping the piston assembly light it will help reduce the force at that point as those calculations if they are correct show. Even if I only manage to save a little weight it should give a surprising degree of benefit and it will also allow the engine to spin up quicker. Manufacturers have been doing that to their std engines as they develop over time to help keep emissions down. So, it looks like the pin boring may be a bit risky, and then using a fine reamer or hone to keep the internal surface nice and smooth may end up using more time than it justifies, and sourcing a lighter pin to start with would be a better plan. Pistons can be milled slightly I would think but less weight saved for the effort with that. I'll just have to console myself with the knowledge the frictional losses will be reduced a bit if I shorten the skirt a couple of mm!  Disappointingly not yet a Jackass |
starion887 Mega Moderator Join Date: 09/06/2006 Posts: 798 |
Haggis Muncher Professional Moderator Location: Haggisland UK Join Date: 01/04/2013 Age: Possibly Wise Posts: 409 Rally Car: VWs (for my sins) |
Thank you for thoughts and input. I had a little play around with some more figures.
I used this site so feel free to quickly call fail if it is bogus! 
http://www.wallaceracing.com/Calculate-Inertia-Force-of-Piston.php So I put the numbers in for a std 1.8 16v Golf engine. Piston assembly: 465g RPM: 8000 Rod length: 5.67 Stroke: 3.4 Results were Inertia Force 3168lbs, Up Force 4118, Down force: 2218 Rod / Stroke ratio 1.67 ------------------------------------------------------------------------------- The std 1.6 8v base engine. Piston assembly: 402g RPM: 8000 Rod length: 5.86 Stroke: 3.05 Results were Inertia Force 2457lbs, Up Force 3096, Down Force 1817 Rod / Stroke ratio 1.92 ------------------------------------------------------------------------------- The proposed hybrid 1.6 8v engine. Piston assembly: 455g RPM: 8000 Rod length: 5.86 Stroke: 3.05 Results were Inertia Force 2781, Up Force 3505, Down Force 2057 Rod /Stroke ratio 1.92 ---------------------------------------------------------------------------- The above comparisons makes me feel happier with no need to go to any great or risky lengths to reduce piston weight. One can see that the std 1.8 16v engine has a lot more to cope with than the std 1.6 8v engine, and my hybrid sits roughly halfway between the two. Both engines use the same 81mm bore but the strokes vary with the 1.6 using 77.4mm, and the 1.8 using 86.4mm. The rod / stroke ratios also change due to the conrod length differences between the 1.8 (144mm) and 1.6 (149mm) engines. And it looks like this also has quite an effect on the forces due to the higher piston speeds of the longer stroke engine. But I could still source a lighter pin and save 20g. 
If these calculations are correct it is amazing to see how a small 50g weight difference can add up to many hundreds of lbs due to the forces when the engine is running.  Disappointingly not yet a Jackass Edited 3 time(s). Last edit at 01/24/2013 08:35AM by danster. |
John Vanlandingham Super Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
bore? compression distance? block deck hgt? piston flush or down the bore? ferfawkssake the juicy bits! By the way, been mixing and match things to get better rod/stroke ratios for oh 25 years or so and the positive benefits has been drastically reduced reciprocation weight.. I can say you can "feel" 150g reduction.. But now you see some others too: reduced piston speed with shorter stroke and the resultant reducing forces trying to sling stuff apart... But now we get to the juicy bits: block hgt, pin hgt (compression), and exactly where the piston crown ends up. Oh compressed headgasket thickness. mm please. John Vanlandingham Sleezattle, WA, USA Vive le Prole-le-ralliat www.rallyrace.net/jvab CALL +1 206 431-9696 Remember! Pacific Standard Time is 3 hours behind Eastern Standard Time. |
Haggis Muncher Professional Moderator Location: Haggisland UK Join Date: 01/04/2013 Age: Possibly Wise Posts: 409 Rally Car: VWs (for my sins) |
Ha ha, I didn't want to give my secret recipe away! If it works I will sell it and I'll be soooooooooo rich I will buy everyone uprated Polo stub axles and hubs, even if they don't have VWs! 
However purely in the interests of entertainment and also if if it all goes wrong, I feel sharing it with others (especially merkuns with their litigious ways), for their input will effectively spread the blame should it fail drastically and the lawsuits start flooding in from spectators getting hit by conrods. 
Ok, 81mm bore with the 77.4mm stroke. Normally this 8v engine uses a chambered head with a 30cc combustion chamber volume and a slightly dished toroidal bowl piston crown giving a static CR of 9.5. However my secret piston has a flat top and a compression height of 32.4mm. Piston at TDC is at the top of the bore, flush with block deck, and the metal multi-laminate gasket is 1mm compressed resulting in near optimum squish. This results in a CR of 11.3 (measured with fluid, not guesstimated). But this could be tweaked slightly by removing or adding thin layers from the gasket. Block height is 220mm. This is the "short block" and not the "tall block" that some of the 2.0 engines use which has a 236mm block height. So do you want to buy some shares  , or walk away now  ?
There is more info but don't want to leak it out just yet and de-stabilise the global economic situation any worse than it is now. But this cheap ghetto engine could be the next thing to save rally, affordable to even the sub prime masif.... Disappointingly not yet a Jackass |
John Vanlandingham Super Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Haggis Muncher Professional Moderator Location: Haggisland UK Join Date: 01/04/2013 Age: Possibly Wise Posts: 409 Rally Car: VWs (for my sins) |
Come on John!! I've just told you the bore, stroke and engine capacity. Do the sums man! You are not inspiring much confidence here.  Disappointingly not yet a Jackass |
John Vanlandingham Super Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Fawk I been reading about our skyrocketing rise in POPULARITY according to the Powers That BE! Oh! got a cuppa, brain is woikin again.. Gotta ask since youse guys get some engines we don't.. schlag: 77,4mm... OK1.6 R4 74-75kW identification parts code prefix: 06B, ID code: AHP (75 kW) Whatcher have against die schöne sechzehn ventiler? Mongo like 16v. John Vanlandingham Sleezattle, WA, USA Vive le Prole-le-ralliat www.rallyrace.net/jvab CALL +1 206 431-9696 Remember! Pacific Standard Time is 3 hours behind Eastern Standard Time. |
Haggis Muncher Professional Moderator Location: Haggisland UK Join Date: 01/04/2013 Age: Possibly Wise Posts: 409 Rally Car: VWs (for my sins) |
Right, you're not far off the script now and after all the maths and numbers above we have established this is a 4 cylinder engine. Groovy.
I would have been a bit pissed off to find my camshaft had two extra lobes. 
Don't mention 16v and VW to me. I come out in an allergy thinking of the badly designed, compromised, overly heavy, expensive to upgrade monstrosity that is the 16v head. With it's single cam timing belt pulley and internal chain and sprockets between the cams making adjustment both time consuming and expensive due to extra verniers. Oh, and vertical shrouded exhaust valves taken as a "performance design" from the Mini 850 engine.
My engine will weigh almost half that of a 1.8 or or 2.0 16v. I see this as a benefit and why I am persuing it. Class capacity splits is the other reason. Disappointingly not yet a Jackass |
John Vanlandingham Super Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Yeah that's was my thinking when for some inexplicable reason fawkin genii at Ford produced the Cossie BOA and BOB 2.9 with an extra stinkin cylinder stuck on so I can bolt the buggers onto my mighty V4...I t'wernt half pissed, as you can well imagine..
Yeah it aint the best-est set up but I look at the valve area of those 2 little 32mm intake valves and they are 27% more area than the 44.5mm intake i have in the mighty V4 so a lot more area yet again over the poxy widdle wee VW 8v intake valve.
'Arf? 'ow 'arf? John Vanlandingham Sleezattle, WA, USA Vive le Prole-le-ralliat www.rallyrace.net/jvab CALL +1 206 431-9696 Remember! Pacific Standard Time is 3 hours behind Eastern Standard Time. |
Haggis Muncher Professional Moderator Location: Haggisland UK Join Date: 01/04/2013 Age: Possibly Wise Posts: 409 Rally Car: VWs (for my sins) |
Aluminium.  Disappointingly not yet a Jackass |
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