Limited Bore / engine discussion (85.5-86mm)

[RFbuilt]

good day guys,

(while i wait for my parts for the 1679 and 1800cc motor)  an unusual tech/theoretical discussion sounds good
specially with the guru's we have on here (i know im enjoying my fair share of knowledge gains here)


lets say we are limited to 85.5mm piston bore (seeing JPM and his custom JEpistons 86mm lets include it)
so lets say we are limited to 85.5 - 86mm bore

goal is to make the most (realistic) power off those bore size, with the displacement unlimited
could stroke it 74,76,78,82 heck even 84 (i think)

point of discussion is - stroke size  - head size (due to the limited bore) - cam choice - rod length choice

lets say piston pin height / comp height we have the choice of 39.6 (A) , 34 (B) and shorter 30mm respectively

(im not sure what JPM/JE ppistons has for comp height)
and to simplify a rather widespectrum of choice, lets limit carb size to the ff:
Weber 44 or 48 IDFs (or dells respectively) and or 48IDA's
-----------------
*how would you configure your limited bore/ unlimited displacement engine 
(hope this is as interesting of a topic as it is to me.. for everyonehehe)

"throwing a wrench into the fan"  gas limit lets put at 96+ron or , 91premium in the u.s
reliability should be no lesser than the common "internet" configuration engine i.e. 1776,1914,2110-2276 motors
use: streetable / strip motor

[RFbuilt]

here's my take :

1905cc / 86 x 82

86mm piston bore , 30mm compression hieght (85.5 barrels bored and honed to size)
82mm crankshaft
040/oem vw cylinder head 35.5in 32 exh. (with good dual springs, portwork etc)
5.600" H-beam rods ( ithink its lighter than Ibeams so thats good)
web110 on 1.25 rockers (should put lift at valve about 0.493") would mean the lift is roughly 35% L/D ratio to valve size
weber 44IDF with vents and jets tuned accordingly
1 1/2 or 1 5/8 merged header
9.4:1 static comp

anybody know if this is a good configuration, why or why not it would work

additionally,
if our factory / new cases usually have a 100mm height from crank mains centerline to barrel deck, we add another 112.5mm ofr barrel length, we get 212.5mm deckheight so here's why i chose the configuration mentioned

resurface the deck (barrel's base on the case) 0.010"
+ 5.6" rods and 30mm piston pin height  and deduce that from the deckheight
would put the pistons maybe 0.040" out of the hole , so a 0.085" barrel shim would give it a good 0.045" quench which i think would be good for a small bore = faster flame front characteristics, maybe letting us run more comp than we can on a given octane, or lesser timing without the sacrifice of power or pinging
the rod length choice and stroke also puts the rod ratio at  1.73 , quite close to rod ratio of a stock length rod on 78 stroke
i guess in my attempt at theory, this could give good piston accel. and signal to the induction ssytem without having too extreme of rod angle at certain stroke rotations

-- im juz theorizing, hope others could have different ideas and configurations, should be fun to discuss rod length choices, stroke etc  an odd out of

[wph]

Your choice of heads would seriously limit the power potential, a quick calculation with Johannes's formula says 136.8hp/5229rpm
with the 0.88x35.5 valve diameter. Suppose you had the same valve sizes as per Mouse motor, you could expect 176.4hp/6741rpm.
With 85.5 - 86 mm bore valve shrouding becomes a limiting factor unless you move the intake valve towards exhaust side.
Camshaft choice depends on your engine's piston CFM demand, valve lift and head flow needs to match it as close as possible.
   



 
 

[TexasTom]

What is the formula used to produce these figures?

[RFbuilt]

awesome WPH,

i figure.. since jpm ran 40mm intake valves then 35.5mm exhaust (33mm initially i think)

and he was 86mm bore..

then  a 86x82  with 40mm intake and 35.5mm exhaust would work i suppose?

i do have a set of 86mm forged pistons with 30mm compression height (pin height)

so  this is interesting (although im waiting for the rest of the 88x74 project to arrive) but will have to finish the 1800cc

so  opinions are apriciated

hello TXT!!

[wph]

What is the formula used to produce these figures?

http://cal-look.no/lounge/index.php/topic,3257.msg44224.html#msg44224

Read through the whole thread, it goes into details of his "Mouse motor" build.
Includes math to engine/head calculations, very simple formulas. Excellent read,
proven design with dynotests to back it up.     

[TexasTom]

Ralf ... Hello! Still worn out from Bug-In weekend.

WPH ...

Thanks very much for the information. I had forgotten that was in the mouse motor thread.

Take care,
TxT

[Udo]

awesome WPH,

i figure.. since jpm ran 40mm intake valves then 35.5mm exhaust (33mm initially i think)

and he was 86mm bore..

then  a 86x82  with 40mm intake and 35.5mm exhaust would work i suppose?

i do have a set of 86mm forged pistons with 30mm compression height (pin height)

so  this is interesting (although im waiting for the rest of the 88x74 project to arrive) but will have to finish the 1800cc

so  opinions are apriciated

hello TXT!!

I would go with 40x35 and a good pauter cam ...

Udo

[RFbuilt]

thank you Udo, thats what i was thinking, go 40x35.5

i think even Jim "stella blue" is on Pauter bumpsticks hehe

will def push for this route,

first : when the 88x74 and 88x69 parts arrive, will make a buildt thread on that, see how it goes, then give this a shot after..

[Jon]

With little hp to spend on springs, I would go with a Raptor camshaft.

[RFbuilt]

oooh thats another option i forgot.. JPM raptors


ahhh im getting excited,

in a week or 2, my parts will arrive and the 88x74 + 88x69 project will start

[wph]

With little hp to spend on springs, I would go with a Raptor camshaft.

Better yet, a cam ground for your specific engine details. Don't know if JP does this kind of work,
I would be very intrested if custom grinds are available.

[Torben Alstrup]

You need to define "realistic"
- For street use,
- or for racing.

The smaller valve solution will limit power to 130 - max 140 hp. And then they have to be very good. On the other hand it will be able to produce a massive torque curve over a wide range of rpm.
The larger valve solution can - naturally - extend the power curve upwards, hence higher numbers.
Wrt. rod length. Max hp, max rpm and you should choose the longest rod available.
- More usable torque at lower rpm and you should use about the shortest rod available.
- best all round length for an 82 stroke is 5,5" IMHO.

Personally I like a RR  in the 1,70 - 1,80 range in general. For race (Drag) I choose the higher numbers.
T

[RFbuilt]

wph -   good idea,  as i can have webcam do custom grinds.. like everyone knows..
         our cam grinder (honda specivic) can do some for me also.. so this is interesting


Torben -  as usual  great input  ,   

           here's what i was thinking
            since crank spigot height to center main bore distance is 100mm
            barrel is 112.5mm   = 212.5
            212.5mm - 2/stroke 
                         - 142.24mm (5.6" rod)
                         - 30mm pin height
                         = -0.74mm  or -0.029"
           
should be enough for a lil barrel shim or copper headgasket to get maybe a tight 0.045" deck

r/s would be 1.75


and my definition of realistic =  3-4days a week use... city/hiway   and maybe 1s a month drag 

[Johannes Persson]

Hello guys,

My recommendation to this topic is:Stroke 74mm
                                                  Bore 86mm
                                                  Valve size 40x35.5mm
                                                  Comp height 30mm max
                                                  Rod length 5.6"
                                                  CNC custom grinded Raptor cam based on timearea calculations
                                                  A serious "Pipe Tuning" very important
                                                  11.5-11.8:1 comp ratio
                                                  IDA 48
                                                  Stepped header 1 5/8-1 3/4"

depending on the heads, a realistic output is 215-218hp at 7500-7700rpm.
The bore and stroke will give a very wide power band with a snappy racing attitude.

The internal parts should be kept as light as possible especially in the valvetrain.

Keep up your good work and I am looking foreward to hear about your results.

Best regards
Johannes

[RFbuilt]

wow.. and to think "first" building the 88x74 and 88x69 for both my beetles

"then" il consider completing the parts, for this theory motor


johannes how bout a private message for the costing/price on the lovely Raptors