Compression Ratios

sander288 "Bolt-on Master"

1,404 posts · joined Aug 2008 · 14 Mar 2009, 1:25 pm

#700780

Was wanting to know how to change or set compression ratios, may seem stupid but I actually have no idea.

And what sought of compression are people running with their engines, using normal pump fuel, or even 98 octane

BMW R65 LS
1967 1300 Deluxe
Swing-axles FTW ❗

Bizarre Moderator The artist formerly known as blue74l

12,789 posts · joined Aug 2002 · 14 Mar 2009, 1:37 pm

#700785

It gets complicated but basically simple

It is your overall swept area compared to your squished area

here is a site that does it for you

http://www.aircooled.net/gnrlsite/resource/specgeninfo/calcs.htm
deck height is in thou the rest metric
go figure
What you need to know is the
cc's (cubic centimeters) of your heads
your deck height (the distance from top dead centre to the top of the cylinder)
bore
stroke

Now it gets complicated
Your bore and stroke aint going to change
69 stroke
85.5 bore for a 1600

A half decent compression ratio that will run happily on 95 octane is 8 to 8.5 to 1

now this is like
40 thou (1 mm) deck
50 cc heads
8.1:1
pretty stock

You can
put spacers oner the barrels to change your deck height
hog out the heads to increase CC
fly cut the heads to de crease CCs

But it is all about squish - a tight deck height and a good shaped head chamber

getting it all right is what seperates the boys from the men

Futue te ipsum!!!

bajachris88 The international telephone dialing code for Antarctica is 6

6,661 posts · joined Apr 2005 · 14 Mar 2009, 3:25 pm

#700800

why such a high octane with low compression compared to 2days cars? Is it still massively detonation prone in the classic motors?
I'm not being a smart arse, itsj ust out of curiousity, cause the modern cars, even some of the 4x4s like the jackaroo run at like up to 10, and others around 9 -9.5

Is it just some ultra sensativity with the classics or sumfin?

(ô_!_/ô) (ô_!_/ô)
69' baja: kombi box, thing spindles, irs, disc front, type 3 rear drums, 2 inch lift kit, 31x10 rears. 👍
New engine in process: 94mm p&bs, 74mm C/w chomol Crank, 35.5x39 SP heads, turbo. Wierd combo, hopeful torque monsta!

trickysimon 100km/h is only 9 seconds away

1,473 posts · joined Dec 2007 · 14 Mar 2009, 4:54 pm

#700803

New cars arnt aircooled. Aircooled engines run hotter 😉

RISKYBUG Seriously Crusin Dubber

187 posts · joined Mar 2008 · 14 Mar 2009, 6:03 pm

#700816

most people who what low comp ratios will use a spacer
or you can get low comp pistons or have some head work done
the get high comp you can machine the head or stroke it?
the high comp you go the more octane you will need
i run 91 ron in my sbug with under 8:1 comp