TheSamba.com :: View topic - Building a 1745 what compression should I be going for?

Tue Nov 03, 2009 10:39 pm

Still learning about all this compression ratio stuff but wanted to get people's thoughts/experiences with CR. I want to build a good mileage 1745 but not sure what CR would be best to shoot for. Also, would like to run regular unleaded if possible and so that might figure into the equation. Lastly, is there a practical maximum to deck height for this kind of engine? In looking at the numbers it seems like I cant get below a CR of 8 unless I really increase the deck height.
thanks,
Lori

Wed Nov 04, 2009 5:21 am

what camshaft are you going to use? what's the rest of the specs?

Wed Nov 04, 2009 5:37 am

Yep compression should be matched to the cam. Last non-standard engine I built, I called up webcam and explained my engine design and displacement and usage and went with their reccomended cam and compression ratio. It worked out well.

Wed Nov 04, 2009 6:46 am

Deck should be in the 0.040" to 0.060" range. Get that set first, then modify the chambers to get your CR. CR depends on cam selection.

Wed Nov 04, 2009 8:40 am

Thanks for the help...I missed the part about the cam being such an important factor but that makes sense.

So far this is what I am planning:

new case, full flow, dual relief
76mm 4340 crank
unitech rods
stock rockers
level 3, Stocker plus heads-ACN (regular valve size, P and P)
119 cam-ACN (Lift - 0.422, Duration - 276 degrees)
dual weber ICTs and SVDA dizzy
lightened flywheel
stock pistons

Have been emailing with ACN and they say this all sounds fine. I will email them for their recommendations as this is their cam. If I keep the deck at .04 to .06 then I am going to have to open up those chambers quite a bit. I was guessing that the stocker plus heads had a volume of around 52 on average. I will ask them about that too.

Lori

Wed Nov 04, 2009 9:58 am

It helps to crunch some preliminary numbers, but remember to triple-check when you are putting it together.

Wed Nov 04, 2009 11:10 am

Heard back from ACN...they recommend 8:1 with that cam. Also the heads run about 53-59 cc so I think I will be ok with around .14-ish shims. Of course I will measure everything beforehand to see what I will need.
thanks,
Lori

Wed Nov 04, 2009 11:59 am

Sounds like a plan. Keep us posted.

Wed Nov 04, 2009 12:12 pm

you can remove some material from the top of the piston to help get you there if you cant get enough cc out of the heads. you'll need a lathe or a machinist.