Compression changes Options

[-JR-]

Having never worked on a air cooled engine before I'm wondering if it's possible to increase compression of these motors in the same way you do with a conventional watercooled engine.

IE - Head shaving or piston changes?

I've got all the bits from a 2L transporter engine that I'm going to put on my 914 EC case motor. However this will mean I will only have 7.3 : 1 compression still. I'd "like" to see it around the 9.0 region but I'm not sure if I can accomplish this without pistons.

Has anyone got some feedback?

Can I pull this off?

Thanks!

[nebreitling]

i'm thinking you're gonna want pistons.

as long as you're at it, build yourself a 2056; stock stroke, stock bottom end, 96 jugs. add hot cam, a little head work, carbs, and you should make 110-120hp.

[Mueller]

if I figured it out correct, you'd have to move the pistons .08" closer to heads

now if your motor has the head gaskets, those can be removed (and recommened), thickness of those are about .02", you can remove material from the top of the cylinder to increase the compression...of course now you'll have to check the pushrods since they might be too long (they might work as is if just building a "beater" engine)

if you can weld, you can close up the combustion chambers to increase the compression....

as far as I know, no easy or cheap method to do this........

[-JR-]

Thanks for the tips so far.

I imagine that I CAN NOT remove material from the head surface to increase combustion because of the design of these motors?


If I remove the 0.08" from the cylinders, will the push rod guide tubes put up with that, or will they have to be modified?


***THis is a beater motor that I'm doing a quick and nasty rebuild on. I only want another 2000mi out of this motor over the next year.


914Guru - Do you have all the cyinder displacment figures for preforming a compression calculation on a 914 2L engine? IO have a calculator if I had all the figures.

Thanks!

[Mueller]

the cylinder heads can be cut, but it's the same effect to the pushrods/tubes, not too sure how much you can get away with, I'm guessing the pushrod tubes would be okay with that modification

each engine is different for the compression, you need to know chamber CC's, the deck height, bore and stroke

[-JR-]

Thanks Mueller. I just thought you might have those specs on a sheet somewhere that I could steal!

I'll have to measure it up myself when I get the parts.

James

[Joe Ricard]

Just measure your deck, Then like me measure about 15 more times. CC the heads. stock are in the 62 CC range. If the deck is to close you will crash the pistons into the heads.

Someone said you can mil some material off the piston crown of bus pistons to lower the dish (which is 10cc BTW) US is 5cc and euro is actually 2cc although they look flat.

[redshift]

I just put a sponge in there to take up some space, and pad the valves.


I don't really, I just thought it'd be funny to say that, but now I guess we all know it isn't, so just remember, sponge based compression change jokes aren't funny.



M

[Jake Raby]

Don't think about increasing CR with the stock cam!! The dynamic CR the stock cam creates is already too much with the factory CR settings!

Thats why a cam change really wakes these engines up in performance, cooling capability and increases the powerband.

To increase CR you can flycut the heads, or decrease deck height. The TIV head is severely weakened when the chamber floor is flycut excessively. In some instances the floor can actually burn away and you gain a compresssion leak around an exhaust stud. The design flaw in the heads creates a weak space right behind the chamber where the exhaust stud is drilled.

The 2.0 head is the worst in this area.

[-JR-]

Thanks Jake. I've heard the 2.0L heads were bad for cracks. I suppose that's the reason.

I have 1.8L heads I'll be using instead of the 2.0L heads. I don't want to spend hardly any money on this engine as I'd rather put it towards the 3.2 I will be putting in it next year.

James

[Dave_Darling]

Yeah, you really want to be careful about flycutting the heads. There was a thread on the Shoptalk Forums "Type 4um" that showed cross-sections of various 914/Bus/411/412 heads. Pretty scary how close the exhaust studs were to the combustion chamber in a few cases.

Don't take Mueller's word on the 0.080", though. You always always have to measure for yourself!! Then crunch the numbers to determine the current compression ratio.

--DD

[-JR-]

Thanks Dave, I've read Tom Slider's web page on compression in which he credits you for some of the feedback...


Type 4 Engine compression

So having said all that I'm leaning towards having the cylinders milled. But as you say I will have to calculate the volume on my setup to know just how far I have to go. After reading that page I am also thinking that a ratio of about 8.5:1 to 8.8:1 might also be more appropriate.

James

[scruz914]

I just put a sponge in there to take up some space, and pad the valves.


I don't really, I just thought it'd be funny to say that, but now I guess we all know it isn't, so just remember, sponge based compression change jokes aren't funny.



M

Sponge Head Miles! (IMG:style_emoticons/default/lol2.gif)