What's your compression ratio? Options

[porsche735]

I am rebuilding my 2.0L....again and want to know what folks are running for compression ratios. What compression tester readings are you getting at first rebuild and then once the engine has "settled"? How long did the engine take to "settle"?

The reason for our rebuild is:

cyl #1 110lbs
cyl #2 115lbs
cyl #3 110lbs
cyl #4 45lbs

Thanks,
Chris

[Brad Roberts]

New engines:

100 around the corners.

After 500 miles and warm:

120 around the corners.

What happened this time ?? And did it just happen at ThunderHill ??

What kind of oil temps are you seeing now with the cooler ??


B

[porsche735]

Not sure what happened, my dad blew it up... I didn't even get to drive it. Are you sure about those pressures? What we had was good then for an engine still breaking in? Greg Chiocco said they had 170psi on all 4 of their's. We didn't measure his with our comp. tester, so his may be off. But gosh, we felt humbled...

Oil temps have been good <275 sometimes with 2 drivers going. Mostly around 250.

I think we need to cc our heads and do the math to make sure we have our compression ratios right. We have just been going with .050" deck height and calling it good. What is a good way to measure cc's of the head? I was just going to get a syringe and fill a leveled head....

Chris

[porsche735]

I am starting to think it is the spark plug wires. We replaced them 2 years ago, but it seems like that may be the only thing left that could be causing our problems.
What do you run for wires? Magnecor? Our's currently, are Bosch.

Chris

[Brad Roberts]

A syringe and a piece of plexiglass cut for the head with a hole in the middle.

Gregs numbers are "illegally" high. Even with Euro P+C's we dont get that number unless the heads have been shaved down "alot". Meaning not within the spec of the rules. I have seen 140.. but only when we had what we fealt where illegal heads (for SCCA and PCA).

I still think 250 is too high on the oil temps (especially with 2 drivers)

Stock Beru wires.. or Magnecor's. MSD wires on the carb'd cars (8mm)


B

[SirAndy]

Oil temps have been good <275 sometimes with 2 drivers going. Mostly around 250.

oil temps are too high, you should stay under 250 for sure,
200 would be much better ...

Andy

[Mueller]

Hey Chris,

I have a buret you can borrow if you need one. I won't be back in town until next Friday if you can wait till then.

[porsche735]

Thanks Mike,
We got new pistons and cylinders, so I shouldn't need it.

What compression ratio is considered stock? I have seen 7.6:1 as stock with 8.0:1 for Euro. I have also heard people say Euro is 8.6:1. Which is it? I need to know because the rules list .5 point above stock. What I have been building to is 7.6+.5=8.1:1. I need to know if I can go more.

What should I build to?

Thanks,
Chris

p.s. Sir Andy, I think you need to get your oil temps above bwater's boiling point to make sure you steam off all of the water to lubricate properly. I always thought that 250 was fine for a 2 driver car. On the hot days I have seen close to 275 before we backed off. Is my logic flawed?

[porsche735]

Here's some info on oil temps from another forum.

http://www.aircooledtechnology.com/dyno/co...g/dtm-tests.htm

http://www.aircooledtechnology.com/dyno/co...g/911-tests.htm

So it looks like 250 during a race is not all that uncommon....

Chris

[Part Pricer]

8:1 Compression, 94mm 2.0L Euro Non-dished

7.6:1 Compression, 94mm 2.0L (USA) Dished

[SirAndy]

p.s. Sir Andy, I think you need to get your oil temps above bwater's boiling point to make sure you steam off all of the water to lubricate properly. I always thought that 250 was fine for a 2 driver car. On the hot days I have seen close to 275 before we backed off. Is my logic flawed?

brad, the "oil-temp master" has repeatedly educated me that the desired oil temp range is between 195 and 210 for our engines ...

moisture in the oil can indeed be a problem but you don't
have to boil the water for it to evaporate.

Andy

[Brad Roberts]

Chris and his dad share a 914 at the track and have gone thru 4-5 engines since I have known them. It wasnt until last year that I found out they where not running a extra engine cooler.

I swear Chris (I dont give a shit about what other people say) you are frying these engines with those prolonged exposures to 250+. The cylinders will warp... the heads will warp.. everything goes to hell quickly.

You need to think about a larger cooler and/or a deep sump for 2 extra quarts of oil. Your case is EXTREME with two VERY good drivers pushing a 914 like very few can. It will take an extreme cooling system to handle what your trying to do.

Oh.. compression ratio: 8:1 for our cars in BP. Its .5 over stock (which reads USA 7.6:1) Part of the reason for this: stock P+C's used to cost more than the Euro sets... so they adjusted the rules to make it a tad cheaper.

Now.. I did some re-reading of some notes I had on compression. Greg is almost dead on with 170 AND Euro pistons. Its still considered a little high. I see 155-160 in my notes depending on how much carbon build-up they have... 170 might show up.

I true deck my blocks (where the cylinder meets the case) and run no shims under the barrels. This allows the piston to fully extend into the combustion chamber. I highly suggest you CC your heads and start doing some math to figure out what you have.

B

[porsche735]

Yep, we need to get a bit more scientific about it, I think. I will cc the heads when we take them off.

Do you lapp the cylinders into the heads?

I hope with all this, we can get to 170 psi. That would be nice. I'll pay more attention to the oil temps. The last event, it didn't even warm up!!!! If the oil is staying hot with different compression values, we can increase the cooler size. We are already running the "tuna can" and get about another 1/2 quart with that plus the plumbing for the cooler etc.

Does anyone have a chart of CR versus psi readings? I know you can't just take 14.7 as 1 atmosphere and divide because the temperature changes.

Chris

[Brad Roberts]

The increase in compression will certainly bring the heads temps up which in turn will increase the oil temp. Your seeing 250 now.. plan on 265 with more compression.

I meant an aluminum deep sump in place of the tuna can. They carry about 2 quarts more oil. Its easy to ad one right now while the engine is apart (also cheaper than buying a larger cooler and playing with ducting.

I do have to block our oil coolers off for certain events.. but usually its first thing in the morning to get the engine up to temp.

Your issue has more to do with heat soak than anything. That is why I suggest a "more oil" solution over a larger cooler solution.

B

[Dave_Darling]

There is no formula for compression ratio versus compression pressure. There are too many variables for there to be a formula.

Depending on some things, increasing the compression ration can actually lower head temps. I don't think that's the case with the stock 914 motors, but it can happen.

--DD

[McMark]

What your comp. tester should read would be atmospheric pressure * compression ratio. So for a car with 8.0:1 compression your max would be 14.7 * 8 = 117.6. To get a compression tester reading of 170 you would need around 11:1 compression. Yikes!

[Dave_Darling]

Not true. The compression pressure is not linearly dependant on atmospheric pressure and compression ratio--it is a non-linear adiabatic (sp?) function. Plus, things like the cam profile and valve sizes have a good bit to do with compression pressures as well. Not to mention liquid fuel going in versus air--because fuel in its liquid state is far less compressible than air.

See what I mean by "too many variables"?

--DD

[McMark]

Seems to me the question is, "How far off is it?" Can you get 170 psi out of a cylinder with 8:1 compression? 9:1? 10:1? Seems to me it would be a fairly close estimate for all but the most wild cams. I'll have to look into it. Hello Google!

[kdfoust]

I dunno but lookup stock CR on a '72 1.7. My mechanic just came back with 170 on all four on my engine. He said the highest he's seen was 180.

[Dave_Darling]

Stock CR on all of the 1.7s except the CA-spec 73 1.7 is 8.2:1. Some of the 1.7 Bus engines had lower compression, I believe, and the CA-spec 73 1.7 ran 7.3 or 7.6 compression.

--DD