Compression tests and CR

[Mark McClure]

Hi,

Just curious about CR and compression tests results. Obviusly the compression result is relevent to the CR. The higher the CR the higher the compression results. So is there a rule of thumb to go by. A well respected local machanic mentioned some today.

From what I remember he mentioned a 3.0 SC with 8-1 CR will give result of 140psi, 9-1 will give 175psi 10-1 around 200psi.

Is there arule of thumb........are there exact figures?

Cheers

Mark....

[A Quiet Boom]

There are to many other factors involved to give good rule of thumb numbers, it's best if you can compare to a known good running engine. Things like pumping efficiency and cam timing (overlab) can have a big effect on compression numbers, even how fast the motor is spun by the starter affects the numbers. The most important information a compression test gives is whether or not the cylinders are equal. If I remember right the last time I ran compression on my radically cammed 12.5:1 drag motor it's numbers were "only" around 190PSI but that was like 3 years ago so I don't remember really clearly. I believe the Haynes manual has compression specs for the various 911 motors, ideally you'd like all the numbers to be close and within the manufacturers recommended range.

[Wayne 962]

Contrary to popular belief, higher CR usually means *lower* compression numbers in high performance engines. This is because these engines require camshafts with significant overlap, which translates into lower compression numbers at low RPM.

However, it is true, if nothing else changes (like putting a big bore kit on an engine), then increased CR will result in higher compression readings...

-Wayne

[Mark McClure]

So does that mean that anyone making making a guess at what should be seen on a compression test is doing just that "making a guess."

I have had a look all through the Haynes manual and can not find any reference to the compression results.

so here is the next part.......I have a 3.0SC 79 with carrera cams. We are still trying to find out what pistons are in it. It has twin plug heads and we where told that it is a 240HP engine. The comp test shows 145-150 on each...so yes quite even.

In people experience does this show a CR of 8.5-1 or higher? The next part is to either pull the thing apart or squeeze my head into the shape of a cone and have a look down the plug hole. It feels like I have already done this!!

Cheers

Mark....

[CamB]

Mark

Did you ever get the engine to run properly?? How does it go??

On to your problem. I could be talking out my butt here, so someone shut me up if I am, but surely it is possible to figure out approximately the cam timing from observing when the valves are being opened and closed (ie remove a valve cover).

This would help you figure out which cams are in it (I have no smart ideas to figure out the compression ratio).

[Superman]

That's it! I was wondering why my compresson test numbers are lower now. Prior to my rebuild, I was proud of CT numbers all falling between 192-195 psi. Now, after the rebuild, I tested them lower (can't recall...in the 160s I think...163-167). Now I realize that it is probably those pesky 20/21/ grind cams.

[Mark McClure]

Cam,

I have identified the cams as the standard 78-81 SC cam. We now have to identify the pistons and therefore the compression ratio. For the claimed 240HP there needs to be more beef to it.

The lastest advice was to change the Magnacore leads as they are interfering with the Microtech computer. So after $300. still nothing. I am a fair mechanic but just have no real time......so I made time. The interference was from the distributor wires running to close to the injectors, timing was fluctuating from 10-40 at a constant 2000rpm. I moved the wire, timed and revised the ignition map again.....PROPERLY this time! Hey presto it runs better. Still not the 240hp.

So the next thing that I have looked at is the throttle opening. Again the butterfly is not opening fully. The linkage is loose and the balance is out. How do these so called professionals sleep at night.........I used to race motorcycles so I have build quite a few engines.....not as many as these guys. But they seam to dive in at the deep end without sorting out the obvious stuff first......

FRUSTRATED.........YOU BET!!!!!!!!!!

Thanks for the calming responses from all

[CamB]

I don't think SC cams are ever going to show 240hp in a 3.0, even with fancy computer control.

I have just had my 3.2 (6.5% bigger - diff from 3186 to 2994) dynoed at 270hp (this would very theoretically be 254hp in a 3.0) and I have S cams, ported heads and 10.3:1 compression (with twin plug). I also have fancy computer control

But, it sounds like the engine has some pretty good stuff on it. Did the seller make any representation about the pistons? To my mind, if it has SC cams that tends to suggest that the pistons may be standard (unfortunately).

Either way, it sounds like by starting from scratch (balancing throttle bodies (don't forget to check the computer knows where open and closed is on the TPS) and timing etc) you are going in the right direction.

Good luck.

[Mark McClure]

OK......sellers listing of the performance of the engine.. Which has been lying around for 4 years.


1. Won some porsche cup races.......
2. Will keepup with the turbo's until they get their legs......
3. goes like ***** off a stick.....
4. my car is lighter so it went really well in mine.....
5. we put a 3.6 in it and the laptimes only increased by .5 sec.....
6. this engine owes us $25K......

and the best one if you are to believe that all of the above are true or close to truth.........

*****NOTHING HAS CHANGED SINCE I HAD IT*******

Before it was put in the mechanic put new rings in it.....maybe just maybe......there was a bit of slight of hand?? I have traced the history and it did cost $8k for the injection system. You don't spend that money without doing the normal things...ie cams/compression.

So maybe there should be a new thread.......entitled......

"What the seller told me......and then the truth"

Cheers

Mark.......

[CamB]

I guess a better question is "how did you figure out it has standard SC cams".

'Cause 240hp is possible, just not with those cams. Porsche managed 204hp with 9.8:1 c/r. The injection system alone can't add that much. But if you had S cams... a different story.

It is possible a switcheroo was performed - I sincerely hope no-one has pulled a fast one on you!! Fortunately, it does seem kinda far-fetched. Did they tell you it is SC cams?

[CamB]

Reading back, I guess with twin plugs, you might have anywhere up to 10.5:1 c/r. Even then, with SC cams and virtually any exhaust I still struggle to see 240hp. I could see 220hp, but that last 10% is a lot...

I dunno - I am not an expert, just offering my limited experience and I don't want to lead you into trouble!

[Mark McClure]

The mechanic removed the cam chain covers and the part number was stamped on the end of the cam 930 105 147 08 and 930 105 148 08 L & R.

I took it back to the guy who raced it and said drive this. The first thing that he said was....."we never managed to get it to tick over this well".

There was an electric air pump added which pumped air into the throttle bodies.........this was to assist tick over. I have disconnected this....result......lower tick over. Conclusion a waste of time having it .....AHA unless something has changed!

Whilst driving it he comes out with.....well it's not as bad as I thought it might be, it used to have a lot more punch....but my car is a lot lighter and your wheels are bigger and heavier!

The dyno reading was 134rwbhp........hmmmmmmm

Cam.......it sound like you have a good setup on your car. Did you do it yourself or have the work done?

Cheers

Mark.....

[A Quiet Boom]

Well here's a little test that MIGHT work but you'll need the engine on a stand to do it. Locate the engine so that one spark plug hole is at it's highest possible point (we're going to fill the cylinder with oil) set that cylinder at exactly TDC. Fill a graduated cylinder with cc or ml (same thing) marks on it with light oil and record the level it was filled to. Now pour the oil into the cylinder until it's just to the threads on the plug hole (you need to have the engine at such an angle that only oil is in the cylinder at this point no air pockets) check the level of oil still left in the graduated cylinder to determine how much oil in cc's it took to fill the engines combustion chamber. Record this number as VC (combustion chamber volume) now look up in the manual the exact displacement in cc's of your engine and divide by six (number of cylinders) this number is VS (swept volume or displacement for that cylinder) CR (compression ratio is equal to (VS+VC)/VC

CR=(VS+VC)/VC

example
VS=500
VC=50

CR=(500+50)/50 = 550/50 = 11
CR=11:1

This method will get you well into the ballpark provided you can do it without air pockets so it's very important to be sure the the sparkplug hole is at the very top, since your's is a twin-plug engine just leave a plug in the other hole. When done round the number to the nearest available CR for your engine. Hope this "backyard" trick helps

[Mark McClure]

It's usefull bits like that which make jobs a lot easier for everyone. I know I should have listened more in physics class!

OK.....with the engine in the car what angle do I have to jack the car up to? How many porsche lovers will it take to hold it there? do I stand them up or stack them in? Can I drive it up over my wifes car to get the right angle........No this may lead to sleeping with the dog.....figuratively speaking that is!

Serioulsy it's is looking more and more like an engine out job........the PO is now looking through his filing to find out what it is suppost to have! Honest John they call him.....why does he have that parrott an eye patch and that wooden leg?

Cheers

Mark...

[CamB]

(edit) AQB and Mark posted again while I wrote this. Good luck Mark, here is hoping the guy comes back with sensible specs.

Mark,

I am very very pleased with the setup on my car (just don't ask what it cost - made worse by every step getting pinged for more $$$ (generally with due cause and fairness - hp costs money)).

I didn't do much (anything really) myself, except a lot of talking, questioning, learning, suggesting etc. The engine was available as a (sort of) package from a very respectable engine builder (he pretty much only builds Porsche engines and transmissions). The important bits were there.

So along the way, I learnt a LOT about the various ways to tweak these things for power. I really only know stuff focussed on what mattered to me though.

As I intend to do all the maintenance myself, I spent plenty of time quizzing on that too.

My engine makes well over 200hp at the wheels and, due to having full EFI, idles perfectly at around 1000 rpm and pulls cleanly to 6000rpm (and presumably 7000rpm). To get to this stage took about 1 full day on the dyno.

With SC cams and EFI, your engine should have at LEAST 180hp (or 160rwhp using 10% trans losses) as this is what the SC had. If you ask me, full EFI must be good for a few horsepower, and if you have headers or SSIs or early exhaust you should get ~15. That is pretty much 200hp, or maybe 180hp at the wheels. Something is wrong if you don't get that.

Higher compression probably could gain another 15-20hp, hence my guess of 220hp if it still has high compression pistons - if it is twin plug it must have had them in the past.

And if the cams were reground then it could be quite a bit more (towards the claimed 240hp). This would definitely require different pistons to allow for extra overlap.

Noah is (modestly) not pointing you towards the excellent results of his modifications to his own engine which show 203rwhp from his cams plus higher compression pistons. You could expect a little more from less restrictive individual throttle bodies and full EFI. While Noah and I would disagree as to what driveline losses to use, this is WAAAAY more than the result you have.

So what am I saying? Well, it is possible you could see 240hp if the cams are reground ones and the engine hides high compression pistons. So...

... how does it drive? Is it smooth? What were the air/fuel ratios like on the dyno? What sort of advance is it running (I seem to have quite a lot - up to 35 deg but generally 30 deg or so max).

Does it make any bad noises? Quite tricky trouble shooting when there is so much non-standard (but very cool!) stuff.

[A Quiet Boom]

Originally posted by Mark McClure
OK.....with the engine in the car what angle do I have to jack the car up to? How many porsche lovers will it take to hold it there? do I stand them up or stack them in? Can I drive it up over my wifes car to get the right angle........No this may lead to sleeping with the dog.....figuratively speaking that is!

LOL, judging by the looks of the plug angle on my SC engine I'd say "teedering on the passenger front wheel, about 45 degrees from level" LOL Seriously though it's almost definately an engine out job. Have you checked total timing on this thing? Like Cam mentioned it should be over 30 degrees BTDC, I like 32-34 for good performance. If it's any less than that you could be giving up quite a bit of HP. You can approximate your valve timing with a degree wheel and a dial indicator but I'm sure you'd have to modify the wheel to fit on a P-car (it's just a round disc with degree markings on it) Using the wheel and the indicator you can measure the degrees at which your valves open and close as well as lift but you have to do it for both intake and exhaust and it's still and approximation due to things like valve lash, not having the indicator exactly parallel to the valvestem etc. Once you have the opening and closing points it's relatively easy to determine the cam specs. The wheel needs to be mounted to the crank pulley and a suitable pointer made from wire to indicate degrees. Common practice on V8 performance engine building to set the cam exactly where you want it.

[Mark McClure]

The ignition timing I have played with from 25 to 32 BTDC. Does this depend upon the octane rating of the fuel being used? and the CR? Is'nt it a fact that you cannot advance too much with higher compression and you do not need to advance as much with twin spark due to the better flame front.

The short of this is that I should be getting at least standard output......even with all of the fancy things inplace if the internals are basic then,as Cam pointed out, there is not much room for improvement.

Assuming nothing has been changed....how do you loose end up with lower compression........could it be a base gasket? If the wrong one was fitted it would "raise the head" (no cheap jokes on that one) any thoughts.

Cheers

Mark.....

[A Quiet Boom]

Oops, yes you are correct about twin plugs needing less timing. And yes a thicker than normal base gasket as well as head gasket will lower the compression volume. Don't forget that overly advanced or retard cams can have poor effects on power as well as improper A/F ratio. I haven't read your other posts on this motor so I don't know the specifics of your setup, look like I need to do a little search and educate myself as to what your combo is before I try to figure out what's going on with the power loss.

[CamB]

I'm too lazy to check back through your other posts - does it have good compression?

(edit) Ok so I looked up the page and found the compression. Well they are even, so that rules out a bad cylinder.

I guess what I am trying to figure out is if it runs ok, just bad power, or if it runs badly and makes bad power...

[Mark McClure]

Cam

It starts fine, has good running....a few pops coming off idle, and a bit of a flat spot around 3K.........revs fine and drives well. The pops and flat spot will be ironed out on the dyno. Since I do not have a o2 sensor it is not easy to setup. I have concertrated on the majors before looking into the minors.

Something that I noticed this morning is that when the throttle reading is 99% ie peddle flat down.....the butterflies are not sitting vertically. There appears to be another 20-30degs to go.

What defines full throttle, when the butterflies are vertical, or is there a point at which any extra opening is not necessary?

The TPS sensor also shows a +17% adjustment. What I will do is adjust the TPS sensor to read 0% when closed with no need for compensation by the computer. I will then adjust all of the linkages.......and test.

Keep you updated.

Cheers

Mark......