Guys, heading to the shop tomorrow to check-in on the car. Can you please explain and help me to understand the compression results below. They sent me a report today. The car is a 1973 914-4 2.0. Factory FI is gone and a single Weber carb is in place. These tests were completed POST proper valve adjustment to 0.006" intake and 0.008" exhaust, and the top-end of the engine was rebuilt a couple years ago.

Cylinder #1
1st Puff: 85
Overall Compression: 155
Leakdown: 10-20%

Cylinder #2
1st Puff: 84
Overall Compression: 153
Leakdown: 6%

Cylinder #3
1st Puff: 80
Overall Compression: 150
Leakdown: 6%

Cylinder #4
1st Puff: 85
Overall Compression: 155
Leakdown: 5%

My lay-person understanding is that the higher the number, the better for overall compression and that all cylinders should be within 10% of each other. I also read that 1st puff should be about 50% of the overall compression. Shop says that while the leakdown on #1 is a bit off, it's not too excessive but could cause her to run a little rough.

Please help. Thanks!

is #1 leakdown obvious enough to hear thru the intake, the exhaust or the crankcase?

First was thought to be carbon build-up on valve face or seat, but claims that with the oil cap off, you can slightly hear thru crankcase.

If these are cold numbers I wouldn't worry about it.

i just asked that to get a direction on where your loss was coming from. what was the leakdown %? i like to run seafoam thru older engines to break up any sticking rings and start removing the carbon build up. i'll probably get alot of slack from that suggestion but the only issue i ever had from that is chucks of carbon on the spark plugs afterwards... heck the old timers used the put water down the carb at constant high idle (not hydrolocking amounts) to remove deposits, works good too.

I will ask tomorrow if the tests were hot/cold and the position of the throttle at the time of test. While the compression looks good (to me), I am worried about the leak down range. They stated that they listed a range as numbers were not consistent on the #1 cylinder.

Am I looking at an engine rebuild here guys?

I'd drive it. Worst case you burn some oil, but I'd bet if you drove it regularly, you would not have any problem with these numbers. The leak down on #1 is probably from the valves with the compressions as closely aligned as they are. As long as you have proper valve clearance you're fine.

Thanks guys. I just can't understand how the compression test numbers can be so good (almost perfect and the average is great) and then have that much of a leakdown.

Perhaps Mark is right.

I read Tom Wilson's how to rebuild aircooled VW engines last night and he mentions leakdown tests and results. He claims that < 10% is a new and good running engine. The he states that 10-20% incidates mileage/wear but should still run solid. In his opinion, > than 30% starts thoughts about a rebuild.

I had compression numbers in the 120/130 range and a 45%leakdown result on one cylinder. It happens.
The engine got rebuilt, but the PO said that it had run fine when he pulled it.
The leak, BTW, was due to a collapsed register which in turn was causing a head leak.

Zach

Hell yes, it's good.

nothing wrong with that engine. drive it!
find some dual carbs. that single weber is OK "just to get by" but it will be difficult to tune for low to mid rpm.

why is it in teh shop? does it run poorly?

I have seen leakdown #'s around 50% only drop 2 hp from a 1500cc land speed T1 making 180 at the flywheel. Jake saw the leakdown while dynoing and was concerned about the heads. The engine was left on the dyno while I re-cut the seats, (which were out a lot). The engine picked up exactly 2hp! Worthwhile on a land speed engine, not so much for a street car.

I worry about leakdown #'s as an indication that something is going south internally, not so much for performance issues. I have known of quite a few engines that were performing great at the track, but got torn into for leakdown #'s around 10%. No significant issues were found and when they went back together performance was not improved one bit.

There are other more relevant and revealing ways to keep track of components going south as well. Keep an eye on your valve adjustment and oil consumption. If the valves aren't getting tighter, or looser everytime you adjust them and oil consumption is good then drive on. And keep in mind that one full round of an adsuster is .040", so a quarter of a turn is .010". That may not seem like a lot, but it is!

On a 2.0 or 1.8 a tightening intake valve can be a sign that the valve seat is sinking into the head. Continue to drive with this condition and the seat will eventually fall out. A tightening ex valve is usually a sign that the seat is eroding. Continue with this condition and you will burn a valve and possibly drop the head of the valve off.

If the valves are getting looser each time then the cam and/or lifter is probably taking a crap.

Heeding early warning signs with the valve adjustment can save you heartache!

Thanks for the insight, Len. That's very interesting that leakdown has little effect on performance.

The compression test displays what peak pressure was reached. That pressure does not need to be held for any length of time. The leakdown test applies constant pressure to a cylinder, so there is more time for the air pressure to escape.

The leakdown test is also conducted at TDC, whereas a compression test has the cylinder sweeping the full length of the stroke. So you may have a little more ring/cylinder wear at TDC on Cyl #1, but as soon as that cylinder drops from TDC, it seals better.

Click to view attachment


Speaking of which... I snapped this photo a few minutes ago of a head that is in my shop for remanufacture.

Sometimes this gives warning, sometimes it doesn't. It certainly gives support to frequent checking of valve adjustment. The thing about a intake seat hammering into the head is that it will continue to seal well enough to run well without a skip. Eventually it gets loose and this is what happens! This is not at all uncommon with 2.0 & 1.8 heads with O.E. seats!

Mark is right about the time element in the leak down. I should point out that high leakdown #'s can hurt idle and off idle performance. The higher the revs the more leak is required to hurt performance, especially valve leak. High ring leak can cause crankcase windage issues that have the potential to rob power.

If that was cold with throttle open, those are good numbers on a used engine.

i realize the cam can effect compression #'s (static compression).

maybe jake or len could input on
1] what comp. range a warmed up, stock cammed, say 9:1 piston, open throttle test should be at.
2] how much a 9550 cam should alter those #'s up or down.

That'll buff out

I had #1 seat in a T1/1835 break ... into a dozen pieces


Run that motor, Chuck

So it looks like the leakdown test was done cold and the compression test was done hot. They are not thinking cold engine and possible carbon deposits on the rings for the loss. With the compression numbers I have, I am not going to worry about it. She runs REAL strong and I plan to drive her

Ordered the dual Weber 40 IDF kit today, along with a new Facet fuel pump, new throttle cable and some stainless fuel lines to replace the old plastic ones under the battery shelf. The car was in the shop for a complete post purchase inspection and it looks very good. She was running rough (lots of hesitation and backfiring) but a proper valve adjustment, cap, rotor, plugs, wires and fuel filter have really woke up the car. Now, all that's left is to get that single Weber carb outta there and tune for the dual Weber 40's. I will keep everyone posted on the final results. As always, thanks for the tips!

The 40s will be a big improvement, but your milage will suffer.

You really cant' take advantage of the 40's without the cam to go with them, but definatey better that the single webber.

I might have missed it but too wide and not staggered ring end gaps and also cold testing is not very scientific just a place to start. I would not be interested in paying for a cold test especially on a aircooled motor unless a big leak is expected.

nice purchase. i keep trying to click on teh signature car. does it look that good in person?

Yep, she's in really great shape. And rust free too!

You must live in a fairly affluent area ... where folks just abandon their Porsches in the middle of the street?