Itinerant Air-Cooled

Hi all,

Does anyone know if there are any characteristics of the RAT 9550 cam & "CS" heads combo that would make it difficult to get accurate compression readings?

My 73 2.0L bus runs great, has lots of power, but the compression numbers are all right around 50. A "wet" test doesn't change it much. Leakdown test was good at 14%-16%. Head bolts are tight, valves are adjusted, throttle wide open. The compression went up about 5psi when I put .004" lash instead of zero. Tried two different compression gauges, no change.

The compression just doesn't make sense to me considering how well it runs! Any insights?

Thanks!
David

dwdelano wrote:Hi all,

Does anyone know if there are any characteristics of the RAT 9550 cam & "CS" heads combo that would make it difficult to get accurate compression readings?

My 73 2.0L bus runs great, has lots of power, but the compression numbers are all right around 50. A "wet" test doesn't change it much. Leakdown test was good at 14%-16%. Head bolts are tight, valves are adjusted, throttle wide open. The compression went up about 5psi when I put .004" lash instead of zero. Tried two different compression gauges, no change.

The compression just doesn't make sense to me considering how well it runs! Any insights?

Thanks!
David

Ask Jake at Aircooled Technology. See if he mentions "valve overlap" or "duration". That may be the reason for your low compression, the valve opening/closing timess are slightly out of phase at slow speed to better breathe at high speed. You may have an intake valve still open halfway up the compression stroke and an exhaust opening before the end of the power stroke.
Either way, drive it. Use the leakdown test as your diagnostic.
Colin

Thanks Colin!

Thanks for the advice! I'll try to get an answer from Jake, although I'm not sure how to do that now that he's shut down his forum. I'll probably try to post a question on his Facebook page. Interestingly, Len Hoffman gave me the same advice you did...i.e. drive it! My problem is that this was the first Type 4 I've rebuilt. I was as careful as possible, checking everything over and over before doing anything....but I still don't have the confidence that years of experience would give!

Thanks again!
David

Did you break it the rings correctly? How many miles on the engine?

I broke in the cam by running it at over 2000 rpm for 20 minutes (as advised by Jake Raby), but I didn't take any specific steps to break in the rings. Wouldn't the leakdown test suck if the rings were bad? How do you break in the rings? There are only 2500 miles or so on the engine.

Colin, your suggestion regarding the overlap and duration makes a lot of sense to me. But honestly, I'm a "why" kind of guy, I like to know why things are as they are. I had an idea, (since I haven't gotten a response from Jake on his Facebook page). It may be a stupid idea, but if I were to remove a rocker, thereby taking the valves out of the equation, could I then hook up the compression gauge on that head and test it then? It seems like that would tell me if I screwed up the heck height/CR calcs.

Okay, So from three different sources, sources I HIGHLY respect, I have the same advice:

Jake Raby said "If you really had 50 PSI dynamic the engine would run rich, be impossible to tune, have no power and generally perform like ass. I'd check your compression tester and the schrader valve inside of the compression tester fitting. If the engine runs well, throw everything else away and drive it."

Len Hoffman said "I would not put much stock in cold compression #'s on an engine that runs good"

Colin said "use the leakdown as the diagnostic. Drive it"

So that's what I'm going to do! I'm not going worry about the compression numbers! (Thank god I don't have to tear the heads off!!)

dwdelano wrote: I'm not going worry about the compression numbers!

We have a performance metric here on the IAC forum. Use it to help break in your rings.
Accurately time your acceleration from 50-60 mph on a level road, let it coast to 50, then time/accelerate 50-70. Now come back the other way and record those numbers to cancel head wind effects on your times.
Post 'em.
Colin

Amskeptic wrote:Post 'em.
Colin

I will do that! This weekend. Post'em in this thread?

dwdelano wrote:

Amskeptic wrote:Post 'em.
Colin

I will do that! This weekend. Post'em in this thread?

Yes, this is where the inquiry currently resides. Your numbers may or may not put to rest any concerns about the output of your engine. Is your bus at a fairly empty weight or is it loaded with thirty years of knick-knacks and tools and road accumulations?
Colin

Hi Colin!

Sorry for the long delay, we were out of town.

So here's my results:
50-60: 5.1 secs both times.
50-70: 11.2 & 10.9 secs.

David

dwdelano wrote:Hi Colin!

Sorry for the long delay, we were out of town.

So here's my results:
50-60: 5.1 secs both times.
50-70: 11.2 & 10.9 secs.

David

Your grade is:

Adequate

Enjoy driving it.
Colin

What gear should you be in for the test? Id imagine you could rip through this a lot quicker if you start in 3rd....

airkooledchris wrote:What gear should you be in for the test? Id imagine you could rip through this a lot quicker if you start in 3rd....

This is not about "quickest possible". This is solely a test of engine torque-to-horsepower output. There can be no shifting, which opens up the car to abuse and the test to individuals' shifting talents.

Fourth gear pull only . . .
ColinTheRippernot

And that makes sense. The differences in gearing from one setup to another will then be a factor in the numbers.

Any hypothetical differences one would expect to see, for example, in a 002 Vs a 091? I know there's a few possible differences in the 4th gear ratio from one 091 and another. The freeway flyer Vs the hill climber gear?

airkooledchris wrote:And that makes sense. The differences in gearing from one setup to another will then be a factor in the numbers.

Any hypothetical differences one would expect to see, for example, in a 002 Vs a 091? I know there's a few possible differences in the 4th gear ratio from one 091 and another. The freeway flyer Vs the hill climber gear?

Absolutely there are differences based on gear ratios. You can figure out road horsepower/wheel torque easily enough. This test is primarily for 091 2.0 with .89 4th set-ups where I was originally comparing Raby engines to a fresh (once fresh) 2.0 factory build in the BobD.

Out in the world, I have found that early buses squirt off from a stop better than late buses, but run out of revs sooner. That thar is wheel torque from the 5.375 rear axle.