Normal B3 cylinder pressure?

[BigElCat]

Has anyone here done a compression test on a carby B3 motor?

The compression ratio is 9.7 to 1 according to the FSM.

I can't find any specs on the actual PSI these cylinders will generate when the motor is in good shape.

[kellen302]

9.7x14.7=142.5ish, 14.7 = ambient air pressure at sea level. I haven't done a compression test, but this seems reasonable.

[Zanzer]

207 normal 149 minimum per the [Haynes] book. Most of the ones I've tested are in the 150 - 175 range if they have no valve, ring, or head gasket issues.

[BigElCat]

"Fascinating"

For every 1000' increase in altitude, atmospheric pressure drops 4% (on the average). I'm at 1000 feet above sea level, so if I figure 14.1 x 9.7, I get 137PSI.

Apparently, the vacuum created in the cylinders draws more air/fuel mixture than would be pushed in by atmospheric pressure alone.

To re-phrase this observation; Normally aspirated engines create a small amount of 'boost'. Can this be true?

Actually, all I needed to know was 149 PSI per cylinder means the B3 is doing okay. Thanks for the input!

[WmWatt]

Sorry but 149 psi means the engine is worn out. The range is 209-149. 209 is no wear. 149 is worn out.

[Zanzer]

149 is low but it'll run. I wouldn't get worried until you start flirting with 100psi. That's when she'll start having trouble firing the mixture. I had one motor where the cylinders were between 120-140, you could tell she was a little tired but still ran well.

Also, don't let low compression numbers fool you into thinking the motor is worn out. These engines are bad for building up carbon deposits on the exhaust valves which will cause the valve to improperly seal and will yield low compression numbers. I've seen this on 2 Aspire motors recently.

[kellen302]

Actually, I was reading in HOT ROD a while back and found that on high revving race engines one of the most important numbers is intake valve duration, because at high velocities the intake charge has a significant amount of inertia and momentum that continues to carry charge into the cylinder just past BDC. So in effect, yes, we do get a teeny weeny eensy bitty bit of boost.

[Zanzer]

The same car running below sea level will pull a better ET than running above. We ran into this a few years back when trying to beat a time set at Houston Raceway Park but we couldn't get our time corrected for our altitude. So yes, the further below sea level you go the more "natural boost" you get

The port velocity and duration has to do with cylinder filling. That's why flow vs velocity is extremely important to remember when porting a cylinder head for NA use.

[kellen302]

I love it, on a forum cheifly dealing with engines that cost less than $300 making less than 70hp most of the time, we're talking about the effects of altitude and intake charge inertia/velocity on creating natural boost to up the compression, lol

[Zanzer]

Like I was telling a friend last night. A motor is a motor and I love them all

And every little bit helps LOL :mrgreen:

[kellen302]

Yea, I know it, at 11k feet in Cusco, Peru, A) walking up stairs for the first 2 days is a b**** B) All the vehcles haven't been retuned for altitude and run hella rich so the smog also sits because it's heavier and You get consumption just walking out the door.

[BigElCat]

So in effect, yes, we do get a teeny weeny eensy bitty bit of boost.

I've never really thought about this before, so I'm not arguing. At 207 PSI, divided by the compression ratio, we get 21.3 PSI at the beginning of the compression stroke. That's a full 50% increase over average atmospheric pressure. The extra volume must be getting pulled in by force, it's not just going with the flow of mother nature.

I imagine the people who actually know about these things are rolling their eyes But. hey. I'm having fun

[Zanzer]

The extra volume must be getting pulled in by force, it's not just going with the flow of mother nature.

I imagine the people who actually know about these things are rolling their eyes But. hey. I'm having fun

Some of it is getting pushed in by ram effect due to velocity and intake pulses between the back side of the intake valve and the length of the runner and volume of the plenum. This is where you get the term "tuned port" because depending on runner length, cross section, and desired RPM operation you can "tune" the runner for desired attributes for your application.

I'm not sure how much this effects a simple compression test but hey, it's fun to speculate...and when I stop having fun I'll have to find another hobby


Also, on a related note: Don't forget to do your compression test with all of the plugs out of the motor in case there is a head gasket issue between cylinders. This helps eliminate cross filling. Also, some people recommend you add a tiny amount of oil to the cylinders when doing a compression check. This may help help boost your 137psi number up a bit.

[WmWatt]

A dry and a wet test will suggest if replacing the rings will improve compression. Putting oil in the cyclinder (wet test) improves the seal.

Am I right in suspecting a low compression engine won't pass emission testing?
Unburnt fuel getting into the exhaust? What effect would that have on the O2 sensor and the action of the computer to correct?

[Zanzer]

You are correct sir Luckily we don't have the sniffer test here (yet). They just look for the CEL and even that is easy to defeat depending on the issue that's causing it to illuminate.

Unburnt fuel in the exhaust will cause EGTs to be lower and the ECU should read this as a rich condition and adjust mixture accordingly....in theory anyway.