I'm trying to work out a knock problem I'm having during light acceleration. It's a 88 305 TPI, and when I did a compression test I got 210 all the way around. I would have to aasssume that someone has been through this motor because 210 seems high to me. I'm running 92 octane at 5btc. I do have some carbon build up in the runners and plenum, but can that be enough?

 

Whats your engine temp? Real or factory guage measured? are your plugs correct? do you have different pistons in there making the compression ratio higher than you think? Do you have different heads?

spark knock on a factory motor thats had a good tune up and is set to 5* advanced doesn't add up. Is this audible knock or is it picked up by the Knock Sensor and displaying on a scanner of some sort?

 

Temp is fine(factory gauge)185-190 running down the freeway after 10 minutes of idling temp comes up, and fans come on at 200. Scan software comfirms, nothting out of the ordinary. I don't know about the pistons, that's why I'm wondering with this kind of compression, that someone before me has gone through the motor. I have had the car for about 8 months and just recently did a tuneup on it using all GM parts from the dealer. I'm using the platenum AC delco plugs the dealer book called for. When I did the compression test, plugs looked fine. I'm wondering if someone had the timing set to compensate for the compression, and then I came along and reset it to factory spec not knowing. It never pinged until I did all this. Can carbon build up on the tops of the pistons cause this?

 

The computer does pick up the knock, I can see it on the scan tool. And yes you can hear it also. It's a one time crack right when you hit the throttle. But it's OK after that even holding your foot into it.

 

I'd say your compression is way high, 210psi equates to about 14:1 compression ratio.

 

Yup, 210psi is way too high. It should be more like 170-185psi if you are going to be using pump gas.

 

what kind of compression ratio do you think I have to get compression like that?

 

Sounds like the CCs may be plugged with carbon.

 

Actually, its probably possible that your camshaft could be very worn out. How is your top end power? If there isn't any, that could be why.

If your cam is really worn, the valves may not be open long enough to bleed off excess cylinder pressure, which could result in a problem like yours.

 

The car runs killer. The top end never seems to end which is unusual for a 305. That's why I'm suspecting some internal changes have been made at some time.

 

WOW....210 is very very high.... I'd agree with dimented in that maybe it has some bad carbon buildup. But across EVERY cylinder? I don't know... Have you tried an induction cleaning on it? And I don't mean one of those wimpy Tire Kingdom induction services....I mean a HARDCORE induction clean. Either what is it they call it...petrol? Or GM top engine cleaner.

 

Believe me it was a suprise to me too. It nailed 210 across the board, and felt like it was going to jerk the compression tester out of my hand. That was a dry test too, I didn't squirt any oil in the cylinders. No I haven't tried any decarburizer yet, but was thinking about trying the GM top stuff. Like I said in my original post it only pings right when I get on it, even when I stay in it, it's only right at the hit. What do you think we are talking about for compression ratio 10:1? Higher?

 

As long as they all read the same, I wouldn't pay too much atttention to the actual number itself.

210 is so high as to be beyond belief. Something isn't right. Even if the motor has been "gone through", that's too much cranking compression. 170-185 would be more normal. So that's yet another reason to ignore the absolute valur of the number the gauge showed, and just ocncentrate on the differences among cylinders. Which if they're all the same, would tell you that the whole motor is in equal consition throughout.... and since a motor rarely if ever deteriorates identically in every cyl, would indicate that everything is probably working right.

I also wouldn't put too much stock in "the top end never quits".... it's a TPI motor, and TPI by design craters at above 4500 RPM, due to the acoustic tuning of the runners. They are tuned to reinforce cylinder filling across a certain band of RPMs, at the expense of "de-tuning" any RPMs above about 1.5 times their tuned RPM which is 3600 RPM. Compared to a TBI 305, maybe it has "top end"; but not compared to something with flow. Again, I'd tend to entirely discount that as any sort of indication of the engine's internal condition; other than if it runs smooth, doesn't smoke, gets decent gas mileage, etc., it would indicate that there's nothing seriously broken in there.

Warming up at all while running down the freeway is highly abnormal. That tells you that your lower air dam is missing. I'd recommend checking that before you do anything else. Your car should run at thermostat temp on the highway, even if it had no fan. It should only get warm while sitting still. Heating up at high speed is a definite sign of a malfunctioning colling system, and should be repaired before you go any further.

Your EGR system should be recycling some exhaust gas back into the intake at cruising speeds. The passages that fed exhaust up there often get all coked up over a period of years. The purpose of EGR is to lower combustion temps under light load, so that lean mixtures and greatly advanced timing can be used. If the EGR system doesn't do its job, the ECM's timing curve will be too aggressive at light load, which will result in pinging. You can usually clean it out with a coat hanger or something. It's a bit of a hassle in a TPI car though because it's under the plenum. But it wouldn't be a bad idea to verify that all of its parts are present and working; vacuum lines, wiring to its solenoid, etc.

 

Sorry, I guess I didn't word it right, but it gets to 185-190 running down the freeway, and when sitting at idle, it takes 10-20 minutes before it gets to 200 and the fans kick in. I'm not having any heat problem at all, I just mentioned it because I knew with the compression someone would ask me about temp. EGR is good, I just replaced it and the plastic lines with rubber ones. Passsages were pretty good, not much more than a slimmy build up. As for the 210, not sure what to think either. I tested the gauge, and tried it with a different one, and same results. When I say the top end never quits, I'm saying in referenc to the TPI. I know they tend to fall on their face around 5000, but for a TPI car, it really goes. I also have a 89 formula bird with a 305, freshly tuned, and the camaro is running circles around it.