# compression #'s questions



## RMTZ67 (Mar 12, 2011)

Ok guys, So my car started running rough for no obvious reason. And since having a ticking lifter for the second time I figured i might end up putting a cam kit and chain.So I wanted to check my compression #'s before I pulled the trigger on those parts.My #'s are as follow.

Dry Wet Dry Wet
1) 151 158 2)155 170
3) 151 162 4)151 160
5) 151 157 6)154 170
7) 151 160 8)154 170

Assuming I did the wet test consistently. I knew nothing about this motor when i got it ten years ago. Other than it seemed to run well. I might have put 5000 miles after the restoration. I have used valvoline racing oil since i got it. Supposed to have a high zinc content but do not know the numbers.Here are photos of my plugs 1st photo is # 1 bank and second is #2 bank in order.

1967 GTO original 400 with 067 heads. assuming its stock?


Questions: 1) how do those comp #'s look?
2)Think its ok to do a cam kit or does it need a full going thru?
3) What plugs do you suggest? has r45s now.
4) what do the plug colors look like? seem good to me.
After checking all that. I am starting to think the points may be causing the miss. So points and plugs tomorrow. I do have some mechanical knowledge but a wise man listens to advice. Thanks for your help in advance.


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## cij911 (Oct 25, 2017)

Your compression numbers seem to be within 10% - so that is ok.

Your spark plugs don't look great to me - one looks brand new and has not even been in a running motor...may was to check the spark plug is seated and/or broken and another looks pretty dark and wet.

Not sure the condition of your carb or timing, but I'd guess if your car was done 10 years ago, then the carb could use a rebuild (gaskets and accelerator pump) - cheap and relatively easy. I'd also verify timing too.

I also assume whoever built your car and motor put a cam in 10 years ago, but I could be wrong.....


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## My65goat (Jul 26, 2017)

Your numbers are pretty even, so that's good. Now the question that comes to mind with me is I also have compression numbers in the 150 -160 range on my 65 gto. The original service manual says compression should be 170- 190 range. I'm assuming that when they rebuilt your engine they lowered the compression for today's pump gas?

And in my engine I'm assuming that they lowered the compression by cam timing ??? My engine was rebuilt with stock type flat pistons and has the original #77 heads with a original style 068 cam. So with that said my compression should be near the original 10.75, which according to the manual should be in the 170 - 190 range. I'm a little baffled as to how my numbers are in the 150 -160 range. 

Can anyone verify if compression can be lowered by cam timing? To be more specific, if a aftermarket cam chain set was installed with the crank sprocket set on either of the advance or retard settings will that lower the cranking compression? 

Is this how they were able to rebuild my engine with factory style parts and get a lower cranking compression ???


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## My65goat (Jul 26, 2017)

Your plugs look so new that they are barely showing a read, that one in particular looks brand new. I believe that they are the proper plug type and application. As far as a cam, I would leave that advice for some of the more knowledgeable guys here to answer. In the meantime try to find out what is causing the misfiring. Check out that valve tick you mentioned.


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## RMTZ67 (Mar 12, 2011)

So heres the latest. Installed new 180 thermostat. cleaned the clean plugs for good measure. Installed new points and condenser took it for a test drive and seems to run close to normal at cruising speed but when I come to a light or stop it idles down and almost dies. like i stated above did this for no obvious reason. Tho there was one change that came to me earlier today. I usually put regular gas with no additives and this time I put supreme with 93 octane. Ran good for a while but on the way home is when this all started. Maybe it used what fuel that was at the bottom of the tank then by the time we headed home it had mixed or used up the existing fuel. What do you guys think of this observation? BTW plugs were only 6 months old and I restored the car but not the engine. never smoked or missed.


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## cij911 (Oct 25, 2017)

RMTZ67 said:


> So heres the latest. Installed new 180 thermostat. cleaned the clean plugs for good measure. Installed new points and condenser took it for a test drive and seems to run close to normal at cruising speed but when I come to a light or stop it idles down and almost dies. like i stated above did this for no obvious reason. Tho there was one change that came to me earlier today. I usually put regular gas with no additives and this time I put supreme with 93 octane. Ran good for a while but on the way home is when this all started. Maybe it used what fuel that was at the bottom of the tank then by the time we headed home it had mixed or used up the existing fuel. What do you guys think of this observation? BTW plugs were only 6 months old and I restored the car but not the engine. never smoked or missed.


No your octane is NOT the issue but you should be running 93 all the time IMHO.

You need to check the timing and vacuum and fuel mixture at idle. Good luck


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## BearGFR (Aug 25, 2008)

Examine your spark plugs carefully with a magnifying lens of some sort. Let us know if you can see any 'shiny silver specks' on them.

Bear


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## PontiacJim (Dec 29, 2012)

My65goat said:


> Your numbers are pretty even, so that's good. Now the question that comes to mind with me is I also have compression numbers in the 150 -160 range on my 65 gto. The original service manual says compression should be 170- 190 range. I'm assuming that when they rebuilt your engine they lowered the compression for today's pump gas?
> 
> And in my engine I'm assuming that they lowered the compression by cam timing ??? My engine was rebuilt with stock type flat pistons and has the original #77 heads with a original style 068 cam. So with that said my compression should be near the original 10.75, which according to the manual should be in the 170 - 190 range. I'm a little baffled as to how my numbers are in the 150 -160 range.
> 
> ...


I would not be too concerned with the compression numbers as long as your engine seems to run good and does not burn oil excessively. The 10% between the highest and lowest is more important.

A cam can indeed affect compression numbers at the cranking speed you spin the engine to test. It is always best to crank the engine will all plugs removed, battery charger on the battery, engine warm, air cleaner off and carb wide open. The closing of the intake valve, duration, and the cam's overlap can change your compression numbers. This is what was done so some of the high compression is lost at lower RPM's and gets picked up at your higher RPM's. The timing gears can also have an effect depending if installed straight up, retarded, or advanced and many cam grinders build in a preset advance, so to truly know your cam's actual timing, you need to "dial in" your cam using a cam degree wheel. 

Keep in mind that Pontiac compression is listed as the optimal compression ratio with the minimum head chamber cc's for that head. Most were not the minimum and actually larger. So your compression is more likely 10.5 or possibly less.


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## PontiacJim (Dec 29, 2012)

RMTZ67 said:


> So heres the latest. Installed new 180 thermostat. cleaned the clean plugs for good measure. Installed new points and condenser took it for a test drive and seems to run close to normal at cruising speed but when I come to a light or stop it idles down and almost dies. like i stated above did this for no obvious reason. Tho there was one change that came to me earlier today. I usually put regular gas with no additives and this time I put supreme with 93 octane. Ran good for a while but on the way home is when this all started. Maybe it used what fuel that was at the bottom of the tank then by the time we headed home it had mixed or used up the existing fuel. What do you guys think of this observation? BTW plugs were only 6 months old and I restored the car but not the engine. never smoked or missed.


My guess would be the carb is flooding. It'll take the gas while running on the highway, but as soon as you come to a stop/idle, it'll load up with gas and kill the engine. It was noted that your plugs looked a little wet - this would explain the wet plugs. Carb probably needs a good rebuild with all internal parts made for ethanol gas, ie needle/seat, accelerator pump, and float. More info here: Ethanol Fuel and Vintage Vehicles - Hot Rod Network Also replace all rubber hose with ethanol friendly hose.


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## BearGFR (Aug 25, 2008)

Flooding sounds like a good suspect to me also. Measure your fuel pressure at the carb inlet. If it's too high, it can force the needle off the seat.

Bear


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## RMTZ67 (Mar 12, 2011)

ya thanks guys. Gonna clean out the filter (quadrajet) then get me a carb kit this weekend. let you know the results. So the engine ran good when i bought the car so I never rebuilt it. is there anyway to know for sure short of taking the heads off If is been converted to todays fuel?


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## BearGFR (Aug 25, 2008)

RMTZ67 said:


> ya thanks guys. Gonna clean out the filter (quadrajet) then get me a carb kit this weekend. let you know the results. So the engine ran good when i bought the car so I never rebuilt it. is there anyway to know for sure short of taking the heads off If is been converted to todays fuel?


If it's a fuel pressure problem, replacing the filter isn't going to help - in fact it can only make it worse. That's why I recommended measuring it instead of randomly spending money to swap parts and "hoping". Back to compression. Yes, a cam change can bleed off cylinder pressure at cranking and at low rpm, but none of that means two squats to the engine when it's running. As RPM increases, volumetric efficiency is going to also increase until you reach the 'sweet spot' for that cam. At that point, VE (volumetric efficiency) will be at its maximum and cylinder pressure will also be maxed - and that's when you can get into detonation trouble. The only way a cam change can avoid detonation in an engine that has too much compression, is to put the point of peak VE high enough in the RPM range that the engine in effect "doesn't have enough time" to detonate. That can work to a point, but it's also going to kill a significant amount of low rpm torque and that can turn it into a real dog.

Inspect those spark plugs really closely under magnification and see if there are any shiny silvery specs on them. If there are, that's going to be bits of aluminum that has been being blasted off the pistons due to detonation. It's important to know if that's been happening or not.

Bear


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