Compression and Spark Plugs

[7upstang91]

As some of you may know, I had received a bad tune for from a horrible tuner who lied to me. I found this out last month. I'm pissed. Anyway I am going to do a compression test before anything else to see where I stand from the bad tune. What compression should I be looking for in each cylinder for my 331? Also why I am there what spark plugs would you guys recommend if they are bad and what gap should they be?


Thank you for your help!

 

[Mustang5L5]

I think consistency between cylinders is more important than actual numbers. But from what i've seen, healthy motors are usually in the 150psi+ range I believe?? I could be wrong as it's been a while since i've dealt with comp tests.

I'd get your numbers and post them up and ask opinions.

As for plugs...depends on combo and ignition. Boosted car? I'd go iridium plugs.

 

[SpartaPerformance]

Compression can range from 120 - 170 but Mustang5L5 is correct consistency is key!! 10% variance between cylinders is optimal.

 

[7upstang91]

Compression can range from 120 - 170 but Mustang5L5 is correct consistency is key!! 10% variance between cylinders is optimal.

I figured consistency was key, just wasn't 100% sure about the range. 120 seems rather low to me, as well as 170 is rather high. 150 is a good number to go off??

 

[project67]

get your numbers and post them up and ask opinions.

 

[Repostyle]

The numbers you get depend on a couple of things, you can't just go by motor size. It depends on heads, stroke, bore, rockers, cam. All those things can change the numbers it "should" have. At this point any numbers are just guesses. The most important thing is that they are all within about 10psi. If they are off by more than that, try and get them to the highest number, since problems like valves not seated properly or bad rings, or leaking head gasket drop compression.

 

[90mustangGT]

Get the engine warm. The difference in temp can make a difference.

Pull all spark plugs. This allows the engine to spin at a higher RPM and gets a better reading, also easier on your starter and battery. Just the proper way by the book as well.

Either disconnect the power to the fuel and spark, or better yet, turn the motor over by jumping the solonoid. Ignition systems don't like to be ungrounded, you don't want to be the ground yourself, nor do you want to start any fires, so disconnect the ignition. If the injectors are spraying fuel, it may alter the test results by the fuel buildup and/or the fuel washing down the rings. If you just jump the solonoid, neither one of these will be active and you will get the best results.

Have something hold the throttle wide open, so the air supply doesn't alter the results.

When spinning the engine over, note the time it takes to build compression. 70psi is minimum for combustion. I've noticed that engines with worn rings tend to take a while to get usable compression. If some cylinders are way down, that is a good indicator you have a problem with that cylinder.

Then there is the leakdown test. There is a tool for this, but you can also hook up a compressed air supply to the cylinder on it's compression stroke and listen to see where the air is going and how much is leaking from the cylinders.

 

[7upstang91]

Get the engine warm. The difference in temp can make a difference.

Pull all spark plugs. This allows the engine to spin at a higher RPM and gets a better reading, also easier on your starter and battery. Just the proper way by the book as well.

Either disconnect the power to the fuel and spark, or better yet, turn the motor over by jumping the solonoid. Ignition systems don't like to be ungrounded, you don't want to be the ground yourself, nor do you want to start any fires, so disconnect the ignition. If the injectors are spraying fuel, it may alter the test results by the fuel buildup and/or the fuel washing down the rings. If you just jump the solonoid, neither one of these will be active and you will get the best results.

Have something hold the throttle wide open, so the air supply doesn't alter the results.

When spinning the engine over, note the time it takes to build compression. 70psi is minimum for combustion. I've noticed that engines with worn rings tend to take a while to get usable compression. If some cylinders are way down, that is a good indicator you have a problem with that cylinder.

Then there is the leakdown test. There is a tool for this, but you can also hook up a compressed air supply to the cylinder on it's compression stroke and listen to see where the air is going and how much is leaking from the cylinders.

Thank you for the advice, I should be able to get the readings tonight. I have the kit ready to go. I also am going to do the test with two other guys whom have performed the test before.

Also Iridium plugs were recommend, anyone second that statement? Maybe have another opinion?

Any advice is more than welcome. Thank you for all advice so far!

 

[90mustangGT]

Thank you for the advice, I should be able to get the readings tonight. I have the kit ready to go. I also am going to do the test with two other guys whom have performed the test before.

Also Iridium plugs were recommend, anyone second that statement? Maybe have another opinion?

Any advice is more than welcome. Thank you for all advice so far!

Anytime.

Copper plugs for our cars. My truck runs irridium plugs, expensive bastards that were designed for the truck, which has coil on plug, runs much more voltage, but goes 100K between tune-ups. You will probally only have poor performance and foul the iridiums on your motor.

 

[7upstang91]

Anytime.

Copper plugs for our cars. My truck runs irridium plugs, expensive bastards that were designed for the truck, which has coil on plug, runs much more voltage, but goes 100K between tune-ups. You will probally only have poor performance and foul the iridiums on your motor.

thanks man! I need to go look into some decent plugs then, could you recommend any? Unless copper is the name and now I feel like an idiot.

 

[Mustang5L5]

Stock Ford motorcraft copper plugs. Cheapest plug on the market, but for some reason the 5.0's love them.

Not like doing a plug change on a 5.0 is hard. No real reason to run anything else if you are N/A.

Now if you had a power adder, then i'd go NGK iridium's

 

[7upstang91]

Stock Ford motorcraft copper plugs. Cheapest plug on the market, but for some reason the 5.0's love them.

Not like doing a plug change on a 5.0 is hard. No real reason to run anything else if you are N/A.

Now if you had a power adder, then i'd go NGK iridium's

NGK Iridiums it is. What should I gap them to on a 331 with a supercharger?

 

[7upstang91]

Stock Ford motorcraft copper plugs. Cheapest plug on the market, but for some reason the 5.0's love them.

Not like doing a plug change on a 5.0 is hard. No real reason to run anything else if you are N/A.

Now if you had a power adder, then i'd go NGK iridium's

Hopefully going to finally get the numbers tonight.
Any specific numbers (spark plug wise), I will replace them tonight, because they are fouled. I mean, I want a part number(I don't want to get the wrong one).

 

[liljoe07]

Autolite 3925's

 

[Cobra1741]

go with the ngk! make sure all the cylinders are pretty much within 5-10% of eachother when testing the compression. i guess that would be optimum

 

[NIKwoaC]

I've been running Autolite iridium plugs for years. No problems here.

Also, AFR may have a "suggested" plug for their heads. You should look into that.

 

[7upstang91]

I've been running Autolite iridium plugs for years. No problems here.

Also, AFR may have a "suggested" plug for their heads. You should look into that.

I emailed NGk a month ago and they told me which ones to get, the only problem is I have to order them from NGK. No one cares them so I was looking for another one I could run for a couple weeks. Turns out Napa was out of those too. Just went with a basic copper plug and gapped them to 30 thousandths.

1: 175
2: 175
3: 175
4: 180
5: 175
6: 180
7: 175
8: 180

These are the numbers I got tonight from my 331. Give or take one psi.
Any thoughts? I thought they seems like very good numbers to me.

 

[liljoe07]

Yes, they're perfectly fine. Quit sweating it. Raise that gap up about .005

 

[7upstang91]

Yes, they're perfectly fine. Quit sweating it. Raise that gap up about .005

Ah, just my old tuner wasn't very good. In fact people have laughed in the past when I say who the tuner was. Anyway, another tuner who is a Ford only tuner told me .030, very well know guy who is booked two months in advance. Thank you for that advice. I'm gunna ask him today about raising the gapping then!

 

[liljoe07]

I didn't read the supercharger part. Leave the gap where it is.