So I decided to check my compression and here is what I got..........

[viperrt96]

I had some time on my hands this past weekend so I thought I would replace the plugs and check compression while I was at it. The car is a 96 RT10 (GenI) and the motor has over 25k on it since I’ve modded it (48K total). In all honesty I haven’t even taken off a valve cover. I race it and I spray it (NOS) a lot so one could say it’s been driven hard. Well I’ve been wondering Just how well has it been holding up? That’s why I decided to check compression and here’s what I got

Odd cyls - Drivers side (Starting in front)
Even cyls - Passenger side (Starting in front)

Cyl # Results
1. 181
2. 181
3. 177
4. 181
5. 178
6. 183
7. 175
8. 178
9. 172
10. 180

Lowest compression is Cyl #9 and it’s slightly over 5% so I’m pretty happy with the results. Car still runs great

 

[Darth Menace]

Shows how much I know........I thought compression was done in ratio. How do you go about reading it?

 

[happy56]

A compression gauge is screwed in where the spark plug normally is .You normally turn it over three rounds and all readings should be within 10% of each other.Just fyi

 

[Mad Max]

Another test you can do is called a leak down test. You can tell where it's leaking.

 

[plumcrazy]

leakdown tells much more about the engine than a comp test does.

 

[ultradriver10000]

There prolly isn't anything wrong with her. I had 600whp on a n20 LS1 in my C5 for 50K miles before she broke. Prolly put 100 bottles through her haha. Keep driving the beast!

 

[plumcrazy]

my engine with 100+ bottles would prove otherwise in 2007

 

[DrumrBoy]

As mentioned, leakdown would be a better diagnostic tool but those compression test results look pretty good to me.

 

[Fatboy 18]

Ive tried to do a compression test and leak down test on my engine but have always had varied results

Ive tried reading the Factory manual but its very complicated
It states the engine should be at working temperature? So are you supposed to run the engine, test the compression on the first cylinder then, put that plug back, then run the engine again, take out plug No2 etc, etc.

The reason I say this is because of the Hydraulic lifters, these require oil pressure to make the valves sit tight,...... Don't they?

So if you start at Cylinder 1,........By the time you get to No10 the engine will have dropped valve pressure?

To perform the Leak Down test, each cylinder has to be on TDC? So you have bought the engine up to temperature, then you have to get under the car with a socket and wrench and try and hope you can gently crank the engine around to TDC? And performing that 10 times is a nightmare!

Is all that correct, or have I missed something?

 

[viperrt96]

Quote :To perform the Leak Down test, each cylinder has to be on TDC?

Yes and in my case I didn't see a need to do so. It is a pain but the easiest way to me would be by starting with number one (TDC) and going in order by the firing order and with a good light you can see the piston come up when the plug is out. And as long as the valves are closed it should be good enough to do a leakdown

When I did my car it was warm - all plugs pulled - coils unplugged and I used a digital compression gage. Cranking the car over a few times until the digital readout stopped climbing (at most 5 cranks)

 

[ViperTony]

Ive tried to do a compression test and leak down test on my engine but have always had varied results

Ive tried reading the Factory manual but its very complicated
It states the engine should be at working temperature? So are you supposed to run the engine, test the compression on the first cylinder then, put that plug back, then run the engine again, take out plug No2 etc, etc.

The reason I say this is because of the Hydraulic lifters, these require oil pressure to make the valves sit tight,...... Don't they?

So if you start at Cylinder 1,........By the time you get to No10 the engine will have dropped valve pressure?

To perform the Leak Down test, each cylinder has to be on TDC? So you have bought the engine up to temperature, then you have to get under the car with a socket and wrench and try and hope you can gently crank the engine around to TDC? And performing that 10 times is a nightmare!

Is all that correct, or have I missed something?

In speaking with the Wizard, he told me to get the engine warmed up. I've let mine idle until my temp gauge is at normal temperature for me. Then I shut it off and let it cool a bit. I just want imagine working with a very hot engine and not stripping the spark plug holes. I'm paranoid about that, lol. But the engine is definitely warm/hot to a workable point.

I pull the fuel pump relay before I start each test. Although, since I'm using a bump start switch with the ignition off I don't think pulling the relay is necessary.

I used a compression gauge on each cylinder, crank the engine 3-5 times and write down my reading. I use a bump start switch that I connect to the alternator. This way I can use one hand to keep the TB's open and the other to work the switch.

It's not the lifters that seat the valve but rather the springs. The lifter pushes the pushrod which in turn pushes the lifter which in turn pushes the rocker, etc. eventually opening the valve. No need to worry about the lifters being oiled up in the compression test.

Regarding the leakdown test, true each cylinder you are testing needs to be in TDC position. There are a number of ways to do this. I use a TDC spring tool from Eastwood. It screws into the spark plug hole and will bump up as I turn over the engine. Using the bump switch I don't have to rotate the crank to get to TDC. Also, a straw in the spark plug hole will also work fine for indicating TDC.

I've done this many times, took pics but never got around to a write up. Someday.

 

[Fatboy 18]

Thanks for the feedback

 

[BoremViper97]

is everyone going to give me a hard time for reviving a thread 11 years old?
or praise me for staying on topic

everr since buying my 97 rt-10 in December 2022 I've felt like the overall power was a bit disappointing. I had just traded a 2008 corvette base model automatic and the corvette was more impressive.
Anway i wondered if my 43,000 mile Viper was just worn out from abuse. So i bought a compression test kit and did it tonight. Also new spark plugs to install afterwards.

I don't know how the cylinders are numbered, but my readings are
168
165
165
154
160
161
164
162
160
156

I did it stone cold; the car hasn't been run in many days (ambient temp was 85-90degrees- central florida at 8pm)
I had my wife hold the gauge while i cranked the car (with all fuel injectors unplugged and the fuel pump fuse removed). I cranked the engine for about 5-10 seconds; maybe 15- I wasn't counting. But i saw the gauge jump on each compression stroke. most times it took 5 or 6 compression strokes to get to maximum pressure.
Typically what i saw was a huge jump on the first stroke- maybe about 80 to 120 psi on the first stroke. then a few more smaller jumps, and on the 5th 6th stroke i stopped cranking because the needle stopped increasing.
I had the gas pedal wide open throttle.

Fun fact; the check engine light was on solid after i finally reassembled it and started the engine. That was a little worrisome and took me five minutes to realize i had left the passenger side rear fuel injector unplugged.

I'd love everyone's feedback please.

 

[Old School]

Those results look fine, well within 10%

Viper cylinders are numbered correctly (unlike Fords) front to back. Since the left bank is farther forward, cylinder 1 and all other odd cylinders are on the left side.

 

[BoremViper97]

Those results look fine, well within 10%

Viper cylinders are numbered correctly (unlike Fords) front to back. Since the left bank is farther forward, cylinder 1 and all other odd cylinders are on the left side.

thanks for the reply.
Ive seen a lot of other posts that report 170's to 180. And of course I'm wondering if mine is worn out from 40,000 miles of fools beating on it.
Or if my numbers are just a little low below it was performed on a cool engine (90 degrees, lol)

 

[Goggles Pizano]

Yes numbers can and usually are low because of cool engine.