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| JeffK | Cold compression test? | ||
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Elite Guru
 
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I bought a compression tester the other night. I asked about a leak-down tester and the guys looked at me like I was from another planet.
Anyway, I started looking at it this morning. The instructions say to warm the engine up to normal operating temp. That is not going to be possible for me with the headers since they do not allow me to get the adapter and the hose in there. I am going to have to remove the headers from the heads just to do this. I'm assuming that the cold engine will give me lower compression numbers. Am I thinking right? |
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Posted on: 2011/1/30 18:25
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| bogus | Re: Cold compression test? | ||
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Grand Imperial Pooh-Bah
 
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good question... never really considered it. I have done compression tests on cold engines in the (way) past; I just don't remember the cause/effect.
I worry about the shop that has no clue what a leak down tester is... that's frightening. |
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Posted on: 2011/1/30 19:08
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The single biggest problem with communication is the illusion that it has taken place. - George Bernard Shaw Education is the best tool to overcome irrational fear. - me |
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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That's what I get for shopping at autozone for tools I guess.
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Posted on: 2011/1/30 19:50
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| BillH | Re: Cold compression test? | ||
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The Stig Moderator
 
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Yes, nothing will be expanded, pistons, etc. You can still look at one cyl v.s. the others. |
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Posted on: 2011/1/30 20:18
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Every man dies but not every man lives. |
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| BillH | Re: Cold compression test? | ||
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The Stig Moderator
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Let me guess, Autozone, O-Riley's, Pep Boys......  |
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Posted on: 2011/1/30 20:20
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Every man dies but not every man lives. |
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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You hit on the head Bill.
I think what I'm going to do is let the car warm up and then try taking the headers off carefully. Should be interesting to see how badly I burn myself... |
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Posted on: 2011/1/30 23:49
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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I hate HEDMAN!!!
I got to 5 of the cylinders. The passenger side header was not cooperative at all. 1 - 160 3- 180 5- 180 7- 180 and 4- 180 I really don't see how it would be phyically possible to get to 6 & 8 with the engine in the car. |
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Posted on: 2011/1/31 1:23
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| CentralCoaster | Re: Cold compression test? | ||
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Senior Guru
 
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Also, heat of compression will be quickly absorbed by the cool heads, reducing pressure. The compression is not adiabatic, the work done adds heat, which expands the air and increases pressure. A car with 9:1 compression with 90% volumetric efficiency during cranking would only give you about 118 psi adiabatically, the rest is from heat.
14.7psi * 9 = 132 psia = 118 psig If you do a compression check on a warm engine, you have to repeat it in the opposite order. This is because the numbers will noticeably drop as the engine cools. Otherwise you'd be misled to believe the last cylinders are weaker. The last one I did, the compression dropped by almost 20 psi before I got to the last cylinder. Also as the battery gets weaker, the engine will turn over more slowly, generating less heat of compression. |
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Posted on: 2011/1/31 7:28
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1985 Z51, ZF6 |
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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As always, excellent informative post CC.
Thank you |
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Posted on: 2011/1/31 19:59
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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Well, I have been debating between pulling the heads and having them gone over or buy this rebuilt L98 I found on Craigslist.
http://raleigh.craigslist.org/pts/2161293330.html I think that I will go the cheaper route and have the head work done in hopes that the bottom end doesn't grenade itself a month later. |
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Posted on: 2011/2/1 15:44
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| CentralCoaster | Re: Cold compression test? | ||
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Senior Guru
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I would do that also, as I hate trusting other people's work. It looks like he put some good parts in there, but you have to ask yourself if it's so great why is he selling it.
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Posted on: 2011/2/1 17:17
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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That was kind of my thoughts too. I think he's selling off a bunch of C4 parts. but I was not sure that I want to trust his ability to put it together. |
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Posted on: 2011/2/1 17:48
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| 86PACER | Re: Cold compression test? | ||
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Guru Newb
 
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Why exactly are you pulling the heads?
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Posted on: 2011/2/1 22:33
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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Pulling heads because #8 plug threads are messed up and I figure with 131K on the clock they could use some freshening up.
Now I'm thinking of doing a mild cam while I got it open. Maybe a hotcam and roller rockers. |
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Posted on: 2011/2/1 22:36
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| Ultraman | Re: Cold compression test? | ||
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2015 Memorial Day Car Show Winner!
 
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Uhh, and while you are at it pull the pan and check the bottom end. If you start adding to the top you will need the foundation to handle it. |
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Posted on: 2011/2/1 23:34
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2010 Grand Sport Coupe Gone but not forgotten... Vote DeSantis in 2024.... Make the IRS go away... |
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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Can I do that with the engine in the car? I haven't researched that yet.
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Posted on: 2011/2/1 23:36
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| CentralCoaster | Re: Cold compression test? | ||
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Senior Guru
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Yes, you can pull the main and rod bearing caps.
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Posted on: 2011/2/1 23:52
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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Wow. I'm almost to rebuilding the entire engine at this point minus the nice clean cylinders and outside.
Thanks guys. Now, should I buy new stainless valves and have the machine shop guy install? I know the stock sizes are 1.94 and 1.6 correct? Looking on Summit they show different lengths. Will he tell me what I need or is there a standard size? If I go with RR's should I get longer pushrods or valves? Should I do new lifters since I'm doing all of this? HOLY SNOWBALL!!! |
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Posted on: 2011/2/1 23:55
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| Ultraman | Re: Cold compression test? | ||
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2015 Memorial Day Car Show Winner!
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Consider carefully how much you want to spend on your car. Most of the money you spend will never be recaptured. If you are going to keep the car and love the car then the money will be well spent. There is no end to what you can spend to go faster!
Then when the engine is strong will the tranny and rear end hold up to the power? I am not trying to discourage you, but these are all things that I have considered in modding my own car. My car is a 90 with 55K and is worth 8 to 10K one a good day. I will be doing exhaust mods this winter and clean up the intake side next winter. Gain should be 40 to 45 hp and 25 to 30 ft/lbs of torque. That's as far as I want to go on this car. If I want more I will take the money and move up to the C6 cars and put my money on the new platform. I'm just saying....... |
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Posted on: 2011/2/2 5:37
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2010 Grand Sport Coupe Gone but not forgotten... Vote DeSantis in 2024.... Make the IRS go away... |
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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Quote:
You're not discouraging me. I appreciate your post and I understand what you are saying. I daily drive my Vette when there isn't snow or ice on the ground so my end goal is to have it become a relatively reliable car that I can drive to work without having to worry about what's going to break next. The previous owner bought it and then started going through a divorce and didn't drive it much for almost 2 years. Then I bought it so now I am working through a bunch of issues. And it seems that everytime I fix one issue it causes another to pop up. So I am now trying to be proactive and fix things before they leave me stranded. I found the cross threaded sparkplug hole not too long after I bought it, paid a shop $300 to fix it and they only F'd it up worse. Now I have to pull the intake manifold off again since it is leaking oil so I figured that I would pop the heads and get those spark plug threads fixed and maybe do a few things to make more power. I don't see myself moving up to a C-5 or C-6 for a couple of years so this is it for now. |
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Posted on: 2011/2/2 12:26
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| rklessdriver | Re: Cold compression test? | ||
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Senior Guru
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JeffK-
With the stock 113 heads... I suggest you have the machine shop cut a 3 angle valve job and then back cut the factory intake valves with a 30* cut. I personally wouldn't spend the $$$ to buy new stainless valves unless your factory valves are unuseable. If you do happen to need valves, buy them from Competition Products. Summit is getting about $8.00 each for their branded imported stainless valves.... which are either Martin Wells, Manley or SI mfgr. You can buy the same valve from CP for $6.00 each. These are what you will need. http://www.competitionproducts.com/St ... ntake/productinfo/I2552P/ http://www.competitionproducts.com/St ... xhaust/productinfo/E2809/ However if you buy them in a set of 16... You only pay $4.37 each. http://www.competitionproducts.com/St ... 32-Stem/productinfo/8430/ If you go with roller rockers you should check the valve train geometry. You may need a different length pushrod. If you do plan to upgrade the camshaft or go with 1.6 rockers make sure you buy and have the machine shop install the correct valve springs, locks and retainers. They will also have to cut the valve guide on the cyl heads down for clearance. Will |
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Posted on: 2011/2/2 14:49
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1984 Corvette. 434 SBC with a Powerglide. Best pass - 8.48@160MPH 1.23 60ft on MT 275/60R15 Radials. 1972 Corvette. LS5 454 BBC with M20 4 speed. Best Pass - Doing good to just pass a gas station. |
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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I wish I had half the knowledge you guys have stuffed in my head.
I got to thinking about the cam swap. With 131K on this engine won't the cam bearing be pretty much worn to match the exsisting cam? I'd hate to put a new cam in there and have stuff go to hell. So can I go buy the LT4 springs and use those with the RR's? Going to work on getting the heads off tonight. YAH! |
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Posted on: 2011/2/2 15:50
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| Ultraman | Re: Cold compression test? | ||
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2015 Memorial Day Car Show Winner!
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Quote:
It's the Borg Collective for C4 knowledge! Resistance is futile!  |
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Posted on: 2011/2/2 23:55
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2010 Grand Sport Coupe Gone but not forgotten... Vote DeSantis in 2024.... Make the IRS go away... |
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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Ha-ha. Out in the garage now... Yah!
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Posted on: 2011/2/3 0:01
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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Update for anyone that is following this thread.
Got the intake manifold off. Wow! that Right Stuff sealer is really grippy. I wasn't sure if I was going to break that seal. I think I found my leaky spot. [IMG]http://i149.photobucket.com/albums/s46/kresge1975/1990%20Corvette/bGVhayBwb2ludC5qcGc.jpg[/IMG] Anyway, I took the passenger side VC off just to see since I knew I wasn't getting the heads off tonight. [IMG]http://i149.photobucket.com/albums/s46/kresge1975/1990%20Corvette/SU1HMDAxMTUtMjAxMTAyMDItMjEwNi5qcGc.jpg[/IMG] The VC gaskets are the ones with the blue rubber inserts. They weren't really stuck fast too tightly. I don't get why I am supposed to look for self-aligning rockers. There are pushrod guides already on the heads. Sorry, I'm tired and can't think of the real name for them. [IMG]http://i149.photobucket.com/albums/s46/kresge1975/1990%20Corvette/SU1HMDAxMTYtMjAxMTAyMDItMjEwNi5qcGc.jpg[/IMG] Anyway, I will probably get the heads off over the weekend and then drop them off at the machine shop Monday. Might be putting it back together next weekend...or the one after that. |
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Posted on: 2011/2/3 3:04
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| rklessdriver | Re: Cold compression test? | ||
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Senior Guru
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Your cyl heads do have pushrods guide plates however.... They are not hardened and were never inteded to locate/guide the rocker arm during engine operation. They were nothing more than a holder to algin the pushrods on the engine assy line when it was built at the factory. If you look closely you will notice a fair amount of clearance around the pushrod.
Your pushrods are not hardened either. If you removed the SA rockers, used regular rockers and relied on the factory guide plates/pushrods.... they would eat each other up in short order. Now that your modifying things you have the option to use real guide plates (with no additional machine work) or retain the SA rockers. Will |
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Posted on: 2011/2/3 13:45
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1984 Corvette. 434 SBC with a Powerglide. Best pass - 8.48@160MPH 1.23 60ft on MT 275/60R15 Radials. 1972 Corvette. LS5 454 BBC with M20 4 speed. Best Pass - Doing good to just pass a gas station. |
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| JeffK | Re: Cold compression test? | ||
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Elite Guru
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I see. Thank you for the information.
In order to make sure I have the right length pushrods I have to wait until the heads are back from the shop and installed and then measure right? |
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Posted on: 2011/2/3 15:33
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