Compression numbers question and new build

[11secondFWD]

Ok first off I decided to start modding my dd 3000gt. My talon is fully done and I am bored with it. It is fast with a billet 16g (10.6) and after the shootout I will be running a 67 mm turbo, a new fmic kit, direct port nitrous for spoolup and a new intake manifold. But I have built 2 dsms already and they are easy to make fast and fun. But I dont really like their looks and they cant compare to the 3000gt in any respect for me.

So I was only going to use my vr4 for daily driver duties. Well I had it for a year, it did great in daily city driving, the winter, and long highway trips. It has been the perfect car. I took it down the track at NG11 and ran 12.4@111 with a 1.7 60ft. I was very surprised and got the urge to add little mods and build another 3S. It has been well over a year since I ran 11s in my FWD TT. The only mods the vr4 had with the 12.4 was a mbc @ 14psi, a cat back, a stage 2 clutch, aluminum flywheel, aluminum crank pully, metal y pipe, boost gauge, and thats it. It has the stock air box and stock downpipe/precats/main cat.

The plan.

SAFC 2
DR650s (old style) at 25psi at least or as much as they hold.
1050cc injectors
supra fuel pump hotwired
e85
my ebay fmic off my fwd tt
A new clutch if I need it
wideband
downpipe

I think that is it other than minor stuff.

I see no reason I cant run 10s in the daily. My other 2 e85 talons run 10s. My FWDTT ran 11.8@123 w/ crappy 60ft and only 18psi on 93 octane.

I feel that with e85 and alot of boost I can run 10s. Austin did it on 9bs and light weight. I feel with dr650s and moderate weight reductions it should not be too bad. I hope I stay with moderate weight reduction. lol. My last 3000gt was 2860lbs, and my talons are 2450lbs. , both without any CF or lexan.

Here is my real concern though. WHen I got the car, I did a compression test on the front 3 cylinders. Its a 91 so I never did it before on a 91. On my 94 I pull the crank sensor wire by timing covers. On this 91 I pulled the cam sensor plug.

A year ago I got like 90psi on 1, 140 on cyl 3, and 90 on cyl 5. That was after 5 cranks and warmed. I said its either wrong or the engine is hurt but I got it for cheap and it was only my dd. Anyway, I drove it a year, it never smokes at all, never had a lack of power, never consumed alot of fuel, never trashed oil, it has been great. I get 26 on the highway. Then I ran the 12.4@ 111 to my surprise at ng11. I was hot lapping and it was consistent. And then I have been racing around town alot too. And I drive it hard and it still runs perfect and still mid 12s when I went back to the track 2 weeks ago.

So I put those numbers in the back of my mind and considered it either wrong reading or that I was doing the test wrong. I re did it today because I am getting ready to do these mods and dont want to start on a hurt engine. Car is still running great, no smoke, fast, ect. I do the test.

Cylinder 1 goes to 60 on first compression stroke, 90 psi on second and levels off at 105psi. Cylinder 5 is identical. Cylinder 3 goes to 150 after about 4 -5 compression strokes and stops.

Now I am asking for help/questions. I have built 2 dsm engines/setups now from scratch and have been focusing on dsms for a while and have got rusty on the 3Ss. Can anyone tell me if pulling the cam sensor is the right way on a 91? Also can someone tell me what could cause the numbers to be low but it running great and IMO reading wrong. I did a wet test and they all stayed the same. I know the rings are fine. No smoking, great power, no oil consumption, no trashing the oil with fuel., ect. I dont know. What do you think it could be? Is there actually a problem with the engine? Why are the numbers like this? I truly think it is just a bad reading or I am doing it wrong. How could it run so good and so long and be that low. That is below the service limit and is way off from each other. COuld the valves be not sealing well but still run great?

I take out front 3 plugs, pull cam sensor plug, screw tester in hole, floor the gas and crank car 7 times when already at operating temp. What could be wrong. I do compression tests weekly in my DSM and never have problems. I just feel there is an error making the readings low. Could I really have 100psi in 2 cylinders and have no ill effects?

Thanks for the help. I feel like a newbie. I know alot about these cars and DSMs and never had a problem but I feel lost here. Mainly because I feel I have been from the 3S scene to long and am over looking something stupid or am doing the test wrong for a 91 vr4. I havent worked on one in well over a year! I never even open the engine bay on this dd.

[11secondFWD]

My initial thoughts are valves or timing belt is off causing the valves to be off. I am not sure what valves are open when and when one is at tdc what other cylinders are at. Its just been too long since I have been inside or rebuild a 6g72.

Also finding tdc on 1, 3 and 5 seemed odd to me. I lined up the single line on the exhaust cam gear with valve cover point and with the double line on the intake cam gear with the rectangle on valve cover. I did number one like that, then I went 16 teeth and then found #3 then 16 more and found #5. I listened to compression as well to make sure it was correct. So that means the firing order is 1,2,3,4,5,6 then? On the 4g63 its 1,3,4,2 so I didnt think it would be in order. Is this right for the 6g72?

Any way. COmpression numbers are between 100psi and 105 for 1 and 5 and 145 for 3.

Leakdown was

#1
94 psi on main gauge and on second gauge it was 87. So that is a 7psi drop which is 92% or 8% loss. The air was flying out of spark plug hole #5 when I was doing leakdown on #1. So I put the plug back in and it was coming out of the exhaust alot. SO #1 is leaking out of the exhaust I assume and the valves on #5 are open when #1 is supposed to be closed causing it to come out of #5 spark plug hole.

#3

both gauges read 100psi. SO I would say there is close to 0% leakage. As it slowly leaks out since no cylinder is airtight, both gauges say the same thing. Also, there is only slight air coming out of the oil cap, so I would say just some slipping past rings thats it. ANd that confirms the 145psi compression reading.

#5
first gauge was 90 second was 84, so its 93% and a 7% leakage. Air is flying out of cylinder 3# hole. When plug is back in, it comes out the exhaust. SO i assume #3 exhaust valves are open when 5 is closed causing the leak

So my conclusion is that the exhaust valves are not sealing well. I do not know if timing marks are 100% right because it seems weird to me that the circles on the outside of the gears never line up and the dowel pins never point the same. BUT the marks on the inside of the cam gear line up with valve covers, so I guess its correct and a valve seating problem.

What do YOU guys think? WHo has rebuilt engines? Who knows which valves are open when? Who has done many timing belts and a leakdown before?


I know it is not rings or HG like I knew before. But can it run this good with exhaust valves not seating? I mean I guess when moving that fast it might not make a huge difference at WOT? I mean I dont see 9b cars going 12.4 with just mbc and catback, let alone ones that are hurt and have this low compression. I need help with advice and if I should change turbos and safely make more power?

I will put pics of gears at TDC #1 just to see if it was correct.

[Jimvr4]

Subscribed to learn how to properly run compression and leakdown tests

[11secondFWD]

Ya that is exactly right then. When 1 is compression, 5 is exhaust so that is why it is coming out of 5 when i do a leakdown for 1. And when 5 is compression 3 is on exhaust so that is why it is coming out of 3 when i do the leakdown for 5.

SO the question is, is it okay to upgrade like this? Is the engine even slightly healthy enough? I mean it runs great. Could it not matter that much or hurt performance when the engine is at much higher rpm, but at low rpm during the compression check, that little bit of a bad seat shows up. But then it not affect at wot and high rpm?

Stealth 316 - 1992 6G72 DOHC timing events

[11secondFWD]

Ya man. Compression test = take plugs out, pull cam sensor or crank sensor depending on year and crank it over more less.

Leak down = find each tdc and pump 100psi in and see what second gauge reads. TDC #1 is when the timing marks on cam gear match valve over on the back side of gear (valve cover side). Go 8 teeth more to find each tdc since it fires 1-6 in order. You can hear air come out too when doing compression stroke. After brushing up on the 6g72 and reading on stealth 316 link above. Its not hard.

[Chris@Rvengeperformance]

Those leak down numbers aren't bad.

Yes an engine may seal up better when actually running.

Sent from my DROIDX using Tapatalk

[speedy25]

Leakdown is always done at TDC. It cant be done any other way since the air pressure forces the piston down. Anything less than 10% leakage is considered good. Any large variances between cylinders is BAD. As you found there will be at least some leakage somewhere. Exhaust valves take the most beating from heat and sometime trap some carbon between the valves and seats.

I've always considered compression tests to be almost useless since they vary so much and only tell you when something is wrong, but never helps you to tell whats wrong.

-SP

[11secondFWD]

Yea I didnt think the leak down was too bad. I just have a hard time in my mind with having 1 and 5 compression only be at 100psi, and everything running so well. And I feel the leakdown being in an acceptable range as weird with such low compression numbers. But I guess with that slow of rpm, whatever is wrong with exhaust valves, is causing a scary low reading but minimal consequences. We can only see I guess how it handles the mods.