Compression test results.
Thread Starter
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Compression test results.
I have spent the last 3 days getting the car all ready for racing season (hopefully the track will open next week), and have been tracing down a timing problem among other things. I was setting the hot valve lash and figured I'd do a compression test to see how things are doing. The shortblock has approximately 10,000 very hard street/strip miles. I am very happy with how the came out. The highest variance between cylinders was 3%.
Results:
Cylinder - PSI
1 - 180
2 - 183
3 - 180
4 - 186
5 - 182
6 - 186
7 - 182
8 - 185
Results:
Cylinder - PSI
1 - 180
2 - 183
3 - 180
4 - 186
5 - 182
6 - 186
7 - 182
8 - 185
Melting Slicks
Joined: Jan 2004
Posts: 2,672
Likes: 1
From: New Cumberland PA
I was thinking you had a problem when I saw the topic.
Guess you were bragging!
Doesn't get any better than that.
Anyway, those are great numbers and very good 1/4 mile times to boot.
Guess you were bragging!
Doesn't get any better than that.Anyway, those are great numbers and very good 1/4 mile times to boot.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Did u change cams?
Originally Posted by VETDRMS
I have spent the last 3 days getting the car all ready for racing season (hopefully the track will open next week), and have been tracing down a timing problem among other things. I was setting the hot valve lash and figured I'd do a compression test to see how things are doing. The shortblock has approximately 10,000 very hard street/strip miles. I am very happy with how the came out. The highest variance between cylinders was 3%.
Results:
Cylinder - PSI
1 - 180
2 - 183
3 - 180
4 - 186
5 - 182
6 - 186
7 - 182
8 - 185
Results:
Cylinder - PSI
1 - 180
2 - 183
3 - 180
4 - 186
5 - 182
6 - 186
7 - 182
8 - 185
Looks like ur c.r. could be higher and still use pump gas if u have good cyl quench. Those numbers would be much higher with a smaller cam/less overlap. Looks like u changed to a tighter LSA cam? Which is ur Lobe Separation Angle - LS or CL? 110 or 106?
Okay i'm no expert so u should do ur own research but D. Vizard says u can run at least 200 psi for street machine and up to 220 psi for street/strip mtr and use the pump gas.
Just my blab'n but looks like u could go to a shorter duration cam and maybe make the same or better power while more tame driving around town. Please don't shoot me because i know those roller cams ain't cheap but when u get down to durations around 270* there is no advantage to a roller.
Well those cyl press do look like the bottom end is nice and tight.
cardo0
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Thread Starter
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Thanks cardo, lobe seperation is 110 installed on a 106 intake centerline. I will not try to push the 91 octane pump gas too far here. A margin of safety is acceptible.
Quench is "good" at .039". I spent $400 on Cometic head gaskets getting it! 
Are you referring to no advantage at .050 duration less than 270? I would beg to differ in that regard. Seat duration is 280/288 with this cam. I ran a comp XR288 hydradaulic roller with .050 duration of 236/242 compared to the current 242/248 and the car runs stronger with the solid roller. The area under the curve and duration at peak flow is higher than a flat-tappet/h-roller. The benefit is keeping the valve in the peak flow of the head longer while using less seat duration. I drove this car 700 miles last week.
Driving is perfectly tame, no issues at all.
EDIT: The Dynamic Compression Ratio calculated using Smokemup.com's calculator is 8.5:1.
Quench is "good" at .039". I spent $400 on Cometic head gaskets getting it!
Are you referring to no advantage at .050 duration less than 270? I would beg to differ in that regard. Seat duration is 280/288 with this cam. I ran a comp XR288 hydradaulic roller with .050 duration of 236/242 compared to the current 242/248 and the car runs stronger with the solid roller. The area under the curve and duration at peak flow is higher than a flat-tappet/h-roller. The benefit is keeping the valve in the peak flow of the head longer while using less seat duration. I drove this car 700 miles last week.
Driving is perfectly tame, no issues at all.
EDIT: The Dynamic Compression Ratio calculated using Smokemup.com's calculator is 8.5:1.
Last edited by VETDRMS; Apr 10, 2005 at 03:31 PM.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Full duration as 270*
:o I should have said full duration and yes this is a general statement as total flow area with vary with manufacture and cam intensity. I don't recall the dynamics of this but do remember 270* full duration is the trade-off point where rollers begin to walk away making more power and by 280* its no longer a match. 
But if ur happy with what u have then i'm happy for u. It sounded like u were asking for advise/opinions and u got mine. Myself i like the lower c.r. engines as they seem to live forever - at least the bottom end. The hi c.r. with hi rpm makes more pwr but seems to break more mtrs at the strip. Just what i seem to observe.
BTW i thought i was still buying/finding 93 oct at the pump? Haven't bothered to look while pumping. All brands down to 91 oct?
cardo0
But if ur happy with what u have then i'm happy for u. It sounded like u were asking for advise/opinions and u got mine. Myself i like the lower c.r. engines as they seem to live forever - at least the bottom end. The hi c.r. with hi rpm makes more pwr but seems to break more mtrs at the strip. Just what i seem to observe.
BTW i thought i was still buying/finding 93 oct at the pump? Haven't bothered to look while pumping. All brands down to 91 oct?
cardo0
Thread Starter
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Cardo: Thanks for your opinion, if I had both the time and extra cash I would go with a belt drive and swap cams, heads, cars..ect ect. I may be leaving a little on the table, but from my research this cam was best suited to my setup.
Yes, in Montana you can only get 91 octane.
I may be leaving a little on the table, but from my research this cam was best suited to my setup.Yes, in Montana you can only get 91 octane.
Instructor
Joined: Apr 2004
Posts: 166
Likes: 0
From: Troy MO
maybe someone could help me with a problem...... I have a 67 GTO 421 with flat tops, ram air 4 heads, and the previous owner milled the block, heads and intake, bringing my compression to 270 psi per cyl. It runs like a raped ape, with enough acetone in the gas anyway, but I have to replace my head gaskets every 2-3 weeks of racing her. everytime blows the gaskets out between the cylinders. I have tried every brand of gasket I can find, and I get 2-3 weeks of race time. I haven't ran the car now for about 4 years, cause it gets old working on it every other week. I just want to run it, have some fun. I have not a clue where I could find those heads again, and really don't want to spend that kind of money anyway. Is there a place to get extra thick gaskets? or something to get the compression down? I did have eyebrows cut into the pstons out of neccessity when I first got this engine, as the valves bent hitting the pistons. It took me a couple months to find enough new valves to repair the heads. Apparently the ram air 4 heads were an after-market no longer made after 69 and are very rare.
Loren
Loren
Le Mans Master
Joined: Apr 2000
Posts: 7,562
Likes: 9
From: B'Ville NY
Cruise-In III Veteran
St. Jude Donor '05
TLChydraulics: check to see if Cometic gaskets has anything for your application. They're not cheap, ask VETDRMS 
, but I've heard of people not having to o-ring their boosted engines (high cylinder pressures) when going with these gaskets.
Very nice pressures VETDRMS
I'm probably going to pickup a compression tester also before I put my engine back in
, but I've heard of people not having to o-ring their boosted engines (high cylinder pressures) when going with these gaskets.Very nice pressures VETDRMS
I'm probably going to pickup a compression tester also before I put my engine back in
Thread Starter
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Loren: You may want to give Cometic a call. A fair bit of the LSX guys use them in boosed applications. I got them to reduce my quench and maybe run a little nitrous.
http://www.cometic.com/
The custom sets are $180, but just make sure you order the right custom dimensions (don't trust the salesmen when he says they will work), or you'll be buying another set...
Mike: Thanks, how are the modifications to the heads going? I would like to see some pictures if you have any.
http://www.cometic.com/
The custom sets are $180, but just make sure you order the right custom dimensions (don't trust the salesmen when he says they will work), or you'll be buying another set...
Mike: Thanks, how are the modifications to the heads going? I would like to see some pictures if you have any.
Le Mans Master
Joined: Apr 2000
Posts: 7,562
Likes: 9
From: B'Ville NY
Cruise-In III Veteran
St. Jude Donor '05
Originally Posted by VETDRMS
Mike: Thanks, how are the modifications to the heads going? I would like to see some pictures if you have any.
Thread Starter
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Mike: Those guys just seem to be way too slow, I'm surprised yoru still going there. I am fortunate enough to be good friends with my machinist, I would go nuts not being able to walk in and check out all my stuff on a daily basis!
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Were are kind of running the same cam, that is if you have XR280R
Solid roller with 242/248. If it's the same you must have a noticable idle. I've set mine at 950 rpm and I was pleased to tweak in the four idle screw until it got a stead 13 inches of vacuum.
I went with 10.7 static with a .021 thick copper gasket.
Solid roller with 242/248. If it's the same you must have a noticable idle. I've set mine at 950 rpm and I was pleased to tweak in the four idle screw until it got a stead 13 inches of vacuum.
I went with 10.7 static with a .021 thick copper gasket.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I have the same cam and 11:1CR I will check my cranking pressure this week. My 355ci cubic in. L48 had cold cranking 230-235 psi with the Sportsman II iron heads and ran like a champ anything above that you are looking for trouble.
If you have 270psi and it is a racing motor then the simple solution is put in more cam, you probably don't want more than about 240psi
If you have 270psi and it is a racing motor then the simple solution is put in more cam, you probably don't want more than about 240psi