Compression test??

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mullinax95

mullinax95

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I was hoping to let Bobby at BJR racing build be a nice 360 but I can't afford it right now. That would be icing on the cake but I have spent about $6000 or so on the car this year. Bobby is a super guy and I wouldn't want anyone else build my motor but him. I will get to it this coming year.

I have been concerned about the compression numbers and thought surely they should be higher that what they are. Like 125 psi or more.

What I have is a 64 273 4bl carb non adjustable rockers with a Mopar .474/.474 cam. The motor was rebuilt in July of 06 is what the previous owner has told me. It had a .420 intake lift cam in it but it was to mild so I changed the cam to the Mopar. I did a compression test on the motor with the .420 cam and the readings were just a shade higher than what I have now. That was a year ago. I don't know what compression I'm running or bore size since I have not removed a head. I did a compression check on the engine after I warmed it up and the readings are low. I added oil to each cylinder and there was no change from the first reading. Here they are:

1= 105 2= 115
3= 90 4= 115
5= 100 6= 95
7= 89 8= 100

It looks like I need to pull the heads and see what is going on. Also find out what bore size and compression ratio I'm running.

I know it's a 273 and I'm not going to get wheel standing horsepower out of it but just want to know why such low readings?

Tell me whats the best move to make.

Motor 003.jpg
 
Depending on the Cams specs..overlap etc. the psi readings might not be exactly what you think they should be based on the compression of the engine. They do seem a bit on the low side though. Usually I'd say if all cylinders are within 10% of each other and it runs good, don't worry about it. 89 vs 115 is better than 20% though.. if you are concerned about it, it might be time to have the heads worked over. Did you have the carb butterfly's open when you did the test? That can sometimes make a small difference.
 
Thanks for input.

Yeah I had the carb at WOT.

I would be less concerned if the readings didn't vary as much.
 
A leakdown test would reveal where its leaking from. If it's the rings then tearing the heads off will do nothing but cost money. See if you can't track a tester down then go from there.
 
street demon said:
A leakdown test would reveal where its leaking from. If it's the rings then tearing the heads off will do nothing but cost money. See if you can't track a tester down then go from there.
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Yeah you are right. I thought about doing this today matter of fact. I'll see if I can round up one.

Do you think the readings should be higher than what they are?
 
Here are some readings I found on another forum that a fellow did on his 318.

1 - 116
2 - 145
3 - 140
4 - 140
5 - 135
6 - 133
7 - 122
8 - 120

I believe mine should be close to those with the motor having been rebuilt in July of 06.
 
I really doubt you have that much compression(small bore and stroke) plus the cam doesn't help the readings. I had 2 318's with 120, 360 with 160 and my 340 now has 205. If your going to build another motor I would spend the extra money for upgrades with the new motor in the back of your mind. If your going to reuse the heads then rebuilding them is a good idea but if your not then I wouldn't.
 
street demon said:
I really doubt you have that much compression(small bore and stroke) plus the cam doesn't help the readings. I had 2 318's with 120, 360 with 160 and my 340 now has 205. If your going to build another motor I would spend the extra money for upgrades with the new motor in the back of your mind. If your going to reuse the heads then rebuilding them is a good idea but if your not then I wouldn't.
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Wow 205 on the 340!

What compression ratio is the motor?
 
11.51 on pump gas. Won't build another that high since gas is getting worse and worse but it runs great right now.
 
I wished I new exactly what compression ratio I'm running. I have a good mind to yank the heads and calculate everything to see what I'm running. Like I said I don't know what the true bore size. I talked to the engine builder and he seemed to busy to answer my questions but he "things" it was bored .60 over. And then he said the heads was good to .510 lift. ???

If I find out that I'm running lets say 9:1 compression do you think if I got it up to 10:1 or lower and do gasket matching, porting that the cam would do better? I have a 318/360 Performer intake and really don't think the intake matches the heads.
 
I haven't messed with 273 very much but it seems your engine has been messed with some so it's hard to say what you have now. 273 had very small valves from factory but not knowing what heads you have changes that. You would have to mill the heads and/or use thinner head gaskets to get the compression up. If you mill too much you will have to get shorter pushrods. I doubt you'll see much gain by gasket matching unless the mismatch is way off. Have you thought about upgrading your heads to a set of 308 heads? If your planning on building the 360 you would be money ahead finding and rebuilding a better head that you could use on both engines.

Also there is always porting your own heads. Most people frown on home porting but I've learned alot by doing it and have become decent at doing it. Porting takes alot of time and not much money and I find it fun to do expescially watching the meter go up while my heads are getting flow tested.
 
street demon said:
I haven't messed with 273 very much but it seems your engine has been messed with some so it's hard to say what you have now. 273 had very small valves from factory but not knowing what heads you have changes that. You would have to mill the heads and/or use thinner head gaskets to get the compression up. If you mill too much you will have to get shorter pushrods. I doubt you'll see much gain by gasket matching unless the mismatch is way off. Have you thought about upgrading your heads to a set of 308 heads? If your planning on building the 360 you would be money ahead finding and rebuilding a better head that you could use on both engines.

Also there is always porting your own heads. Most people frown on home porting but I've learned alot by doing it and have become decent at doing it. Porting takes alot of time and not much money and I find it fun to do expescially watching the meter go up while my heads are getting flow tested.
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Very good advice. I will ponder on it and find out what heads I have. I didn't see the w 308 in the chart.

http://www.mopar1.us/headcastnumber.html

308? Maybe I'm over looking it. What size valves would they have?

thanks
 
They are on 360's in the late 80's to early 90's. They have a great exhaust port and pretty good intake port. The valves are 1.88 and 1.60. Those are small enough for your 273 but probably bigger then what you have. The chambers are around 70cc and you could get smaller with some milling.

These number are with stock valves and valve job....

Stock ported(home ported)
int. exh. int. exh.
.100 58.0 42.9 62.2 48.0
.200 123.3 78.4 121.3 79.2
.300 171.6 110.9 183.4 114.1
.400 193.8 129.9 218.9 142.6
.500 208.6 136.6 227.8 155.7
.600 214.5 139.8 228.1 158.9

Superflow 600 bench @28inches
 
those heads on that 64 273 are deffenitly not orignal,,64 /65 use a special botl pattern intake,,,
 
fstfish66 said:
those heads on that 64 273 are deffenitly not orignal,,64 /65 use a special botl pattern intake,,,
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I know the casting number on the block was 64. I have had the top end of the motor apart for a cam change air head me never checked what casting numbers is on the heads. Do you think the 1.88 intake 1.60 valve heads would be to much? I don't know the true bore size since it might be bored over. But if I get 1.88 heads then later I could use them on the 360 to keep the velocity up.
 
Im Not Sure About 1.88 Heads,, But I Remember An Artice In I Think Was In The Old Direction Connection Race Manual,,,about 318 Heads On A 273 Having Some Sort Of Chamber Size Difference,,,but It Appears You Have Some Sorta Differnt Heads,, Maybe They Are Newer 273 Heads Sorry I Dont Have More Info,,
 
Popped a valve cover today and got the casting number. 2658920-J

2658920 273/318 1.78inatke 1.50exhaust
 
Took one of the heads off and found that the motor is indeed bored .060 over. So I have a 281 I believe LOL!

I more than likely get the heads milled down to clean them up and go with a thin head gasket so that will add compression.
 
I would think talking to BJR would have helped clarify why you dont have teh pressure. The cam is way too big, and probably not installed the way MP intended, unles it was degreed. The things that cause cylinder pressure are static mechanical ratio (not what the book says, what they measure and compute to), cam timing in relation to the piston in it's travel, and rings/valves sealing. Milage, time since rebuild, temps, etc, all mean nothing. The cam is too large, and going with larger valves will hurt it more. You need a cam designed to create cylinder pressure. Not necessarilly bigger, but better. I'd also bet that thing has a true static ratio of about 8:1... Anything uner 100psi is a red flag for me. I would want at least 125psi, preferrably closer to 180. But with low static, and that cam, no way will you get near even the 150 mark.
 
moper said:
I would think talking to BJR would have helped clarify why you dont have teh pressure. The cam is way too big, and probably not installed the way MP intended, unles it was degreed. The things that cause cylinder pressure are static mechanical ratio (not what the book says, what they measure and compute to), cam timing in relation to the piston in it's travel, and rings/valves sealing. Milage, time since rebuild, temps, etc, all mean nothing. The cam is too large, and going with larger valves will hurt it more. You need a cam designed to create cylinder pressure. Not necessarilly bigger, but better. I'd also bet that thing has a true static ratio of about 8:1... Anything uner 100psi is a red flag for me. I would want at least 125psi, preferrably closer to 180. But with low static, and that cam, no way will you get near even the 150 mark.
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The cam that was in the car was a Elgin .420 intake and sounded like grandma's car while running. Same cylinder pressure as I have now with the Mopar .474 but sounds ten times better and runs about the same..lost a little torque down low.
 
It sounds like the exhaust valve seats are beat out because of unleaded gas. I had a '69 273 do the exact thing yours has done. It ran great after a valve job and intake/carb package them about a year later I noticed that it didn't run like it used to so I did a compression test and it was all over the board and low like yours. I pulled the heads and found the exhaust valves buried into the heads. I replaced them with '75 318 heads and my compression was 125psi.


Chuck
 
moper said:
At least it sounds good...lol.
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Listen....the motor will smoke the tires and chirp second gear. Yes it sounds good and runs good but I did lose some bottom end torque but gained more on the higher side. The cam was installed straight up and no I didn't degree it because I've never have been showed how. I have never had a motor with such low compression psi and want to know what compression ratio I had and let BJR help out by milling the heads etc... I'm taking the heads and intake over there Friday.

I'm biting my tongue because AdamR wants us to watch our manners and sometimes we take things wrong. But for you to say "At least it sounds good.." is nothing but saying my motor does not run good and is lame BUT it sounds good. Instead of saying things of that nature how about saying "You know what I would do this" or "I would run this cam because that one is to large and you would be more satisfied with the results".
 
OK.

Found out when I took the heads to Bobby (BJR Racing) that my 273 heads were shot. The valves was moving around in the guides and not sealing properly. So Bobby set me up with some 302 heads with new valves, springs, etc.. I changed cams and went with a .441 / .441 lift which is smaller than the .474 / .474 that I chose to run. I will have a nice running 282.41 (bored .060 over 273) with roller rockers now. By the computer I should have peak HP of 270 and peak TQ of 306.

Heads 002.jpg


Heads 003.jpg


Heads 005.jpg
 
Mullinax,
They look very good on your engine and it looks like the mateing surfaces are as they should be without the intake gasket.
Good job! I'll get your rocker shaft spacers out to you Tuesday, have fun with them as Im sure you will.
 
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