Author Topic: olds 350 Low compression?? or normal?  (Read 306 times)

bmanrrt

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olds 350 Low compression?? or normal?
« on: October 06, 2018, 10:45:43 PM »
Hi everyone. This is a discussion from the second gen 1970-1981 forum that I really should have started here. I did not intend on a duplicate post but this is the best location for it.

Ok so I recently bought a 1979 TA with an olds 350 rocket in it. It ran but poorly so I have Timed it, tuned it (mechanic not me), replaced all the plugs, wires, cap and rotor etc. This all helped but it still has very little power. Feels like a tired motor.

I did check the casting # on the block which is 395558 Code 2. According to the chart I found this is a 68-70 350 block.
The Heads are #8's which from what I am reading are complete Junk heads.
I am compression testing the cylinders, dry test only for now, and I did not test #8 because it was a nightmare getting the plug back in that one due to a huge head bolt and the AC unit. Compression test is as follows
#1- 100 psi
#2- 100 psi
#3- 100 psi
#4- 90 psi
#5- 100 psi
#6- 100 psi
#7- 100 psi.

This seems low but is consistant. I have read somewhere on the interweb here that this motor new has about 105 psi, but my common sense bone seems to question that because it seems so low. The pressure is pretty consistant across all the cylinders though.

Any thoughts?
Thank you
« Last Edit: October 06, 2018, 10:50:02 PM by bmanrrt »

Grand73Am

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Re: olds 350 Low compression?? or normal?
« Reply #1 on: October 07, 2018, 08:37:28 PM »
Seems way low to me. I did a couple of compression tests on my 403 a few years ago, dry and wet, and it tested on average around 150.
Steve F.

aussieta

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Re: olds 350 Low compression?? or normal?
« Reply #2 on: October 08, 2018, 12:54:05 AM »
most motors need around 130-150
dont know anything about olds, what chamber size are olds #8 heads
1978 Y84 W72 WS6

Aus78Formula

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Re: olds 350 Low compression?? or normal?
« Reply #3 on: October 08, 2018, 05:05:11 AM »
Some of the early 350 had dished pistons to lower compression prior to doing it with the heads in later 70s models. You may have both, numbers are all fairly consistent too.

bmanrrt

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Re: olds 350 Low compression?? or normal?
« Reply #4 on: October 08, 2018, 11:53:06 AM »
Thank you guys. I appreciate the help.

Aussieta they heads are 79cc, considered to be absolute JUNK by those who know..lol
Aus78, I think you may be right here:

The compression seems consistant across the cylinders,  but it runs like a tired old motor. I understand its choked down to start by the epa, but something is not right still. The combination of dished pistons and #8 heads might just be the problem. This might be an older block (68-70) with dished pistons and #8 heads (73-76) with 79cc Chambers.  I'm tracking down a borescope now to see if I can tell. If that is the case, do you think I should rebuild the motor so I can change the pistons? or Just heads? I appreciate all your help and I look forward to your opinion. Thanks

Re: olds 350 Low compression?? or normal?
« Reply #4 on: October 08, 2018, 11:53:06 AM »

kingwil

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Re: olds 350 Low compression?? or normal?
« Reply #5 on: October 08, 2018, 04:30:57 PM »
       Yes, the consistent numbers of 100 psi is low.  Your #8 heads that you mentioned are 79cc is giving you low compression, probably around 8:1 or thereabouts.   Since you have the 68-70 350 Olds Block, you might consider looking  around for some #5 or #6 heads (from 1969 or 1970) and see if you can mill them down a bit to around 64 ccs.  The #5 and #6 heads had somewhere around 68cc to 70cc combustion chambers.  With some 2" intake valves and 64cc combustion chambers, you might be able to get the compression up to 9.5:1, depending on the dish cc of the pistons.  Some of the pistons are around 20 cc's for the dish.  With 64cc heads, you can probably get the psi up around 160, and with some power-tuning wake up the 350 Olds.   
79 Nocturne Blue 403 Firebird TA
72 Viking Blue 455 Cutlass Convertible

bmanrrt

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Re: olds 350 Low compression?? or normal?
« Reply #6 on: October 08, 2018, 10:26:42 PM »
Thank you Kingwil. I kind of have been looking around for heads. I see a lot of aluminum heads for $1300 for sale but the decent vintage heads for these are a little hard to find. I am also probably not very good at finding them either. I am a little apprehensive of buying used heads because I really am inexperienced and I could easily buy someone’s junk. I did find some rebuilt heads on RockAuto, the picture says they are 7a, which from what I have read are good heads with 67cc chambers, but I would like to confirm with them somehow that that is what I will actually get if I order them. https://www.rockauto.com/en/catalog/oldsmobile,1968,cutlass,5.7l+350cid+v8,1217427,engine,cylinder+head,5304

olds403

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Re: olds 350 Low compression?? or normal?
« Reply #7 on: October 16, 2018, 08:56:32 PM »
You can find Olds Pre-72 heads on classicoldsmobile.  Just ask.  We spent ~$1100 for a set of heads rebuilt/pocket ported and we spent $2200 on a set full race. All this year.  I would not feel good about putting any more money in that motor.  Perhaps, you do a leak down test and ensure all the rings still seal well. 

The only way to run Aluminum BB heads on the SB is to use RPM intake (not fitting under TA hood) and/or port matching the 20yr old out of production Holley street dominator and welding up the ports to get enough material.  Plus, you will not be able to run mechanical fuel pump and have to redo your entire fuel system with BB heads. 

Not trying to steer you one way or another, but I've ran small block Oldsmobile motors in 79 Trans Am's since 1985.  Everything I post I have done or used myself. 

olds403

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bmanrrt

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Re: olds 350 Low compression?? or normal?
« Reply #9 on: October 17, 2018, 10:07:59 PM »
Thank you 403. I appreciate your info. Any guy who has been running olds motors in their ta for 30 years, knows allot about what I am trying to learn. I like your advice about the leak down test. I will do some googleing and see if it is something I can do myself. I have been looking at heads from http://oldsrocketparts.com/  I do not think they are BB heads but I could be wrong. I will go to classicoldsmobile to see what I can find. I am still aprehensive about buying used heads, but If nothing else I am sure I will find some good info. Thank you very much for the help and the direction.

olds403

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Re: olds 350 Low compression?? or normal?
« Reply #10 on: October 18, 2018, 05:37:03 PM »
Any aftermarket aluminum are B.B. heads.
The irons I sent a link above are some of the best.
It all comes down to your budget

bmanrrt

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Re: olds 350 Low compression?? or normal?
« Reply #11 on: October 18, 2018, 05:51:10 PM »
Got ya, thank you. I might have found a set here locally too, Waiting for the dude, to get back from vegas to check, or something like that. I have my fingers crossed because its the same guy who was probably going to be the one rebuilding the motor for me also. He said he could do heads now, but he is moving shops so a rebuild will have to wait untill the begining of 2019. I really appreciate your help. I found some of 7a's on rockauto a week or 2 ago, I wanted to confirm that they were 7a's. the listing didn't state, just the picture, so I wanted to be sure before I orded those. I looked again the other day (very briefly) and they were not there anymore. I'll look again and I'll scour through the site you sent me as soon as I can. Thanks again.

olds403

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Re: olds 350 Low compression?? or normal?
« Reply #12 on: October 19, 2018, 12:11:10 AM »
You really don't want any shop doing Olds heads.  I had a set done by another shop and they were not good.  Hands down - Champion Racing Heads in Florida does some amazing work.  Its about $100 each way in shipping, but well worth it.  If you look at my 424 stroker motor they did those heads. 

bmanrrt

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Re: olds 350 Low compression?? or normal?
« Reply #13 on: October 19, 2018, 09:40:48 AM »
Thank you again 403. I do not think the plan is to have him modify the heads. I think he just has them. I have allot of faith and trust in my friend (customer for years) and He has allot of confidence in this guy who builds his engines. He apparrently is an excellent engine builder. I would love to check out your setup, did you send me a link already? I will keep Champion Racing heads in my rollodex. $100 in shipping is nothing to insure it is done right. I checked more on rockauto, but I guess those heads they had are gone now. snooze and loose...lol

Re: olds 350 Low compression?? or normal?
« Reply #13 on: October 19, 2018, 09:40:48 AM »
You can help support TAC!