1972 350 Rocket Help
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Joined: May 2018
Posts: 13
1972 350 Rocket Help
Hello everyone,
I've having a bit of an issue with my 1972 Cutlass 350 Rocket. I had the motor rebuilt quite some time ago and finally had it installed in my car. The overall build is as follows:
Block Bored over .30
MSD Distributor
Cam is Lunati Voodoo w/ advertised duration 262/268, lift .499/.510
Performer RPM intake
Holley 770 Street Avenger Carb
#8 Heads (Unsure of amount of work done)
Stock Pistons
Hooker Full Length Headers
2.5 dual exhaust w/ Spintech 6000 Pro Street Mufflers
10 Bolt Posi Rear End with 3.73 gears
The transmission is a TH400 with a 2400 stall. This will be changed soon as I have an overdrive transmission with a 2000 stall just waiting to be installed in the car.
Essentially the car runs and drives fine. Just had it out cruising on I-95 a few days ago. My only issue is that I was expecting better performance "off the line". I've been doing research on this site and have read that the #8 heads with the stock pistons could be an issue. I do have a set of #5 heads that I picked up dirt cheap but I'd like feedback from you guys before I spend anything as I know many of you have in depth experience with building Olds and I'm still learning.
All I'm looking for is a fun driver with a little stop light action from time to time. Maybe 350 - 375hp if possible? Thank you guys in advance for your help!
I've having a bit of an issue with my 1972 Cutlass 350 Rocket. I had the motor rebuilt quite some time ago and finally had it installed in my car. The overall build is as follows:
Block Bored over .30
MSD Distributor
Cam is Lunati Voodoo w/ advertised duration 262/268, lift .499/.510
Performer RPM intake
Holley 770 Street Avenger Carb
#8 Heads (Unsure of amount of work done)
Stock Pistons
Hooker Full Length Headers
2.5 dual exhaust w/ Spintech 6000 Pro Street Mufflers
10 Bolt Posi Rear End with 3.73 gears
The transmission is a TH400 with a 2400 stall. This will be changed soon as I have an overdrive transmission with a 2000 stall just waiting to be installed in the car.
Essentially the car runs and drives fine. Just had it out cruising on I-95 a few days ago. My only issue is that I was expecting better performance "off the line". I've been doing research on this site and have read that the #8 heads with the stock pistons could be an issue. I do have a set of #5 heads that I picked up dirt cheap but I'd like feedback from you guys before I spend anything as I know many of you have in depth experience with building Olds and I'm still learning.
All I'm looking for is a fun driver with a little stop light action from time to time. Maybe 350 - 375hp if possible? Thank you guys in advance for your help!
Last edited by 72oldsman; May 27, 2018 at 06:36 AM. Reason: Edit.
Old(s) Fart
Joined: Mar 2007
Posts: 50,730
From: Northern VA
So you have stock 1972 low compression pistons with the soup-bowl sized dish and #8 heads with 80cc chambers. Your CR is under 8:1!
Keep in mind that even with the stock 7A heads, which had 64-ish cc chambers, that motor was only 8.5:1 CR. If you used the thick FelPro blue head gaskets, that drops CR by about another quarter point or so.
Couple this with an aggressive cam and your motor is the poster child for mis-matched parts. Sorry to be so blunt.
Old(s) Fart
Joined: Mar 2007
Posts: 50,730
From: Northern VA
The #5 heads have the same size chambers as the original 7A heads. Even if you used the original style steel shim head gaskets, you'll only get 8.5:1 CR with those pistons. That's going to be a lot of work for minimal improvement. If you don't plan to change the pistons, I'd talk to cutlassefi here about a cam better matched to your combo.
Registered User
Joined: Nov 2008
Posts: 8,472
From: Central Fl
OP- you should ask your shop why they picked the parts they did. This falls on them if they picked them.
Just an FYI for all, there are more shops out there than you’d think that can’t figure compression ratio. They look in a book and see what it SHOULD be with a given piston but don’t take into account other variables. Compression ratio is one of the first things learned how to calculate over 40 years ago.
Last edited by cutlassefi; May 27, 2018 at 08:54 AM.
Thread Starter
Registered User
Joined: May 2018
Posts: 13
Yeah about that shop. I went in inexperienced and trusted a mechanic that built a few engines for family members previously. He's no longer around so I can't do anything there. I remember him specifically saying the compression would be 9.5:1 with this build. Right now I understand that I'm going to have to spend funds to rectify the issue. My question is what option would give me the performance I wish to have. If I swap pistons can I keep everything else the same? Dump the #8s for the #5s and still swap the pistons? I'm also ignorant in terms of engines but I'm trying to learn so I don't end up in the same boat again. Again, thank you for your help!
Registered User
Joined: Nov 2008
Posts: 8,472
From: Central Fl
He’s off by almost 2 full points. Right now you could be down around 7.5:1. Again that just reiterates what I said about the inability of too many shops.
Your call on what to do. If you really have some $$$ in the heads then change the pistons, but you’ll still have to cut the 8 heads. But if that’s the case then do the new Mahle piston. It’s a much smaller dish plus about .015 taller, not to mention lighter and with a better ring pack.
If the heads are std issue then just swap heads. You’ll need to have the 5’s cut a fair amount though, they’re normally about 68cc’s, not 64 like most think. You’ll need be around 55cc’s to be close to 9.0:1.
Either way I think you’re really in for both a piston AND head swap to get to your stated 9.5:1.
I guess it’s a good thing your guy isn't around anymore huh. I for one am glad he’s not.
Your call on what to do. If you really have some $$$ in the heads then change the pistons, but you’ll still have to cut the 8 heads. But if that’s the case then do the new Mahle piston. It’s a much smaller dish plus about .015 taller, not to mention lighter and with a better ring pack.
If the heads are std issue then just swap heads. You’ll need to have the 5’s cut a fair amount though, they’re normally about 68cc’s, not 64 like most think. You’ll need be around 55cc’s to be close to 9.0:1.
Either way I think you’re really in for both a piston AND head swap to get to your stated 9.5:1.
I guess it’s a good thing your guy isn't around anymore huh. I for one am glad he’s not.
Last edited by cutlassefi; May 27, 2018 at 09:13 AM.
Thread Starter
Registered User
Joined: May 2018
Posts: 13
Thank you Sir. If it needs a piston and head swap to get performance then that's what I'll have done. At this point I just want the performance I expect. Once it's running how I think I'll be able to move forward with body work, paint, etc...
Out of Line, Everytime😉
Joined: Dec 2006
Posts: 10,053
From: Melville, Saskatchewan
You need to find out what pistons are in there for sure. Flat top pistons will work great with #8 heads. If you go with early heads like the #5 heads, Cutlassefi brought ultra modern 10cc forged pistons with Mahle that give perfect low to mid 9's street compression.
Registered User
Joined: Feb 2013
Posts: 15,371
From: Phoenix, AZ
They can't be stock pistons if the block was bored .030".
They could be stock-replacement pistons, though, which would be way bad as those not only have the huge soup bowl dishes, but are short on compression height as well.
They could be stock-replacement pistons, though, which would be way bad as those not only have the huge soup bowl dishes, but are short on compression height as well.
Thread Starter
Registered User
Joined: May 2018
Posts: 13
So,
The engine is apart and there are flat top pistons with the #8 heads. The cam has a wiped lobe and the engine is being completely cleaned and a new cam will be installed. Is there any other advice you guys would offer or suggest I check? Not looking for anything more than a fun driver that can decently launch from a stop light from time to time. Thank you all.
The engine is apart and there are flat top pistons with the #8 heads. The cam has a wiped lobe and the engine is being completely cleaned and a new cam will be installed. Is there any other advice you guys would offer or suggest I check? Not looking for anything more than a fun driver that can decently launch from a stop light from time to time. Thank you all.
Out of Line, Everytime😉
Joined: Dec 2006
Posts: 10,053
From: Melville, Saskatchewan
I would CC the heads, they may have been milled. The two pairs of untouched #8 heads CCd at 77 to 78CC, supposed to measure 79cc factory. Those are probably Speedpro flat top forged pistons, which sit around .025" in the hole but would still double check that measurement. Along with 77cc chamber, that puts right around 9 to 1 with a .040" thick head gasket. I would add a 2" or 2.07" intake valve with the bowl opened with a bowl hog cutter at minimum. Also make sure the restrictive lip right under the exhaust valve was removed as well with a bowl hog. Your #8 heads will flow better than untouched #5 heads with the above mods. I assume the valve springs match the cam? Did you go with the same cam?
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