Hot Rod Forum banner
1 - 13 of 13 Posts

·
Registered
Joined
·
6 Posts
Discussion Starter · #1 ·
Hello I have a stock bottom end 89 350 tbi crate motor with a comp cam 280 magnum camshaft it has 230 duration at 0.50, .480 lift, and 110 Lsa. I just bought 906 vortec heads that have been milled to 56cc combustion chambers and have z28 valve springs for a max lift of .525 the studs have also been pinned and it has been bowl blended behind the valves I would like to use scorpion 1.6 narrow body sa rockers as well
I am pretty new to engine modification and I’m not sure what the compression ratio would be and if I will have to run premium also not sure what the pushrod length would be but I can figure that out I think any information or extra tips are greatly appreciated as this is my first time changing heads and I would prefer not to mess something up also is there anything else I will need to change to make this work thanks
 

·
True Hotrodder
Joined
·
1,950 Posts
More than likely. These engines depending on use seemed to range from 8.5 to 9.5 mostly dependent on the head and piston combination. With 56cc heads you upped the numbers pretty good. Other people here can spin your head with exact facts on these engines but I'm just trying to answer the question asked. Also don't get carried away with RPM on this as you've also set yourself up for some broken rocker studs. I'll never understand why people want to pin the rocker studs on late model heads. The metal and casting isn't the same as a 60's era head.
 

·
More for Less Racer
Joined
·
20,546 Posts
Actual compression ratio is going to vary here since we need a little more information....head gasket thickness and bore diameter, and was the piston dished or flat top?
I'm not familiar with what piston is used in the '89 TBI crate...

12cc dish and .015" thick x 4.100" bore gasket....10.51:1
5cc eyebrows flat top and same thin gasket, ....11.49:1
if you used a thicker common composition gasket at about .040" thick x 4.166" bore....
Dish piston = 9.82:1
Flat top piston = 10.65:1

Which head gasket you used is going to make a huge difference, 3/4 of a point....
 

·
Registered
Joined
·
6 Posts
Discussion Starter · #4 ·
More than likely. These engines depending on use seemed to range from 8.5 to 9.5 mostly dependent on the head and piston combination. With 56cc heads you upped the numbers pretty good. Other people here can spin your head with exact facts on these engines but I'm just trying to answer the question asked. Also don't get carried away with RPM on this as you've also set yourself up for some broken rocker studs. I'll never understand why people want to pin the rocker studs on late model heads. The metal and casting isn't the same as a 60's era head.
Thank you by the way they are dish pistons and the heads were modified this way before I bought them
 

·
Registered
Joined
·
6 Posts
Discussion Starter · #7 ·
Actual compression ratio is going to vary here since we need a little more information....head gasket thickness and bore diameter, and was the piston dished or flat top?
I'm not familiar with what piston is used in the '89 TBI crate...

12cc dish and .015" thick x 4.100" bore gasket....10.51:1
5cc eyebrows flat top and same thin gasket, ....11.49:1
if you used a thicker common composition gasket at about .040" thick x 4.166" bore....
Dish piston = 9.82:1
Flat top piston = 10.65:1

Which head gasket you used is going to make a huge difference, 3/4 of a point....
Thanks this is very helpful on 10.5:1 do you think it would need to run mid range or premium gas or would it be fine on regular
 

·
True Hotrodder
Joined
·
1,950 Posts
I wasn’t being sarcastic if that’s what you thought I just wanted to know why you thought I should take them back I’m guessing because they are pinned and because vortec heads are known to crack is there any other reasons though?
I work a day-work job for a high performance engine shop. If they need me, I come in. The fact that almost all late model stuff is thin to start with is a real problem. The manufacturers don't worry about you or me being able to hot rod the parts, they just want it to live through the warranty period and they have looked at everything to reduce weight and improve their CAFE numbers. I have seen a number of late model heads that guys have pinned the rocker studs instead of milling them down and installing a treaded rocker stud - and they over-rev them, miss a shift, hit it with the bottle and the rocker bosses crack off right where they are drilled at - every time. Even milling them isn't like the old heads - those had plenty of meat to work with - the new ones just don't.
 

·
Registered
Joined
·
6 Posts
Discussion Starter · #12 ·
I work a day-work job for a high performance engine shop. If they need me, I come in. The fact that almost all late model stuff is thin to start with is a real problem. The manufacturers don't worry about you or me being able to hot rod the parts, they just want it to live through the warranty period and they have looked at everything to reduce weight and improve their CAFE numbers. I have seen a number of late model heads that guys have pinned the rocker studs instead of milling them down and installing a treaded rocker stud - and they over-rev them, miss a shift, hit it with the bottle and the rocker bosses crack off right where they are drilled at - every time. Even milling them isn't like the old heads - those had plenty of meat to work with - the new ones just don't.
Ok thank you sir I’m glad you told me that unfortunately they were pinned already or I would have done screw in studs I’m hoping by keeping the lift at 480 and after lots of research today I will be going with the 1.5 ratio rockers like stock in hopes to keep some pressure off of the studs I got the heads very cheap and already had them inspected by a speed shop so I will be keeping them as mild as they can be and hoping the pins don’t ruin them like you mentioned I’m not exactly sure what to do about the rpm but I think I might be able to figure something out to make sure the rpm doesn’t get too high with an aftermarket ecu hopefully with all this into account I can avoid failure otherwise I’ll be looking for new heads but anyway thank you very much for the conversation and advice
 
1 - 13 of 13 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top