Head suggestion 1978

[stevehendo34]

I have a cracked block 305, so I picked up a 4 bolt main 350 today to build for my boad.
I would like to get the 200HP the 305 had.
The 350 has the crappy 624 but it dose run.
I was thinking even my 305 heads would give me a lot of low end grunt on the 350?
Any sugestions on a NEW SBC Head that would work good and bolt up and give good low end.
Will a new style Vortec head bolt up or will I have to just change intake?

This is all confusing to me I am not a V8 guy--rice rockets are my thing.

 

[81 Checkmate]

Re: Head sugeston 1978

Re: Head sugeston 1978

When swapping motors they need to be the same year vintage to make it simple.
Old parts on new block and install.
Most likley you have a 2 pc rear main!

Crappy 624 not sure what your talking about?

You can use votec heads but you will need a vortec style intake also they are differnt from a 1978 model GM motor.

Hope this sheds some lite for ya.

 

[Fishermark]

Re: Head sugeston 1978

Re: Head sugeston 1978

Do not put the heads from the 305 on the 350. Will they bolt up? Yes. But they do not flow as well and your compression will be too high for your purposes on the boat. Best bet would be to upgrade to set of cast iron Vortech heads with new intake. If you do not want to change your intake, then any heads from the appropriate vintage 350 will work better than your 305 heads.

 

[stevehendo34]

Re: Head sugeston 1978

Re: Head sugeston 1978

old Century 19ft weight unknown,
OK I'll find some GM Vortecs.
What lift and duration cam should I use.
Flatop or dished pistons. I can probably find both--obviosly will effect CR.

 

[joewithaboat]

Re: Head sugeston 1978

Re: Head sugeston 1978

old Century 19ft weight unknown,
OK I'll find some GM Vortecs.
What lift and duration cam should I use.
Flatop or dished pistons. I can probably find both--obviosly will effect CR.

What are your hp goals, what amount of money you want to spend?

 

[dollarten]

Re: Head sugeston 1978

Re: Head sugeston 1978

old Century 19ft weight unknown,
OK I'll find some GM Vortecs.
What lift and duration cam should I use.
Flatop or dished pistons. I can probably find both--obviously will effect CR.

Stay with the dished pistons, don't' want to get the compression ratio to high . It has been documented that the dished pistons will have more downward pressure from the flame front during ignition . The flame travels outward and hits the dished area of the piston in order to give more downward pressure from combustion . In a flat top piston engine the flame travels outward and some of the flame front hits the side of the cylinder wall and sends combustion pressure directly to the cylinder wall and loosing combustion pressure in the process I know its kind of complicated description. In an engine where there is a high dome on the piston , now we are talking different parameters but marine engines are more concerned with torque than horsepower >>>>>>>>>>>>>> On your camshaft , when I had access to the Mercruiser parts manual I found out that the 350 and 305 engines had the same camshaft part number .I the looked in the service manual and looked up the cam specs and found out that Mercruiser used a GM 350 HP camshaft which is identical to the Mecruiser cam . I would stick with this camshaft as it it always better to under cam an engine as to over cam it / Hope this helps you

 

[dan t.]

Re: Head sugeston 1978

Re: Head sugeston 1978

Heads: the 993 is the best old style(pre 87) head, otherwise go for the vortec
Pistons; the very best piston design is the reverse dome, it keeps the compressoin reasonable ( 9.2-9.5) and will still give you quench.
I personaly prefer cast, hyperutectecs are good but you must run a huge gap on the top ring, forged are good and very strong but noisey.
Cams: depends on what you want the motor to do, personaly I would find a newer block with the factory roller set up.
Check out comp cams website and do some browsing.

 

[Silly Seville]

Re: Head sugeston 1978

Re: Head sugeston 1978

EDIT: After long reply, realized OP already has a block available. My point is moot.

 

[stevehendo34]

Re: Head sugeston 1978

Re: Head sugeston 1978

"I found out that the 350 and 305 engines had the same camshaft part number" I'll just move it.

The TYZ 350 I got is realy a low CR Smogger with dished pistons, so Maybe I could use the 305 412 heads and keep it below 10 to 1 with some mods to the heads untilll I find some Vortecs @ a decent price. We have 94 gas in Wisconsin and most of the time the boat is pushed by a kicker pulling planner boards. I am only Looking for original HP 198 or or so. This would be a $0 dolllar project if I could do this. I got the 4 bolt TYZ 350 free from friend that scrapped his 77 chevy 1/2 ton a year ago.

Thanks time to quit thinking and start doing.
Going to my friends buisness tomorow to use his fork lift to pull the cracked block 305.

 

[joewithaboat]

Re: Head sugeston 1978

Re: Head sugeston 1978

Not sure about the info you are getting on the reason to use dished pistons, or the 993 which is a terrible casting... but what ever.

At any rate keeping compression ratio at the proper level is important.
Making 200 hp with a 350 will be easy regardless of which head you use.

Re using a flat tappet cam in a different block has a pretty low success rate. If you try... using new lifters will increase the chances that it might develop a good wear pattern.

A 4 bolt block will not be needed! Maybe sell it and use the funds to get your build as close to zero a you can, if that is important to you. I got 300.00 for the last std bore one i sold.

good luck!

 

[Bondo]

Re: Head sugeston 1978

Re: Head sugeston 1978

so Maybe I could use the 305 412 heads and keep it below 10 to 1 with some mods to the heads untilll

Ayuh,... It's yer boat, 'n ya can do whatever ya want,... I Wouldn't...

The original 350 heads on the motor now are a Better choice, with only 200hp as the target....

 

[dan t.]

Re: Head sugeston 1978

Re: Head sugeston 1978

So Joe With a boat where do you get your chevy engine info? All my books recommend the 993 casting,have you actually even seen one? Fyi they are a heavy casting with 76 cc chambers, 1.94 intakes 1.5 exhaust with hardened seats,and if you check the intake ports you will find a smooth passage without the dimple where you can feel the bottom of the rocker stud. I have used these heads with great success on several medium performance 350s.

 

[joewithaboat]

Re: Head sugeston 1978

Re: Head sugeston 1978

So Joe With a boat where do you get your chevy engine info? All my books recommend the 993 casting,have you actually even seen one? Fyi they are a heavy casting with 76 cc chambers, 1.94 intakes 1.5 exhaust with hardened seats,and if you check the intake ports you will find a smooth passage without the dimple where you can feel the bottom of the rocker stud. I have used these heads with great success on several medium performance 350s.

Dan,

Its been a long time and and although i am not old my memory is not what it used to be... I to have lots of books on small block chevy heads but dont have them in front of me, they are packed up in boxes from my last move. I worked in a machine shop for a couple years in college so I've seen and handled an awful lot of sbc heads.

Anyway... as i recall they are actually on the list of heads to avoid.
Open chamber
Small valves
Commonly found on "Trucks"... sometimes 400's which means they have seam holes "BAD"
some would have very small intake, which would be 1.72 not 1.94 if so they would even have over size stems... again truck engine

I have never seen someone suggest or seek out an open chamber head before. The only good thing i could see about them is they would be an early Pre lightweight casting but are late enough to have induction hardened seats.

Some dirt track rules mandated open chambers in lower classes to limit hp... LOL

 

[stevehendo34]

Re: Head sugeston 1978

Re: Head sugeston 1978

The Problem:
I have a low compression TYZ 350 rated @ 160 HP with a 4bl carb on it and 624 heads.
If it was yours how would you get it back to 200 HP like the original 305 mercruser.
I thought the main reason for the low 160 HP was low stock compression of the 350--much less than the boats 305.

 

[joewithaboat]

Re: Head sugeston 1978

Re: Head sugeston 1978

The Problem:
I have a low compression TYZ 350 rated @ 160 HP with a 4bl carb on it and 624 heads.
If it was yours how would you get it back to 200 HP like the original 305 mercruser.
I thought the main reason for the low 160 HP was low stock compression of the 350--much less than the boats 305.

Quick internet search shows conflicting info on the 624 casting... can you post the entire casting number and maybe a picture of the chamber. Is this the heads off the 305 ?

Putting a decent camshaft in the engine will be where most of your power will come from.

 

[Scott Danforth]

Re: Head sugeston 1978

Re: Head sugeston 1978

I have a set of heads and intake from my '88 350 I will sell cheep. I switched to vortecs amongst my other changes.

 

[stevehendo34]

Re: Head sugeston 1978

Re: Head sugeston 1978

My obtained 350 1977
--the compression was 8.5:1, or 8:1.
--It is rated at horsepower 160-165 @ 3800 rpm,
--Torque 270 ft/lbs. @ 2400 rpm.

Heads 462624
1.94" / 1.50" valves
72cc chambers
people say the suck because the always crack but mine are currently OK.

I was planning using the stock 5.7 mercruser cam from the 305 with new lifters
They are both the same via #3 book about 260 on the intake 270 on exaust.
--will this alone get me back near the 200HP that the original 305 had,

The 350 1977 TLZ 350 has extreamly low CR 8:1.
I think this is my biggest problem?
What is CR on a 5.7 Mercruiser
--how would you get my 350 to the same CR as stock Mercruiser?

 

[stevehendo34]

Re: Head sugeston 1978

Re: Head sugeston 1978

I found out a Mercruiser 5.7 is 9.3:1
My 350 is 8:1, or 8.5:1
At rebuild time how would you solve this?
Pistons

 

[dan t.]

Re: Head sugeston 1978

Re: Head sugeston 1978

The solution is flat top pistons, a stock 4 valve relief piston with the heads you have will give you 9.3 with a .040 compressed thickness head gasket. 9.2 with a steel shim gasket. I f your old lifters are ok you should keep them in order so they are matched to the lobe on the 305 marine cam, they should only be changed if there is a problem with them or if they have gotten mixed up.

 

[joewithaboat]

Re: Head sugeston 1978

Re: Head sugeston 1978

I found out a Mercruiser 5.7 is 9.3:1
My 350 is 8:1, or 8.5:1
At rebuild time how would you solve this?
Pistons

Correct, with the pistons. Get on Summit racing and use the menu on the left to focus in on your combo, it will show you piston options.

here, these would be for .030 over. Note some are single some are sets of 8. http://www.summitracing.com/search/...480-in/Wrist-Pin-Style/Press-fit/?Ns=Rank|Asc

I though you were going for the near zero $$ build and reusing the entire 350 as is.