350 Engine advice--GOOD UPDATE!

[n2ostroker]

This is my first post on iboats. I'm fairly new to boats as far as working on them as well as GM stuff. Grown up as a Ford guy. Not new to hotrodding or engines though. Been reading and found plenty of useful info on here.

Here is what I have. Ran into a 1986 Thompson 19' cuddy cab w/ a 230/v8 and Alpha one outdrive. I'm pretty sure it's the Sea Rge model but not positive. All my title says is 86 Thomson 19' with 8' beam. Regardless it had a fresh 305 in it. Found out the first time out the previous owner hadn't actually had the boat in the water after getting it back to Ky and let the block freeze. He didn't know about and I only gave $500 for the boat so no big deal. I built a 355(2 pc main) and swapped over all the 305 parts except the flywheel due to the 355 being internally balanced and my 305 was a 1 piece main with an imbalanced flywheel. The balancer was a 0 balance on the 305 so it worked fine. I know the 305 heads are the 58cc chamber with 1.84 intake valves. So the compression is high with the flattop pistons but with 8* intial and 93 octane with Lucas octane booster it runs great with no detonation. I also run a 1 step colder plug to help out. I had some random popping through the carb but I'm pretty sure its just some water in the tank from the boat sitting. Put a new filter/separator on for this weekend so we'll see.
My problem is the 305 heads have pretty small runners. I think it is limiting my max rpm under load. Holeshot is great as it is now. I can hit about 4400rpm trimmed right which is around 51-52mph on the speedo not gps. Still it moves pretty good. I'd like to get a few hundred more rpms up top for the smooth days on the water and to cruise closer to 4000rpms without being close to full throttle. As well as run some cheaper gas next year.
What heads would you guys recommend? Is there a particular casting number I should look for? I'm assuming the intake is the same as a 350 of that yr but may be wrong. The motor runs a Rochester 4bbl which I'm not familiar with but so far it works great. Played with Holleys and Edelbrock/Carter AFBs. I was looking at the Vortec stuff and can get them fairly cheap but don't think I really need them jsut to play for another year till I get something else. Also I think they had just put a replacement truck cam as the cam and lifters were new when I switched them over. I'd like to switch to the comp cams 256h if any one thinks it would be worth it?

Also, yes I reatained all the marine spec parts off the old motor. No auto parts on here except for the flywheel and freeze plugs. The boat is freash water as well.
The hull and transom are solid as a rock and the boat over all is in great shape minus the teak being weathered, vinyl a little dirty, and the gel coat needing shined up.

This site has been a lot of help on gaining knowledge in a fairly unfamiliar territory so far. Thanks in advance for any advice offered.

 

[Bondo]

Re: 350 Engine advice

Re: 350 Engine advice

Ayuh,... Welcome Aboard n2ostroker,....

If I were buying Heads for a SBC,...
The Vortecs are the answer,+ the Only Answer.....

I believe they only come in 2 casting #s,...
The 906s that I have, have a bigger exhaust seat, which helps with cooling the exhaust valves....
The others are I think 662s, which have the smaller exhaust seat,+ the Hot-Rod guys love'em...
They claim the bigger seats cause flow restrictions,...
But that doesn't happen at the rpm levels We run our boats at, so I see the 906s as the Better Choice...

 

[John_S]

Re: 350 Engine advice

Re: 350 Engine advice

I agree with Bondo, the vortecs would provide the best overall hp increase. It will be rather pricey for one season of use. Along with the heads and intake you will need self-aligner rockers, and center-bolt valve covers. Because of the 64cc vortec chambers, and your flat-top pistons, and 0.030 over, you will still be on the high side of compression ratio. You should be able get rid of the octane booster, but might need 91 octane, if lucky 89. Given the truck cam would estimate about 270hp @ prop.

While it might not meet your performance goals, the minimum you need to do is bring it closer to a stock marine 350 of that era. With the flat-tops, the marine 350 had heavy duty 76cc chamber heads. This allows for regular grade octane. Selling your boat as it is, will be hard in $4+ gallon world. A new buyer will not be happy with having to buy 93 octane + booster. I am not sure what your current hybrid is producing, but with the 76cc heads and current truck cam, would estimate 240hp @ prop.

In both cases above you could gain an additional 20-25hp by picking a slightly larger than stock marine cam. Also, some tuning of your current Q-jet may be required.

BTW, I am assuming you want more mph along with the extra rpms. If you just want rpms, drop the prop pitch by 2".

PS: Welcome!

 

[n2ostroker]

Re: 350 Engine advice

Re: 350 Engine advice

Thanks for the quick replies and the welcome guys. A buddy of mine owns a machine shop so the Vortecs heads, valvecovers, and rockers would all come together and probably pretty cheap or a trade for work. The only thing I'd be left buying is the intake. Being that all my friends are performance guys with and few owning shops I could pick a new one up close to cost, same with the cam and a decent set of valvesprings. I'd probably have $300-400 into the whole thing. But an old set of 350 heads would be close to free. I guess I got the boat cheap enough I'll check into the Vortec stuff a little more.
As for compression, the 64cc chambers should put me around 9.8:1. 76cc heads would be around 8.7:1. I like a little higher compression anyway cause of the better response. I haven't tried without octane booster on the current so I don't know for sure if it would detonate or not. Just didn't want to take the chance.
Guess I have a good winter project in front of me.

Yeah I would like a few more mph and the higher but easier cruising speed. I know I won't, but I'd like to try and keep up with my buddies Checkmate 242 w/454 on straight smooth water. Or at least be able to run with him at his cruising speed (55ish mph on his speedo), my top speed. My other buddies 275 Baja 496 HO is way out reach, besides 70 something mph in this little boat might not be too fun.

As for the q-jet, any suggestions for tuning?
I like the idea behind the q-jet. Just don't know the best route for tuning.

One more thing, should I remove the hydrofoil off the lower unit?
I've read good and bad about them. Some have said to get a quicker plane. Some have said it killed the top end. Some both or neither. With the torquey high compression I have this thing planes really quick the way it is but the foil is on.

 

[wca_tim]

Re: 350 Engine advice

Re: 350 Engine advice

I would lose the hydrofoil... my boat had one on it when i got it and it ran faster when it was gone.

quadrajets are good carbs, and can work great. There's some books around about how to tune them if you can't find someone that knows them. I haven't messed with one for a good while. Some others on here may be of more help in that regard.

If you happen to live in eastern north carolina, i"ve got a servicable set of 350 heads you can have. it is time for them to go... maybe even out to the curb if necessary.

 

[John_S]

Re: 350 Engine advice

Re: 350 Engine advice

Given your sitituation, then vortecs are the way to go. I actually struggled with the idea of even mentioning the 76cc heads. The low-cost, expensive gas, and one season is what prompted the suggestion.

If you want some interesting calcs to look at, calculate stock vortec compression ratio. They use the -10cc dished pistons. From the static, calculate the dynamic, if you have the cam specs. I think you will be surprised how low it is compared to the advertised.

Keeping the Q-jet with vortecs will limit your intake choices. With the Edelbrock brand, it will limit you to stay in the Performer line, vs RPM. If your Q-jet is on its last legs, than consider a carb change too. As far as tuning, the merc service manual helps out there. The main difference between a 305 and 350 calibration is the secondary rods and spring wind-up. Lots of info on the web on tuning.

As far as keeping up with your friends, not much chance of that. The vortec heads are only going to give you 30-40hp over what you have. Depending on hull, thats only a few mph improvement.

PS: I agree, drop the foil. Search on "Smart Tabs" if you need planning help.

 

[MikDee]

Re: 350 Engine advice

Re: 350 Engine advice

Ok, you don't say what size prop you have, if it was an orig. 230hp/305 it's probably a 19" pitch, and the rec. rpm range for that engine is 4400-4600rpm, and a 50mph boat as it was, considering I had an older 228hp/305 in (probably) a heavier 20' SeaRay bowrider that would do 50 with a 19" pitch prop, I'd say it's not the heads holding you back! There's something else going on!, that boat should do 55 easy with a 355. I suggest you go over that engine real good, it's got the guts to do more then that, even with a "plain vanilla" stock cam.

 

[John_S]

Re: 350 Engine advice

Re: 350 Engine advice

Be careful with similiar engine to different boat comparisons. OK for gross comparison, but not for minor differences. Prop pitch without the gear ratio, is not valid. I believe the standard alpha ratio of those 5.0L was 1.62. If you plug those numbers in a prop slip calculator, it shows next to no slip. So if Mikdee's speed is accurate (gps?) it would make more sense if he had 1.50 drive ratio, giving a more realistic 10% slip. While a weight comparison is important, exact hull design is as well. The amount of deadrise and any pad makes significant diffences. There are many 350 powered 20' boats that only hit the low 50's (gps), especially of older vintage.

 

[n2ostroker]

Re: 350 Engine advice

Re: 350 Engine advice

The q-jet is fine. So I'll stay with it. Been reading up on the tuning of one and they quite a complex piece. Though I think tuning for a boat should be a little simpler since you don't have constant throttle transition going on.
If I were to switch carbs should I look into a different intake than the Performer? I'd read about the dual port and liked it till I saw the price. I just wouldn't think the gain would be worth the price at that point though I may be totally wrong. I also didn't see an app. for the Vortec setup. Next to that is the Performer rpm, a larger plenum/runnered performer. Then a single plane which I think would be pointless at the rpm range I'm looking at. Though the old Torker single plane I used run out performed my Peformer rpm.

I have no idea what prop is on the boat. I've looked for #'s and can't finding any markings on it yet... I'll pull it this weekend and see what I find. It's a 3 blade and shiny. I'm guessing it's a 19"pitch stainless.

As for the something wrong with the motor. Highly doubt it. I've been through it. I just built it... I added 50 cu in, around a 1.5 compression and used a head with 155cc intake runner and a 1.84" valve. It's choking, no doubt. I was actually surprised it pulled as good to the rpm it hits. The boat planes almost instantly and pulls to 43-4400rpm then hits a wall. Kinda like torquey truck. Has a bunch kick down low and nothing up top. Even then it's at 50-52mph. It's not a speed boat so I'd say it's not the most efficient hull design for going fast anyway. I'll pull the foil and see how it acts. I've read most people lose 2-3mph adding one. So I'll see if I gain or lose anything without it.

I was going to mention dynamic compression but without knowing the cam specs I didn't want to touch on that just yet. Without knowing duration specs and lobe separation it's hard to tell. But if it is a 114-116 lobe sep cam it probably still has a fair bit of squeeze in the mid 4k rpm range. The 9.8:1 static with the Vortecs and their more efficient chamber 89 octane shouldn't be too far out of reach to run next year.

Thanks again for all the suggestions and help. I'll see how it acts this weekend after playing with the carb, a new fuel filter, and removing the foil.

James

 

[MikDee]

Re: 350 Engine advice

Re: 350 Engine advice

Be careful with similiar engine to different boat comparisons. OK for gross comparison, but not for minor differences. Prop pitch without the gear ratio, is not valid. I believe the standard alpha ratio of those 5.0L was 1.62. If you plug those numbers in a prop slip calculator, it shows next to no slip. So if Mikdee's speed is accurate (gps?) it would make more sense if he had 1.50 drive ratio, giving a more realistic 10% slip. While a weight comparison is important, exact hull design is as well. The amount of deadrise and any pad makes significant diffences. There are many 350 powered 20' boats that only hit the low 50's (gps), especially of older vintage.

I know those Thompsons, because I like them, & almost bought one yrs ago before the SeaRay it had a 260/350 Merc in it, but No cuddy windows They are lighter, with less of a Vee hull then my 79' SeaRay bowrider, besides all the info I had at the time from the Clymer manual, the guys at the marina, and other sources, stated my 305 Type 1 drive was 1:50 gearing, same as the 350, and I used a Merc. Black Max 19" aluminum prop, but mine was a slick bottom unpainted lake boat when I got it.

Shoot, One challenge I had, 3 kids in a 16' Boston Whaler with an 85hp johnnyrude laughing at me, till I walked past them, with the thru-hull bellowing, you should have seen the shock on their faces surprised me too

 

[n2ostroker]

Re: 350 Engine advice

Re: 350 Engine advice

The hull is a narrow deep v center comes up then wide v out. I haven't really looked under at the stern to see its shape. I'm 99% sure from what info I've found it has the 1.62 geared Alpha. When testing last weekend with just me(175lbs) and a half tank it ran that speed. Add my dad (160lbs), stepmom (~210lbs) and 7 more gallons of fuel the planing and accel were identical

I'll also try to find a gps to get a more accurate figure on speed.

 

[MikDee]

Re: 350 Engine advice

Re: 350 Engine advice

The q-jet is fine. So I'll stay with it. Been reading up on the tuning of one and they quite a complex piece. Though I think tuning for a boat should be a little simpler since you don't have constant throttle transition going on.
If I were to switch carbs should I look into a different intake than the Performer? I'd read about the dual port and liked it till I saw the price. I just wouldn't think the gain would be worth the price at that point though I may be totally wrong. I also didn't see an app. for the Vortec setup. Next to that is the Performer rpm, a larger plenum/runnered performer. Then a single plane which I think would be pointless at the rpm range I'm looking at. Though the old Torker single plane I used run out performed my Peformer rpm.

I have no idea what prop is on the boat. I've looked for #'s and can't finding any markings on it yet... I'll pull it this weekend and see what I find. It's a 3 blade and shiny. I'm guessing it's a 19"pitch stainless.

As for the something wrong with the motor. Highly doubt it. I've been through it. I just built it... I added 50 cu in, around a 1.5 compression and used a head with 155cc intake runner and a 1.84" valve. It's choking, no doubt. I was actually surprised it pulled as good to the rpm it hits. The boat planes almost instantly and pulls to 43-4400rpm then hits a wall. Kinda like torquey truck. Has a bunch kick down low and nothing up top. Even then it's at 50-52mph. It's not a speed boat so I'd say it's not the most efficient hull design for going fast anyway. I'll pull the foil and see how it acts. I've read most people lose 2-3mph adding one. So I'll see if I gain or lose anything without it.

I was going to mention dynamic compression but without knowing the cam specs I didn't want to touch on that just yet. Without knowing duration specs and lobe separation it's hard to tell. But if it is a 114-116 lobe sep cam it probably still has a fair bit of squeeze in the mid 4k rpm range. The 9.8:1 static with the Vortecs and their more efficient chamber 89 octane shouldn't be too far out of reach to run next year.

Thanks again for all the suggestions and help. I'll see how it acts this weekend after playing with the carb, a new fuel filter, and removing the foil.

James

The 305 heads choking? I don't think so, I had 2 -305's so far, one was in my 88' Monte SS a mild HO version (which could pull 5500rpm), and the other was in my 20' SeaRay (the one I mentioned previously) and I never had a problem getting rpm's out of them, I didn't "hit any wall" so to speak? With that compression, and 1.84 valves, it shouldn't be a problem getting rpm, the standard chevy 350 valves are 1.93, only .010" bigger. Maybe your timing, or cam is not up to snuff?

Lose the foil, at least for now, it should pick up some speed.

 

[n2ostroker]

Re: 350 Engine advice

Re: 350 Engine advice

The 305 heads choking? I don't think so, I had 2 -305's so far, one was in my 88' Monte SS a mild HO version (which could pull 5500rpm), and the other was in my 20' SeaRay (the one I mentioned previously) and I never had a problem getting rpm's out of them, I didn't "hit any wall" so to speak? With that compression, and 1.84 valves, it shouldn't be a problem getting rpm, the standard chevy 350 valves are 1.93, only .010" bigger. Maybe your timing, or cam is not up to snuff?

Lose the foil, at least for now, it should pick up some speed.

I don't think your understanding. If a head is set up for a smaller motor and all you do is add cu in your effecient operating rpm will be lower. Assuming the 305 heads were effecient at 4800 with the cam be used, I just added ~16% more cylinder volume. With the same cam, heads and intake a 355 will be moving the same air as the 305 at 16% less rpm. I should be making more hp and a ton more tq now than with the 305 but at 5-700rpm less rpm. Throw in the fact that the 355 has a 4.030" bore and the 305 has a 3.78" bore. Then the 305 head has a smaller combustion chamber, intake port, intake valve, and quench area for a smaller bore. Things are all out the window when guessing what rpm range the motor work at efficiently.

This ^^ might make sense. It's not a perfect comparison but it's geneal idea.
Same reason if you take a large lumpy 7000rpm cam in a 305 toss it in a 350 all the sudden it's less lumpy and only needs 6000rpm. Or that Chevy, Ford, whoever made heads with small and large ports for a particular size engine and desired operating rpm.

I read your post a couple of times and I am still uncertain what it is you want to see. Are you looking strictly top end speed?

I have to replace the heads so I don't worry about burning up parts anyway. I needed suggestions. Vortecs worth it? Would standard 350 heads be dissappointing? Better cruise rpm with less throttle input. Little better top end with ability to get the revs up. Wanting to get my game and parts together for winter and don't want feel the need to change things later. I'm tickled with the boat now for the cost and performance but I don't want to feel like I'm giving it all its got to run 4-4200rpm. If it'll go to 4800 then I'm comfortable reducing throttle to 4k. Where now it feels like 4k is about 90% throttle.

Sorry about the long post and thanks again for responses. Not all forums welcome newcomers as well.

 

[MBAKER]

Re: 350 Engine advice

Re: 350 Engine advice

Even with flat tops, the 64cc heads will get you under 9.5:1(closer to 9.2 if I remember right) with a 0.040 head gasket, and thats assuming they are a true 64, which they are likely bigger. With a good chamber design like the vortec and if you watch your quench distance I dont think you would have any trouble running 89 octane with that setup.

There is no doubt the smaller runners and smaller chamber would start to hinder the 350 but at what rpm is the question. The cam timing would play into it too. The bigger 350 bore will tend to breathe easier at higher rpm, but I think the small chamber is hindering it a little. Bigger valves would help it breathe too. The smaller valves might not be a restriciton at peak cam lift, but you have to look at low lift numbers too. The bigger the valve is the more area there is for flow, not only at full lift, but all the way through the lift curve. So when the valve is only 1/4 open you have a bigger window to pull in air than a smaller valve. More air = more power.

To do it right with vortecs, you would need everything from the head up, valve covers, rockers, intake, etc. But its also possible in some cases to have the Vortec heads redrilled to take the old pattern (pre 86)intake manifold that you already have. Just have to make sure the gaskets and intake cover the ports on the heads since the vortec has taller intake ports. Also have to check if you are running a cam over 450 lift that the retainers dont get into the valve seals, if so you have to machine the valve guides down as needed.

Having messed with cars for years the other option I see that never gets mentioned would be an all new head casting like the Dart S/S or World Products SR or SR Torquer head. They are small runner heads *yet bigger than the 305) close to the vortec but a much beefier casting. They would take all the bolt on pieces you currently have without trouble, and the world head is available in 67 or 76 cc chambers, Dart in 67, 72 or 76. They woud be a direct bolt on and in the end may be cheaper than redoing a set of vortecs and buying all the misc stuff to go with them.

 

[Uraijit]

Re: 350 Engine advice

Re: 350 Engine advice

you've got 305 heads on a 355. So you're moving 15% more air through them now...

The valves are only 2% smaller than 350 valves. If your 305 were sucking every bit of possible air through those heads that it efficiently could, then you'd be correct that a 15% increase in displacement would equate to a 15% decrease in flow efficiency. Unfortunately, that's not the case. A bone stock (or even slightly modified) 305 isn't even CLOSE to maxing out those heads.


Surely something else is amiss. Clearly these heads are not going to have the best flow. And Vortec heads would make a world of difference in actual flow potential. But methinks you should be looking elsewhere first. If your cam isn't pulling its weight, it won't matter what heads you throw on there.

 

[Bondo]

Re: 350 Engine advice

Re: 350 Engine advice

Without knowing duration specs and lobe separation it's hard to tell. But if it is a 114-116 lobe sep cam it probably still has a fair bit of squeeze in the mid 4k rpm range.

Ayuh,...

If you install a Cam with the LSA up around 114? to 116?,....
You'll have a Boat Anchor instead of a Motor in short order.....

Marine Cam LSAs are kept between 109?,+ 112?.....
That keeps Reversion for Destroying your motor....

 

[MBAKER]

Re: 350 Engine advice

Re: 350 Engine advice

not necessarily... just because the engine is bigger doesnt mean it gets completely filled with air on every intake stroke. The 350 will only pull more air through the head up to the point the restriction becomes too great. At that point you are only getting the amount of air the restriction allows during the time the cylinder can fill. Think about restrictor plates under carburetors (or extremely small carbs). They only allow so much air in which reduces HP.

The 305 head is likely creating a lot of low end torque because at a given rpm (say 2000) the 350 is pulling more air through the same runner, this increases intake port velocty which helps ram the air/fuel mixture into the cylinder enhancing cylinder fill. Small ports are good at that, but as rpms increase the head becomes more of a restriction and cant flow enough air to --COMPLETELY-- fill the cylinder in the time the valve is open which starts to hurt power.

 

[Bondo]

Re: 350 Engine advice

Re: 350 Engine advice

Ayuh,....

You guys are kinda beating a Dead horse,...

My Engine Guy, who's a Race motor builder tells Me that the Exhaust is the choke point on a typical SBC.....

That said,...
In Boats,... The Exhaust Is in fact the Major choke point.....
Unless you're willing to spend Thousands on True marine Headers,....

Even the center-riser manifolds+ Thru-hulls are a Major restriction.....
Mainly because of the open plenum of the manifods.....

 

[Uraijit]

Re: 350 Engine advice

Re: 350 Engine advice

Again, though, you're assuming that the 305 was maxing out the heads' flow potential, and that now that you've got a larger engine, you've crossed the threshold. That just isn't the case. Of COURSE Vortec heads will offer some improvement. I'm not trying to dispute that. But you need to have a cam matched to your heads, if you hope to get any real improvement out of that upgrade.

My advice would be to still go with the Vortecs, but I'd pay particular attention to the cam profile as well.

It's also possible that you've got TOO MUCH octane going on, and that the Premium gas, coupled with an octane booster, coupled with colder plugs, could be preventing you from getting a complete burn.

I'd start by backing off the octane booster, and see if you get any knocking. If not, maybe even try going back up to the stock plugs. You want to have the least amount of knock compensation, without risking knocking/detonation.

Anything more is just wasted money, and wasted power.

 

[MikDee]

Re: 350 Engine advice

Re: 350 Engine advice

Just a few points to ponder, the 305 is actually 3.48 bore, and 3.48 stroke, (tiny pistons, & the 350 crank), Yet the early pre-vortec 305 puts out 230hp, and the pre-vortec 350 puts out 260hp and I'm pretty sure that's with the same external equipment, camshaft, & compression ratio, But with bigger valve heads, & almost 50 cu in more, that's only a 30hp increase, and they both are made to reach full hp between 4200-4600rpm WOT.

Uraijit, & MBAKER, have the right idea, Vortec heads carry a lot of baggage to adapt, over the other aftermarket heads, but "whatever floats your boat"

I see this all the time over in the S-10 Forum, the smallblock spinoff 4.3V6 (which incidently has the same 4.00" bore& 3.48" stroke as the 350, a bigger bore then the 305 ) has these same issues with a Vortec V6 head swap, by the way the max lift on the Vortec heads is about .480" from what I hear, more then that you need machine work, OR, they make beehive springs, & thinner aftermarket retainers, to give you much more lift, but it just costs more $