Page 1 of 3

"290HP" GM crate 350 details?

Posted: Wed Oct 18, 2017 11:04 pm
by KnightEngines
Got a ski boat 350 in, it's shot to hell.
Gonna grab another 350 crate, throw some flat tops & a cam in it, pocket port the heads & stick a performer & 650 holley on it.

Before I get it all in a few details would be helpful to figure out how far into it I gotta get:

Where's the stock deck heights on these things usually at?
How good is the deck finish (good enough to run .015" steel shim gaskets?)
What's the piston weight roughly?
How bad is the stock valve job?
What are the valve springs like?

I'm hoping to be able to pop the heads off, swap out the pistons for flat tops of similar weight (to avoid balancing), use thin head gaskets to close up the quench, mill the heads to get it to 10:1, pop a suitable hyd cam in it, touch up the valve job (just with a stone), tidy up the bowls & short turns, cut the guide tops for more clearance & screw it back together without thinking too hard or wasting time or $.

It's for a good circuit race customer, it's his family ski boat, trying to get it back on the water with a bit more grunt as cheap as possible.

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 12:24 am
by pdq67
The old original, L-48, 350SS engine was right at 10 to 1 with flat-tops, stock .020" thick or so steel shims, a DITH piston of right at .025", and 64 cc heads so watch your CR closely.

I forget if the pistons were 2 or 4 notch jobbers though. Don't forget that a lot of the, "rebuilder", pistons are more like .045" to .050" DITH.

Add a 2101 Performer and a 750 Q-Jet along with a good old CC 260HE, or maybe the 268HE hy-cam and go.


Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 12:36 am
by KnightEngines
It's going to be a goodwrench crate, the cheap ones - that's all I can get easily!

Depending on deck height I reckon I can get 9.5:1 out of it with flat tops, thinner gaskets & milling the heads down a bit.
Got 1.6/1.5 ratio rockers for it from the customers parts pile, that can't hurt.

We'll top it with a performer intake & a 600 square bore - again from the spare parts pile.

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 12:55 am
by 68post
Why not start with the better 350 vortec roller cam crate (because - no fuel pump mount ?) ?? Two versions - 2 or 4 bolt and I think that is only a fifty $ diff. !
It has several pluses.

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 1:06 am
by KnightEngines
Coz I'm in Australia & the 290/350 cheap ass crate is the only thing I can get easily!
Even that is costing $4200.

Still cheap compared to fixing the old donk, heads were cracked, 2x bent rods, block rotted. The only things that could be saved is *maybe* the block & the crank, rest is scrap!

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 1:17 am
by 68post
That's unfortunate, but sounds like you have a plan. Should be a big improvement with your upgrades.

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 1:21 am
by KnightEngines
Got some info back from my supplier, it's actually a GM12499529, not a goodwrench.
*Should* have flat tops in it already & 8.5:1 comp rather than 8:1.

Fingers crossed, then I don't even have to take the pan off, .040" off the heads & .015" gaskets should get me to 9.5:1.

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 3:06 am
by F-BIRD'88
On that motor and for your boat purpose the cam will work better if you advance it a bit.
factory it is 1deg retarded 115-113 centers 114 LSA... You want to advance it to a 107 to 108 intake Centerline 107-121 centers have moved it 8 degrees and it is now 7 degrees advanced.
it will have a better torque curve for a boat and perform Better.

The 350-290HP heads stink. but you can make them better by agressessive porting. Get in there and GET serious.
If you do you can get the sub 200cfm flow up to 230 ish 180 ish ex even with the stock valves.
A high perf multi angle valve job is a bonus. But do not use super narrow drag race motor valve seat widths as it is a boat motor.

ya you can flat mill the heads 76cc to about 70cc. The pistons are -15cc dish and usually .025" or a bit more below deck at TDC.
the cr wil still be lame even with flat tops added and a .015" head gasket

it will be 8.67:1 with shaved 70cc stock heads and a .015" gasket

it will be 9.59:1 with shaved stock heads and flat tops and .015" gasket.

Find a 64cc high perf head to port for this....way better result.
A popular common 083 casting is just right Port it agresively and it will rock
These were stock on some mercruiser 350's
Also found on GM TPI L98 350's camaro and corvette.

A 062 or 906 vortec head is a GREAT CHOICE too.

With a 64cc head and stock pistons and .015" gasket the cr will be 9.1:1
Shave the heads to 61cc and the cr is 9.5:1 with .015" gasket.

you will get a better end result by starting with a 64cc head and spend less money.

The crate motor 224-224 .450-.460" 114LSA cam is not bad and gets better with re phased for the boat.
107-121 centers as above.

A common 224-234 .465 .488 112 107-117 cam is a good one too for this.
Crane HMV278-2 278-290 222 234 114 109 119 .467 .494 is a good one too.

REPLACE the stock valve springs with a common high perf Z28 spring or like same
Even that stock cam gets better with these as the stock springs are lame above 4800rpm.

I'd just buy the short block base LM-1 350 short block motor version.
it is the same motor minus the high perf .450-.460 cam and Crap heads.

Buy your own 224-234 112 .465-.488 cam for LESS and get 64cc heads for it and BUILD A BETTER boat motor for LESS.

You will save enough to buy NEW 062 or 906 vortec heads for it and use the stock pistons.
The boat will go a LOT BETTER with vortec heads.
This is why mercury Marine Mercruiser uses them on their 350's.

The 350-290 long block crate motor is POOR $$$ performance value for this.

The .015" steel shim head gasket either seal up or it don't.
Install clean and dry on clean dry surfaces. NO SEALER or any kind is needed.
retorque the head bolts after initial run in.
I have never had one fail in 40 years on production and USED far less than ideal decks.
The $25 GM GMPP cylinder head bolt kit is great for this as the smaller hex head allows restoring the heads
with the rockers on.

The ONLY sealing required is some black permetex on the head bolt threads.
Again NO SEALER on the .015" head gaskets is needed NONE ever.
They either seal up or they don't.

if you detonate KNOCK PING the motor any ALL head gaskets will fail.

If you do shave the heads,,,, any head and get a new surface ANY ALL head gaskets seal BETTER
reguardless of thickness. The block decks can be ugly but the head decks have to be good.
This is true for any all SBC head gaskets. No sealer is ever needed on a SBC head gasket.

Again find a 350 core to rebuild or by the GM 350 short block.
Find or buy some 64cc high perf heads and rework these.
EG: the mentioned "083" castings.

L-31 350 VORTEC heads are GREAT on these motors. 062 906 castings.
Require a vortec intake use a RPM style hi rise intake reguardless.
use a 750 cfm carb on it. ANY 750cfm carb. Even the stock Mercruiser QJET carb is GREAT on this.

You will give the customer a great performing high perf 350 sbc boat motor, with confidence and make motor money too.

Keep the real compression ratio under 10:1 for Marine pump gas.
10:1 + is TOO MUCH for a marine motor on marina pump gas. its not a street cruiser.
it is a boat motor.

if you build your own short block go +.001" extra on the piston to wall clearance.
( marine cold block cold water temp with hot pistons needs a hair more Piston to wall clearance than a street motor) +.001" looser is JUST right for a boat motor.

DO not build with excessive compression ratio. The motor will fail on marine pump gas and you will redo it for free No profit in that. The Power is in the heads, cam and HI RISE intake and 750-800cfm carb,

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 6:02 am
by KOLI11
What I did when I had to put a new engine in my ski boat, is that I purchase remanufactured 350 vortec from Eagle spares located in Dandenong Victoria. All I did was put a new intake manifold on it, and rest from the old set up. I have had it 3 years now, and 300 hours as I do a lot of skiing. It is absolute ripper motor. I put a edelbrock carburetor on it, best carby for marine application hands down and dont need a choke. However, make sure you coat the edelbrock carbs as they are prone to corrosion. I learnt this from an old marine mechanic who had been building ski boat engines for about 30 plus years. They used the original cam that cam in the trucks, if you would like more performance then get yourself a cam with about 212 at 0.05 duration and use a single pattern to maximise the torque.

One thing that suprise me is that it only use 10 litre per hour while skiing, now that is a cheap ski ride.

The 290 hp engine is a donk in boats. I have heard a lot people using it and not happy with the down low torque. The cam has a duration of 222 @ 0.05 which will destroy the low end torque. Ski boat needs all low end torque needed.

Good luck

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 6:50 am
by Carnut1
I think it is funny Tony asking for advice. If you get the better engine with the flattops what I would do if I had your shop to be cost effective would be flat mill, bowl hog, touch up vj, aggressive burr bowls, ssr, open pinch a bit, back cut valves, reassemble with .015" steel shim. Heads should go from about 205 cfm to 235-245 cfm probably 265cc ports. A half set of 1.6 rockers on intake just for fun. Thanks, Charlie

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 7:17 am
by Truckedup
Living up here by the big lakes I know guys with boats....Number one problem with modified engine appears to be detonation damage...A load of bad octane fuel and the pistons turn inside out...Might make sense to keep cylinder pressures on the conservative side ......

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 7:37 am
by DaveMcLain
I think I had one of those come through the shop about a year ago. I built a 350 Chevy for a guy with a ski boat and he gave me the old engine that he took out of the boat with terrible freeze damage to the block. Everything else from that engine looked like brand new, perfect crank, rods, pistons as well as heads. Everything looked very normal for a 2 piece rear main seal 350 Chevy but the heads were interesting. They were a 624 casting but they were built using all of the Vortec hardware, rockers, valves, retainers etc. The pushrod holes were round and not slotted too.

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 9:20 am
by KnightEngines
Charie - I'm pretty much just wanting to know what to expect when I tear into it to avoid frustrating suprises. I can make it go no probs, but I'd like to know more about the starting point.

Boat runs water cooled log pipes - gotta be careful with cam.
It'll never see over 5000rpm.
Stock heads with a little rub will be fine - last motor was a flogged out goodwrench that would have been lucky to be making 200hp.
If I can make 330-360hp the owner will be giddy.

Fbird - you dan get cheap votech heads no probs, but I cant! - they were never fitted to anything sold here & imports are minimal. Junkyard 350's for $200 dont exist, you'll pay $700-1000 for a good std bore 4 bolt bare block. Machining & parts would push the price of a short motor past the price of a complete crate engine.

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 10:07 am
by autogear
The 260hp crate engine has a smaller cam (essentially the 300-horse cam) and is usually cheaper than the 290hp crate. Something to think about perhaps.

Re: "290HP" GM crate 350 details?

Posted: Thu Oct 19, 2017 10:38 am
by Keith Morganstein
Expect a crude mass production valve job that usually seals up good enough.
Pistons .020" in the hole.
Nothing fancy typical 70's / 80's stock passenger car stuff.