My Volvo/Mercruiser Thunderbolt setup

[Speakrdude1]

Hey All. I had to re-register my screenname for some reason. My old password wouldn't work any more. I tried over and over to get the "email me my password" thing, nothing, nada. So, same screen name with a 1 added. Now I am a "New User" again!

So anyway, I have added the Thunderbolt IV ignition system to my 70's model GM 350, 10:1 compression engine. Just in case the Mallory electronic (still with Mechanical advance timing) was causing part of my lean condition.

While I have the base timing at 700 rpm's at 8 degrees BTDC, the max advance does what it is suposed to do which is max advance of 32.

When I set the timing to 8 degrees BTDC at idle, it really wants to slow the idle speed down, hopefully its not laboring to much.

But it is a tiny bit slow to start when warm. Other than that, it runs great. Real crisp acceration. Good hole shot, for a 6000lb boat anyway.

Should I leave the timing alone now and focus on the idle mixture? I had it set pretty good with the old Mallory ignition.

Is there any possibility that the old 70 model GM 350, with high comp heads, flat top pistons (truck engine, I believe) needs 10~12 degrees at idle and a max curve of 32 or less?

Maybe one of you more experienced guys know of some older specs.

Thanks
Jim

 

[Don S]

Re: My Volvo/Mercruiser Thunderbolt setup

Don't recall any posts about a lean condition. What is it doing? What carb do you have?

 

[John_S]

Re: My Volvo/Mercruiser Thunderbolt setup

If you do need to go to 10 degrees initial, you will want the total to stay at 32. No adjustment in the thunderbolt modules. The V8-22's are common though (22 degrees advance), and are easier to find used.

Your compression ratio is on the high side for a heavy boat. I assume you are running 93 octane.

 

[Speakrdude1]

Re: My Volvo/Mercruiser Thunderbolt setup

Hey Don, Glad to see you still here.

I've removed and replaced #7 intake valve now twice and #4 once. I verified my timing each time but with my Mallory electronic, mechanical advance I couldnt be sure.

Each time I would find a spark plug less than finger tight on that cylinder. I was thinking I wasnt getting the plugs tight enough and they would work themselves lose, thereby causing the lean condition and causing the intake valve to "Tulip" and 0 compression was the result on that cylinder.

But, after speaking with the machine shop, the thought it was probably due to the High Compression heads and lean fuel condition causing the heat build up in the clyinders. The end result was a Tuliped valve and a loose spark plug. They also instructed me on the proper direction to install washers on the "old-style" plugs. I did have a real-bad stopped up racore fuel/water seperator. It actually died on the way back to the dock it was plugged up soo bad.

Ok, that was then, this is now,

After 4 weeks of the latest valve replacement, the correct washer placement on the plugs, rebuilding carb (quadrajet) replacing jets with 74's / metering rods(had 70's) veryfying no intake leaks, etc, so on..

I fast idle around alot, take up on plan for a bit, back down, idle some more.
Next 3 weekends, check all the plugs for tightness, all tight. Check firing by removing one plug wire at a time, all good. I have purchased some lesser expensive fuel filters, drain and replace weekly. Almost all crud free now.

Last week I replaced ignition with Thunderbolt just in case. Runs good, but concerned with the 8 degrees. Dont wanna labor engine but dont want to max advance to much either.


Have been running regular octane gas, no noticeable pings.
So far so good, just do not want to remove a head again.......

Any suggestions?
Jim

 

[Don S]

Re: My Volvo/Mercruiser Thunderbolt setup

If your filter was all plugged up, then that will sure lean things down at max load/rpm. While you may get the proper WOT rpm, you could be just BARLEY getting it, thus the lean burn and tuleped valves.
That's why I try to avoid raising compression ratios, upping cam performance etc. on an old style 350. Especially when that single engine is pushing a 28' boat. The engine is on the edge to begin with.
Not sure about putting those washers on the sparkplugs. Sounds like a patch used to cover a problem, not fix the problem.
I would take it for a good run, shut it off and read the plugs and see if they show lean, overheat, or whatever, and adjust from there. But I would sure remove the washer from the plugs. That also reduces the depth the plug reaches into the combustion chamber.

 

[Speakrdude1]

Re: My Volvo/Mercruiser Thunderbolt setup

Do the plugs not require those washers? Remember, these are the old style heads. No taper holes. I didnt think I could use those without them.

 

[Don S]

Re: My Volvo/Mercruiser Thunderbolt setup

Oh, Ok, I thought you meant you were installing washers on plugs with tapered seats.............. Disregard.

 

[Speakrdude1]

Re: My Volvo/Mercruiser Thunderbolt setup

So Don, Just so I dont melt something too soon, how long do you recommend that I run it WOT to check the plugs for? And, should I just pull a sampling of them or all of them?

I should be able to get 4~4200rpms. The last I checked with my GPS, I could hit 32mph at 3800 rpm's.

 

[Don S]

Re: My Volvo/Mercruiser Thunderbolt setup

You better be getting closer to 4600 rpm. 42 to 4600 is book, and the closer you are to 4600 with that big a boat, the better off your engine will be. You are just flat overloading it at 4000 rpm max. Speed means nothing, rpm is engine loading.
I'd run it for maybe a half hour to an hour at cruise speed. That should make the plugs show how they are going to be. Shut it off and drop the anchor, don't idle around for too long, or the plugs will show it if it idles rich. You want to see them after a run, that's when they go bad. Then pull the plugs and look at all of them. They should all look ok, but don't forget, you was only loosing 1 plug, not all of them.

 

[Speakrdude1]

Re: My Volvo/Mercruiser Thunderbolt setup

So I may be over-propped as well? Good thing I have another boat, same size, same year, in my shop. Best thing about it is its all factory. Think I will trade props.

 

[EricR]

Re: My Volvo/Mercruiser Thunderbolt setup

Are you sure that thing is actually a calculated 10:1? If so I would think it a detonating monster! I would not run such high CR in a marine application with iron heads. Even the hot rod dudes will tell ya 9.5:1 is about has high as you want to go on pump gas with iron heads.

I have an S10 with a flat top pistoned 355, I run a fairly large overlap cam (224/230* @ .050") and I swapped 76cc heads for 58cc's. My pistons (flat tops) were .045" in the hole and I used .039" compressed thickness head gaskets. With the 76cc heads I had 8.34:1 and now I am at 10:1.

Now I picked up nearly half a second off my ET in the 1/8th mile but it sure can't be used to tow my boat on a warm day anymore! I cut back from 37* total to 32* (HEI) and it still spark knocks bad under load. I am going to shorten the slot on the vaccum can to limit that under part throttle, and I am running 93 octane. This is in a 3200 lb shortbed truck with 3.73's.

I have a 400 shortblock that came out of a jersey skiff. They had planed the heads to the max, that thing was at about 10.5:1, and run on mid grade marina pump fuel. It broke the ring lands off four of the 8 pistons, and blew a head gasket in such a way it took a chunk out of the deck betewen two cyls. The piston crowns are peppered with slag from the detonation.

Now I know I am new to this forum, you have been around a long time Speakerdude, and have probably built a lot of small block chevies but man I do not think 10:1 in a 6000lb boat is not going to give you longevity!

I would cut back the CR, and stroke it to a 383 to get the same result.

 

[Speakrdude1]

Re: My Volvo/Mercruiser Thunderbolt setup

Thanks for you input on the compression ratio Eric. Believe me. The high compression part of this restoration (resurrection, according to Don) was not intentional. The boat came with the 70's model Chevy Truck Frankenstein block. All I did was take to the rebuilders and have them rebuild to marine specs, cam etc. I had no idea it had high compression heads untill I started having a few issues.

Now I am mearly trying to get it to perform without melting. Max horsepower in the 1/4 mile are not much of an issue here.

This project is just about done and I have already started on the next two. I need this engine to be reliable for the next owner. I just need to be able to idle without fouling all the plugs and causing that nice black ring around the waterline and a simple crusing speed at 3800 RPM's with a max rpm of 4200~4600 without "Tuliping" Intake valves or any other self-destruction.

If you have any simple ideas that could help without pulling or rebuilding, that would help. On my last incident with repairing the Intake Valve for the 3rd time, I almost swaped out the hi comp heads for some normal ones but the rebuild shop talked me out of it.

Jim

 

[John_S]

Re: My Volvo/Mercruiser Thunderbolt setup

"If you have any simple ideas that could help without pulling or rebuilding"

Run 93 octane pump gas. I can't see how 87 would not be giving you problems. With the boat engine behind you and enclosed, you don't hear knocking or pinging as easily as a car. I wouldn't switch back to lower octane, until you determined it was something else.

Your actual dynamic compression will be less than 10:1, because the intake valve doesn't close, until part way through the compression stroke. At least that is the case with the standard hyd marine cam with stock valve timing. Probably still true with a "standard" truck cam. That doesn't get you out of the woods though. Just means the "standard" advertised marine 9:1 and 9.5:1 (vortec) ratios are actually lower.

 

[Speakrdude1]

Re: My Volvo/Mercruiser Thunderbolt setup

By-the-way, It was rebuilt with an RV cam for Marine use, If that makes matters better.

So how much does 93 Octane gas at the Marina cost now-a-days? Hmmm....125 gallons x $4.50 = $562.50 ouchhhhhh