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BBC 502 600hp jetting recommendations

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BBC 502 600hp jetting recommendations

Postby gasman5.0 » Sun Mar 13, 2011 8:10 pm

A buddy of mine has three BBC 502 500hp in a 38' boat. He pulled the motors with 500 hours on them had them freshened up {rings and bearings} and also installed Elderbrock's POWER PACKAGE TOP-END KIT #2094 which is said to make 600+hp. The kit consist of 77489 Victor Jr. 24° Rectangular Port Cylinder Heads, 2902Victor Jr. 454-R intake manifold, 2262hyd. roller cam
Duration at 0.050" Lift: Intake: 236° Exhaust: 245°
Lift at Cam: Intake: 0.368" Exhaust: 0.376"
Lift at Valve: Intake: 0.625" Exhaust: 0.639"
The motors have 850 holley carbs 4150 models on them and he has no idea where to start jet size wise. The carbs had some strange jetting "to me any ways" primary were 73/76 ...secondary 92/95. any suggestions on where to start.
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Re: BBC 502 600hp jetting recommendations

Postby In-Tech » Sun Mar 13, 2011 8:24 pm

Amazingly enough, those 500hp carbs actually work pretty good even after engine changes. Start with the stock Merc jetting, chances are you'll find it needs more primary jet and less secondary and a 45 pv if they are not already. You need to put wide bands AFR meters in the boat for tuning, remove afterward. I assume it still has the cmi headers? Make sure those heads have inconel exhaust valves too.
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Re: BBC 502 600hp jetting recommendations

Postby gasman5.0 » Mon Mar 14, 2011 7:37 am

What's the deal with jetting being spread on the same metering block? The few 4150 automotive carbs I have messed with always had two main jets same per a meter block. I'll be sure and pass this info to my buddy, I told Him I really didn't have any idea about a marine motor carb jettting but he was lost so I suggested pri 82/sec 90 with 4.5PV for a starting point? I have a WB-O2 I can loan him.
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Re: BBC 502 600hp jetting recommendations

Postby Patriyacht » Mon Mar 14, 2011 8:05 am

Jetting comes staggered from Mercruiser. Apparently they think it gives better mixture distribution.
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Re: BBC 502 600hp jetting recommendations

Postby gasman5.0 » Mon Mar 14, 2011 8:53 am

Patriyacht wrote:Jetting comes staggered from Mercruiser. Apparently they think it gives better mixture distribution.

I can't imagine this true, Unless Merc was to dyno tune each motor. All three carbs had the same jets and staggred the same.

How does pri 82/sec 90 with 4.5PV sound as a starting point?
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Re: BBC 502 600hp jetting recommendations

Postby EthylCat » Tue Mar 15, 2011 9:50 pm

Here's how I do it.

600hp=about 300lb/hr fuel
300lb/hr fuel=3510 cc/min fuel
3150cc/min fuel divided by 4 jets=787.5 cc/min per jet

My jet flow chart says that a 83 jet flows 780cc/min on the low side 795cc/min on the average and 811cc/min on the high side

So, that would be 83 square. But, you want this thing to idle and cruise leaner than that so you add a PV.

A PV is worth around 6-8 jet sizes so that would give me approximately 76 fronts with the Pv and 83 rears .This would be for something around a 12.5 afr Boats may want to run richer than that. (according to the original jetting, I would assume so)

A 93 jet flows 1250cc/min according to Holley, so your original jetting is equal to 350lb/hr fuel and 700hp@12.5afr

If you go with an afr around 11.7:1 you need about 360 lb/hr fuel. This would explain the original jetting numbers and spread.

A clean idle and low cruise(76 fronts) yet extremely safe rich afr(93rears) when duty calls and you need to show off for the girls.

If you want a happy medium 325lb/hr go with something around 78 fronts and 90-91 rears.

Good luck.
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Re: BBC 502 600hp jetting recommendations

Postby Patriyacht » Wed Mar 16, 2011 7:40 am

gasman5.0 wrote:
Patriyacht wrote:Jetting comes staggered from Mercruiser. Apparently they think it gives better mixture distribution.

I can't imagine this true, Unless Merc was to dyno tune each motor. All three carbs had the same jets and staggred the same.

How does pri 82/sec 90 with 4.5PV sound as a starting point?



Check a Merc. parts book. This is well known in offshore boat circles
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Re: BBC 502 600hp jetting recommendations

Postby gasman5.0 » Wed Mar 16, 2011 9:43 am

Ranken Tech wrote:Here's how I do it.

600hp=about 300lb/hr fuel
300lb/hr fuel=3510 cc/min fuel
3150cc/min fuel divided by 4 jets=787.5 cc/min per jet

My jet flow chart says that a 83 jet flows 780cc/min on the low side 795cc/min on the average and 811cc/min on the high side

So, that would be 83 square. But, you want this thing to idle and cruise leaner than that so you add a PV.

A PV is worth around 6-8 jet sizes so that would give me approximately 76 fronts with the Pv and 83 rears .This would be for something around a 12.5 afr Boats may want to run richer than that. (according to the original jetting, I would assume so)

A 93 jet flows 1250cc/min according to Holley, so your original jetting is equal to 350lb/hr fuel and 700hp@12.5afr

If you go with an afr around 11.7:1 you need about 360 lb/hr fuel. This would explain the original jetting numbers and spread.

A clean idle and low cruise(76 fronts) yet extremely safe rich afr(93rears) when duty calls and you need to show off for the girls.

If you want a happy medium 325lb/hr go with something around 78 fronts and 90-91 rears.

Good luck.


Thanks for the info and taking time to explain. =D> That's what I love about the place...there is usually data or a method to responses. I'll suggest to him the 78's / 91's as a starting point and tune from there. I talked with my buddy and the original jet numbers that I posted were incorrect... it had these jets {Pri-75/78}{Sec-89/93} . Looks like his orginal 500 hp jetting matches very close using your formula for 600hp. He mentioned the boat owner wasn't really concerned with top end but more crusing although I know they will run WOT a few times.
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Re: BBC 502 600hp jetting recommendations

Postby EthylCat » Wed Mar 16, 2011 12:49 pm

Yes the jetting seems the same but we are still dealing with two very different air fuel ratios. Same jetting = a very rich 500hp or a leaner 600hp with a lot less room for error. Do these engines cool with lake water or are they closed systems?
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Re: BBC 502 600hp jetting recommendations

Postby gasman5.0 » Wed Mar 16, 2011 1:58 pm

It's an open cooling system. If I use .65 BSFC 500hp I come up with 3412 cc/min and the factory jetting using my chart is roughly 3398 cc/min. perhaps marine motors of this type use a much higher BSFC than your typical hot rod motor. When using .65 BSFC 600hp = 4095 cc/min does this sound correct?
Last edited by gasman5.0 on Wed Mar 16, 2011 2:52 pm, edited 1 time in total.
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Re: BBC 502 600hp jetting recommendations

Postby F-BIRD'88 » Wed Mar 16, 2011 2:34 pm

Take one of the three upgraded motors and dyno test it to dial it in.
Preferabily with the marine manifolds installed.

The stock default mercruiser staggered jetting will likely be way off with the new single plane manifold.

Think of the cost of rebuilding 3 damaged expensive 502 BBC's from incorrect carb set up as compared to a dyno session.
The new engines combo will want a different timing curve too.
It will need much more initial base timing at idle (to idle correctly) and quite possibly less timing at WOT high rpm. The mercruiser ignition won't be right for the new combo.

It will save you time, money, wasted fuel etc etc by dyno testing this new combo compared to trying to attempting to get
($$$$ 3 motors $$$$) dialed in out on the lake. 1 motor is enough to deal with. How will you keep and eye and ear on three? It ain't going to happen.

The carbs may need more than just rejetting to get them right. (are the air bleeds staggared? R/L ) Do the venturri boosters have (staggered) distribution tags or reliefs ? Flip a carb over and look.
If so, what worked for the stock 502-502 dual plane manifold will not work with the racey single plane manifold. Dial it in on a dyno with a proven dyno carb and then make the marine 850 carbs run the same.

The new combustion chambers are smaller than the origional. The compression ratio is now higher.
May/will need better fuel. Head Chamber to piston dome clearance and VTP clearance must be verified.

The fuel flow of the 3 upgraded motors is now a lot higher. A fuel system delivery capacity check is required. Starting at the in tank pick up tube(s). A upgrade may be necessary. If you short these new more powerfull engines on fuel you will melt all three in a matter of seconds, out on the water.
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Re: BBC 502 600hp jetting recommendations

Postby EthylCat » Wed Mar 16, 2011 3:02 pm

Your math is correct.

Take note as well to what F-Bird is saying. There is more to this combo change than switching jets.

I'd tell you to bring one over and we would strap it to the DTS, but that's a long haul.
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Re: BBC 502 600hp jetting recommendations

Postby gasman5.0 » Wed Mar 16, 2011 3:20 pm

F-BIRD'88
The boat owner had the motors built by a reputable speed shop according to them. My buddy is a close friend of the owner and he pulled the motors and is the one who is installing them and doing the labor & carb work. I know he doesn't have a clue about jetting carbs or reading plugs. I agree that it would be well worth the money to run the motors on a dyno first considering what a pain in the a$$ it is to install/remove them. I personally have no dealings in this deal other than being asked about jetting. He called me and said he was going to go up on all of the jets by 9-10 sizes. I said what....10 sizes! Wait.. let me check into marine jetting I do not have a clue about marine engines.
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Re: BBC 502 600hp jetting recommendations

Postby F-BIRD'88 » Wed Mar 16, 2011 3:28 pm

This not near the same as a street motor or drag car. Advise your friend that you don;t know the exact best jetting and that he is crazy to risk 3 motors by guessing out on the water. Lead him to a dyno shop preferabily with HP marine engine testing capability and then tell him you want nothing to do with it.

Having been in your exact position on the same issue in the past, this is your best move to help your friend.
When your friend guess's on jetting out on the water and tries to feel his way thru it, $$$$ it does not end well $$$ X3

I have a good friend who found this out the hard way in a very similar senerio on twin SBC's. There was no telling him.

From just the limited info on the carbs It don't look good.
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Re: BBC 502 600hp jetting recommendations

Postby cfm » Wed Mar 16, 2011 5:54 pm

F-BIRD'88 wrote:If so, what worked for the stock 502-502 dual plane manifold will not work with the racey single plane manifold. Dial it in on a dyno with a proven dyno carb and then make the marine 850 carbs run the same.



The Merc HP500's came with a Dart Single Plane intake manifold.

As Patriyacht mentioned the carbs come with staggered jetting.
By my notes they came with the following:
Starboard Primary = .082" = #75 Jet
Starboard Secondary = .105" = #93 Jet

Port Primary = .093" = #81 Jet
Port Secondary = .104" = #89 Jet
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