RPM loss on engine rebuild
 

I rebuilt my 420 HP engines and noticed a RPM loss on the top end. It went from 5000 to 4600 RPM. In doing the rebuild I replaced the stock 099 GM heads to the Dart iron eagle heads and switched intake manifolds from the stock Dart single plane to the Edelbrock RPM dual plane intakes.
Using Crane hydraulic roller rockers 1.7
I have a the thunderbolt ignition and timing set to 30 or 32 advance. ( I don't remember )
I have a crane cam (Intake 359) &( Exhaust 372)
Prop is a Hydromotive quad 4 22 pitch.
750 Holleys
Gill Exhaust
Compression is great on all cylinders.

How much does the advance in timing effected the RPM loss in this situation. If I increased the advance to 34 what effect on RPM would it have?

Thanks

I would definately increase the timing to 34*.

Which Crane cam do you have??? Those cam numbers don't make sense.

Here is a link to the cam specs:

http://cranecams.com/index.php?show=...41&lvl=2&prt=5

Did you degree the cams in ? where did you set them ?

Degree of Cams -not sure. Left the bottom end untouched. Just upgraded the top end.

Sorry I thought you changed cams. What size intake runner on the new heads

308CC intake runner
129CC exhaust runner
Intake Valve Diameter 2.250 in.
Exhaust Valve Diameter 1.88 in.

The 2.25 valve may be acting like more cam if the stock heads were 2.19. I would try 34 degree of timing like Griff said.

Agreed, on timing that'll buy your some ponies and crisp it up- but listen carefully for detonation.

Well....as long as you degreed and checked everything carefully and everything else is up to snuff,

- id go to the heads and the manifold combo as the reason dual planes usually sacrifice a bit on top for midrange and low end as compared to a single plane. Not so much so in a really high output config 600+ but in a lower HP config this has been true for me.

sorry I dont know your head specifics I've been running Brodix heads the last 20 years almost exclusively, the head/cam/ intake combo is a huge factor in a engines performance

Has your combo been independently tested?
or Did you get a bunch of good used parts here, and

How did you select this combo of parts?


Uncle Dave

My engine builder said I should have used the Brodix hv2000 in stead of the Edelbrock RPM Performer Manifold. Would switching intake manifolds really give me an extra 800 - 1000 RPM?

No way.

listen to your engine builder

Most people run 34-36 with a set up like yours. It's best to use a piston stop to find TDC. On engines with a lot of use, the ring on the dampener can move. Iron Eagles do flow well and should make more hp. I would go back to that Dart intake as well.

Did you ever bump the timing up????

I agree, go back to the Dart intake. The Edlebrock may make better torque down low, but will loose to the Dart in the upper rpms.

ck make sure that throttle opens all the way...you never know..Rob

Another thing to keep in mind, given your cam and components, your engine probably makes it's peak hp in the 5600-6000 rpm range. Re-propping or re-gearing seems like a good idea.

Fixx
 

you have too much cam for your engine,,changing intakes is not going to help at all!!!!! you will still be at the same rpm in the big end.. before you change anyting try putting the old carbs back on..you went from 800 cfm carbs to 750 cfm when you did all the up grades..bump the timing and try the old carbs first..

check your spark plug's after a hard run,,i bet their white...what jets are you using in the carbs?? in the 800 cfm carbs i think the stock jets were 72-84?..you put in a bigger cam and better flowing heads,, the engine will need more fuel which i think you may be lacking. my guesstimate would be a 850 cfm carberator would put you @ 5200 rpms and which is the max rpm for that engine..

I always kept the 750 Carbs never went to 800 or 850.

Fixx
 

is the list # on the choke horn 9022? did you jet up?

I agree. With that cam, I would prop for 5500 RPM. I would prob try 34 degrees total timings, or maybe 36. Of course make sure you are jetted properly. 420 mercs should have the 800CFM holleys if they are the original carbs. I believe stock they were 81's in the primary, 91's in the secondary, 6.5 power valve in the primary side. With your heads and cam, and turning 5500+rpm, you may want more jetting than a stock 420 merc. Of course the air bleed size comes into play too. Plug readings can be tricky with todays fuel.

PS, I'd go back to the dart intake. But it wont give you 1000 RPM. However, bolting on a supercharger might!

I have a Holley 750 double pumper with 30 & 50

OThe size of the Accelerator pumps is irrelevant. Jet sizes, air bleed info, and powervalve setup is what would give us more info. The list number off the airhorn would be good too.

Carbs
 

This is a link to my carb
http://www.holley.com/0-80528-1.asp

Nice. BUT WHAT SIZE JETS ARE IN THEM

How do I find that out?

If the jets have never been changed by somebody that does it regularly you will have to remove the fuel bowls and look at the numbers on the jets. They are stamped into the brass. Be mindful that you will probably need gaskets if they havent been off in a while. If you have had Jim or another mechanic ever do carb work they sometimes write the jet and powervalve size on top of the bowl.

Good luck!

Fixx
 

you need to do a run with the boat to determine if the jets need to be changed which they probably do before you pound the intake valves out of the heads..keep in mind that you can only go x amount of jet sizes before you have to raise the cfm on the carburetor so the engine will achieve the correct air to fuel ratio..

From the way it sounds you may want to have a experienced tuner tune them before you grenade the engines.

The jets are 73 front and back and power valve is 6.5.

Im gonna say that is no where near enough fuel/jetting for what you have. Especially if there is only a power valve on the primary side, and not the secondary side.

If it were me, i'd put 82's in the primaries, and 92's in the secondaries, and keep the 6.5 power valve, then go for a run and do plug checks. You may end up wanting a 4.5 pv with that large cam. But get some bigger jets in that thing.

Fixx
 

if you have bees running your boat like this i would als recheck your valve adjustment..pay close attension to the intakes..if thehe tip of the valve stems are higher then the exhaust you might want to look at intake valves..

I'v been running the boat like this for two years with no problems or issues. I just have low RPM of 4200. So you guys think my problem is related to my carbs not feeding enough fuel?

CARB TO SMALL IF ITS A 750? IF I was you have a engine builder check your valve train to make sure its right, thats a cam guys use to up grade there from hp 500s to 600 plus hp. put the darts back on timimg set at 34 to 36 mine is 36 on my 650hp set up. send carbs to nickerson or CFM tell them what cam and heads intake they will set carbs up. like I went from a isky cam a lil bigger, dart intake,merlin marine hrads 310, dry headers carbs tried three a 850 BG made 637, nickerson 1050 made 650hp last is a custom cfm and intake 670hp. thats with a 509. so you should be at 550hp anyways with the right carb and make sure the cam was degreed you loose hp there. and the merc 454 420hps had 800 holleys with out being on a dyno its harder to get it all. I dynoed my motor every winter we made changes then fine tuned jets out on the water. my 850 was 86/86 and 6.5 pv all around. hope you have not leaned your motor out running like that. oh my red line was 5800. Iwould think you would want to run it about 5600. with a 24 ,25 p prop when done. good luck