454/420hp REBUILD Compression
 

I'm embarking on re-building my 454/420hp motors. I will be using the 990 iron GM heads having the bowls and ports re-worked and using Gold Crane Rollers. ALSO, Bore notching the cylinders!

Before I finalize on a Cam, I want to have the heads flowed to match the cam selected. I intend on using a Hyd Roller set-up . Probably around 575 - 590 lift and mid 230 duration with @ 112-114* lobe separation. I believe this cam would be similar to the hp500 roller cam.

Other components include:
Dart HP500 intake
CMI e-Top Headers
800cfm Holley Marine Carbs (mechanical secondaries)
Crane or MSD ignition systems.
Forged Pistons w/ Balanced Crank assembly

Is 9.5 : 1 too high?.

My engine builder suggested going as high as 10:1 knowing I will be filling up the boat on the trailer using 93 octane!
However, that scares me unless I carry 5 pints of 104plus when 89 octane marina gas is used.

Re: 454/420hp REBUILD Compression
 

Mtgbiker,
It has been my experience that one of the biggest mistakes a marine engine builder can make is “chasing” compression ratio. In your application, the difference in round figures for a full point of compression will net you approx. 3-4% power. That’s 20 HP on 500 and 10 Hp per .5 point. Arguments are made regarding SCR vs. DCR, IVC angles, etc. but until you pull it at max VE/peak torque you wont know the threshold of detonation. Point is, there’s lots more HP to be made elsewhere without the consequences.
Bob

Re: 454/420hp REBUILD Compression
 

Bob, I emailed you about 60 days ago and you suggested intake wedges, port and bowl work for these heads.

I am in complete agreement with you about creating safer hp. The bottom line is that my builder will honor me as the customer when I tell him what I want and I would appreciate everyone's input. It's better to do my "due Diligence" then be left with the consequences after the build. :D

Re: 454/420hp REBUILD Compression
 

Has anyone used a merc (Dart) HP500 carb intake on a warmed 454 with re-worked GM 990, 188 or 088 iron heads ?

Re: 454/420hp REBUILD Compression
 

MTGBIKER. I upgraded my HP500s last year and have a few spare parts to sell.
2 of each:

CAM

LIFTERS TRAYS DOG BONES

THERMOSTATS AND HOUSINGS

Let me know if you are interested in any of these?

Thanks
Shawn

Re: 454/420hp REBUILD Compression
 

[QUOTE=shawn]MTGBIKER. I upgraded my HP500s last year and have a few spare parts to sell.

Shawn ,

What cam did you install in your motors?
Carbed or fuel injected?
Are you running Gil Exhaust or headers?

Thanks, Dan

Re: 454/420hp REBUILD Compression
 

Dan, the cam is 238 @ 620 and 248 @ 629, thay are carb motors with Gil exhaust. They made 599HP at 5600 on the dyno.

Shawn

Re: 454/420hp REBUILD Compression
 

Excellent Hp. Its nice to back it up with Dyno numbers too :D

Are you using the factory heads and intake? If so, any mods? Or did you change them to a aftermarket set-up?

Dan

Re: 454/420hp REBUILD Compression
 

9.5:1 is on the edge with iron heads. it could live with enough
cam but i would back it down a touch.

Re: 454/420hp REBUILD Compression
 

I used the stock heads and had them ported and polished. I also had the intakes matched to the heads and installed a V6 module in my TB ignition. I have spooled them up to 5800 - 6000 once in a while.
My CR is 9.3 and I just use 93 octane.

Re: 454/420hp REBUILD Compression
 

mtgbiker,
I would stick to around 9 to 1 with iron heads. How much rpm are you going to run with these engines? If this build is targeted to around 5200 to 5500 max, then your induction system is pretty big and port velocity is not going to be good. This can be crutched with a small cam to improve velocity but you would be better suited with going with a set of merlin oval ports on your mills and an air gap intake.

Chris

Re: 454/420hp REBUILD Compression
 

Sutphen 30,
Must be missing something here, nowhere in my post did I make any reference to or suggest a "low compression" motor?

Re: 454/420hp REBUILD Compression
 

I spoke to the engine builder yesterday and he still wants to go up to 10:1 compression on these mills. He has assured me that with a CUSTOM GRIND cam and re-worked heads that the motors will not detonate. But he also wants me to run 93 octane!!

He actually got defensive when I suggested no higher than 9.5:1. Of course "I'll do what you want me to do" was his final position. I do trust him since he's had his shop for 28 yrs but 9:5 to 1 is where it will be !!

Re: 454/420hp REBUILD Compression
 

Where would you suggest I back down to?

Dan

Re: 454/420hp REBUILD Compression
 

I'm running 9.5 with aluminum heads and I find I have to use 92 octane. I wouldn't run 10 with iron heads. With today's crap gas, all you need is one tank of bad gas and you're done.

Re: 454/420hp REBUILD Compression
 

Parti nailed it. Say you get caught at a marina that has 89 only. I would run no more then 9 to 1. Your .5 a point will not yield anymore then roughly 20 more HP per engine and is not worth the chance of popping an engine.

Chris

Re: 454/420hp REBUILD Compression
 

Yea, I have friends who have to have someone follow them around with a pickup full of barrels of race fuel so they can siphon it in when they need it. Build for 89 octane unless you are full race. 9-1 as was said.

Re: 454/420hp REBUILD Compression
 

low 9's

Re: 454/420hp REBUILD Compression
 

I've been keeping an eye on these posts, hoping I'm not digging myself a hole. I'm ordering parts to rebuild my '98 Mercruiser
7.4 MPI (L29)...thus far, I've got the Edelbrock aluminum oval port 110cc heads and HP rods for full floating pins (3/8 ARP bolts). I'm looking at Speed Pro H-426CP pistons that have a .100 dome...that should give me a 9.19 cr.

I live at the end of the earth, and the gas is 87 octane. I was thinking the aluminum head would give me a cushion, but now I'm wondering if I should order a flat top piston.

I have a 6000lb, 25' cabin cruiser. I'm looking for mid 40's at around 4800 while cruising at around 3200. I couldn't imagine WOT for more than 60 seconds.

At this point I'm going to keep my stock cam and thru-prop exhaust. I got a Bravo-3 drive to replace my Bravo-2, but haven't installed it yet. In the future I'd like to upgrade to the GLM aluminum exhaust, or maybe the stock 496 exhaust.

I'd really appreciate some advice on piston choice. I've already been advised to go with a steel crank, but that's probably not going to happen. I know I'm talking to the go-fast crowd, but my engine doesn't.

Re: 454/420hp REBUILD Compression
 

Merc built most of their old engines with 8.8 to 1 compression with iron heads. They ran just fine with 87 octane. You should be ok provided you engineer the rest of the system properly. Make sure the engine runs cool: 140 dgree thermostat, large oil cooler. Be conservative with the ignition timing: 34 dgrees or less total. Make sure the carb is jetted right. Lean mixture in a modified engine is certain death. Under prop the boat a little to take a bit of the load off.

My engines are more highly modified 540's putting out 650+ hp so I think they're generating more combustion chamber pressure even though the comp ratios are close to the same. I wouldn't expect you to need as much octane as I do.

Re: 454/420hp REBUILD Compression
 

[QUOTE=PatriYacht]Make sure the engine runs cool: 140 dgree thermostat, large oil cooler. QUOTE]

PatriYacht,

What about running no thermostats since the waters where I live usually hold 80* surface temps thru summer?

Or stats that are 100-120?

mtgbiker

Re: 454/420hp REBUILD Compression
 

Guys, check my post on the 330/ 454 build thread on here. I know the slightly larger roller cam helps with the detonation, but I run 9.2 comp with the cheapest 87 gas I can find. It makes peak power with 36 degrees of timing, and I've had it to 42 in the boat looking for detonation just to see if it was close, but found no pinging at all.

Re: 454/420hp REBUILD Compression
 

Your Dyno numbers are respectable! You hit the right combo with your components. Good luck with the motor(s).
Dan