BadDog

Building an HP 500 clone right down to the camshaft and blue intake, carb etc. My only difference will be Lightning Headers and compression ratio. I still have an opportunity to change the C/R.

If I bump C/R to 9.0 or 9.25 will 89 or 92 octane be safe?
How would this effect the jetting, I expect I would need to bump all jets up by a size or two or is that not enough of a change to require re-jetting?

I know that plug readings should be done but with todays fuels it is more of a black art than anything else. I am building to HP 500 specs as I had all the pieces parts and would be able to got right from the Merc book when jetting and setting time. No point in reinventing the wheel.

kneeslider74

Loaded question!! Best advice is to stick to what works ie. Merc specs will give you reliability. That being said proceed with caution!! You need to consider timing, temp, and head material. If the heads are aluminum no problems up to 10:1 with stock timing 89 oct./ if they are steel and stock timing 9:1 is the limit I would run (as you mentioned todays gas SUCKS) However you can push the timing a bit more at 9:1 which can improve top end hp. I am NO motor gu ruu best to get specs from merc. Post a thread and ask for a merc expert, they are on here. If you are looking to gain some power by upping the C/R you wont see noticable gain with the stock heads unless they are HP as well. Stock style heads have rather large cc chambers most time around the 110 range ie.454 marine tall deck. and the other factor is the design of ports inhibit flow compared to HP heads. Think of it this way, com. increase the intensity of the explosion and at the same time you cant flow air and fuel in, it becomes unnecessary to have more of an explosion than the heads and cam can fuel and exhaust.
Jetting is another tough deal if you have axcess to a dyno DO IT. Another option is to install an fuel air gauge or EGT set up. DYNO DYNO DYNO DYNO Here you can get the most out of timing fuel mix and find out exactly where you need to match your props to optimum RPM range.
92 oct would be safe @9.25 stock timing
Compression doesn't affect jetting by itself, flow affects it ,more air =more fuel / more flow=more fuel & air
Everything is related to flow and that is why when you build you should match cam to heads and intake to cam. they are all designed to work together. I have seen guys spend thousands of dollars on components that are not matched and all they end up with is an expensive motor that runs for sh*^% and you cant tune.
Cheap Hp can be gotten by improving flow, match and grind and polish intake to heads by placing gasket and boring out to match gasket on both sides. Do the same w/ Exhaust gasket and headers. You can do this yourself cheap and get much more than a compression bump will give you.then you can mess with your timing all you want.
When in doubt refer to the specs and the experts!!!!!!!

getrdunn

iTrader: (7)

This is just my opinion and experience here however I certainly wouldn't be affraid to bump the comp up as long as you stay under 9.5 to 1. I have never experienced problems with that as long as a little common sense is used and use 92 or 93 when ever possible. When you get stuck with marina gas just don't drop the hammer as much or as long. Hard thing to do on the bud light but not a bad idea. Comp as you know makes power. In regards to the jetting I would think you'll end up to close to stock jetting. I ran the same headers with 9.5 to 1 years ago and loved it. The only difference was I used the 465 hp cam and had dart oval ports that were ported and flowed very well. If I remember right I staggered the jetting but was right very close to stock jetting. It may have even been slightly under. Like you said plug readings are the key. If you wanted to be on the safe side go up two jet sizes first time out on the front and rear and do your cruising checks at 3K or so and then WOT. I know people read plugs different ways however I like to start out with new plugs and get at cruising speed as soon as possible and run for a few minutes and immeadiately shut down and check. Checking all plugs is important the first time around and typically the hotter one or two there after when making changes (most often #8) but not always. Repeat at WOT. With that jetting you won't hurt anything from the get go. You going to run 34 total advance? Hope this helps a little. BTW your going to love the sound of those lightenings. They have a nice bark to them. LOL...

BadDog

Agreed, all sounds like good advice. My initial plan was to do that exactly, just stick to Merc specs. When given the opportunity to change C/R it seems like I may want to deviate a little but not enough to upset the whole apple cart, hence my question on IF changing from 8.75 to 9.0 or 9.25 would be a game changer. Heads are the same as the HP 500 with 2.25 valve on intake side. HP 500 ignition module and all other parts are same as HP 500.

getrdunn

iTrader: (7)

This same set up went on the dyno at More Power after mine came off. Same cam, cubes, intake, carb, heads. It was a 10 to 1 comp engine however ran a tight lash solid roller set up. Dyno headers it made 492 HP. It was a bone stock 9022/800 cfm holley. It was dyno'd for more of a break in procedure so there wasn't much fine tuning because of the headers. The EGT's were a little high though but the plugs looked great.

kneeslider74

Do you know what the cc's are on the heads?
Are you looking to get a little extra? If so refer to the matching of intake and exhaust!
What about a dyno?
On my 454 500hp we got 38.6hp gain in jetting and timing and it didnt cost much just dyno time.

getrdunn

iTrader: (7)

Like K-slider said and given the dyno results of the extra comp with same set up it probably isn't worth the worries. You'll have a nice reliable package as is. Just know if you do decide to go 9 to 1 or a little over you will be safe.

getrdunn

iTrader: (7)

I would guess he has 119 cc chambers. He can also pick up some power with a little bowl work and bore notching. But then he gets back to having to deck the heads to maintain comp. The gasket matching certainly helps in some applications and others we haven't noticed much. I use to run Dart rec ports with the raised floors (Valako) and epoxied one intake to match and the other one we didn't. There was no noticable difference between the two. That goes years back.

kneeslider74

Yep seems like he gets it at least. Not sure if it was mention earlier, but like you said take too much off the heads and good luck fitting your intake as well!
I have the same problem, always wanting more hp and now I tend to do the little things like porting polishing carb work ect. much cheaper and doesnt adversly affect performence or longevity. wish I had a dyno and a crane.

BadDog

Heads are 119cc but block was line bored and using 20 thou longer rods with change in pistons. I am relying on my builder to cc the heads and measure everything up. From that he will either cut the heads and or select a non stock head gasket thickness. It is up to him to get the CR at what I request. I think I will spec it at 9.0 and call it good. He has the tools to mill the intake if needed.

Whole point of this project is de-powering from a B&M 250 Supercharged engine. Going with a little milder cam and non forced air. Lowers my insurance and reduce maintenance interval and expense. 80 MPH was a tad much for my Force 235 hull anyway.