head bolt pattern gm

[stonehedge]

Running a basically stock 99' mark V 454 motor. I had it rebuilt this winter and put new pistons in. Broke it in, and the comp is 170 straight across on all cyl's. (test-fresh batt, all plugs out, stock cam, high torq. starter, motor not warm, but not ice cold, throttle wide open)
That high needs high octane, correct? thanks

[stonehedge]

When I was breaking it in I ran 50/50, half race 110 with lead and 92,(to be safe) fyi

[1980Cougar]

That high needs high octane, correct? thanks
Your compression test is not really a good indicator of your final compression ratio. Different cams will give vastly different compression pressure readings.
You need to find out what your engines mechanical compression ratio is (ie 9.5:1 etc). That shouldn't be to hard if it is full of stock parts.

[stonehedge]

Its bone stock except for the pistons, and they are only one size up. How would I go about finding this info?

[stonehedge]

Does anyone know the mech. comp ratio on a GM 1999 454 Mark V truck replacement motor?

[SmokinLowriderSS]

Does anyone know the mech. comp ratio on a GM 1999 454 Mark V truck replacement motor?
I would be willing to be it was about 8.5 to 9:1 max.
Were the new pistons flat-tops, just .030" overbore ("one-size-up")? If the pistons that went in were flat-topped (no compression-raising dome) with your cylinder head chamber volume (usually available online if you have the head casting number), the thickness of the head gasket, bore size, stroke length, it is fairly simple to calculate a compression ratio. It will only be as accurate as the numbers fed it so is BEST if the chamber has been measured (cc'd) but the "designed" size will get us a figure to work with. To be most accurate, you need to know how far down in the cylinder the piston stops at TDC also, but we can still try to work a ballpark figure if you don't know this.
Post it all up & we'll try to help.
If the pistons DO have domes, I need the dome size (volume in cc's) as well.

[MudPumper]

you only need pump gas.

[steelcomp]

Running a basically stock 99' mark V 454 motor. I had it rebuilt this winter and put new pistons in. Broke it in, and the comp is 170 straight across on all cyl's. (test-fresh batt, all plugs out, stock cam, high torq. starter, motor not warm, but not ice cold, throttle wide open)
That high needs high octane, correct? thanksYou're on the edge...165-170 max for pump gas. What you're measuring is true cyl pressure, albeit cranking pressure. Your static compression ratio doesen't really mean anything, as it's just that. A static number. It's the dynamic cylinder pressure that matters, and how the piston and cumbustion chamber work together, or don't. That's what dictates what octane you're going to need, not a number. Some piston/chamber combinations can't tolerate any compression at all before they detonate...some can run very high static numbers and never detonate. I'd run premium pump gas, and listen carefully for detonation, or do a good plug read. Keep the timing conservative, and don't run it lean. If you want to run it hard, and get more power, run some race gas to be safe.

[Aluminum Squirt]

Let me start off by saying that I am NOT a professional (or even amatuer for that matter) engine builder so treat it for what it is, one guys opinion on a website: I agree with Steelcomp, static CR's mean nothing except for an approximation for your buildup, choosing a cam, pistons, timimg etc. It never hurts to run quality gas. Tune your motor carefully and you will not have to worry about your gas too much and you may even find a couple of HP you didn't know you had. Start out with some conservative timing and good plugs. From there I'd make sure you are jetted either right on or a little fat and then I would work on your timing and listen for detonation. If you jetting and timing are tuned right you can make it run on whatever gas you want within reason. You can time it real conservatively and run low octane gas all day or you could jet and time it on the edge and run some high octane stuff and probably get some more HP, your choice. Just base it on the type of boating you do. Do you always fill up where there is high octane or race gas or do you get stuck and have to use some garbage gas because you stay out on the lake for a couple of days? Do you run hard all the time or is it more of a family boat with an occasional blast of WOT? A lot of this is dependant on your set up, ie what carb are you running(injected maybe?), what type of ignition (vacuum, mechanical, locked out etc etc), is it hot where you boat etc. If your pistons are the same profile but you went to the next oversize (.030), your CR went up because the pistons are bigger and you are forcing more volume into the same size combustion chamber.
I recently read a very interresting article about calculating your actual CR. It was much more involved than calculating cylinder volume, head volume, gasket thickness, dome size, etc. It actually factored in the volume of your cylinder starting at the time that the intake valve closed, meaning that the piston was already part way up the bore when the valve closed and that that was the point for which you should calculate the volume of the cylinder and use that for the basis of your calculation for determining overall static CR. Kind of involved but an interresting point non-the-less. The above scenario was obviously totally dependant on the cam profile. As the cam gets wilder, the static CR need to go up to compensate for the lowered volume of the cylinder at the time the intake valve closes. Now that I've completely muddied the waters, re-read the first paragraph and act accordingly. Just my $.02-Aluminum Squirt

[stonehedge]

Thanks a million, that is some very helpful info. I will put some of this info to work today. thanks again