obnoxus

Ok,, I found my low compression culprit,,, a spring keeper is slightly out of position and was holding the valve open.

So here is my question, I have decided to do a complete stock rebuild,,, 1995 502 MPI,,,,,,,,but can I have ANYTHING done to the heads to improve flow ie: bigger valves, bowl or gasket porting without having to mess with the computer.

I have Gil headers and K and N arrestor and still the motor was running on the rich side,,,, no heat line on most plugs and a nice black film on the piston tops that flaked right off with a finger nail, so all in all it is about where it need to be, but favoring rich ( ok in my book!!!! ) just curious on what flow would do to the mixture.

Also,,, without digging through magazine articles,,, what was the timing base and total that they were stiing 502 MPI to above stock?

JimV

iTrader: (1)

I would be concerned how the keeper moved to hold the valve open, Maybe valve float or a rocker to retainer clearance issue?

Modifing your GM heads can result in some hefty hp gains if the work is done properly. The exhaust ports are very sensitive because the lack of short turn radius. Done right and you will see a 40 to 50 cfm increase or 250cfm@ .650 lift. The intakes with a 2.300 valve will flow 360+ for an increase about 45 cfm with some real port work. This will yield about a 40-50 hp gain. Here is the sticky part, If you truely get that kind of airflow gain you will problaby have to change your fuel curve. What I'm saying is get airflow numbers from who's doing the work. If someone tells you that they need to enlarge the intake flange to the gasket, your in trouble. The flange is already too big. A valve grind and bowl port on the exhaust is waisting your money and wont gain anything. One other thing, If you are changing your pistions and are doing some needed combustion chamber work, get a piston with some dome on it so you don't come up short on compression. The chambers will net 125-128 cc's when finished. With bore notches minus a couple more. I really don't know what the hp limit is for stock EFI systems. I'm sure someone on the board will know. my two cents

blown1500

Obnoxus!
Jim V sounds right to me. I prefer aluminum heads, AFR's first. I would compare cost of the work above to the price of AFR or some other quality brand-DO NOT GO TOO BIG.
The stock efi can go considerably bigger but you need to consult with someone like Jim at Arizona Speed and Marine about the program. Accell is supposed to get back to me about their new system. If it turns out to be as good as I hope, you could just get lucky!!!

Vinny P

I thought that the intake plenum is one of the most restrictive parts in these motors. What is the sense of having good flow numbers on the heads unless the intake can keep up? Has anyone tried Extrude Hone for increasing intake flow? To me, it sounds like all extrude honing does is a de-bur of the casting, but it can't change any port shape for increased flow.

blown1500

Checkmate, the plenum is some restriction, but the low lift flow in the head is worse. Extrude Hone forces material that looks like thick mud but is very abrasive through the passage you want to improve which will not do much to change the shape but will smooth the surfaces and will enlarge the passage. Best I have done is have a good porter shape the ports and then send to Extrude Hone for finish. Killer results!! I have only used this where you are limited to a certain part, stock appearing like some car racing, etc.
Jim V sounds very knowledgable and, for what we normally need in a boat application, I want to talk to him.

obnoxus

But any of these I think would cause me to have to remap the ECU yes?