2.0L 135 fatblock heads on 2.5L 200?

[LakeFever]

Well today didn’t go as planned. I did manage to pull one of the small chamber heads off but didn’t install any. I also tried to CC it and my plate decided to crack and leak everywhere before I could get an exact CC although I did get close. The small chamber box stock 787 head is pretty much spot on 34CC I think it’s slightly under that. Maybe a half cc or so difference I had the air almost completely out when the plate broke and that was it. My stock 200 488 head is 38CC. To add to my day I broke my compression tester by dropping it on the floor which is the main reason I didn’t install these small chamber heads yet. I’ll get those tools sorted soon and get back to this but season is quickly coming to an end and I’m going boating as many times as I can before hand so that’s going to be my priority this week for certain.

[NZ Sidewinder]

[LakeFever]

34 cc if my memory serves should net around 145psi

[NZ Sidewinder]

I've just had a better look at it
Actual cc of a small top hat that's 15mm deep is a bit over 35cc
Plus the void in the spark plug.
You are right about the 140psi.
There is very little difference in volume between a .100 cut large top hat and a stock small top hat.
.100 cut on the stock 43cc head gives 145psi
I'm about to cut either .040 off a small stock chamber or around the same off a .100 cut big chamber head.
Looking for 160psi.
Stock omc triples have been running factory 150psi for centuries

[tlwjkw]

Stock omc triples have been running factory 150psi for centuries

i seem ta remember some of tha old ones that were happy with 80lbs.... quite awhile back!!!

[NZ Sidewinder]

I've got factory shorty stingers, a Shorty Hustler, long shaft Stinger plus a couple of shorty 49 cube 70hp omc fishing motors that all have 150 on 2 or 3 cylinders.
I race one that started at 145 psi and is now 135psi.
I've not come across a healthy 49er with 80psi.
Maybe a 56 cube motor might run with 80psi

[NZ Sidewinder]

I've got factory shorty stingers, a Shorty Hustler, long shaft Stinger plus a couple of shorty 49 cube 70hp omc fishing motors that all have 150 on 2 or 3 cylinders.
I race one that started at 145 psi and is now 135psi.
I've not come across a healthy 49er with 80psi.
Maybe a 56 cube motor might run with 80psi

[tlwjkw]

it was tha big V6 at tha time... i didn't do omc back then but was helpin' a friend who did... i checked it for him and thought i found his problem!.. he said nope, thats jus fine!!!!..... like said a long time ago.

[NZ Sidewinder]

Yes I've seen those low numbers in v6 omc.
That might be why they can run those rediculous 5/16 head bolts.
It's fortunate that as they strip, they create a nice hole for a 10mm thread tap .

[LakeFever]

After my last run I decided to stuff these heads on and see how they go. I managed to cobble my compression tester back together and did a quick before and after and it picked up 15lbs of compression with the head swap and going from a 1.2mm head gasket to a 1.1mm as well. Nothing huge in terms of compression I guess I’ll find out how that increased quench works out. Leaving everything else the same as last time out. Same prop, ecu, all of it.

[NZ Sidewinder]

What was the original psi?
I went from 120 to 145 by cutting stock 43cc heads 100 thou.
1mm head gasket.
Chambers around 34cc.

[LakeFever]

My gauge is a suspect so I’m leery trusting it because the needle is parked at 30 and indicated 160 so that would make it 130psi with the 38cc heads and it indicated 175 with the small chamber heads. I ordered a new tester and will get the final number again but I can’t swear the 130 number is accurate

[NZ Sidewinder]

Yep
If there is one certainty it's the lack of in compression testers.

[LakeFever]

Pow!!! These heads woke the living heck out of my motor. Fantastic mod for my set up.

[NZ Sidewinder]

Compression is the cheapest hp
Well done