Cam and engine Gurus.. Need a blessing or condemning

[gbeltran]

Re: Cam and engine Gurus.. Need a blessing or condemning

More like 9:1 with an early 80"s head. If you want the higher ratio with a flat top you will need something with a 64cc combustion chamber, most, if not all, heads from the early 80's will have a 76cc chamber. With the small cams you have listed though that 9.75:1 is probably going to be too high for todays fuels. Both the 262 and the 256 cams are going to work better with something in the 8.5 to 9.0 to 1 compression range. If you want the higher ratio you should step up to a set of NEW vortec heads and an intake manifold to match. It will require a bigger cam to keep the dynamic compression in check, and that probably isn't what you are after with your outdrive.

 

[Friscoboater]

Re: Cam and engine Gurus.. Need a blessing or condemning

Makes sense.

 

[wca_tim]

Re: Cam and engine Gurus.. Need a blessing or condemning

I'm running an xm270H with 10:1 compression ratio, 200 cc world products 64cc chamber heads and -16cc dished blower pistons in my 383. the block was zero decked for a quench of about .045... I have never had any issue with pump gas. I'm running aftermarket manifolds with long risers and would think I'd be pushing the possibility of reversion with anything less. that's a dynamic compression ratio of around 8.5:1 if i remember correctly and gives a cranking compression of 190-195psi on all cylinders. I just upgraded to an airgap rpm and modified 750 cc holley double pumper and it's running great (picked up a few mph).

The 256 cam with 9:1 ish compression ratio sounds like a good fit as gbeltran mentioned above. New vortec heads (a bargain!!) sound like a great addition to your engine! Note that if you leave your compression lower like 8.5:1 and add vortec heads and a cheap intake (edelbrock performer or something like that), it'll still make a bit more power and you'll be able to run about any old pump gas you like... Sometimes It's hard to find clean 92 or 93 octane on the water...

If you go with more cam and higher compression, i would think you'd be getting into more of a grey zone with your exhaust and reversion...

 

[Friscoboater]

Re: Cam and engine Gurus.. Need a blessing or condemning

I went with the 256h and stockers for the heads. I need to think of this for the family.

 

[wca_tim]

Re: Cam and engine Gurus.. Need a blessing or condemning

I went with the 256h and stockers for the heads. I need to think of this for the family.

Good for you! I just listed a bunch of power adders for mine for sale in a few places... it's got enough the way it is that if I put much more p[ower in it it won't be suitable for the daughter and her friends to use so I'm going to buy myself a car toy tomrrow and spend the boat parts (alpha ss drives, sternjacks, afr heads, etc, etc, etc stuff that will sell for good prices) cash on it and look for another bigger boat. What good is a overly fast boat if it is the only one I have and it's no longer easy to use with the family (ie daughter...)?

if you need an intake, I thought I saw a performer in the classifieds over on scream and fly for cheap... there wa also a couple of cheap (but presumably rebuildable) 650 cfm holley spreadbores. just a thought for those on a budget. most folks over there are really straight up to deal with.

Enjoy it and let us know how it turns out when you get it together

 

[n2ostroker]

Re: Cam and engine Gurus.. Need a blessing or condemning

355
9.75:1
ported 305 416 casting heads
Performer RPM intake Edelbrock 750 carb
stock center rise manifolds/4" risers
Comp XM262H cam and kit

Will idle down to 500rpm if I want. I idle at ~650rpms. Just seems to be happier. Great response, decent economy, pulls to ~5000rpm. Great cam and definitely not too big.

The mid 80's Mercruiser 260 used a 64cc chamber head so a flattop piston .025 in the hole with a reg .039 gasket you would be 9.5:1 or so. Check the casting on the head and reference it. Watch your pistons though sometimes rebuilders will use a 1.54 comp height piston which drops the piston .045 in the hole and kills quench and compression. Stock is a 1.56 compression height.

You'll like the 256h. Its far and beyond a "rv" cam. It'll idle almost stock and make gobs of off idle torque.

 

[jtybt]

Re: Cam and engine Gurus.. Need a blessing or condemning

The 256 would be a good choice for less than 9:1 compression. It would be a straight drop in without need to alter springs.


The 64cc heads from the 80's were probably from a 305 CI .

 

[ringmaster72]

Re: Cam and engine Gurus.. Need a blessing or condemning

Without reading all of the posts.......Have you considered a roller cam? It is expensive, especially if you have to get the conversion lifters, but in my opinion well worth it. You can get a lot more lift while still keeping peak hp in the 5k rpm range.