Marine 273 build
Howdy FABO, first time poster here... have enjoyed the terrific knowledge base for a while and figured it was finally time to register.
I, along with a few friends of mine over at
CorrectCraftFan.com have some vintage Chrysler muscle in our vintage inboard ski boats. Ive seen everything from Poly 318's, various LA/LM's (273/318/340) as well as some of the RB big block (426 and 440) engines installed in older Correct Crafts- primarily mid-60's to early 70's vintage. There are a few differences in the marine stuff from the automotive- namely exhaust, ignition and carburetion limitations... but a lot of the same parts and knowledge can be leveraged- which brings me to today's question.
A few of us are looking at modifying our 273's. Marine 273's (at least the ones we're familiar with) were rated at 200hp from the factory. The ones Ive seen all came with 4bbl AFB's, and they used (larger chambered) 318 heads (like the 675 castings). They move our 1600 lb, 16' boats pretty well- to the tune of ~45mph or so... every bit as capable as the 289 and 302 4bbl Fords (~210hp) that were offered during the same time period.
Anyways, while swapping out to a 318 or 340 is always possible, we're looking at what it would take to extract a bit more out of the 273 without adding cubes... so we're looking at head, cam and intake options.
Heads We are thinking that something with smaller chambers will help get compression up from the stock ~8.5:1. Also, something with smaller valves would keep us from having to notch the cylinders. Based on what Im reading, automotive 273 heads (315 or 920) or the later 302 castings might be good options. Are any of the above more preferred than others? I assuming porting would be recommended with any of the above?
intake The one intake that has been identified to work well is the LD4B. Are there others worth considering? It sounds like the regular Performer is less than revered, and I *think* the RPM is a bit taller than the LD4B... is that true? We do have height limitations in order to close the motorbox- but I know the LD4B fits.
Cam This is the trickiest, as our vintage boats are all reverse rotation. Reverse cam blanks are in short order, but we do have access to 2 options that we know of. Both are hydraulic flat tappets.
1. Gross Valve Lift: .410/.411, Duration @50: 198/199
2. Gross Valve Lift: .429/.444, Duration @ .50: 209/220
(This is essentially the reverse rotation version of the HO 340 cam from what I understand)
The latter worked great on a built up 340 (approx 330hp). We also tried one on a 273 with the stock (low compression) heads and it performed a little better- but not drastically. Might have been too big? Which of the above options would work better for lightly modded 273?
Carb Are the original AFB's the best option? What type of CFM should work best? I need to pull the model numbers off mine to see how big it is. Would a Holley 450cfm work better?
This isnt the first time we've modified engines in these boats, so just a few things to keep in mind... We dont spin them very high (5200-5400 is about the max we'd prop them to spin at WOT). High-ish compression is ok (we've had a that ran great at 10.x:1). "Off idle" or "low end" power is largely unimportant to us, as the prop slip coming off idle gives us a lot of instant rpm (2500+) just like a high stall torque converter.
Oh, and one last question... since my buddy has the following parts already collected, what do you think he's looking at for hp once he gets it assembled?
[FONT="]273 +.030 with factory replacement 10.5 domed pistons from Egge
[/FONT][FONT="]Cam: 429/444 (210-220) rev rotation marine cam[/FONT]
[FONT="]Ported 273 heads (2465315)[/FONT]
[FONT="]Edelbrock D4B intake[/FONT]
As everyone loves pictures, this is one of the Chrysler powered Mustangs in our group. This one came with a 273 that was swapped out for a 340... one hot little ride.
Thanks in advance for any and all feedback!