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Crossflow upgrade for jet boat

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Hello all,

I’m not sure if this is the right place for this question so if not, admin please point me to the right forum and I apologize in advance.

I’ve got a stock 250 crossflow in my (older) jet boat and would like a bit more power. The engine is from an 85 falcon I think, and has a C2 head and Weber carb. It tops out at 3950 rpm which I am guessing is around 130hp on my impellers (2 stage HamiltonJet 752 with 19 degree bullet front and 17 degree rear).  I really want a bit more zip, and would like to get to 4350 rpm, or even 4500.  That would require 177hp or 195 hp respectively ( jet power is proportional to the cube of the engine speed). So I need a high torque engine. The engine will never see 5000 rpm - that’s almost 270hp on my pump.

Is this economically viable ( I’d like to keep to around $2k in parts)?

Can I get there with a cam swap, and sundry parts? Do I need a new inlet manifold, and if so, should I go to a 4 barrel carby (450 Holley or similar)? One key restriction is I am keeping my log style water cooled exhaust manifold, so extractors are not an option. 

My other plan is to drop in an used AU or XR6 engine as I can pick one of them up for about $1000nzd here in the South Island of NZ and reuse the exhaust manifold but I’m not too keen on playing with or remapping ECU to be honest, or messing with drive by wire conversion on the electronic engine.  But I know it can be done, and the XR6 is a good engine, but that’s not my desired path. I’d prefer to warm up the crossie as it is a good, relatively light engine.

I’ll add some pictures tomorrow when I figure out how to post them. Thanks in advance for any ideas or feedback.

regards Rich

 

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Hi and welcome, sounds like a cool little project. I'll start off with I'm probably not the best person to ask about crossflow stuff but I can sure help with 4.0 info, a couple of things straight off, the 4.0 has a different bellhousing pattern to crossflow (I know sometimes that doesn't matter in a boat if you are driving the jet off the front of the crank) and the exhaust manifolds are not interchangeable with crossflow but if you are using the original engine mounts a little bit of fiddling will make them fit a 4.0. Up to au you can still run a distributor (and with a little bit of modification a crossflow dizzy will fit a 4.0) ba and later don't have a place for a dizzy, you can get carny manifolds for a 4.0 and with a crossflow dizzy that will eliminate having to run efi, if you are willing to play with the engine a little bit an ed/ef head (casting no 94da) or ed/ef xr6 head (casting no 94dt) will bump up compression a little and if you source yourself an ea camshaft you will have a nice little torque monster there is a heap of info on 4.0s if you look in the 6 cylinder section for "Thoms 4.0 thread" there is a lot from myself and other members, as far as uploading pictures of you are using a phone to browse the forum the tapatalk app is the easiest way, if you are using a computer uploading photos to hosting sites like imageshack and then linking them across is your best bet

Sent from my SM-G960F using Tapatalk

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Don't you hate it when you're 99% done on your long post and your battery dies....

Aaaanyway, here's the short version:

Thom is onto it for bang-for buck, however that would really only apply if starting from scratch. If you want to use what you have (and keep all the marine gear, etc) then you can apply the same principle as anyone does when hopping up a stocker: CCCE.... Cam, Carby, Compression, Exhaust.

Cam: go for something with a narrow LSA under 110 degrees. This boosts the mid-range at the expense of top-end, which you're not after anyway. Crow does a couple like that; a speedway grind 141550S that has a very narrow 106 LSA, 0.510 lift and 219/224 duration at 0.050". One up from that is the 14686 with 108 LSA, 224/224 at 0.050" and 0.506" lift. These would be very good for getting out of the hole and on plane.

Carby: take estimated horse power and double to get CFM rating of carby. A 450 would do well.

Compression: as long-stroke, short-rod motors are sensitive to timing and compression, best to be on the safe side and stay under 10:1. Those cams I suggested aren't wild by any stretch so compression doesn't need to go up to compensate.

Exhaust: limited to mods on your manifold such as rounding off corners, opening up restrictions, de-dagging casting flaws, etc. Don't underestimate what gains you can get by doing this; the old red/black motor in my ply ski boat gained 4 knots top speed just by opening up and smoothing out the inlet manifold runners. It was a bit of a shocker to start with but you can find gains everywhere.

Your RPM limit is sensible and doable with otherwise stock parts. Valve springs and push rods are the weak link and should be upgraded if staying above 4000 for long periods. Stock rockers are fine on cams around 0.500" and smaller. Get a good harmonic balanced too.

That's all I've got

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The boys are all over it. Below is a little 2 barrel engine I recently did for a customer in Brissy. 500 Holley - Ultraflow manifold. Open chamber head with 1.86in - 1.60 ex. Bowl job and basic 3 angle seat cut.
Bottom end was 250/250 deal with a tighe 140t solid. 240/240@.050 and .510 lift. Comp was 11:1.
It's a good street engine that makes solid torque and hp without having terrible manners.
You could pair this combo back a bit and you would be at your target. The small Aussiespeed 2 barrel manifold and a 500 Holley with something in the 215@50 and around .500" would be good for a stock valve size application.


8e83c943b6a36118f81cc90f0caf82d6.jpg


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This is really helpful, thank you all for the feedback so far!  Talking to the manager at Hamilton Marine here in Christchurch he recommended staying around 9.5:1 compression ratio for longevity and tolerance to NZ fuel.  Does the Aussiespeed 2-barrel manifold make a noticeable difference over the stock one?

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They are a massive improvement over the stock junk.  I had a before and after graph on my business page but the link was with post img and now I can't retrieve it.  From memory it was 20hp gain and about 40 ft lbs.  

when you say poor fuel over there - what octane rating is it?  Just remember an alloy head will take more comp than an iron head and the c2 has a very good chamber shape (being a closed chamber) and is less prone to pinging. 

If you have to keep the comp below 10:1 and your looking at a cam upgrade, make sure you look at the cam grinders recommendations. Camtech cams are good at providing recommended comp.  If you follow their guidelines you won't go wrong. 

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If you have adjustable timing gear you can always advance or retard the cam to get the peak torque where you need it. So don't be too scared of getting a cam that says it performs at 5500 RPM because you can just put in a couple of degrees advanced and bring that torque down to the 4500 RPM Mark.

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Having been through cam selection and what not with my own kiwi xflow i can say that camtech in aus were great to deal with but import tax etc kills them.

I have used a few cams from Kelfords over the years with good results (old starlets and corollas) and am on my second cam from them in my xflow.

I recommend emailing kiel@kelfordcams.com and letting him know what your up to. Bloody good bugger and real helpful.

I have the old towing/lpg torque cam in the shed, might be a bit weak for your needs though.

Good luck with it, most engine shops look at you like you got the pox when you want to spend money on a crossie haha.

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Flying into Queenstown on wednesday next week. If you get a cam before then I can bring it over for ya.

LEGEND!! And that is what a good forum is all about, genuine blokes trying to help each other out. We are so lucky on this forum coz some other forums are all just negative crap and blokes having a pi$$ing competition to see who is better than the other ☹️

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Fantastic offer Crazy2287, really appreciate it, but I don't have anything ordered.  Enjoy your vacation!  I'm chatting with Kelford Cams in Christchurch right now and looks like he has something similar to the Crow speedway cams so I will see if that might be a solution.  Although I think I will be looking for an aussiespeed manifold in the nearish future.

To answer the older question, octane ratings here are 91 and 96.  I'd like to use 91 for cost, but in all reality, petrol is the cheapest thing about jetboating :)  I imagine the same thing is true for speedway.

 

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if you ask anyone ... they'll tell you to use 96 fuel.. or better. (plus octane booster even)

you can get far more timing in, and compression ratio for better power for essentially free(doubt the fuel used per year difference would be justifyable at all) 

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