290 Prop Selection question again
 

dyno'd first engine and hp looks like 480 with torque at 525. because the dyno is an antique i dont have answers to the many questions you may have however the guy said the peak hp was reached around 5-5500rpm. my plan was to prop for 5000rpm at wot. i have 24" bravo 1's now (ran 60 gps @ 4400 with 7.4's) and feel that i should be able to run 26". i have bravo 1 drives at 1.5 ratio. any thoughts based on this info?

Did you trim it or was that with the drives all the way in?

start with the 24's

hey BBYSTWY i was trimmed out no tabs in the water probably pretty full of fuel and had a friend with me. liberator221 i will keep the 24's but I'm really wondering if this extra 300 hp will allow me to run 26" props and increase my cruise to 60 ish @ 3500 and move my wot speed to the low 70's at 5000.

You will never run 60mph at 3500rpm with a 26. My boat ran around 48 at 3500 running a bravo 1 prop with decent slip numbers.

Run the 24 and see how it does before you start buying props.

Run the 24's for a base line. If you were only pulling 4400 RPM prior you will probably be in your 5000-5200 now.
If you feel you need 26's I have a set you can try before you go prop poor.

No way to really give any really good advice without some real dyno numbers or at least the engine specs

i was bummed that i couldn't get a dyno print out. anyway, i kept the stock bore and stroke, upgraded connecting rods, hypereutectic small dome pistons. supposed to be about 9.8:1 compression. roller cam 230/236 @.050 and .547 lift, went with afr aluminum 265cc heads 112 combustion chamber, stainless roller rockers, Edelbrock rpm air gap intake and quick fuel 800cfm dbl pumper. it ran easily to 6000 rpm. seems to be a nice combo and again hoping to run in the low to mid 70's

Is it the 7.4 with 2 bolt mains and cast crankshaft??

My old dock neighbor had a similar build in a Formula 302 that ran mid to high 70s. He also started with 7.4s. I believe he had many relatively trouble free seasons with the boat and it sounded great.

Pratt's heads were bigger (305's or 315's as he was anticipating 540's down the road), cam was bigger, and ran a dart single plane intake. Ran great until he hydrolocked/punched a hole a cylinder (bad exhaust flapper coming off plane). That block was sleeved and they have since been upgraded to 489's with Eagle rotating assembly. The original stock 7.4L (Gen 6) blocks/rotating assembly held up fine but he limited the RPM to 5,200 WOT.

The blocks are GEN V 4-bolt mains and yes using the stock cast crank. Running the standard bravo 1 outdrive, do any of you guys run engine rpm above 5000?

I would prop it to run 5000 rpms for your average boating conditions and then under ideal conditions with a light load, it would prob run up to 5200-5300rpms.
I ran my old 454mag with upgraded cam and roller rockers to 5450rpms with no issues. My 525SC ran 5300rpms at WOT. My 502mpi's run up to 5100rpm in ideal conditions.

That will be the plan. Thanks

Thanks for your info and offer. The set of 26" you have, are they extra? IF 26" actually worked on my boat, would you sell yours? Thanks again

No, not really extra. I have a set of stock 26's, a set of Mercury labbed 28's and stock 30's if I ever install my Whipples. I'm still tuning and experimenting with drive spacers and don't know what I like yet. lol Just have them to trial so you don't end up buying what you don't need.

Sounds good. I didn't think so but if you don't ask you don't know. I was just over to Mercury's slip calculator trying to figure out where I might be and really if I'm not able to run my current 24" to 5500 rpm or the 26" pitch to 5000 rpm I am afraid I am going to be a little disappointed. That puts me right at 74 mph with 10% slip. I don't know if that is a realistic slip number either. Really not sure what to expect from an approx 300hp gain. In my mind that's quite a bit. Almost like adding a 3rd engine. I am going to look around for another shop with a dyno. I feel as though there might be a few extra hp in there and I really want the graph. I do know that currently the air flow ratio is running in the low to mid 14's. He said with my compression ratio that that would be fine. I was thinking it supposed to be around 12.8-13.5 ish. In the real world maybe this is 1 hp. I don't know. I would like it to run a little "fatter". Thanks again for your offer with the propellers. I may take you up on that this spring. Are these your primary props or are the 28's?

You are 14s at what rpm/load? If WOT don't run it till you fatten it up. Hopefully you didn't damage anything.

I don't know the "load" applied I can only tell you it was a run on the dyno. Not sure that answers your question or not. It was back firing a little at lower rpm and would the carb would catch fire and shut down. Some of that I think was the timing because he advanced it a couple degrees and it stopped. The more I talk about this the more I don't feel well. We ran from 3000 to 6000 probably 7 times or so. No smoke from anywhere and it still sounded good after all was done. I realize that last statement is not scientific but no unusual noises. Going into this I was thinking that the 800 cfm dbl pumper Quick Fuel carb was going to be a little big.

I don't want to get too deep in the woods here but he did pull a couple plugs after a couple pulls and they looked fine.

Was the a/f ratio on the dyno derived from a wideband 02 or like in old days before wideband 02 sensors from fuel flow meter and airflow meter ? If the latter, the printed a/f ratio can be way off from actual and thus usually meaningless.

I think its an updated 02 setup. I only noticed a sensor in the exhaust. Lets say that it was run lean and there was damage. What am I looking for? Hole started in a piston? Valve issue? I do have the Inconel exhaust valves.

If I were having engine built, I would try to get real Dyno numbers and a printout, 5000-5500 is too vague for my liking. Is there someone else local that can dyno them for you?

for instance, if you make peak power at 5450 at 5000 you may be 75-100 HP less, try to get real numbers!

I feel exactly the same way. I want these set up "perfectly" with timing, carb jetting, etc. Too much invested not to know. I did find another shop to perform a dyno tune and setup with actual numbers. I've never had a dyno setup before and found out that a Dyno set up is a pretty expensive endeavor. Anyway, I will get the other one done and running and bring both to this other shop.