Originally posted by Jungle Boy
Sea level will give you about 250 - 300 RPM over the 3400 ft I was stating, if the conditions are right. I have raced in both and we try to keep acurate records. Cas is right; Try to choose a cam that will perform up to and peak at around 4500/4800 rpm. Seen a few guys that have dropped a hotrod car SBC into their jetboats and they performed REAL bad. An RV cam could be concidered too, since your motor isn't built to run into the 5000 rpm plus area. I've pleasure boated and raced only small blocks and still think they are great for the lighter jetboats. 200 lbs less weight at the pump makes a big difference.
Got to get the HP in the expected RPM range. I suppose the first mistake is to overcarburate. 600-650 ( i suppose) is just about right. 2nd mistake is to misjudge the HP absorption curve. 3rd mistake is to misjudge the exhaust. I am going to use the Comp XM270H or the XE274H. Impellers are easy to change. So easy in fact that i might just carry one with me like i did on my I/O...one for skiing and one for cruising. Got to keep the lift below .5
Exhaust will be nicson logs. To what i aint sure yet.
mikeT
xe274 would be a pretty good choice. Peak TQ is at 5000 but there is very little change starting at just over 4000. The dual pattern cams like that one usually work pretty good in a jet.
Match up a B, maybe even a BC and it'll go along pretty good, depending on the weight of the hull, of course.
Putting a set of 2.02 heads would do wonders too.
it is done. See here mikeT's disposable motor
XE274H installed straigh up (thankyou W...)
pre-79 4bolt 350 balanced engine +- gram....flexplate
76cc heads 1.94/1.60
9-1 compression
cheep hyperutectic pistons
10/10 crank
HEI ignition
Cheep Air Gap Manifold
600cfm square bore
Nicson Logs
OT extentions
American Turbine SS B
7qt oil pan/baffles/doors
Maybe a 19' PF, light-medium layup.
Stickers and decals to make up for the lost CID
I still think i am looking at about 5000rpm WOT and 63mph. I am told the RPM may be a little higher, i dont see it. I dont think there is enough HP there to do it.
Still need some stuff...i aint got the carb or the mounts yet, or the extensions off the logs. Little other stuff.
I do appreciate thems that are encouraging, and thank them. I also know thems that are discouraging a SB have very good and valid reasons to do so, and i thank them too.
time....time....time....
mikeT
Mike, hopefully you have included some funds in the budget for a dyno session.
No i havent. Never thought about it. Never done it or seen it done. It would be the ONLY way to find out what it is really going to do.
I was thinking that the impeller would be a good enough dyno for me. Matter of fact i was relying on it. Let me look into it. Thanks.
mikeT
Originally posted by miketsouth
....I was thinking that the impeller would be a good enough dyno for me...
Mike I don't think anybody out there can truly vouch for the accuracy of those impeller absorption curves. Are they accurate to +/- 1%, 5%, 10%.....?
At least if you dyno the engine, you will have one set of hard data to work with. Won't hurt to know that you have optimized the engine combo, either.
what's the specs on the xe270h? I went to the Comp Cams website and couldn't find that number. Found the xe268h and the xe274h but nothing in between.
There's 270h though.
Mike T give me a ring. 316-794-8616. I've got the mounts and the carb.
My mistake XE274H. I was going to use the XM270H.
mikeT
Originally posted by Cas
what's the specs on the xe270h? I went to the Comp Cams website and couldn't find that number. Found the xe268h and the xe274h but nothing in between.
There's 270h though. http://compcams.com/Technical/Search...umber=12-246-3
Originally posted by OLDSQUIRT
Mike I don't think anybody out there can truly vouch for the accuracy of those impeller absorption curves. Are they accurate to +/- 1%, 5%, 10%.....?
At least if you dyno the engine, you will have one set of hard data to work with. Won't hurt to know that you have optimized the engine combo, either.
I dont know about their absolute accuracy. I am pretty sure that one could use engine RPM to see if changes made in jetting, timing, or other things were working, as long as nothing else changes.
I have asked many people what RPM their stock 454 BBC turned a berk 'A' and they all say the same thing 4600. This goes with the chart. Of course it couldnt be TOO accurate. Most people have a tach that is not THAT accurate.
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