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COMP. RATIOS

J

Joe

Well-known member
Can any one tell me what ratio would be good for airboats. I am building a new buick 455 and the guy at the machine shop said I shouldn`t run anything more than 8.5-1,I was thinking 10-1. Am I setting my self up for a disaster? any help would be appreciated.
 
Compression ratio is not the main issue it's cylinder pressure. I try and run all my iron heads between 9.5 and 10.5 that mostly depends on the size and weight of the boat. All my aluminum heads I run between 11.0to1 and 11.75to1. Once again the main things to consider is how much weight your hauling and the gear ratio. A 2:1 gearbox can't run as much compression as a 2.68 gearbox and no way do you run big compression in boats over 14ft. The camshaft really dictates cylinder pressure more then your compression ratio! Also pushing the quench tighter allows you to run more compression without going into detonation. If you want tell me what size, weight and gear ratio of your boat and I can give you a safe compression ratio number for the cam you should run!
 
I`m running a 14 foot combee glass haul I have no idea how much my boat weigh`s, a guess would be loaded 2000 lbs.I`m currently running a 2.37 but plan on putting my 2.68 box on as soon as I can get the S blades.My heads are stock 1970 heads 69cc if memory serves me right. If you can suggest a cam as well that would be GREATLY appreciated. Thanks for your input,your opinion is valued!
 
Where do get the books at?I`d love to learn,but nothing beats experience,and Thunders got a HOLE bunch. Any other input would be appreciated from any one.
 
Our Cadillac engine do fine at 10-1 with ZERO deck and polished chambers. Cam does dictate cylinder pressure as WT said. On a 5000 RPM engine, we run about a .550 lift hyd, with 238ish @ .050" on a 112. We do have a stroke advantage and are limited by the Edelbrock intake on rpms so we shoot for 5000 and work down as far as the power band goes. What type of intake, what head flow and what rpm range are you looking for?
 
I`m shooting for 5000 max,edelbrock dual plane.Haven`t had any thing done to the heads yet so don't know the flow.According to all I have read these things made 510lbs of torque at 2800 or 3200 stock.Cant remember for sure.I`m not really looking to build a monster just a bear. If I went with 500 lift would 10-1 be ok?
 
You will be dictated more by duration and lobe separation more than lift. Should be fine on similar perameters we use. 230 something and maybe a 110 for yours for more overlap since we don't know how it'll react exactly. Slicking up the chambers and zero deck are a must. Best detonation preventative I know of. You should plan on a little head work like one piece valves (may as well go a tad bigger) and at least some minor bowl blend and some work around the guide bosses, especially if you can do it at home as it's free power. If not, spend the coin to have someone spend at least 4 to 6 hours on the port cleanup, it'll be money well spent IMO. A good set of springs and rockers are money well spent also. I despise any stock rocker that B,O,P & C ever made.
 
Thanks,I planned on at least slicking up the chambers and maybe some more,I don`t want to get to crazy.Pardon my ignorance but what exactly is zero deck?
 
Thanks for the info. How does 508in.508exh.231in 234exh @.050 and 110 sound with 10-1. Am I on the right track?or should I go bigger or smaller? Sorry for all the questions,I`m still learning :D all so would you go single or dual spring on the valves?and how about stainless valves
 
http://kb-silvolite.com/index2.php

Check out the tech info. Good reading and like I said just one opinion, but it sounds like Cad PP is of the same thinking! Mine ended up minus .006" deck (So pistons sticking up above block deck), polished combustion chambers and valves. 224@.050 at 110 L/S Mech. flat tappet. Just got the block back so still have to do a trial assy to see if I clearanced enough.
 
thanks cold,so quench is the area between the top of the piston and the head,right. I have one more big question,TA Performance says not to use hypereutectic pistons in a Buick cause they are brittle,is this true,or would they be fine for my application?
 
I have buddies that have run Hyperutectic for years and love them and I also have buddies that are stuck in the seventies. I would read all the tech articles KB has and talk to as many people as you can and make up you mind.
 
opinions are like a--h-les, but i think everyone would say if you ping (detonate) a hyper. it wont take it.
but if you have a good engine builder who knows all aspects of your engine, then you should let them decide piston composition.
im sure the pros will chime in soon. im def. not one. :roll:
 
You can make you quench tighter with a KB then about any other piston! I have built alot of KB piston motors and in airboat applications I use them often! In endurance motors I like the high silicon pistons JnE, SRP, Wiesco and KB. In airboat motors don't use the lower silicon N2O pistons like Ross. Remember your not turning any RPM's and your not using N2O. The less your piston grows at temp the tighter your bore to piston clearance can be and the tighter your piston to wall is the tighter you can make your piston to head (quench)!
 
Hey Thunder, I've been reading your post for years now and I'm continually amazed at the amount of knowledge you have concerning engines and airboating. As a matter of fact, quite a few people on this site have a wealth of knowledge and experience with engines and airboats. Have you ever thought about writing a technical manual on building performance airboat engines? I've been reading engine books for years and would love to have a "THUNDER Engine Bible".

Maybe it could be a collaborative effort between Thunder, Potter, Thurman & Felber, " How to Build Perfomace Airboat Engines, LSx, Cadillacs, Small block Chevys, BBC & Aircraft" All that information at your finger tips in one location, sign me up as your first sale!

I can see it now, Chapter 1 LSX, Chapter 2, Quiet airboats, Chapter three Engine weigth vs boat size, how to optimize for performance. If you run a SBC on a 14 ft boat, you shoud have XXX HP, your lobe seperation angle should be XXX, your quench area should be XXX, Step 1, Step 2, Step 3. How about a chapter for beginners. "How to drive an airboat". "When you lift off the gas in deep water with a open hull boat, watch for the back wash so you don't swamp". Maybe that's a whole different book.

Anyway, it's just a thought.......
 
I would pay $26.95 for a copy of the Car engine book. I flipped through the Cartech BBC interchange manual and it's a pretty light read. Don't know how much they charge but much less information than I have learned from "Southernairboat" members. getting off topic because you could do AC engines book, Hulls and riggings etc.

Waterthunder and Felber as well as others validated my purchase of KB Hyperutectics, and they gave me the information to build it to it's best potential within my budget. I think all the information you need is in previous threads. Really looking forward to getting this thing running! I tried to post some pics from the KB site but wrong format. They make a stepped piston for the Buick 455 and I was surprised the distance between the top of the pin and crown of the piston. What a difference between it and my short little long rod 489 pistons! :lol:

T-rex, I guess that's the problem with being your own engine builder! You have to learn from data, ask peoples opinions (like you said everyone has one, and don't argue quench engines with old Mopar guys! :wink: ) work it into your budget without going over too much and hope you don't make too many mistakes. don't run 42 degrees advance with a Hyper. and learn and understand why. Actually part of the process I enjoy, and since I haven't built any REAL Hi-performance engines since the '80s I guess I can say this engine will be built with the new ideas! BEEN PULLING WRENCHES FOR 30 YEARS AND IT'S JUST A JOB NOW, but I'm enjoying this project! :lol:
 
I'm amazed at the intellect of this site there are a ton of very experienced and intelligent people on this site both car motor and A/C. I have been on a ton of sites where people try and disprove people to prove how smart they are. On Southern everybody is open to all information take it in and learn from it or expand on it. I love this site people like Felber and Potter and Thurman can always teach you something new. I guess there are not any know it all's that have never built a motor. Everybody on this site is a do it yourself person that has actually done it many times! Myself and Felber are both tired of arguing with people who have never even seen the inside of a motor I'm sure Potter and Thurman are tired of this also. The internet is a pain in the @$$ accept for here because like I said earlier everybody has real experience or truly want's to do it their self and that's when it's fun! I hope the type of members stay the same and Southern grows but doesn't change!
 
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