blue thunder

I find myself in a position of trying to figure out how to do this successfully. The reason I have hypers is I had to bore engines out to .070 over three winters ago during rebuild. KBs were the only ones I could find that would accomodate. So engine have ran flawlessly at 9:5.1 compression for 3 seasons. Now I've purshased 2 sets of 525sc take offs including carbs and all. But come to figure out this may be unwise thing to do because of the hypers and all the failures the 496ho have when trying to run boost. I presume these failures are due to poor thermodynamic characteristics of the alloy relative to forged and also the top ring is set high.

Yet I remain unconvinced that I cannot make this work with the boost referenced carb approach. This is were I need input from those in the know. Here are some thoughts I am considering:

> All boosted hyper failures I've read are fuel injected. Fuel injection is a digital output process as opposed to a carb which is analogue output. Carb are way more fool proof is my point. They do not have the discrete changes a FI setup would.
> Failure of a well tuned hyper engine would happen due to the top of the piston getting too hot at extended heat soak. So are the failures we've seen on well tuned FI engines only after lengthy flogging. One I read today Bob B. mentioned he ran for 7 minutes at wot then boom. I never run that long 15-30 seconds would be about it, then cooldown.
>I planned to run 4psi max. This would be 12.1:1 compression. If hypers really are going to fail doing this and heat is the culprit.... how about I add innercoolers but leave the boost at 3-4psi? More $$ but if it works I'd be happy.

What do you guys think? My engines are at the end of the line anyway at .070 over.

BT

BillK

bt,
My biggest concern would be the top ring gap, especially with the KB pistons. I think you can make the pistons last if you stay out of detonation, but the KB's are known for pulling the top ring land off if they dont have enough gap on the top ring and when you add more combustion chamber heat with the blowers, you are going to have to pull the engines down and increase the gap. You will probably need an additional .005" or so gap to keep the rings from butting with the boost. If you are going to do that, then you might as well get a good set of pistons and be done with it for a while.

Just my opinion,

Bill Koustenis
Advanced Automotive Machine
Waldorf Md

blue thunder

Thanks for the response Bill. The second year after I built these engines I tore them down again because I was on the low side of ring gap. Now you can drive a bus through the gap. .034-.036 on the top and .028 or so on the second if memory serves me. Both much higher than what was specified by KB. I was also wondering if the 496ho were failing due to end gaps, just forgot to mention that is my first post. I also have cometic head gaskets, a tight quench and faria exhaust valves which should be plus' over the 469ho.

Any more input?

blue thunder

Alright, I looked it up on the KB site. NA marine end gaps, 4.32 bore is supposed to have .034. I was more like .036+ since I was erroring on the high side. Unfortunately I can't find my actual measurements. The spec for supercharged marine was .038 which new I would have been at or just under. I should be there by now with 3 seasons running. Ring ends shouldn't butt I'd say.

RichardCranium572

I bet they butt.

About 10 years ago I built a whipplecharged efi small block for a full sized blazer. Since KB had just about any compression ration and pin height, I decided to give them a try. I had used them plenty of times in some hot street as well as circle track environments. I thought I played it plenty safe and had .040" top ring gap. It lived through the dyno tuning. No knock, afr 11.5:1 at wot(only that rich due to no intercooler). Just before I was to deliver the truck, I took it out in the country for a wide open throttle pass, not just a quarter mile...I had to check if it was going to live in the real world. It didn't, about 140mph it blew part of the top off of one piston. Took the block and had to weld a few of the chambers up too, expensive experiment. They are good pistons and priced right if used in an environment where they will live. ie hot street, drag or circle track use with high octane and short track. The only way I see them living in a marine environment is if the waves are big enough and you're in and out of the throttle allowing that momentary cooling when off the throttle. Making the horsepower is easy, making the horsepower and controlling combustion heat is the key to life in an endurance marine application. Using a piston that does not reflect the heat and then using the oiling system is all part of heat control of the internal components as well as combustion heat runaway.

Personally, I'd just order a set of forged pistons from cp, je, diamond, probe, anybody really. It might take 4-6 weeks to get them since they're not shelf stock(I haven't looked ,maybe somebody does have the 4.320" you need), ordering custom is not that big a deal and then you can get whatever you want...ring size combo, distance between lands and top ring placement, cr, etc.

blue thunder

Thanks for the reply Richard. Interesting story. How much boost were you pushing? I presume they were aluminum heads since you were able to weld them up. I'm also curious if that rich afr could have been a contributor to preignition at the upper rpm? Seems like an aweful lot of fuel.

I think if it was a matter of just ordering A set of forge pistons it would be a done deal. Unfortunately everything is times 2 in my case. Puts a whole different spin on things.

hoozeyurdaddy

the money your wiling to spend on intercoolers will cover the cost of good pistons, If there are any good parts in these engines you are putting them at risk of a major come apart, If your willing to gamble, go for it, odd's are not in your favor.
thats a lot to ask of a piston that does not work in any supercharged application. I ran the 525SC set up with 8.75:1 and 6lbs boost for 5 years with no problems, good forged pistons and 28 deg timing.
just my thoughts
larry

blue thunder

Can anyone suggest a good forged piston for boost applications that can be had in a .070 over, 4.320 bore? This seems to be a problem.

Also, I assume going from hyper to forged the bore gets opened up slightly, so this should work out ok?

BillK

bt,

Any of the piston manufacturers will make you a blower piston in that size.

As far as the clearance goes, it is actually determined by the piston, not the bore size. A piston that requires .005" clearance will be .005" smaller than the finished bore size. So for instance, a forged piston designed for a 4.000 bore would measure 3.995" if it required .005" clearance. You do not make the bores bigger for forged pistons. If the bore is worn, it will have excessive clearance.

I will try to remember to look on Monday to see if anyone has a .070" piston as shelf stock. What compression do you want to shoot for ?

Bill Koustenis
Advanced Automotive Machine
Waldorf Md

fastlane40

Blue thunder you have 496's correct ?How are you going to fit a 525SC setup on the 496?Nothing will fit.Unless i'm missing something here.If i am i'll go and sit in the corner.