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Piston and Math help please...

5K views 67 replies 7 participants last post by  Greg T 
#1 ·
I recently came upon two sets of gorgeous pistons. (20 all toll) (new) They are forged BME for SBC 4.125" bore, 23°, compression height of 1.200". I haven't had to calculate this in so many years that I just lost my touch. So, given a 9.020 small block, what rod/stroke combo would I need to use this piston? I understand it may be .005" or so either in or out of the hole, but my brain is slipping. Any help would be great. Also, the pin diameter, according to Bill Miller, is BBF. ??
 
#39 ·
Eric.....This is the internet and theres no room for the truth here. Just....stop:D



Cheap and a valve train at 9K don't even belong in the same sentence. A few hundred in springs, a few more hundred in titanium valves, about a thousand on a shaft rocker system, another thousand for 7/16 push rods, lifters, etc. Then you still have to get a cam long enough to make power at 8k and work ok with compression low enough for pump gas.

The last and and only 358 I used made 680 at 9200 and cost $32k and it still wasn't better than my 410 at 8K and $15K. The 358 required refreshes at 1500 laps and cost $7500 while the 410 goes back at 3000 laps.
 
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#43 ·
got you on the rods eric the same guy has several sizes of rod widths.

As for the 400 there is the issue of compression. How much compression will that motor make in a 3.75 stroke. with those big domes.

The engine with 3.75 stroke will land with 11.5:1 to 15:1 depending on heads and the amount of dome on the pistons. Im not so sure about it going to 11.5:1 with those pistons thats assuming they are only 10cc domes.

I dont see why either could work but seems to me the short stroke combo can be done cheaper. You dont need titanium valves or super valve springs. Cheap heavy springs are not that much. Same for the rockers lots of none shaft rockers have gone to 8K. Now for a circle track motor doing 500 laps/miles per race the setup would be completely different. Hell the stock heavy duty stamped steel rockers can hit 7K and did when gm sold the 302.

Not everyone has a need for the best of the best. Lots of cheap stuff will go really high rpm without flying apart. Now rods can be the one thing that does not last but lucky many companies make them in many different sizes.

As for the thick spacer bearings i have seen a set on ebay a dew months ago but didnt buy them. I think they can be found. I know i was looking and saw someone was selling them as nos on there website. A good search on the net will turn up a set im sure.

Im super cheap and know that the cam is a none issue plenty of them on the market lunati has a nice selection of really big cams for the drag race market. Lots of racers run on very tight budget so they need to keep cost down and make things that are not perfect work.

I know a guy that runs a 307 and turn it to 8K all the time every weekend his car is fast and uses mostly stock parts he buys every 307 he cans and swaps parts around as need to make it work making it work as cheap as he can. Car runs fast and has been on the track for 20 years only spend a couple hundred a year for parts since he cant afford any more.

Its nice if you got 30K to spend on a motor but people did things like this long before titanium and other parts are on the market. So there must be a way to make it happen.
Well, my kid's other motor that scattered was a stock crank, stock rod, iron headed wonder when he first got it and he used to set the tach on 7k during burnout contests for 2 solid minutes. And the beat it like an ugly step child on the way home. That was a 350. Then he put the NKB heads, Howards rollers, Lunati 250ish cam, and custom Q series QFT. Spun it to about 6800 on the dyno runs, made some nice numbers on the first run, and would probably still be going strong today if the fuel pump wouldn't have had to defecate in the middle of the second run. After the complete teardown and analysis, it was determined that the front 4 pistons wadded up like golf balls. He said it was still running on the back 4 until he hit the kill switch. So, yeah, we don't need all the top shelf parts to make a good running bullet. Care at assembly I believe is more important. Balance, tight clearances, and a surgeons care is what makes them run.

P.S. These are .250" domes. The valve reliefs look quite deep, but that is relative to the dome. So they may not really BE that deep. These could easily be a 12cc or so piston. Bill Miller couldn't tell me without assembling and CCing a cylinder. I guess he didn't want to do that for free. I don't blame him.
 
#45 · (Edited)
Yes, ... even considered a slight bit weaker due to the head bolts being so close to that large bore....sometimes cracks happen from bolt hole to bore, or from steam hole to water jacket opening.

Anyone who truly wants to try for a bulletproof package doesn't start with any stock block,327, 350, 400, whatever,... it has to be aftermarket.
 
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#46 ·
Well, I just got off the phone with him and he's decided he wants to make liars out of his nay-sayer buddies. They laughed when he bought a 327 expecting to get 400hp to the wheels. The motor he bought is a complete rebuild, so he wants to tear it down and send out the block for testing, decking to 9.00", crank checked, get some aftermarket H beam rods and forged slugs with .200" domes (aiming for about 11.5:1 SCR so I'll map that out later) and then have it all balanced. Gonna get top shelf bearings, AFR 195 competition heads and custom solid roller set up. He's got 4.56 gears in the truck and with a good converter I think he'll have a fun truck. I'm relatively sure he'll surpass his goal.
 
#47 ·
just my opinion, but I think H-beams are expensive overkill, no sense putting an race Heavy duty rod in between a stock small journal crank and a stock block when the Sportsman type stock appearing I-beam rod will easily handle the load....and be a good bit more budget friendly.

H-beam rod weight could lead you into an expensive balance job with that little crank too.

The TRW forged L79 327/350HP domed piston is still available for a budget deal, TRW L2166NF with a .125" 5.3cc dome, but it only gets you 10.85:1 on a zero'd block. If you go this route, note that these are pressed pin only, not a problem at the power level but would be a problem if you mistakenly already bought bushed rods ;) . Same piston was in 365 and 375 HP engines also.

Either solid flat or solid roller with the AFR 196's will get you to the 550 hp crank level at a 7200-7400 rpm power peak.

This is probably one place I'd say the Performer RPM might be the best choice...best single plane is probably the Weiand Team G as it has the smallest runner cross section of al the race manifolds.

Has he got at least 3500 stall ??
 
#48 ·
Thanx much! I'll pass that along to him. I forgot about the weight difference with the H beams. I'm sure we'll use the RPM Air Gap as usual and a solid roller either from Lunati or Comp....custom grind. Extremely happy with mine from Comp. Degreed in perfectly. I'll have to double check the flow numbers before deciding on lift. For pistons I'd like to be at about .200" dome as I roughly calculated about 11.5. Some of the .250s I've checked out are listed as 12.5:1 which would be pushing it a bit too far and would require more trips to the air port. I am also considering a 6" rod for a bit more dwell time if a 1.375" CH piston is available, or close. We can deck it to match the piston height if we wait til all the parts are in-house. I see Lunati offers a 4.030 with a 1.383 (I don't know either) for a height of 9.008. That would work. I'm getting a head ache! At least he's made up his mind and we can begin the accumulation of pieces!
 
#49 ·
ICON/UEM/Keith Black has a 1.385" height 12.7cc hollow dome #IC818, .400" tall dome 12.4:1 comp@64cc head.
Also have 1.385" height 9.7cc dome #IC817, .250" tall hollow dome 11.9:1 comp@64cc head.
And 1.385" height 6.3cc dome #IC791, .163" tall solid dome 11.3:1 comp@64cc head.

These were all for a 6.0" rod and all are forged 2618 material
 
#53 ·
Great info. And the smallish cam kinda surprises me. Looks as tho he may have quite a bit on the table. I would think in a small cube something in the mid 230s on the intake would be the ticket. I've read a lot of conflicting stories about duration split these days, also. I'll need to speak to a good designer for that. I was always taught that head flow dictates lobe split. If the exhaust flows less than 70% of the intake (average) an 8+° split is called for. If it flows about 75%+ then a 4 to 6° split is good and if the exhaust is 80% then you use a straight grind. Going with that, I designed my 388 cam this year on my heads, in which the exhaust flows 86% of the intake and I used a reverse split. 245°/243° @ .600/.600 lift on a 112° LSA.
So, I was going to calculate the AFR heads and work as such. Recently, I see mag articles that say exhaust flow doesn't matter!
 
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#54 ·
He was looking at it last night and said there is a .010 stamped on the crank, but isn't sure which journals to which it refers. I'm guessing with no indicators it means all the journals have ben cut. Have to wait til the crank is out and mic it all up.

He did pull a brand new Comp 268 cam and lifter set out of it last night. That'll go on Ebay. Still has the assembly lube on everything. I was looking at shooting for about 8.5:1 or 8.75:1 DCR for pump fuel but that's going to be the last of the planning when it comes time to grind up a stick.. I know we are wanting to start with about 11:25 or 11.5 SCR, high flowing heads and a solid bottom. I'll speak to the grinder about RPM range etc. when we get to that stage. Obviously, if the peak power number we're looking for can be made in a lower RPM, THATS what we'll shoot for. We'll just have to see how everything plays out. First order of business is to mic the crank, shop for rods, have the crank checked at a shop, and if everything looks good we get some pistons. And then it begins! ;)
 
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#56 ·
You can make your goals with those heads without having to resort to .700" lift cams, etc. 11:1 compression, a solid flat tappet in the 235 to 240 range & a little over .500" lift will get you close without going much over 7000 rpm.These are not NKB'S. They will make way more power with alot less cam & still have a decent lowend for driving around on the street.A good head doesn't need excessive duration & lift to make power.
 
#57 ·
I agree that cam suggestion is more than a bit much, with a power peak over 7700 rpm and the torque peak at 5600 rpm according to DeskTop Dyno and Performance Trends Engine Analyzer.

Something solid/solid roller in the low 240's intake dur@.050" and 110° lobe separation seems to provide the broadest usable power curve.
 
#62 ·
If the stock rods don't already have ARP rod bolts you may want to just plan to replace them with better aftermarket I-beams, as the cost of ARP bolts and resizing gets close to the same cost as just replacing them with much better parts.
The stock rods are 1053 steel, have who knows how many million cycles on them already, and have inferior 11/32" bolt size(the small journals all had these smaller bolts, compared to the 3/8" bolts in large journal rods). The bolts are also responsible for cap location/alignment,

Aftermarket I-beams are either 5140 or 4340 high strength steels, have no run cycles on them, have a much stronger capscrew fastener in 3/8" size and hollow dowel locators for the caps.

If you've got to really stay on a budget, new sportsman rods and the TRW 350HP L79 piston(5.3cc solid dome) is about as cheap as it gets. Take it to a .005" shy deck and use a .026 or .028 gasket for .031-.033" quench and you'll get 11.25:1 compression.
 
#63 · (Edited)
Well the rods have to go and that's why I was asking earlier about the shims. The large journal rods have larger bolts than the small journal rods. So, I was thinking a set of 350 large journal rods because of the bolts, and, I believe the beam is a bit heftier as well.

Eric, saving money on the rods and pistons is what I had in mind so that there would be more money to put into heads. BUT, not at the expense of durability. If those L79 slugs will have no problem with the HP range and rpm we're looking for, then that is a viable option.

hcompton, we were checking out the 195 AFR competition ported heads for the flow ratings. My idea, again, was to get maximum flow runners at a minimal lift range. The AFRs seem to fit the bill. They are flowing at .400 to .500 what most heads need over .600 to get. I believe for a durable HOT street engine, unless you use top shelf exotic valve train parts, you need to stay in the .600 or less range. Seems to me the spring pressures needed to control large lift would take it's toll on anything less than very expensive parts.

Again, if we can make a small cube motor run well, run strong, and surprise quite a few people with a dyno pull of about 420, 430+ or so to the wheels and able to joy ride around town, it will be a success.
 
#64 ·
If staying with the stock 5.7" rod length, the TRW/Speed-Pro forged replacement for the factory L79 piston will handle that kind of power without a hitch....they would be fine well over 600HP NA and if the rings were gapped for it they would also handle another 200+ nitrous on top of that. The only downside is they have a old school half round dome shape that was made for the old Fuelie head, the dome may not clear a modern chamber shape, and the intake valve notch mat not be big enough depending on what your chosen head has in it.
Look up TRW L2166NF images to see what I'm referring to.
Might not be worth the hassle to you if rework is necessary, might be better to just get a modern piston even though cost is higher, if you have to pay to modify the old school stuff it probably would equal out and the new stuff would be plug and play.

I wouldn't use a factory large journal rod just to get the 3/8" bolts... plus you then have to find the thick wall bearings.SCAT and Eagle both have a small journal stock appearing I-beam that has 3/8" capscrews and a press fit pin in the $240-260 range. Eagle's is SIR line and 5140 material and SCAT's is ProStock line and better 4340.
 
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#66 ·
I believe he wants to replace the rods, not knowing if theyre actually 50 years old or have been replaced at some time in history. As long as the rods will be replaced I'd like to see a 6" rod in there. We have a fine scale so matching the rods to what is in the now may not be a problem. Once we get a piston out we'll weigh that up too so we can work on getting one close or matching something that looks good. We'd like the guy doing the balancing to have the least amount of work possible when he gets the stuff.
 
#67 ·
This should come off as a really good, powerful,& efficient motor.You appear to be going about everything the rite way & taking consideration of everything that counts on a good build.Starting out with a good head makes alot of difference.You don't have to crutch the build up with extreme cams, or, sacrifice anything for the use it will see most of it's life.
Throwing in the biggest cam you can find along with big heads, etc very rarely works or makes the best power except for in the small minds of ppl who don't know any better. When you have a head that flows really good at all lifts, it really makes a difference as the motor's power is linear.It is constantly changing from off idle to max power.The lower lift flow does matter & doesn't need extreme lift/duration to keep the valve open longer like a less efficient head would.
You're also considering all the little things that some find insignificant, but, actually make big contributions in the final picture like quench, weight, rod length,& balance.
 
#68 ·
We always try to use the best stuff we can afford at the time. I learned at an early age that tossing cheap parts together just doesn't make it. I'm a believer in doing something right so it only has to be done once. I'm hoping this will turn out to be eye opener to the critics out there. ;)
 
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