First, let me admit that I’m useless when it comes to using the search function here on the HAMB. I was hoping to find an all encompassing string of Pontiac engine information like that Olds Rocket thread, but I didn’t. I’m building a ‘62 Pontiac 389 for my Stude pickup to replace the ‘59 389 that’s in it now. It seems I didn’t think it through when I ordered the master engine kit from Northern Auto. The pistons supplied in the kit combined with the heads I have (69cc) make for about an 11:1 compression ratio. Taking 10 ccs out of the chambers only gets me down to 10:1, so I looked for some dished pistons. I’m thinking I can use Std bore (4.120) 400 Pontiac pistons since my block had to go to 0.060 to clean up. Summit and Jeg’s show KB hypereutectic pistons (KB346.std) for 400 Pontiac applications. They state that they have valve reliefs for the 14 & 20 degree valve angles. The compression height is close enough at 1.710 vs 1.700 for the pistons in the kit. Same 3.75 stroke, same rod length. I should end up with around 9.2:1 Any reasons why they won’t work? Am I overlooking anything? Let’s hear from the Pontiac fans.
Unless your cam is of very short duration, and the overlap is large (over about 112 degrees), 10:1 is on the edge of being usable. The Stude engine that I'm building (?) is about 10.4:1, and it's a street car (blue wagon). Come on, don't wimp out here, build an engine with some power !!! Mike
Is it really worth the effort to go from 10:1 to 9.2:1 ? assuming you can get 10cc out of the combustion chambers???
I’m having trouble visualizing 10 ccs, but the last time I ground on some chambers and measured them it seemed like I had to remove a lot of metal to get a cc or two change. I’m still looking at options. The KB pistons have about 0.010 more compression height (1.700 vs 1.710) if I believe the specs, and/or I can use a thinner head gasket to put it around 9.4 - 9.6. It’s a truck, and that means that it might end up with a trailer hitch and actually doing truck stuff like my old F100.
If your worried about compression ( I wouldn't be though) Use the 400 pistons, 71' up heads and intake and you'll have 8.2 to 9 compression depending on which head number. BTW the intake fits 65' up, heads are 65' up also so the only real difference is compression from 71' up. Pontiac used the heads to change compression. ..
Sounds like a great project. Your engines do have different bolt patters ya know. You want to be here: https://forums.maxperformanceinc.com/forums/forumdisplay.php?f=418
I had the same problem and cured it with a set of 0.060" Comtec gaskets from Butler. They were expensive but a lot easier to deal with.
I’m trying to keep the early Pontiac look and not deviate too much from the “as-built” look of this old custom pickup. If the 400 pistons will work, and I can achieve a workable CR without a lot of grinding of the chambers, then I can drive it with the ‘62 heads and my 3x2 setup until I’m ready to go ahead with my future plan to rebuild and reinstall the Rochester F. I. that the truck came with. The F.I. will require the earlier ‘59 heads.
Yeah, I got a brand new Wilcap adapter with the ‘62 389 when I bought it (cheap). It puts a Chevy automatic on the ‘61-64 block. My research found that Wilcap no longer makes the early Pontiac adapters so I grabbed this one when it popped up. Did you have flat top pistons and 69 cc chambers? Any idea where the CR ended up?
Two things you don't want to do to a Pontiac.. Use dished pistons, and modify the combustion chambers. A lot of engineering went into those machined chambers. Use the Cometic gaskets. Just watch the intake fit. Might need thicker gaskets there.
This would be on a stroked 455 and it lowered the compression from 11 something to 10. I don't recall the exact numbers but I am using flat top pistons. I live at 5500 feet elevation and have run 10:1 here on regular gas without a problem. It looks like you have the transmission adaption sorted out. I learned that one the hard way when I put a TH400 behind a '63 421 tri power in my old '62 Studebaker Champ pickup. Who knew that ugly could go that fast?
What am I missing here? Maybe I’ve read too many magazines. Ever since they decided that leaded gas was horribly dangerous for humans, hot rod magazines have been recommending 9 - 9.5:1 compression max for street engines running cast iron heads and pump gas. I know some engines are more detonation resistant than others, I ran my 392 Chrysler with a 10% over 6-71 and 10:1 compression on pump gas with no ill effects. And with a 4 or 5 spd and 4.11-4.88 gears you can get away with a lot of compression but that ain’t the case here. Here we are talking automatic trans, 3.23 gears and 30” tall bias ply tires lugging a 3200 truck around the streets at 35-55 mph mostly. Add in the 100 degree California sunshine and you have a recipe for detonation, especially with crappy California gas. Right now, with the factory advertised 10.25 CR, it’ll rattle pretty good until the poor old StratoFlight decides to kick down. Octane booster helps that. The simple Summit compression calculator says that 400 cu in with 69 cc chambers, 0 deck, flat top pistons, .060 gasket ends up with 10.9:1 CR. I don’t think that’s a practical number for a daily driven truck. There is the cam profile to consider (Comp XE262H), but I still believe it’s a point and a half too high for my application. I certainly do appreciate any and all comments, keeps me thinking.
My machine shop guy is supposedly looking into that for me but I haven’t heard back. Doesn’t that kill quench? Or do the dished piston tops kill it about the same? I’m not that familiar with the dynamics of cylinders and combustion chambers.
Bob, raising the head with a thicker gasket or taking .060 (if that’s what it take) off the pistons would have the same effect on quench. EDit: I mis read, I thought you were going with a head gasket .060 thicker. But an 060 head gasket will still take you out of a good quench area.
Look at Butler Performance or Icon piston, Icon has a good piston with 14cc dish that works well in 400 engines. I have built a couple engines for restored GTO's using them. They both run really well on regular gas. Pontiac over rated the compression ratios by about 1/2 point on most engines, so use the calculators or a good know site for accurate numbers, http://wallaceracing.com/cratio0001.htm Wallace Racing has about the best information and calculators you find for Pontiac's, http://wallaceracing.com/index.html.
That is the calculator that I used when I built my engine, unfortunately I failed to record the information and so cannot replicate the calculation. All I remember for sure is that it came out at 10:1 which was my target. Remember that I am at altitude and the air pressure here is about 80% of what it is at sea level; this has a rather major effect on cylinder pressure/detonation.
After looking around, I landed on the Butler site, their pistons are the ones I was considering. I got a response to my question from them this morning. I’m going up to talk with my buddy at the machine shop this morning. Since I got my Pontiac powered pickup I’ve been all over the Pontiac sites reading everything I can find. Some interesting info out there.
Hey, Bob; Just a reminder: Please consider keeping quench at ~ .045-> max of .060. Quench is considered "mechanical octane". If you have to dish the pistons, I'd only consider doing that in the open-non-quench-portion. Also, static compression isn't all that important, but dynamic compression is. & dynamic(running) is usually way lower that static. Which affects pinging, detonation, et-al. Cam events control dynamic comp ratio. Octane of course, does play a role. Most of the older engines didn't have all that much decent - or useable - squish, which was, & is, a problem. Stacking or thick gaskets, shaving the whole piston-tops, hogging out combustion chambers(esp in the non-critical side) reduce/eliminate squish, & drivability. Good luck. Marcus...
I guess the dished piston option is off the table. The block is already bored a hair too much to use Std 400 pistons. I’m about ready to say f_kit to the Pontiac and install the 392 Hemi that’s been languishing on the stand waiting for a home…..
Your over thinking it. Throw some .010 over 400 pistons in it, run premium fuel and drive it. Simple as that. Built many pontiac motors, most over 10.5 compression and none under 9:8, never used anything but premium fuel at the pumps, no octane boost, no race fuel etc. Many of those motors now have over 50,000 plus miles on them and still running fine on premium pump gas and all but one were not stock. .
You would only remove material near the center of the piston, leaving the rim near the edges intact. If these pistons have domes, you would just reduce the size of the domes, and nowhere else. If the pistons had an advertised dome of 10cc's, you would simply completely remove the dome. On a 4-inch bore 10cc's is about one point of compression, so you'd need at least that, if not a little more.
Hey, Bob; (Slight)Piston Knurling & maybe smallest oversize file-to-fit rings? Old time-tested solution. For todays' thinwall mills, probably not. For older well-cast iron mills - why not? W/o insulting, just how many miles will this mill get, over what time period? Knurling should be good for ~50k(or more) esp w/todays' excellent oils & filtration. Another possibility would piston-coating the skirts, but only for a small amount. Not excessively cheap, however. Hope you get this one figured out w/o too much headache. Marcus... ps: 1st gen hemis like Studes(& vice-versa), & they're ~ the 3rd easiest mill to drop in.
I stopped by the machine shop and Steve, the machinist floated the idea of knurling the pistons. Willy, the old master, wasn’t there so I’ll call him tomorrow.
Bob, this book might help you learn some more history and tips. Available from www.BOPengineering.com for a pretty reasonable cost . Tom Hand
Thanks Tom. I have that book in my collection, don’t know why I didn’t think to get it out and read it. I have my answers, now I have to make my decision and get back to work.
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