Swap pistons for quench?

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Spadman

Spadman

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This post is for advice on whether to spend about $450 just to improve quench.

I disassembled a 360 short block I built 30 plus years ago to clean it before finishing it and putting it in my 1970 Dart. I didn’t think of quench back then and put H405P flat pistons in it that are .050 in the hole and about 9.5 to one. I’m fine with the compression but realize I can do better with KB232 quench domes. They calculate to be .012 in the hole and still about 9.5 compression. I have home ported J heads milled to 59 cc. They are not heart shaped and so still have significant room in the chamber for the quench dome. Is it worth it to swap out the pistons while I have it apart? Goal is a great running street engine for 93 octane. I’m using a MP P4452992; hydraulic flat tappet 238,238 at 50, .474 lift, 110 lsa.
 
If the compression stays about the same and all you're adding is quench, I vote no.
 
9.5 to 1 static dont need quench! now you up compression to say 11 to 1 a lil quench is good insurance against detonation... but at 9.5 spend the money somewheres else!
 
I agree with above. Also, 30years ago we did not know (or just started to know about)
quench area. Don’t be hard on yourself. I did a 360 .030” over and street drove it hard with 10.74 to one compression. Keith Blacks. Street fuel was better then. 34 degrees (approx) all in and little detination.
 
Wouldn't the quench pad have to be around a 0.100" out of the hole to get some quench?
 
Syleng1 said:
I agree with above. Also, 30years ago we did not know (or just started to know about)
quench area. Don’t be hard on yourself. I did a 360 .030” over and street drove it hard with 10.74 to one compression. Keith Blacks. Street fuel was better then. 34 degrees (approx) all in and little detination.
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Right! Even the factories didn't know about all the benefits, because they routinely put engines together with closed chamber quench heads with pistons .100" or more in the hole at TDC.
 
273 said:
Wouldn't the quench pad have to be around a 0.100" out of the hole to get some quench?
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I think that's close. Seems I remember reading somewhere the chamber was on the order of like .092" deep where the quench pad would be. So, either use a quench dome piston and mill them for proper clearance, OR use a closed chamber head with zero deck height flat tops.
 
Use a thinner metal head gasket and raise the compression. You'll need to increase the Pressure for that cam. Or all you'll have is lumpty lump and no power increase. I hate engines that chug a lug in gear. They should sound like horses in a gallup. If your gonna cam it give it pressure.
 
Thanks for your replies. I really appreciate the help I find on this forum.

Seams like I may be shooting low on compression. My heads are milled , maybe forty thou, I don’t remember. I will go ahead and measure what is left in the area where the quench pad would be. The piston’s quench dome is advertised as 50 thousand tall. With .12 in the hole and a .039 fel pro it would be about even with the head surface. The depth of the chamber would be quench measurement. If I by chance could achieve about 30 quench, what compression could I run with 93 octane?

I did not find a suitable step dome piston yet that didn’t put me in a close to eleven. I’ll measure and report back.
 
Checked my heads and they need some additional work. I just measured the cc’s in one chamber so far, but depth of the chamber in one head varies from 30 to 40 using straight edge and feeler gauges. I planned to clean them up from 30 years of sitting so I can even that out to with a clean up cut and grinding some from the one hole that’s 30. The other head is about 60 in all four. Pending how the cc looks like I will need to mill around 20 more on that side. Oldmanmopar says I need more compression. What can I run with say 35 quench in iron heads on 94 octane.
 
Spadman said:
Checked my heads and they need some additional work. I just measured the cc’s in one chamber so far, but depth of the chamber in one head varies from 30 to 40 using straight edge and feeler gauges. I planned to clean them up from 30 years of sitting so I can even that out to with a clean up cut and grinding some from the one hole that’s 30. The other head is about 60 in all four. Pending how the cc looks like I will need to mill around 20 more on that side. Oldmanmopar says I need more compression. What can I run with say 35 quench in iron heads on 94 octane.
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More compression than "WHAT"? How do you know you need more compression unless you measure what you have now?

Also, what are your plans for the car? Street car? If so, the last thing you need is to push the boundaries of pump gas driving around hung up in slow or stalled traffic in 90 degree plus weather. There are plenty of ways to tune a lower compression engine to run. Don't get hung up on this without knowing exactly what you have now.
 
Thanks. I’ll finish the heads before I look for pistons. Based on the one 59 cc measurement with the current pistons I’m at 9.45 assuming 60 cc after cleanup, but they are flat and 50 in the hole. With a .039 gasket, plus .050 deck clearance and .035 in the chamber; that’s .124 total. That number seemed horrible and I started looking for a stepped head piston, with greater compression height. Except for the KB232, the others I found increase compression. That’s why I said I went to the compression question.
 
Spadman said:
Thanks. I’ll finish the heads before I look for pistons. Based on the one 59 cc measurement with the current pistons I’m at 9.45 assuming 60 cc after cleanup, but they are flat and 50 in the hole. With a .039 gasket, plus .050 deck clearance and .035 in the chamber; that’s .124 total. That number seemed horrible and I started looking for a stepped head piston, with greater compression height. Except for the KB232, the others I found increase compression. That’s why I said I went to the compression question.
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9.4 is dead perfect for the street.
 
If your not knocking your neighbors pictures off the walls , you don't have enough compression.
Yes I'm responsible for breaking that valuable vase. lol
 
Are Alum heads ever in your future?
I would spend the money and coat the pistons in a heartbeat.
 
Probably not for this engine. I started it years ago in my youth with porting the J heads and I’ve never ran it. I’m building the day 2 car I would have done back then if I had the money. I want to experience it like that for a while, now thirty plus years later. This was an original 340 four speed car. I plan to also build a 340 for later if that ever comes. Life is uncertain and I’m not a spring chicken. I have all the parts, intake, exhaust, untouched X heads. Will I go for Trick Flows? Maybe. Roller rockers? Maybe. Got to get this engine in the car for now. BTW; I’m curious about the coating the pistons suggestion. How does that fit in?
 
Two coatings, friction improver on the skirts and a thermal coating on the tops. not a universally accepted upgrade, I do it whenever possible.
 
J-c-c said:
Two coatings, friction improver on the skirts and a thermal coating on the tops. not a universally accepted upgrade, I do it whenever possible.
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I knew about the skirts, not the tops. Thanks.
 
Post #5 as a blanket statement is incorrect. I had a V8 engine, 8.5:1 CR, no quench, that pinged badly on 98 octane, the highest octane available.
 
Don’t forget the rebalancing cost with new pistons. I also vote no for chasing quench when you already got parts.
 
1MeanA said:
Don’t forget the rebalancing cost with new pistons. I also vote no for chasing quench when you already got parts.
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I found that combined advertised pin and piston weight of the pistons I was considering were within a few grams of advertised stock. Even checked with the maker when there was conflicting information. That said, looks like I will go with my long time motto “When in doubt, take the cheapest way out”.
 
I have read on this forum that some guys have used H116-CP's with good success without having to re-balance. I believe the weight difference is 4 grams lighter for .030 vs stock. May want to check with manufacturer.
I have not used them myself......yet.
 
JoePole1 said:
I have read on this forum that some guys have used H116-CP's with good success without having to re-balance. I believe the weight difference is 4 grams lighter for .030 vs stock. May want to check with manufacturer.
I have not used them myself......yet.
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That’s one of the pistons I looked at. It’s a flat top with greater compression height (but still .027 in the hole) than the 405 and only one set of valve reliefs for only 5 cc’s. About 10.5 to 1 with 60 cc heads and .039 gasket. I haven’t confirmed the weight. Problem is it’s a flat top so no quench with my heads.
 
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