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post #16 of 26 (permalink) Old Feb 25th, 20, 12:20 PM
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re: Piston-to-head: what's ideal? What's safe?

My SBC 350 I ran .013" piston down the hole and .015" (.028" piston to head) gasket for 35,000 miles Shifted it at 7600rpm max.
I changed to a thicker gasket for .051" piston to head and the thing ran exactly the same.
But when temps got into the 85-90 degree days I had to add a little heavier spring on the advance weights to slow my curve down a little with the .028" piston to head clearance.
I did not need to do that with the .051" deal. The head was 186 casting milled to 54cc

SO not as finicky at .051"
This was a cast piston USA made 345NP which I think is pretty darn close to the same silicone content of the 4032 stuff.

I never had piston to head issues at .028" Stock GM rods and bolts Standard bore.

I friend ran his 357" at .043" and ruined a cam and did not clean out all the debris.
3 months later at the races he gets a rod knocking right out of the hole but has the will to win and stays in it.
He won and the piston did hit the head and tightened the top ring groove, did not break the piston but did bend the rod very slightly.

Stock rod and same USA made 345NP pistons.
That one cleaned the carbon off the head.
Did not hurt the head. 601 casting iron 305HO
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post #17 of 26 (permalink) Old Feb 25th, 20, 1:19 PM
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re: Piston-to-head: what's ideal? What's safe?

Quote:
Originally Posted by BillyGman View Post
I must be mistaking. Let me take another look. Perhaps it is the 4032 stuff my pistons are made out of. Maybe I'm getting the two alloys mixed up. I have the Mahle flat top BBC coated pistons with an 1.120" pin height for a 4.6" bore.

To answer your question, I'm getting ready to start digging into my current engine combo. There's things I'm not content with.
4032 is usually the street/strip pistons like SRP and such, 2618 is the race pistons. If your motor is really old, it could have the 2618 pistons in it. I THINK the 4032 lines of pistons started to gain traction in the early to mid 90's..
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post #18 of 26 (permalink) Old Feb 25th, 20, 1:32 PM
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re: Piston-to-head: what's ideal? What's safe?

Quote:
Originally Posted by BillyGman View Post
Perhaps it is the 4032 stuff my pistons are made out of. Maybe I'm getting the two alloys mixed up. I have the Mahle flat top BBC coated pistons with an 1.120" pin height for a 4.6" bore.

I just looked in the 2019 Mahle catalog and the only 4.600 bore flat tops are 4032

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post #19 of 26 (permalink) Old Feb 25th, 20, 3:11 PM Thread Starter
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re: Piston-to-head: what's ideal? What's safe?

Quote:
Originally Posted by jeff swisher View Post
My SBC 350 I ran .013" piston down the hole and .015" (.028" piston to head) gasket for 35,000 miles Shifted it at 7600rpm max.
I changed to a thicker gasket for .051" piston to head and the thing ran exactly the same.
But when temps got into the 85-90 degree days I had to add a little heavier spring on the advance weights to slow my curve down a little with the .028" piston to head clearance.
I did not need to do that with the .051" deal. The head was 186 casting milled to 54cc

SO not as finicky at .051"
This was a cast piston USA made 345NP which I think is pretty darn close to the same silicone content of the 4032 stuff.

I never had piston to head issues at .028" Stock GM rods and bolts Standard bore.

I friend ran his 357" at .043" and ruined a cam and did not clean out all the debris.
3 months later at the races he gets a rod knocking right out of the hole but has the will to win and stays in it.
He won and the piston did hit the head and tightened the top ring groove, did not break the piston but did bend the rod very slightly.

Stock rod and same USA made 345NP pistons.
That one cleaned the carbon off the head.
Did not hurt the head. 601 casting iron 305HO
Jeff, that's some very interesting info you've shared there. Thank you. So from what you;re saying, it sounds like you were getting a better and more effective "squish" action from a .051" piston-to-head, eh?

Quote:
Originally Posted by bracketchev1221 View Post
4032 is usually the street/strip pistons like SRP and such, 2618 is the race pistons. If your motor is really old, it could have the 2618 pistons in it. I THINK the 4032 lines of pistons started to gain traction in the early to mid 90's..
Good to know. My engine isn't that old.

Quote:
Originally Posted by BillK View Post
I just looked in the 2019 Mahle catalog and the only 4.600 bore flat tops are 4032
Very good Bill! Thanks very much.

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post #20 of 26 (permalink) Old Feb 25th, 20, 3:39 PM Thread Starter
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Re: Piston-to-head:what's ideal? What's safe?

Quote:
Originally Posted by Monk View Post
Billy, if you don't mind me asking, what is it that you're not satisfied with about your 632?
The whole combination for one. It's a typical POS crate engine. The difficulties with it began the first day I brought it back from picking it up at the Yellow freight company, and uncrated it. (3 rocker arms just laying there loose and not even tightened down, with .100" valve lash, and being run like that on the dyno with gouges on the rocker arms and the wrong poly locks installed).

Had to send the engine back right away after I met the builder's demands to send them pictures to prove the damage to them before they would even agree to take it back. Then the cylinder heads leaking coolant out of the header bolt holes even after I used thread sealant, due to the machinist breaking into the water jackets while drilling the bolt holes. The builder wouldn't make good on that. A slap in the face for me after spending $17K on this engine.

Just eye balling some of The scorpion rocker arms I can see that they don't even come close to resting on the middle third of the valve stems, and at least one of them even looks like it's close to hanging off the end of the valve tip, (terrible assembly practices!!). Then there's the issue of the challenge to find header gaskets that will establish a good exhaust seal since the exhaust ports are rectangular and the headers are round, (as almost all header are). All these issues, some of which can be considered minor, all together on top of the merlin heads not being the greatest design anyway, and would be considered mediocre at best.

So after the crap I've seen I don't trust that anything is right. I'll be yanking the heads off to have some closer looks. First of which will be to measure how far down the pistons are in the hole. I don't like not knowing exactly what I have anyway. And I might decide to go with new heads. Not Merlin either. Maybe brodix Dragon Slayer with 365cc ports. I'm kinda on the fence about wether or not to leave out the stud girdles. It looks like it will be a hassle to adjust the valve lash and then tighten down the girdles without the lash settings being moved. Maybe I'm all wrong on that. I never messed with stud girdles before, but they look like they might be more hassle than they're worth for my application. The cam is a solid roller, but the engine power peak with the cam is only 6,000 RPM. So although the total valve lift is .705"/.708" I'm not so sure the stud girdles are needed due to the low RPM range.

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post #21 of 26 (permalink) Old Feb 25th, 20, 4:11 PM
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Re: Piston-to-head:what's ideal? What's safe?

Billy,
IMO run the girdles. Itís not just about rpm. Harmonics, deflection, stability and robustness all come into play when running a higher lift cam. Brodie sells two piece girdles for their heads. One for the exhaust and one for the intake side. This eliminates any stud movement that an improperly chosen/setup one piece girdle might cause.
Iíve never had any issue lashing the valves when using girdles. When set up properly they wonít change your adjustments at all. Brodie two piece girdles are so easy to install!
Great piece of mind for street or track use.
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post #22 of 26 (permalink) Old Feb 25th, 20, 6:21 PM
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Re: Piston-to-head: what's ideal? What's safe?

Tighter than .039, pistons hitting the heads, I have something for those that subscribe to that.....HEEEERE'S YER SIGN!
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post #23 of 26 (permalink) Old Feb 25th, 20, 6:30 PM
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Re: Piston-to-head: what's ideal? What's safe?

[QUOTE=BillyGman;11240106]Jeff, that's some very interesting info you've shared there. Thank you. So from what you;re saying, it sounds like you were getting a better and more effective "squish" action from a .051" piston-to-head, eh?



On that combo it worked out and I have that same short block and same pistons today in it's 3rd vehicle since I had it.
Pistons still .013 down the hole block never bored and crank never turned.
Now running the 10.78 compression and .039 gasket currently ran that just fine for a couple years with a 268H on a 106LSA 57cc chamber heads Ported and worked over iron 601 heads.


It now has a 280H cam on a 110 lsa ..needed more top end power.
I had the RPM but the 268H would get there slower.
It is funny both cams on a rear wheel dyno were really close to making the max HP at same rpm.


268H max HP was 5700 rpm and the 280H made max HP at 5850 rpm.
After the HP peak the 280 really carried the power farther.
I tried different shift points at the track and 7000 rpm was the sweet spot on the 280H. on the 1-2 and 2-3 shift.
The 268H was an odd duck the 1-2 at 5500 rpm and hold till 6000 in second for best et.
35 total timing made 3 HP less than 38 total at the wheels with the 268H on 106 LSA.
35 I felt was fine and pretty conserative.



I built a 355 for a buddy with edelbrock 5089 heads (i ported them also) and the 280H and it also wanted to be shifted at 7000 rpm for best et.
That one was built on a 106LSA.
That 280H is 230 @ .050 and .480" lift.. Running crower cam saver hydraulic flat tappet lifters.
And 91 octane fuel.. if that matters. The 355 with 5089 aluminum heads wanted 41 total timing for best et and the most MPH.
Buddy got I think 12 passes that night with a lot of tuning and experimenting.



I can run 87 octane in my 350 if i keep my foot out of it.. you can get 1/2 way into it and no issues.
But don't hook the pontoon boat behind it with 87 octane.
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post #24 of 26 (permalink) Old Feb 25th, 20, 6:45 PM
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Re: Piston-to-head: what's ideal? What's safe?

H beams and .045 hopefully idiot proofed this 383

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post #25 of 26 (permalink) Old Feb 26th, 20, 12:22 AM Thread Starter
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Re: Piston-to-head: what's ideal? What's safe?

Many of us shooting for pump gas use only, usually have a static compression ratio number as the traget we shoot for. Nobody wants to drive a car on the street, and have to pay $9, $10, or $12 per gallon for race gas because the engine in question pings all over the place on 93 octane pump gas. Unless of course you're a Jay Leno type who earns a seven figure yearly salary. So ofcourse the static compression ratio target number has to be considered when we're choosing the piston-to-head gap to use, since it will ultimately become one of the variables that determines the static compression ratio that your combination will end up with.

When you're starting a build completely from scratch, you'll have more control over that, since you can decide on dome height of the pistons, and head chamber volume too. But be it a partial build, or a build up from scratch, the piston to deck space will be a factor. However, if it's true that having a good "squish" action going on inside the chamber during combustion, increases efficiency, and reduces the potential for detonation, ("pinging") then striving for the ultimate or the best piston to deck space IMO should be one of our goals, (for pump gas builds in particular).

I suppose that what we need to add to the mix is what Scott mentioned about certain applications perhaps requiring, or being optimized by slightly different squish heights too. But from what I'm hearing from some of the examples given, (especially from Jeff) indicate that this isn't an exact science, and/or that a larger squish space than the accepted .035"-.040" might very well be optimal for many pump gas applications. But this is just me speculating. Lets face it, it would be pretty tough for most of us, (if not all of us) to spend the money and the time to perform back to back tests on the dyno and at the drag strip, as well as on the street, on different piston to wall clearences just to see what works best for pump gas use.

Even if we had nothing better else to do with our time and money, any conclusions derived from such tests would still only apply to the specific vehicle combo that ws used, unless perhaps multiple vehicles at various race weights, with multiple rear gear ratios were used. Which makes such an endeavor even more unrealistic. But it still serves as some food for thought when the details of different combinations are shared, and what end results were brought about by those combinations. If nothing else, it can sometimes help us explain why certain combinations that we might have tried in the past might not have worked the way we had planned.
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post #26 of 26 (permalink) Old Feb 26th, 20, 10:33 AM
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Re: Piston-to-head: what's ideal? What's safe?

. Lets face it, it would be pretty tough for most of us, (if not all of us) to spend the money and the time to perform back to back tests on the dyno and at the drag strip, as well as on the street, on different piston to wall clearences just to see what works best for pump gas use.

Even if we had nothing better else to do with our time and money, any conclusions derived from such tests would still only apply to the specific vehicle combo that ws used, unless perhaps multiple vehicles at various race weights, with multiple rear gear ratios were used. Which makes such an endeavor even more unrealistic. But it still serves as some food for thought when the details of different combinations are shared, and what end results were brought about by those combinations. If nothing else, it can sometimes help us explain why certain combinations that we might have tried in the past might not have worked the way we had planned.[/QUOTE]




BillyGman makes good points here.


I was on the very lucky side of the fence in all my testing.
I got lucky by having a job pretty far from home that paid well and let you work a lot of hours if you could get the job done.
I was on 24 hour call quite often ..as people did not want the pager and i would take it and the money that went with it.


70,000 miles driven in a year was the norm. the wife did not like the school system where we lived and in 11 months she had driven 67,000 miles on one of our vehicles.
I had a lot of vehicles in case you got bored driving one of them.


I got to test all kinds of combos and gear ratios and exhaust pipe diameter and muffler and pipe combos.
It helps to be a welder/ fabricator and have many tools also.


I did not need to pay people to do things for me so I got off cheap.


With my higher compression deals 10.7-11.9 I found if i tuned the timing curve under a load like in high gear going 30 MPH and apply some of the brake and apply throttle If I the engine was not pinging i was good.
I would advance until it did ping so i knew where I was at then dial it back until all was gone and a couple more degrees removed to be super safe.


This gave me total timing and I wanted it all in by cruise rpm.
I ran gears from 2.29 to 4.56 and did not need to change total timing and pulling trailers I did not need to change total timing.


So vehicle weight did not really matter as I was on the safe side.
Could I have went .05 seconds quicker with 2 more degrees timing ..maybe. but I will give that up easily for something I drove all the time and worked it hard.


My 57 chevy has the 350" I have had since 1992 and it along with all the vehicles that engine has been in get to pull boats and trailers many times the trailers are loaded much heavier than the tow vehicle.
I do not change the tune much.. but I am not racing either when trailer pulling.
I do actually change the step up springs or power valve when towing.
I change from 9.5 to 7.5 that way I am not in the power mode as soon and I get a bit better MPG.


I try to drive with max vacuum signal not wanting to make the power circuit come in.
Never had any issues driving hundreds of thousands of miles like that.


Many say weight and gear ratio has a lot to do with how much compression etc. but i found for me it really did not make any difference.
Was it conservative tune ..was it my heads are crap compared to AFR180 competition heads.
So I am not packing the cylinders as full.




That is another big thought I have had for years.
DO My high compression deals work because my heads flow much less than some really good aftermarket ones.
Would I have big issues with my compression and tune with a head swap.


Would I be in the crowd that states 9.5 compression is max for the street on pump gas?


I do not know the answer.
Ported iron heads vs AFR eliminator in a real back to back in a dual purpose vehicle.
How would the tune change if it did at all???
I have friends with those heads and some with other aftermarket heads.
I still beat up on them with my ported old iron stuff with heavier vehicle and less gear so I have not went there for my own vehicles.


Some heads that do flow well BBC oval 2.19-1.88 on 396" with 11:1 compression 280H cam 210 PSI cranking pressure.. runs fine on 91 octane 38 total 18 initial.
In a 4200lb truck that pulls his tractor and brush hog.
Never any issues.


It would be nice to know all the answers.
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