There been a mixup between my engine builder and me on what pistons to use. He rebuild my engine with the 340np 030 cast pistons. I whanted the hyper version of that piston. He will ofcourse fix that or give me money to compensate for the error. But as I did not see this until the engine whas re-installed to the car ( with no heads ), I having second thouts about take it out again. It will take some time to do this all over again. And the summer in Sweden is short :/ He say that I will be fine with those cast piston and the engine combo i will use. What do your experince say? This is my engine combo. 355 scat 9000 crank scat 9000 i-rods 345np 030 cast pistons LT4 hotcam 1.6rr canfield 197cc heads Converted lt1 intake Headers dual 3inch Car is a manual -91 corvette I will not use F/I And I shift around 6000rpm. Thanks for your time..

just don't spray it, it should be fine.

Yeah no spray will be used.
Im a bit worried if those pistons will be more sensitve to detonation than the hyper version?

Thanks

Less sensitive to detonation.

Much more sensitive to excessive RPMs however. Use a rev limiter if you can. Set it to no more than 6200, if you want it to live a long time.

Ok.
Sounds good about the beeing less sensitive to detonation, as I need to get this tuned right away.
With my cam and the intake I got my power should peak at around 6000rpm.
You think that will be a probleme?

Should be OK.

Check your deck clearance before you put the heads on; I'll bet that those are going to be .045" (just over 1mm) down in the hole. Your compression won't be whatever you think it is.

Ok, I whas a bit worried that those pistons should be junk considering my engine is pretty much build for RPM ( the cam and the intake ).

The heads i use are with 64cc chambers. Mmm what do you mean about the compression? I thougt it should land between 9.5:1-10:1 somwhere??

Regards

64cc heads, .045" deck clearance, .039" head gasket (such as FelPro 1010), 6cc valve reliefs, .030" over 350:

9.3:1

That's a pretty small cam, the motor isn't going to make power at or above 6000 anyway (probably will top out around 56-5700), so just don't run it way up there, I don't think the pistons will be a problem.

Well i do use 1.6 rrs so it will lift .525, but perhaps that not extreme. hehe i dont know

Good news about the pistons, so I can get a good nights sleep...

regards

Those cast pistons are so far in the hole your engine wont have any good compression. The quench is going to be untouchable. Rule of thumb is not to go over .060 of quench. With a stock deck clearance and gasket thickness you will be way over this desired quench. Deck clearance can vary if the block was never decked and most head ccs are usually off like a few points. They could really measure out to 67 cc. .035 of quench is usually desired since this helps prevent detonation and helps performance. Im pretty sure those pistons are meant for a decked block. You can end up with a bad running engine with lots of detonation with that much quench. Theirs more to compression than just slapping parts together to reach a desired compression. You have to get the correct pistons to get the adequate quench. I would suggest having the block decked if it hasnt already or get different pistons.

That dont sound to good
Part number STL-345NP30 has ,"Compression Distance (in): 1.560 in."
From sumit.. If that tell you something??

The cast pistons your are reffering to have a compression height of 1.540. Look it up on the Federal Mogul website. The hypers are 1.560. If you want to be absolutly postive you need to measure your pistons in the hole.

No matter what the catalog says, if your TRUE compression ratio really matters to you, you MUST measure the deck height. The specs are almost always too ambiguous to be useful. Plus, blocks vary widely, even before they've been worked on.

As stated, the "ideal" quench, which is the distance from the flat part of the top of the piston to the flat part of the head deck, is in the .035" - .040" range. The most common head gaskets are about that thick, so "zero decking" the block to the pistons, such that the pistons are exactly level with the block at top dead center, gives the "perfect" result. The higher the compression, the more important it becomes. However with the motor you're building, you're not pushing any kind of limits on that, so I doubt that it will be critical.

The reason your deck height will be about .045" is because "rebuilder" pistons ALMOST ALWAYS locate the piston about .020" lower than stock (1.54" compression height vs 1.56"). The people who make them must think that it's better that the pistons not come up and hit the head, as compared to giving up a little compression. Stock deck clearance is about .025", and those pistons have about .020" more, for a total of about .045".

Measure all 4 "corner" cylinders. You might even get a surprise in the difference between them.

Whery interesting. Does this also apply to the overbore 345np30 pistons. I cand find those pistons on the federal mogul web catalog?`

So what you are saying is that in my case my comp will be lower then what i first belived? As you said around 9.3:1? Due to the .020" lower than stock?

i dont follow what shaggy56 is talking about "...035 of quench is usually desired since this helps prevent detonation and helps performance." Will i get trouble with those pistons? Perhaps time to talk to my engine builder again ..

Regards

Yes it applies to those pistons. I'm pretty sure they have 1.54" compression height.... which is -.020" compared to stock.

With the low compression you have there, you'll be fine. Don't worry about it. As I said, the higher the compression, the more important it is; but correspondingly, the lower the compression, the less difference it makes. It's all about the combo, not just one part.

I havent order the heads yet, shall I get even smaller chambers on the heads then 64cc to not loose power or low end response on the engine?

That would be good.

With aluminum heads, you should aim for close to 10:1 compression. Something around 58-60cc would be ideal.

I have read this numerous times and thought it would apply but you suggest it wont in his engine. With his deck which could even be more that .025 and even a shim gasket he would be at .060. But of course that using a nominal measurement not his actual deck height. With a thick gasket his quench would be way out of bounds.


Im quoting out of an engine builders book. If you have Performance Trends Engine Analyzer see what happens if you try and punch in a quench higher than .060.

Yes I've seen alot of engine builder books too. And I don't mean to discount what they say; only, that you have to put things in their proper perspective.

The lower the compression ratio, the lower the tendency toward detonation, in the first place. Meaning there is little or no need to worry about preventing it. The higher the compression, the more of an issue it becomes.

Which is why things like stock motors with dished pistons, where the quench might easily be .125" or more, still don't have detonation issues.

can i help this with a thin headgasget? or do i need to get the hole rotating assembly out again, and get new pistons, rebalance the hole thing?

Not really... if it was me, I'd leave the short block alone, order heads with smaller chambers, and move on.

Here straight from the horses mouth.

United Engine and Machine Co.

Heres a random block someone I know got. It shows how much deck heights vary. This is a good reason to deck a block.

#1: .032"
#2: .033"
#3: .027"
#4: .029"
#5: .029"
#6: .029"
#7: .029"
#8: .027"

well the engine is out and on its way to the engine builder. New TRW forge pistons is going in. Now I dont have to worrie..

Thanks for the help sorting this out..

excellent, good to hear...pistons are one thing i've learned to NOT cheap out on...

A new question popped up that relates to this.
My block is bored out 030 and then honed. Will any forge pistons that is +030 fit? Or do you need to rehone the block to get the right piston to cylinder wall clereance that the manufactor recomend?
I sure my engine builder knows this, Im just curious mysefl

THanks

Hard to say....

Measure your bore as it is now, accurately. If it's no larger than .030" overbored after honing, then at least your bores aren't already too large. Too large would be bad, because then you wouldn't be able to use it at .030" over, you'd have to bore it to the next oversize and buy that size of pistons.

But, with forged pistons, the EXACT bore clearance isn't as critical as with any type of cast, and also the desired clearance is larger. So for example (just pulling numbers out of the air) let's say your bores are at 4.031", and the current pistons require a bore clearance of .000" - .002", and there's .001" of clearance now. In other words, your machinist got it right on the money. Let's say that your new forged pistons require a bore clearance of .004" - .010", and it turns out that they're at .005" to the bore as it is now. Obviously you're "in spec", and no more work is required. You'd still be "in spec" if there was .008" of clearance; it might just be a bit noisy when cold, until it warms up and the pistons grow. On the other hand, you might find that your new clearance is now .002", which will require some more honing.

There's no way to know for sure until you have the parts in your hand. But, I'd suspect that your bores will be at least close, and quite possibly able to be used, as they are now.

thanks for the answer

The block is bored 030 and then torqueplate honed to use moly rings.

So the honing process is not only used for making the cross hatch marks in the cylinder bore to let the rings seat. But ALSO a way to take away material from the cylinder bore to keep the right piston to wall clerance, depending on what kind of pistons you will use? like a "fine" over-bore if you whant?

More questions.
If you take a piston that is +030 and whant to run a .004 cylinder wall clerance, Is that piston EXACT 4.030 big and then its up to you to hone out material to get .004? Or is that piston 4.030 minus .004?, and based on a "perfekt" 4.030 cylinderbore?
Regards

Devil..

Depends on the pistons.

That's why it's standard procedure to NOT hone a block until you have the pistons in your posession.

But for the most part, for American cars anyway, the pistons are "undersize", and the bore should usually be finished within a thouandth or 2 of the "nominal" size. But not always.

ok.

But in my case the engine where honed for the 345np030 pistons that I now will not use. it would be a shame if i need to bore it out to .040. I guess I just need to find a forge piston with the same ( or withing spec) piston to wall clerance as those pistons. Or messure the cylinder bore to se how big they are and then decide on what pistons to get.
As I look around for pistons, I do however see that the recomenden piston to wall clerance for most of the forge and many of the hyper pistons has got a pretty big span. From, lets say, 005-0015 etc etc.. So Perhaps Im within spec after all..
Thanks for your help so far. Im learning big time here