OK, I'm having a little troube, I have a 350 TBI engine than I'm not able to get all the specs where I need them. I also don't have time or money to change the bottom end or heads so here the specs I've come up with so far and from all the reading I've done here I got a few issues.

1994 5.7 350 bored .030 over
stock crank and rods
Speed Pro Hypereutectic teflon coated flat top pistons 5cc
Sealed Power Chromemoly Piston Rings(SLP-E-251K030)
Edelbrock Performer TBI intake manifold (3704) bored to 2"
Holley 502-6 TBI
Crane 2030 powermax cam (now called 114122)
Full length headers - Corvette
1.5 Roller Rockers
14096217 300hp crate motor heads 64cc

So now for the problems.

The heads have been milled .025, I'm trying to come up with the CR and DCR using the Pat Kelly calculator. Here is the problem I come up with. If I use the head gaskets I bought (GM 12557236) .051 thick it does not take into account the difference in CC's of the milled heads, so the CR and possibly the quench is off.

Anybody have an idea what the new CC's might be,? Heads don't have valves inn them yet so now way to CC them.


Anyhow, guessing at 62cc here is what I come up with..

CR 9.8
DCR 8.25
Quench .066

Am I looking at disaster? I really would like to find a combination that lets me run 87oct or 89 at most. Is that even possible with this combo?


Change of cam? Enlarge the combustion chamber? Leave it as is? Need more info?

Thanks
Randy

 

Might want to throw on some valves, and cc it. or ask the machinist what he thinks it'd net. I don't know the formula off the top of my head to guesstimate the volume change.

Was the block decked?
What are the cam specs at .050"?
What's the gasket bore?

Why did you deck the heads, if you're thinking of enlarging the chambers? Which heads are those, I don't know that p/n off the top of my head?

I would put in valves, and cc the chambers. Re-do the math, with a proper deck height (measure). Then, assuming you're too high for comfort (right on the edge now), grind open the chambers a bit. Over do it.

Also go to a shorter gasket, why the heck would you want a .051" gasket anyway??

That would optimize quench, and the CR.

 

maybe his heads were warped and it required that much of a machine to level them up.

 

The .051 gasket is to bring the CR back into a more octane friendly number, but thats what messed the quench up.

The heads were milled before I bought them so dont know why it was done.

The heads are GM crate motor heads -> GM Performance Parts 300 HP Crate Engine Cast Iron Cylinder Heads: NAL-10159552 - summitracing.com

I've run the number with a guess at the CC volume and a smaller gasket and the CR comes out to over 11:1 and the DCR over 9:1.

i guess I will have to hog out the combustion chambers and install a shorter head gasket unless there are any other ideas, just did ot wnat to spend a fortune on machine work.

 

What is the part number of the pistons. Common rebuilder #H345NP pistons have a reduced pin height 1.54" vs 1.56" stock, making the piston to head clearance at least .045".

I'd change the pistons for some -13cc dished pistons.

If the bottom end is already assembled, you could exchange the heads for some decent 76cc heads at a local machine shop. casting numbers to look for a 441,920,336,487, 993 Avoid 624 and 882 castings.

 

Pistons are Speed-pro STL-H345NCP30

Don't tell me that! I already have a big enough quench issue.

So if I don't have the block decked, even without a gasket I have a at least a .045 quench, or .015 deck and .045 is .060 correct?

How big a deal is this "quench" thing anyways, I understand that it reduces detonation, but are we talking high HP race engines or daily drivers or any engine with quench type heads?

If I understand it right, its about the turbulence caused by the squishuing of the air against the flat part of the cylinder head, and large clearances don't create enough turbulence.

May have to go back to stock heads as I really don't want to take the bottom end apart.

Are there any tricks to geting away with high quench numbers?

 

haha, sucks eh? I found that out as I was building my motor. I got it decked.
Keep in mind, the factory uses .025" deck height, and a .015" gasket. .040" quench - FROM GM. So if they do it...

Google quench, it'll give you a schwack of info. It makes more HP, better mileage, less detonation, etc etc.

Those are flat top pistons, which are good for quench. flat tops, and 64cc heads are pretty common. Usually goes with a decked block though

Go over your blocks deck surface and check for flatness. You may want to deck it for other reasons anyway. If it's flat, and you REALLY don't want to tear it down, you're stuck with less than ideal quench, so you have to lower the CR ratio to keep away detonation. Those factors all point to less power.

Here, check my sig, I think I listed what I did...

 

If I run the numbers you have about a .029 gasket for 10.1:1 and a quench of .044 Assuming a 5cc relief on the pistons. Hows that work out for ya?

What octane are you running, and what could you get away with?

Looks like its the milled heads that are killing me, have to assemble one cylinder so I can see what the CC is and go from there.

 

One more thing, since the measurment we are looking for is a .040 optimal, that refers to a piston top to bottom of the head height, so if we measure the center of a dished piston we have a pretty good gap.

So is is safe to assume that with the lower the CR, the larger a quench you can get away with without detonation?

If not then the dished piston/quench measurments have me really confused.

 

This is my setup. Not ideal quench, but close.

At 4000' altitude, iron heads, 92 octane gas, it runs pretty well. I think i've got less than 36* timing on it though, 32* right now.

I tried running 87 octane once, by accident. It ran REALLY bad. It was also cheap gas, and my cam was wiping on me, so I don't think it was a fair comparison.
10:1 on iron heads - means premium fuel, basically.

You're right, quench has more and more of an effect at high compression ratios. High, being over 9:1. That's when there's a difference.

Some motors respond to quench, some don't. Old style dish pistons don't really, pop up pistons don't.

Flat top (valve reliefs), and reverse dome pistons, do respond to quench. Just about all cylinder heads do, the newer designs like vortecs will respond more so, than an old fuelie head say.

here's a good site on it, and one of the ones I was hoping you'd find

United Engine and Machine Co.

eg "If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will likely create more ping at 9.5:l than you had at 10:1"

 

Thanks, that site is actually one I read last week when I first was looking for "quench" it does a pretty good job of explaining it.

I'm not building a 1/4 mile machine, but do want all the HP I've paid for, which means I am willing to give some up to keep from paying more (buying new pistons ect. Bought the block for cheap already built so ahead of the game already)

So now the question is assuming you want to leave the components I have in place and just deal with it, for arguments sake lets say its a theroetical 340 HP engine.


If normal timming was say 36 deg and I needed to drop it to 30 deg to remove detonation, what type if loss should I expect? 10hp 25hp 50hp?

Not that I advocate doing this, i just need a frame of reference to justify spending more time and money on this project. Its always hard to leave potential HP on the table.

 

here is another interesting article I found, funny that it lists some pretty famous engine builders that recommend that a .040 is the smallets quench you really want without some really tight clearances everywhere else. Also I assume that as an engine ages the quench should be larger as piston rock and other geometry changes take effect as bearings wear and such. So maybe a .050-.060 is not too bad for a street engine that won't get torn down again ever unless something breaks. JMO


linky -> Piston Head Clearance Guide - Tech Article - Chevy High Performance Magazine

 

4.030 3.48 .045" -5cc 62cc 4.166"x .051" Your cr is 9.28:1

 

should be 4.030 3.48 .015 5cc 62cc 4.090 .051 CR= 9.88:1

.015 bieng the deck height, and 5cc bieng the piston volume and 4.090 bieng the head gasket diameter.

 

CC the heads. It's easy. All of this is based on a "guess" of the chamber volume. Which means: (quoting myself) "If it ain't been measured, it ain't real".
Measure the actual deck height. Measure the actual combustion chamber volume. You might be suprised. Then run some numbers. A couple CC's in the chamber and an extra .005" in deck height will change the numbers a bunch.
To CC the head you need: vaseline, a piece of glass (from a broken window) and some syringes from the farm supply store. ($8 total). If you can't figure out how to do this, someone else should be assembling the engine.

 

I used some scrap plexi glass, and a syringe I bought for $4 from the vet down the street.
But I think you know how to do that.

randy, you said you have .015 deck height, how do you get that? .025" stock, +.020" since the pistons are 1.54 vs 1.56 pin height = .045"
again, assuming the block wasn't decked.
Use a metal ruler, stick the skinny part on the block surface, bring a piston to TDC (choose 1 or 8, or both), lay the rule across the piston pin centerline, and slip feeler gauges under.

My block, with those same pistons, and a .010" clean-up mill, netted me a measured .015" deck height, rather then the mathematical .035". Go figure. Yea, so measure.

 

I like the old CD and vaseline trick as it already has a hole in the center


As for the deck height I was assuming the .010 that I was "told" was milled and assumed the pistons were 1.560 so that would net .015 but it may be .035...


I'm looking for my dial indicator now... Will post in the AM.

 

Well best I could come up with was .025 deck height, but it was really hard to set TDC with the dial indicator, boy is that baby touchy Plus the possible variance in the straightedge.

I tried to take that issue out by putting a dot on the straightedge so I could use the same spot for the dial indicator and dial caliper, I put no pressure on anything. After setting the dial indicator at zero with it touching the head at TDC, I then inserted the straightedge using the dot for reference I got a reading of .145 I then used the dial indicator to Mic the straightedge at the dot and got .119 so subtract the thickness of the straightedge and the depth in the hole should be .026

So that being as close as I could come I'm betting that since the pistons are Speed-Pro 345NCP-30 at 1.540 and a known deck should be .025 , and sonix with the same pistons saw a .020 difference from what should have been, I'm going to set the deck at being .025

Does this sound reasonable or could I really have been off by .010 or more? I also just went back and tried it with two other straight edges and got within .003 of the same readings.

Seems the deck was never milled, the guy I bought this from was referring to the pistons being .010 different, even though their .020 .


Anybody disagree? Any better way to set TDC?

 

Instead of trying to find the exact point that the piston is furthest up in the bore, instead find the two degree wheel points on each side of TDC where the piston is say .050" down the bore.
Rotate the crank a few times to check for a repeatable reading.
True TDC is exactly half way between these two points on the degree wheel. Move the pointer to split the difference and rotate the crank again (in the same direction) without touching the dial indicator.
the .050" down points will now be exactly the same degrees either side of TDC. Now your ready to measure Piston deck clearance @ TDC, 'cause you know where TDC really is.

 

What if I don't have a degree wheel? That kind of what I did without a degree wheel, kept moving the zero mark until it would touch zero without passing it and then go back down. think thats close enough?


Also given that quench height, its not ideal but should be OK with a .015 gasket, looks like the chambers need to be hogged out to 68cc from just under 64, that will give me a hair under 10:1 SCR and 8.25 DCR, that should keep me in mid grade gas, you think?

 

Or forget the dial indicator, lay a feeler gague in there and slowly turn it past TDC. When you get to one that just barely bumps the straight edge at TDC subtract a couple thou and you've got a pretty reasonable deck ehight measurement. I did this for years before I had a dial gague.

Assumng they are .025" in the hole and you're where you think you are I have a suggestion......

A set of 76cc heads should be easy to scare up to replace your current 64cc heads. I mean, either should be available for about the same money.... buy one set, sell the other for about even money. GM put the 76cc '993' heads on ZILLIONS of their universal replacement 350 crate motors and they're decent heads. 882s are basically the same thing and even more common, but they are thin castings compared to the 993s are prone to cracking.

With 76cc heads and a GMPP .028" composite head gasket you'll be in the high 8s. Mill them down just a smidge to 72cc and you'd be at 9.2:1, all with much better quench distance than you have now.

 

you're method of measuring with the dial indicator is far more accurate than what I did. A few thou plus or minus is ok, you're not building a full out race motor.
.025" is quite reasonable. Use a .015" gasket. Port the chambers up a bit, and polish 'em up, run premium.

Or use some 993 heads, and run regular gas. The heads you have now are nothing really special, "downgrading" to 993 heads wouldn't be so bad, if the main point is to run cheaper gas. Higher compression gives more power, and makes you run more expensive fuel, which is it that you want?

Might be able to make some $ swapping to the 993 heads.

 

Tried it your way and its much easier to do than the dial inicator, anyhow .025 would not touch and .026 bumped it just a hair, so .025 it is . Thanks for the help.


Now I like the idea of the .015 gasket, that makes a perfect quench. The Felpro 1094 seems like a logical choice.

I'll see if the machine shop I deal with has any 993 heads that they would like to trade. Barring that, any idea if the current heads I have could be bowled to 72cc? I was going to do some bowl cleanup anyhow.

Using the calculator, 72cc and all the above info gives me a 9.5:1 SCR and less than 8 DCR. Sounds good to me, would that get me by with 89 OCT? Seems like it should, maybe even 87 with a spot on tune? (TBI)


Looks like things are not a desperate as it once looked. Knowledge and a little measuring are a wonderful thing

Thanks for all the help... I'll update this as I get more info.....