I am putting together a mild 355 (350hp/350tq) and have come across a dilemna, given that i haven't purchased anything but the block i can vary piston relief volume (by selecting FT, dish, or dome pistons). I can choose the combustion chamber volume (by selecting large chamber heads like 191 or selecting small chamber heads like 083). Finally i can select the head gasket, and change the deck height of the block.

Given all these variables i could have several different combination that would all yield a streetable 9-9.5 compressions ratio (for use with 87 octane and iron heads).

So here is my question given a fixed compression ratio:
will a large chamber/small piston volume or a small chamber/large piston volume yield better performance?

-Zander

 

The general rule of thumb as I understand it is to have a flat top style piston (as opposed to domed) in conjunction with the appropriate sized combustion chamber to yield the desired compression ratio. Domed pistons tend to shroud the flame front to a certain extent and leave opportunties for detonation. Optimum quench is said to be around .040".
Keep in mind that whatever static compression ratio you choose to run, it has to be compatible with camshaft duration, in particular the intake valve closing event.

 

Best - small chamber heads (58cc) - reverse dome piston (not dish, but reverse dome, more $$)

Good - Medium chamber heads (64cc) - Flat top piston

Bad - Large chamber heads (76cc) - Dome pistons

Zero deck the block either way, you'll want .040" quench distance.

 

Just to be clear, quench is the distance between the top of the piston and the bottom of the cylinder head, correct?

How about a large chamber (72cc) with flat-tops?

I found a set of forged 3.5cc flat tops with a 1.560 compression height that i would like to use (if possible to save me from having to buy new ones). with a std deck block that puts the pistons 0.020-0.025 in the hole, combine that with a 0.028 head gasket, and 72cc chamber and i get ~9.5:1 cr. and that yields a quench distance of ~0.050, which isn't far off 0.040

 

50 quench is large. Stock from the factory is 58 I believe.
35 to 40 is the desired quench value.
Get a shim gasket. It's compressed value is 15.

 

I think i'd rather deck the block 0.010" and still be able to use a composite head gasket. Isn't the factory quench more like 0.065", given a factory head gasket is somewhere around 0.040" (not sure where that information came from) ?

-Zander

 

Factory quench is around .040". .025" deck, .015" gasket.
.050" is acceptable, but you can use a steel shim in your case. Why would you avoid it? If the heads or block aren't warped more than .002", use one.

72cc and 3.5cc dish work ok I guess.
Just realized you're in Calgary. We are at 4000' elevation, and as such, can get away with much more compression. I had 10:1 with my last cam, and now 10.3:1 with my solid flat tappet cam, on 91 octane.

Have you picked your cam yet? I happen to have a comp xe268h collecting dust in my garage (with lifters).

 

You should try to see which cam you want to run and if it works within the range of static compression ratios you can get (gaskets, decking etc). 9.5:1 isn't much if you have have a massive cam but it may not work on 87 octane if the cam duration is small.
The XE268H 268/280 110LSA 106ICL which is a decent sized cam will give you a dynamic compression of around 7.6:1. For a performance street engine that's considered the low end of the scale however it should be happy on 87 octane. That cam might prefer closer to 10:1 static but probably premium fuel.

 

That's exactly what I ran it with.... 9.5:1 on 87 octane should work pretty good. If it's a non-roller block, etc etc.

 

I haven't picked a cam yet, however i was hoping to base this motor on a roller block. I figured if i could pin down the rest of the components then i would simply browse summit for a while a find a cam that would yield a streetable (87 octane) DCR. Although i'm not 100% certain what the best range of dynamic CRs is for a 87 Octane, Iron headed, 350. Any suggestions? Seeing as i'm only looking for 350hp/350tq i shouldn't need anything larger than say a 214/224 cam.

 

Ok, if you've got a roller block, power to you.

"only" 350HP eh? Riiight.... Ok, don't take this the wrong way, but just because a magazine does it, and makes it sound easy, doesn't mean it is. 1hp/cube is not easy. You need good heads for that. If you get good aftermarket heads, then you're right, a mild cam should do the trick. But don't be too upset if you fall short. You're looking at around 280RWHP.
What heads did you have in mind?
Were you planning on going carbed, or sticking with some type of FI?

 

Hehe, dont laugh, but i have a twin-TBI setup sitting on my desk right now that i would like to use. I'll run it off a MSII standalone computer.

Honestly 350hp is not a conrete goal, just a number that i can base questions off for you guys. Anywhere around that should meet my needs, this is just a fun street motor.

As far as heads go i was leaning towards either Iron eagles, PBMs, EngineQuests, or one of the other entry-level preformance castings out there. maybe i can even find somthing for sale on TGO.

I doubt that i can squeeze 1hp/cu.in out of any of the stock GM casting i could find at a junkyard (no i can't get 113s, or L31s at my jyard)

-Zander

 

Twin TBI?? Well, whatever floats your boat... I'm not sure how picky that is to cams, hence why I asked.

Enginequests and whatnot would be similar to factory castings. Iron eagle 180s or so would do it. I'd grab vortecs though, but then you have the manifold problem...

Stock casting? No. That's why you port it I'll have to show you my beast sometime....

 

"I'll have to show you my beast sometime...."

now that sounds creepy ;-P.

Honestly the twin TBI setup is unique, and it cost me absolutely nothing!, i got the TBI cores from my junkyard for $1.00 and i had the edelbrock dual-quad manifold lying around. After a lot of superclean (the most effective degreaser i have ever witnessed) and some silver dupli-color enamel the things look wicked. once i find my digital camera i'll throw up some pictures. Not to mention that combined the system should flow 1000CFM through the 2TBs, so it's very versatile.

Anyone have some insight as to the ideal dynamic CR for iron and aluminum heads running 87/89 octane?

 

Here's a couple for you. You may have seen them.

http://www.chevyhiperformance.com/ho...075/index.html

http://www.chevyhiperformance.com/te...ild/index.html

http://www.chevyhiperformance.com/te...ild/index.html

I haven't done a DCR calcualtion for either of them yet.

 

The aluminum headed 355 with 9.4:1 static has about a 7.8:1 DCR.
The vortec headed 355 with 8.7:1 static is around 7.2:1 DCR.
Both use the same XE268 cam.

 

Thanks everyone for the replies so far. One more question.

Whats the smallest quench distance i can safely run? for arguments sake, say 113 heads <0.500" lift cam.

SkinnyZ, any chance your located in Calgary aswell (i came across a post wherein you made a Calgary reference)

 

Given a clean sheet of paper, I would shoot for no more than .035 quench with stock style/ hyperuetectic cast pistons that maintain a tight fit to the bores. .030 would be my personal target if I weren't revving it over 6000 rpm.
FYI, on my new GM crate L31 longblock, the quench was .049 (.023 in the hole with a .026 gasket).

As long as you get your desired squish and CR, I don't see a big benefit either way with dished or flat pistons. Stay away from domed.

 

.035 is extremely tight. That and you can't get an undecked block that low. 40 is the lowest with the thinnest of shim gaskets that i've ever found on a virgin block.

 

Right, you're unlikely to get below .040 with an undecked block. But he said he could deck the block.
.035 would be tight for loose fitting forged pistons that will rock at TDC, but for close fitting hypers I would still shoot for that. Of course he needs to be realistic about how high he intends to rev the motor. .035 will be more than adequate for 6000 rpm with a .005 piston/bore fit.

 

I hear that .040" is as far as you want to go. The other posts confirm that there is a spec between a hypereutectic piston and a forged piston vs rpm. That's something you'll have to consider.

Zander, as to the Calgary reference; Been there, worked there and I like it there however I live a little further east... I hope to pass through again this summer. Is Edmonton the closest dragstrip?

 

.040", noted.

Race city is actually open for one more season, but next year Edmonton will be the closest track, regrettably.

 

I run my Vortec headed (XR276HR) 350 86 IROC everwhere I can. Edmonton/Calgary included. That's 3500 kms from Ontario. I'd love to drive cross country getting 20+ mpg highway and then nail down a 12 second ET at either track. (That is premium fuel however!)

 

Thats alot of cam for 20+mpg, especially with an auto tranny! most impressive, i'm hoping with a smaller cam (214/220), EFI, and a stick i can get 25mpg. but time will tell.

Whats the rest of your setup?

-Z

 

350 std bore
Vortec heads
RPM Air Gap
670 Holley
XR276HR(straight up)
MSD dist and 6AL box. 34* total timing. 16* initial. Adjustable vacuum advance.
1 5/8 Hedman LT headers/single exhaust
700R4/TCI Street Fighter lock up convertor.
3.70 BW rear gear
26" Comp TA's
The rest of the car is fairly stock. Still managed a personal best of 12.9 @ 106.

 

damn nice !! have you modified your vortecs at all (screw in studs? ported?). whats your CR at (you mentioned premium)?

-Z

 

The last rebuild involved an upgrade to Comp 1.6 Pro Magnum rockers which meant modifiying the Vortec push rod holes. At the same time I added Comp Bee Hive valve springs, competition valve job, screw in studs (3/8) and guide plates.

My SCR is about 9.9:1.
DCR is 8.2? ( I have to check)

 

well isn't the stock gasket about 0.039" compressed thickness? the shim style ones like the felpro 1094 is 0.015. probably a good combination along with stock deck clearance.. but i wouldn't think that this is the factory quench.