I calculated the compression ratio for my 383, and based on posts in another thread, I think I'm a little on the high side (11.7:1 :eek:). I have the standard Felpro LT1 head gasket (9966PT). I assumed a .039 thickness in my calculation, but really have no proof that the gasket I have is in fact .039. What options do I have for thicker head gaskets to lower the cr? Or can I slide with the 11.7:1. I do have access to a good tuner to pull timing out if needed.

 

OEM LT4 gasket is .049 thickness. You will loose your quench, but less compression.

:)

 

Right, the upgrade is the .039 gasket. That's the one I used. You'll need more than 93 octane to run at 11.7. I stopped at 10.75. According to my engine builder, pulling timing has a greater effect on power than the lower cr.

 

camshaft selection would be the determining factor IMHO. I saw the comp cam that your are running (I have the same one) but I can't tell you if it will detonate or not. Maybe Comp cams can tell you if 11.7 to 1 is too much with that cam, those heads and pump gas. Good luck.

 

I hope one can run pump gas on 11.7...I will be lucky if mine will be that LOW!

:D

Seriously, LT1/4 engines can run higher compressions, but as stated above, camshaft selection (how big it is and thus bleeds off the pressure, etc) is major factor.

There are basically three easy gaskets to choose from, Impala-.027, Fel Pro .039, and LT4 at .049. An .049 ought to drop you cr quite a bit...


[Modified by No Go, 8:41 PM 12/15/2003]

 

Thanks for tips guys. Did a quick calc on the .049 gasket and CR lowers to 11.3:1. That definitely helps. Now that I look at my data sheets again, it looks like I was really at 11.6:1 before, not 11.7 that I mentioned above. Not that it matters much. I definitely want to be able to run pump gas. The cam is the Comp Cam 503. I chose what I thought would be the biggest cam I could go without any major sacrifices to daily drivability, although this isn't exactly a daily driver. I didn't want to go above 230 duration on a 112 lsa.

Even though I already bought these gaskets, I'll definitely give that LT4 gasket some good thought. My tuner buddy doesn't seem to think 11.6 is too high.

Oh, and I think you lost me when you said "loose your quench". What exactly does that mean?


[Modified by Jeff Hickman, 10:00 PM 12/15/2003]

 

If you are gettin right down to it, you should cc the heads to find out the volume of the chamber for sure. I know that they were ported and they may have been milled. If that is so, then you may not know what the volume is. Also, did you calculate the deck height? (if it was decked to your reciprocating assy. it is probably 0) You can also account for the ring land volume. Don't forget the valve reliefs either. There really is alot to be considered before you "know" what your compression really is. I can't really explain quench, but as I understand it, it is the distance between the flat part of the piston, to the flat part of the head or combustion chamber. I believe that the smaller that distance, the better your quench. Quench helps burn efficiency and it can also help reduce detonation I believe.

 

Yeah, I think I have it all covered. My head chamber is 58cc. Yes, they have been milled. The block has been decked .020. I will admit I had to do some estimating. I tried to measure deck clearance with a feeler guage (straight edge on the deck, piston at tdc). But it is apparant the piston is rocking a little in the cylinder, so I had a little different measurement between the top and bottom of the piston. I think I used an average. Per catalog, valve reliefs in my pistons are 5cc. And yes I did look at the volume that is around the piston in the clearance between the piston and cylinder wall, down to the top compression ring. All that combined gave me 11.6.

 

Let us know what you do and how it turns out. Good luck. I hope it works, because I will be running more compression than that.

 

I'll try to remember to look you up when I get on the road. Hope you aren't in any hurry. This is my winter project so it will probably be April before I'm driving it.

 

Hey Jeff,

Believe it or not ... Maintaining a tight quench distance is so important to combustion and reducing detonation ... That putting in the thicker head gasket will make the engine more likely to detonate, not less. This is despite the lower compression ratio with the thicker gasket.

Not to sweat it though, you can make 11.7 run without detonation and make great power. The reverse flow cooling of the LT1 will help. First make sure that you have a flow balanced set of injectors (so no cylinders run lean). You can have the cylinder head's exhaust ports and valves ceramic coated to keep things cool, and then if you want added insurance you can run Evans NPG+ coolant (note: I would only recommend this for LT1/LT4 reverse flow engines).

http://www.evanscooling.com/html/npgPls2.htm

 

Jeff are you meaning that your pistons are in the hole .020"? I know you mentioned that you shaved the deck .020" but typically LT1 blocks are taller than the blueprint spec of 9.025. I have seen them as tall as 9.035 and most off the shelf pistons are made so that they are at zero deck with the block at 9.00". If your pistons are .020" in the hole I calculate a 11.55:1 compression ratio. That is very livable in your engine, especially with the cam that you have chosen.

With your cam selection your dynamic compression ratio is right at 8.9:1 while it is on the high side for a street motor it won't be a problem with the LT1's reverse cooling system. If your pistons are truly .020" in the hole you don't want to use a .049" head gasket as that will give you a squish demension of .069" while its not ideal its not that bad either seeing how bone stock factory GM engine is considerably more than that. If your pistons are at "0" then you can use that .049" head gasket.

There are lots of LT1s out there with 12:1 CRs. Check out the F body forums. However these guys are running the Comp 306 and larger cams.


[Modified by tjwong, 9:59 PM 12/16/2003]

 

TJ: The deck is at 9.005 based on measurements made by my machine shop. So I trust that number. I guess I just assumed 9.025 stock. I dind't have the block decked so I don't know where it started at. I'm a little confused though. I think by catalog my pistons should be .005 in the hole, which agrees with what you are saying about being at 0 deck at 9.000. But when I physically measured it, I got somewhere around .015. That is the one measurement I want to do over though. My dial indicator was on the blink and I have a new one showing up today from summit. I may not have been at true tdc. I don't think I would have been off by .010, but who knows.

 

I'm not quite sure I understand the concept, but I'll try to dig up some info. I would think going to a thicker gasket would make it closer to approaching the stock setup. If my block was decked .020, and I went with a gasket that was .020 thicker than stock (.059", doubt one exists), then wouldn't I be back to the stock "quench distance"? This is more likely to deto than sticking with the .039 gasket?

 

Jeff having the correct SQUISH or Quench demension improves combustion by causing turbulence therefore enhancing combustion. This happens when the piston comes up to TDC, the gases are "squished" against the flat part of the cylinder head and moves the gases into the open combustion chamber violently.

Having improved combution also retards detonation. For maximum performance having the correct quench demension creates the greatest turbulance. But it should not be less than .038" on a engine with a 4" bore and .007" piston clearance, .040" is acceptable for all small blocks. At .060" is not acceptable. I got all these numbers from my machinist, he has built hundreds of performance and race engines in the years he has been in business so I can trust him.

You can calculate your deck clearance using this formula: (stroke/2) + Rod Length + piston compression height

For instance a 3.75 stroke with 6" rods and a JE Piston with a compression height of 1.25" would be:

(1.85) + 6 + 1.125 = 9

So if you know who made your pistons and if they are off the shelf, get the part number and look up the specs. All you need is the compression height, you know your block deck height that is a given. From there you can calculate actual deck clearance. If the above pistion spec is true for your engine then your pistons are .005 in the hole.

 

OK, sounds like I should stick with the .039 gasket. I will re-check clearance tonight if I get a chance.

I don't have a whole lot of info on my pistons - I didn't order them. I couldn't even find anything about "BRC" on the web. But my machinst found something in a catalog. Maybe he can give me the compression height. If they are only .005 in the hole, then I'm guessing the CR will be even higher than 11.6.

 

Also remember your doing the programing and you'll make your peak power right on the edge of detonating in regards to your AFR and timing.
With that in mind, which ever compression you go with you'll end up tuning it to that.
I would stick with the .039 for that exact reason your doing the programming.
If you were paying someone you wouldn't have $ or the luxury for the finer adjustments.
JW