Compression question 383 LT4 58cc head Flat top
Thread Starter
Pro
Joined: Nov 2005
Posts: 536
Likes: 0
From: Bardstown Kentucky
Well I ran the numbers last night with the calculator that d84mclain posted. My cam card for the 467 says to install the cam at 109* intake center line.
With the flat tops I came up with 11.8 static and around 9.5 dynamic. I cant remember exactly, I am at work now and cant run the calculator.
With the 16cc dish piston I came up with about 10.4 static and around 8.4 dynamic. The dish piston sounds like the way to go with this cam running on pump gas.
The thing is I ran the calculator with the Comp 468, with flat tops, and the static was the same at 11.8 but the dynamic went to 7.5. I was really tired when I did this so you guys may want to check my calculations. I dint have the cam card for the 468 so I used the 109* I/C from my card. The LSA is the same for both cams at 113.
Edit
I just got my calculator to work.
With the 468 cam, Intake dur 292, Ex 300, lobe sep 113, ILC 109
383 flat top 5cc valve relief
58 cc head
.039 thick head gasket
4.125 gasket bore
Deck height .020
I got Static comp. 11.35, Dynamic 8.28,
Should I run the flat tops and send back the 467 for the 468?
With the flat tops I came up with 11.8 static and around 9.5 dynamic. I cant remember exactly, I am at work now and cant run the calculator.
With the 16cc dish piston I came up with about 10.4 static and around 8.4 dynamic. The dish piston sounds like the way to go with this cam running on pump gas.
The thing is I ran the calculator with the Comp 468, with flat tops, and the static was the same at 11.8 but the dynamic went to 7.5. I was really tired when I did this so you guys may want to check my calculations. I dint have the cam card for the 468 so I used the 109* I/C from my card. The LSA is the same for both cams at 113.
Edit
I just got my calculator to work.
With the 468 cam, Intake dur 292, Ex 300, lobe sep 113, ILC 109
383 flat top 5cc valve relief
58 cc head
.039 thick head gasket
4.125 gasket bore
Deck height .020
I got Static comp. 11.35, Dynamic 8.28,
Should I run the flat tops and send back the 467 for the 468?
Last edited by 96lt4c4; May 9, 2007 at 09:41 AM.
Racer
Joined: Dec 2003
Posts: 454
Likes: 0
From: Double Oak TX
First and foremost, you need to take some measurements and see where you will end up. 2 or 4 CCs can mean a big difference in the static. The deck height plays a big role. Theres a HUGE difference in compression from .020 down in the hole, to zero. So, with that being said, I would measure as precisely as possible, which includes CCing the heads, make sure you end up with something in the neighborhood of 11.5, and the have a cam ground to keep the dynamic around 8.5 or 8.6. And its not the overlap thats gonna make or break the dynamic, its the intake closing. The cam you're looking at, the 467, doesnt have enough intake duration to get the dynamic where it should be. So, to get the intake valve closing at the right time you would have to retard it, and you dont want to do that if you can help it. So, drop the compression slightly, tailor the cam slightly, or run the compression high, and put more cam in it. The one thing that WONT work is high compression and a mild camshaft.
“A” and/or “the” proper quench (tight) adds to efficiency and allows for more compression. David Vizzard wrote a pretty good article on the effects of compression and quench stating typical results of making various changes but emphasized the importance of proper quench as just in itself, it promotes a small increase in power.
Smaller camshafts by their nature are counter intuitive to the beginner as the typical perception is smaller must be safer/better but being cam timing is a key aspect of dynamic compression, it just doesn’t work that way in higher compression setups.
In the beginning I was very aware of many of the aspects pointed out in this string but with a little experience under my belt learned to crank out the numbers before the crank even goes into an engine. Then everything gets double checked again and again.
It’s all in the combo and getting the most of what you got. Not only should you preplan, but also put a good amount of money aside for tuning and refinement. That's what seperates the men from the boys.
Melting Slicks
Joined: Jul 2006
Posts: 3,356
Likes: 1
From: Chesapeake Va
^^^^ All very good points. The biggest thing is precision. Precision in you measurements, precision in your actual machining, etc. Run your quench as tight as you feel comfortable with, and get you static where you want it by either increasing combustion chamber size, or fly-cutting the pistons, etc. It costs a little more, but you can have custom pistons made to get the dish exactly where you want it. I also recommend a custom cam. Off the shelf cams are ok, but they are just a guideline, a place to start.
Thread Starter
Pro
Joined: Nov 2005
Posts: 536
Likes: 0
From: Bardstown Kentucky
Well its looking like the way to go is with the comp 468 and the dish pistons. My static compression would be 10.5 and Dynamic would be 7.6. If I deck it to zero static goes to 10.9 and dynamic is 8.02. Also I posted that the heads were 58cc, they are 54cc.
These numbers sound a little better?
These numbers sound a little better?
