Hey guys,

So just dealing with the guy from the machine shop in regards to my 408 ls build. Specs will be as follows:

4.0 stroke
4.125 bore
68 cc Stock Ls3 heads with upgraded springs to support higher lift cam,
Forged everything
Ls9 4.1 bore head gasket and .051 thickness.

Cam I would like would be a 242/250 612/614 114 lsa cam, or something there abouts.

I am just wondering what cc of pistons I should run to get a compression ratio that is in line with street and track use that will be condusive to 91-93 octane? I tried one of the online calculators, but piston deck clearance I couldn't figure out and this is the first motor I am building myself. Also, will there be any piston to valve clearance issues? Will the cam be condusive to street driving? Any help is appreciated as well as any other feedback.

Going into a Datsun 240z with a T56.

Thanks!

 

way too many variables here. first you need to know how much the piston is out of the block. best way to do that on a custom engine is get a bore gauge rig and measure the piston to deck clearance at tdc.

also a 4x4.125 motor is a 427 not a 408. an ls9 gasket is 4.085 so it will not work with a 4.125 bore. if you are using a stock lq block you will not be able to go 4.125 anyway. for the cam its best to contact someone like martin or tick and tell them your goals. they will then spec you a cam based off that.

for now figure out what the actual bore and piston height is then calculate the compression. you may be able to raise it a little with a thinner gasket. you also didnt say what pistons you are going to run. in general 11.5:1 is about the safe limit for 91-93 many people push 12:1 but at that point the cam selection really becomes critical to keep dynamics compression resonable to control detonation. with the right combo 12:1 is achievable on 93

 

Hey, thanks for the quick reply. I made an error in the original post, pistons are 4.030 bore not 4.125 and my question was in regards to pistons, I want to know which ones will clear and which to use. I just read a post elsewhere that said 10.8 to 1 is pushing the limit for CR on 91 octane. Not to criticize, just trying to learn how this all works. I would like to use Mahle pistons but I am worried about getting too low or too high of compression. The Mahle -3 was what I was thinking but not sure how to calculate this. I also live in Canada so not sure if this makes a difference for tuning or not.

Thanks

 

I have some new wisecos for sale with a 8cc dish and valve reliefs for those heads. Compression with a stock gasket and that 68.5cc chamber should be 10.5:1. With a .040 gasket they would be 10.8:1

 

am using +5cc domes in my 408 for a cr of 12.5.1 to 12.8.1. with a 68 or 64 cc not done the top end yet.

 

Static CR is fixed. Dynamic CR will depend a lot on the cam timing, intake close event in particular. My 418" build ends up at 11.45:1 static and 8.15 dynamic (we have 92 here in SETX). My Wiseco's are -15cc dished with 59 cc heads. Also helps that you have a light car...

 

A stock ls7 pushes 11:1 and an ls2 does 10.9 so that's in no way true. I can try to run the numbers on that can when I get home but are you sure that's what you want to go with? Where did you come up with those specs? Just because it fits doesn't mean it's going to be a good match.

If you want to calculate compression with the Pistons at -3 you would simply enter that into the piston tab. It should say piston type: select dish, then -3cc. All the other stuff is pretty self explanatory however you can do that until you know how far the piston comes out of the block. On a stock engine it's about .007 out which is -.007 in the calc. But since you have all new parts that's pretty useless. As mentioned above and briefly what I touched on is that dynamic is more important than static when it comes to detonation.

In other words one cam with 12:1 compression might be fine and 12:1 compression with a slightly different one could ping to hell

 

Ok, so after more research on this subject, I am even a little more confused. My goal for this build is 500rwhp. I think my static compression that I would like to do is about that 10:1 ratio, just so I can keep the 91 octane and have lots of room for timing advance.

Wiseco makes a -15cc piston (model k445x3) that on the website looks like with my 68cc heads and the piston relief in the piston, will give me 10:1 compression. Thoughts?

Now with that being said, is my cam big enough to support this? I want a streetable cam, but again want the 500 rwhp. Don't need tons of torque as the car is pretty light. Again, feedback?

Thanks again to all the responses!

 

With that setup you have I get a 10:1 ratio on the nose BUT that assumes your piston is .007 out of the hole which I have no idea if it is so you have to measure otherwise you can't know for sure.

However 10:1 is low for a street ls engine. These aren't sbc's that want 36 degrees of timing at wot and ping on pump at 10.6 compression. With this build I would up compression to 11:1 plenty safe for 91 and will yield better fuel economy throttle response, and another 15-20hp over 10:1. You can easily accomplish 11:1 by doing 8cc reliefs in the Pistons or there about and then go to a .040 cometic gasket as opposed to the ls9. The cometic will lower the quench of the engine allowing you to run more compression safely. A thinner gasket running a tighter quench on 11:1 compression will be less prone to detonation than running a thicker gasket on 10:1.

Now with all that said that's all great but you need to figure out deck height as that dictates gasket selection and overall compression ratio. I wouldn't worry about the cam specs until you get that sorted out. Or contact a sponsor and have them do a complete specd package. That cams looks like it would work well in a 408 though. If you are going I piece it together on your own, the head selection is far more important than a cam. I would get heads and set quench then find a cam to fit. Even with the worst cam in the world a well breathing top end will outperform the best cam with bad heads. With 8cc reliefs though you shouldn't have much of a problem with any cam

 

Red Bird this sounds ideal for me! I like the idea of the -8cc pistons and the 91 octane. 11:1sounds like the way to go. As far as heads are concerned I was going to run LS3 heads with a mild port on them. Other then upgraded springs and the hollow valves I want to keep them stock and in budget. I would also like to assemble it myself. Now as far as the deck height is concerned how do I measure this? Thank you for all your help this has been great to learn all about this.

 

well the deck height even at .000 deck or .010 out will only change your scr .2 of a point or so so that is not much to worry about. but 11:0 scr, low to mid 8.xx dcr or so with a .040 gasket will be a good combo. i have ran 9.0 dcr on 93 so no reason low to mid 8's shouldn't easily be doable on 91. and that light car will accelerate quicker than a barge 4th gen and be less likely to detonate also.probably sounds stupid to some but that is true. try accelerating in 6th at a low rpm on a hot day and that is where your knock will happen, engine under a big load and not accelerating quickly.just my worthless $.02