1999TransamWs6

iTrader: (21)

I was wondering which heads i should go with either AFR 205cc or AFR 225cc. I have the stock bottom end and am going to eventually go with a 383 stroker motor and doint want to buy heads twice. I want 11:1 CR and most people arent shure if i can achive that with the 72cc 225 heads. Also my cam specs are 231.8/237.1 .589/.598 on a 111.9 LSA and i dont wnat and piston/valve clearance problems so please respond with any advice asap because the 225cc heads are ordered but i can change my mind up till the 24th and its the 22nd now so please respond asap. Also i heard if you mill the heads too much the intake manifole may not work and i dont want this to happen from over milling. Thanks

Greg Fell

iTrader: (9)

If/when you go with a 383, you can order the pistons to suit the head and desired compression ratio. Flattops or mild pop ups even. You shouldn't have piston/valve problems because the pistons will have valve reliefs.

Patrick G

iTrader: (14)

If you mill the 72cc 225s enough to give you good compression, you're going to sink the valve enough to require valve reliefs with the cam you've purchased. As long as you don't mind fly cutting the pistons of your stock short block, you should be good enough to hold you over for your 383.

1999TransamWs6

iTrader: (21)

Yea i dont want to fly cut so should i just go with the 205cc heads with the 66cc chambers and mill them to 11:1 CR? Then will that alow me to not have to flycut?

Spinmonster

You should use cometic MLS gaskets in thinner sizes to obtain your CR down to .036 or so. For a given thickness you will raise the CR more with gaskets. Also 11:1 is easy to get to with a stroker and 225's.

One more thing: That cam will run on 93 with 12:1 CR due to the dynamic CR being lower from the delayed closure of the intake valve. You will pick up almost 15rwhp and get awesome low rpm punch compared to 11:1. The stock LS2-c6 runs 10.9:1 with a 207 cam on a 117.

Get the 225's.

1999TransamWs6

iTrader: (21)

Can the 225cc be milled that much to achieve 12:1 CR? Also with all that milling wont i have piston to vlalve problems? This is still on the 346 remember 383 is in some time.

Patrick G

iTrader: (14)

You can't mill the 72s enough to get 12.0:1 without compromising flow and manifold alignment. Also, 12.0:1 on 93 octane can be done, but it's a touchy situation. Often, you'll have to back the timing off from ideal to keep it from rattling. You may run faster with 11:1 and full timing, than 12:1 and backed off timing. Your motor will always make its best power with ideal timing.

cyphur

iTrader: (10)

you can always wait for the small chamber 225s.

PREDATOR-Z

True for NA motors, but not when it comes to juice
Absolutely, but that doesn't necessarily mean max timing before detonation.

Oh, and TTT, get the 205's

Spinmonster

You can run 28 degrees timing and 12:1 CR with that cam on 93 and it isn't touchy.
For a complete expanation of this read this in better detail:
http://members.uia.net/pkelley2/DynamicCR.html OR:

Guys, this topic is being killed to death. You get knock from dynamic not static compression. If you use a big cam with lots of overlap, the dynamic compression is drops due to the intake valve closing later. This is called dynamic compression. The static compression you speak of is set by the size of the chambers, gaskets, pistons and the chamber volume.

Now if you have a stock cam in a stock engine set-up and you put in a bigger cam, you just reduced the dynamic compression. The dynamic CR can drop 2 full points just from a cam change. In the static calculation, you measure the CR with the piston at the bottom but with cams with big overlap like the one listed above the intake valve closes so much later that the A/F mixture being doesn't start getting compressed because the cylinder can't start building pressure untill that valve closes but the piston is already on its way up. You then changed the effective volume of the cylinderr since you only measure from the point the valve is closed. Hence the compression drops on a big cam.

You adjust the static compression to compensate. You can run that cam on 12:1 static CR with 28 degrees timing with no issues at all. AND SPRAY IT TOO.

On a 383 the compression with 2cc valve reliefs will be 10.6:1 with stock heads. You can raise the CR by .8 by using .036 cometic gaskets instead of the stock .054's. You also get the reduced quench area but still maintain .029 clearance to the piston in the quench area. AFR claims ytou can get a full point from milling the thick deck on that head.

Here is the page to read in more detail how you do the calculation. http://www.kb-silvolite.com/calc.php?action=comp

Just because the static CR is 12:1 doesn't mean that you have anywhere near that CR by cylinder pressure. A lot of guys running around with cams like the T-REX and have less compression net to the motor than stock because they never comprensated for the drop in dynamic CR. You get knock from dynamic not static compression but if you leave the compression at 10.1 because you can't raise the CR to 12.5:1, you just left 35rwhp on the table and the motor is really seeing more like 8:1 dynamically. Can you get that from timing? No. One thing for sure you will still be able to run 12:1 with 28 degress of timing with that cam. Use auto-tap and you will see there is still no knock. By the way, that is at high rpm for the power increase but where it really pays dividneds is the low rpm torque increase.

Phil97svt is running a cam near that size on 12.2:1, 28 degrees timing, on 93 octane. If you go with a cam with more overlap, you will drop the dynamic compression even more. It isn't iffy on knock. He sprayed a 150 shot for a 10.00 pass---348 forged motor.

You are actually running less dynamic compression than stock if you run that cam without increasing the static comression. I run 11.3 now on a stock LS2 with 1.8 rockers with the intake valve closing much earlier.

Getting back to the original post, if you want the bigger chambers TEA has successfully opened the 205's runners for over 230cc's. The 205's are a good place to start if you want the extra step of porting the AFR's later.

Patrick G

iTrader: (14)

I agree and disagree with the above post. Yes, static compression and dynamic compression are two different subjects and have their own affect on octane resistance.

FYI, the dynamic compression of a stock 98-00 F Body LS1 is 6.9 to 1. Probably not as high as some might think. My current setup is 11.0 static and 8.6 dynamic. I have more octane problems now, not because of my 11.0 static compression, but more because of my 8.6 dynamic compression.

But here's where I differ: Dynamic compression has different effects on octane resistance over the given rpm range. At low rpm, low air speed allows more cylinder pressure to bleed off giving the motor more octane resistance. But as rpm rise, the velocity of air rises and less air bleeds off during the overlap cycle. So a motor might be a little lazy and less octane sensitive at lower rpm, but more crisp and subject to detonation at higher rpm. That's the crazy thing about running high compression motors on pump gas. You can't just look at static compression, nor can you just look at dynamic compression...you have to look at both and the rpm range you plan on operating in.

Spinmonster

I agree this all is predicated on the ability to get the air in and out of the engine in an efficient manner but he is talking about an optimum motor build-up with great heads. You also can't even begin to compare the overlap from the TR 231 to your 215 cam which I assume is on a wide LSA for the FI. You are definitely running a punchy set-up but the cam he is talking about bleeds off a lot more cylider pressure and he should be at least 1 full point over the cam you run at 11:1.

No way I can run more compression with a 204 (probably 206 with the 1.8's) cam on a 116 than you....but I do.

Good feedback Pat.

1999TransamWs6

iTrader: (21)

Thanks for all the input so do you think with my cam 12:1 with the .041 gasket i can achieve all this with no piston/valve clearance issues because i dont really want to take the whole motor apart to do this and i dont have the tool to do it plus i dont want to balance my rotating assembly after the flycutting from changing the weight of the pistons. I changed my order to the 205cc heads from AFR with 66cc chambers i just need to get them milled how much for 12:1 CR if it can be done with no flycutting

Spinmonster

Thought you were building a 382?? If so the pistons will have reliefs. If not you have to check the clearance when you put the cam in. Cometic have a 2-3 week lead time for manufacture. That cam with .041 gaskets is around 11.2:1 and I don't think it will fit but it is only .013 from the stock thickness so it is possible. Call thunder racing for their recommendations with their cam and thinner gaskets.

For the higher compression wait until you do the lower end. The piston reliefs will allow the milling and thin gaskets.

1999TransamWs6

iTrader: (21)

so should i just get different gaskets if it doesnt fit and this will solve the problem?I talked to thunder racing and there answer is "I dont know" i just dont want to cut pistons ill deal with lower CR untill i do the bottom end in a year or so.

Spinmonster

Get a set of stock MLS gaskets till you do the motor