Its been a while since I played with C.R. numbers but I don't believe the gasket thickness effects the Dynamic CR. Just the Static CR up or down. Dynamic is altered by valve event timing. In the event that your Static CR is too high, pushing 11:1 or so, then a thicker gasket will drop the Static but the Dynamic will remain the same regardless.

 

According to my Machine Design textbook (yes, I'm a nerd engineer and like to read textbooks), it looks like the dynamic compression is calculated with the same formula as the static compression, except you just subsititue the dynamic stroke length instead of the nominal stroke. In my case, the dynamic stroke reduced to 2.85" from the nominal 3.48" stroke (using 5.7" rods and the cams 58 degree intake closing angle.) Once I had the 2.85" dynamic stroke, I just substituted it into the orginal static compression calculations, which include gasket thickness.

 

Here is one of my favs for compression ratio, head-scratching, pencil sharpening, eraser burning.
United Engine & Machine. Calculators on the left of page.

 

Honestly, I think you would be crazy not to use a 383 rotating assembly. The cost is essentially the same since you are replacing your rotating assembly already, so it's free cubes! And you may only want 300-400 horsepower, but a couple thoughts on that: 1. the 383 will make more power lower in the RPM (this is a street motor, so more low end torque is a plus). 2. Just because you think you want a certain power level now doesn't mean you wont want more later, or find out that that 350 hp number isn't as powerful as you want it to be.

Personally, I have a motor roughly in your power range and I always want a little more power. But I also like to drive my car like I stole it haha. If you really don't care about power and just want something to cruise around in, then the 350 is just fine! It just seems a shame to have the block apart and buy a new rotating assembly and give up nearly free cubic inches!

 

I'm w/you. If you have to replace the crank, go a little bigger and stronger.
That way when you want a little more power.
You don't have to worry about durability!

JMHO

R

 

 

That's funny. I made a spreadsheet yesterday that does exactly what most of their calculators do. I checked my numbers against the CR calculator and they match exactly. If I change the gasket thickness, it changes both SCR and DCR.

My initial plans were that if I went to a 383, then I'd just get a complete rotating assembly from CNC Motorsports, or similar. If I decided that I would stay with a 350, then I didn't plan on buying a new rotating assembly. I talked to my machine shop and they'll turn a crank for $100. So really, I could use my current rods, turn the crank, and buy the KB pistons, rings, and bearings. I'll have to do a cost comparison to really see what the difference would be.

I love having HP, but I don't want to get into a position where the HP starts tearing up other components like my rear end, which I understand the 80 aluminum diff isn't the strongest.

Thanks for the advice REELAV8R! I'll definitely remember that when I start putting it all together.

 

Your thinking is sound^^^^^^^^^^

If you need a new rotating assembly than definitely go 383.

If not stay with the 355 using the OEM rotating assembly especially at the power level you are shooting for. I reconditioned my forged OEM L-82 crankshaft, reconditioned the OEM L-82 rods, etc during my build of the OEM L82 in 2014. Based on what I personally knew about the 355/383 decision, my expert builder's advice and long commentary sessions with him, and what I read from many of the other SBC forums including F body camaro/firebird/chevelle/nova etc, I stayed with the 355 and extremely pleased I did. I was and still shocked at the power my L-82 355 makes on the street. If I wanted more power, which I do not, I would NOT be going 383 unless I had to replace the crank. I would look to 400+ CI SBC for more power, not a 383, since the cube difference is very small.

The split consensus is/was on other forums either the 383 will change your life versus the 355 OR the difference on the street between a healthy 355 and a strong 383 is VERY little. Some would say barely noticeable who have built BOTH (key word here...experience with both in the same car) to a very slight advantage for the 383 (.10 sec 60 foot time...direct quote) versus their healthy/strong 355.

Bottom line, need a new rotating assembly, 383 all day long! Want 475+ Gross HP 383 all day long since it is easier to hit that power level with a few more cubes....Other than that build a strong 355 with a shorter duration cam (mine is 219/225 which gives strong bottom end and BIG mid range torque), great heads along with moderate compression and you will have a great combo for the street.

 

This is a stronger argument if starting from a forged L82; he's got a cast L48 crank; a modern cast SCAT 383 crank is still going to be significantly stronger than the cast L48 crank, so might as well get the extra 33 cubes along with the stronger crank.
Plus like you said, everyone THINKS that the 383 makes a huge difference and desirability and selling price go hand-in-hand. (Will you get back the extra $200 for a cast Scat 383 crank? Surely.)


Adam

 

Which is more fun, driving a slow car fast or fast car slow?

Which would you rather have, a 'hot' 350 or a 'mild' 383.....

 

Better question is how different is 450 Gross HP whether from a 355 or a 383? About 300 RPM difference across the area under the curve for HP and Torque.............They are both going to be fast, regardless.....

There is no getting around or away from that the difference is 33 cubes between a 383 and 355!!! It's not 80 cubes, 100 cubes or 125 cubes...it is 33 cubes. Don't expect miracles folks!....In other words, it is 5.8 Liters versus 6.3 liters...Holy smokes .5 (1/2) liter bigger for the 383 versus the 355...Give me a break............

I have said this before and will again, unless the 383 build is optimized for the extra cubes, a strong 355 could and would actually be faster....The 383 has more potential but that is where the advantage ends unless taken advantage of.........relative to a strong 355

 

Adam,

He is looking for 300-350 Gross HP motor...He does need a Forged 350 crank and certainly not a 383 crank for that type of power target......His cast crank is good for 400 Gross HP....

 

I've been playing around with the calculators as well....

Changes in SCR and DCR are equal with a head gasket change. As an example:
SCR is 10.36 with .041 gasket
SCR is 10.80 with a .025 gasket
That .44 difference in SCR is also a .44 change in DCR. Others can correct me but a gasket change will only go so far in regards to an SCR/DCR change but its mostly an SCR thing as it can either get you into or out of a ping/detonation condition, Any unwanted change in DCR from a gasket change can be made up for with a change in camshaft IVC.

 

According to David Vizard a stock cast crank can be used with confidence up to 350- 375 HP. So if you ever see yourself exceeding that HP level then it would be advisable to get a new scat steel crank or some such. The stock crank is also pretty poor for windage and plenty of HP is soaked up by it slicing through the oil and the air in the crank case. Scat cranks will have a larger fillet radius journal and crank cheeks which makes it considerably more resistant to cracking though that area.

I crunched your numbers for a 355 and it comes up with 9.64 CR with a DCR of 8.09. That's pretty good. With aluminum heads you could push that up to 8.5 DCR and still be good.
I figured 11.7cc's for those 12cc dish pistons, that's what mine cc'd out to.

Make it a 383 with that cam and your up to 10.31 CR with a 8.69 DCR. Bit high on the DCR. But you would also get different pistons so hard to say what it might be without knowing that.

If you're considering going below .035-.038 on the quench, don't do it. .040 is plenty and getting closer is of no advantage when weighed against the risk of striking the head with the piston.

 

I strongly disagree with you on this. I am running .034" quench on my 406 I spin to 7500 RPM and I don't hit anything. I have a pair of 500 horse 383's in a friend of mine's boat that spin to 6000 RPM that are set to .030". I did a huge amount of testing on this on the dyno, track and street and found that .040" was about the minimum that the quench aided in detonation protection and .050" to .070" actually made it worse. I have gone as tight as .025" on smaller engines. On my race engines I used to tighten the squish until the piston and head would just start to shine the aluminum up on each other and then back off a couple thousandths. The tighter you can go without hitting the better! There are other things about quench that I never see anyone discuss such as quench to bore percentage. On a low RPM engine you can run run as high a 55% quench to bore but On high RPM engine that becomes a pumping loss and will hurt high RPM power. On high RPM engine you want to take it down to a max of 40%.

Smokey Yunick found that a tight squish was benneficial back in the 1960's!

Mike

 

He is and he isn't; if he's going to buy any of the new gen of good aluminum heads that have been recommended on here, he's going to have a quite hard time not exceeding 375 HP. A stroker kit, with a much stronger cast Scat crank (I NEVER recommended he buy a forged crank; you're making a nice strawman there.) isn't going to cost much more than just new pistons, ring, rods & rod bolts; and if he IS getting a new crank, getting a 383 crank costs the same as a 350 crank. +33 cubes is almost +10%.

He needs to decide: no crank or new crank and with a good set of heads and that cam it's going to be hard to keep the motor down at 350 -375 crank HP. Better safe than sorry, and if he's buying a new crank anyway, he should get the extra 33 cubes for "free". (This is why I said the calculation looks different for people who are starting from an L82 forged everything vs an L48 crank with a 350-375hp safe limit.)

-Sometimes the decision YOU made, might not be the right decision for someone else.



Adam

 

 

Mike, I'm sure you're running top-shelf rods and rod bolts, though. Would you recommend the same .034" quench for someone buying a $900 stroker kit with Chinese Eagle rods and unknown rod bolts? (A legitimate question; I'm assuming the answer is no, but I'm legitimately curious.)


Adam

 

If he is going with some good aluminum heads along with that HR cam, then yeah he will be around 400 hp. In my opinion, the stock crank (assuming good condition bearings) would be fine. Especially because the OP clearly doesn't plan to drive it hard.

Good points here for sure! I think you hit the real decision on the head: 1. Go the cheaper route (machine the stock crank, replace the pistons and rings, get some aftermarket heads) and have close to 400hp, but be basically at your power limit. Or 2. get a 383 kit (may not be that much more expensive once you calculate it because machining your stock crank isn't free! You can get a decent stroker kit for about $1,000, and that will include pistons, rings, crank, bearings, flywheel/flex plate, etc...) and have the extra cubic inches (more power, and future power potential), stronger crank, more low end torque, etc...

If you are SURE you won't be wanting for more power with a 350-400 hp motor (keep in mind that is flywheel horsepower, you would be putting something like 275 horsepower to the tire) than I would go straight for a budget rebuild on your crank, new pistons, and some good heads (DEFINITELY look at the Jegs brand heads, they are Profilers and cost $999 a set).

If you think there is a possibility that the above will eventually not be the horsepower you want, NOW is the time to go ahead and do a stroker crank while the block is apart. Right now, it's only a little more money to for a 383. If you build the 350 and decide you want a 383 later, it will cost alot more because you are buying new pistons, rods, and crank to replace the ones you already bought, plus the cost of gaskets and fluids AGAIN.

Personally, it's an easy choice to me: go 383 and don't look back. But only you can decide what is right for you and your car and your wallet.

 

Jim, he can also install the cam a couple of degrees retarded if he gets into trouble with the DCR, right? (I get a 0.12 decrease in DCR for every 2 degrees the cam is retarded.)
A Cold Air Intake would be the best way to deal with being just over the edge of detonation with premium fuel in hot weather, IMHO. Every 8 degrees in IAT temp drop is a 1 octane equivalent increase in detonation resistance.

Adam