This is going to be kind of long so bear with me:
I am in the process of sorting the pros and cons of my next engine build. I have maxed out the "stock" longblock setup and need to move on. The choices I have in displacement is the real question now. I have the heads, exhaust, and cam that I am going to use for the project. (FLP longtubes, CC306, and reworked LT1 stockers that have been very well done).

My question lies in the effeciency of each engine. I am wondering about the power production per cubic inch of each engine (assume equal compression, the same flow from the heads, and exact same cam and exhaust). I am imagining on a budget I can squeeze more hp per cube from a 355 than from a 383, and more hp from a 383 than a 396. If I can squeeze 1.3hp/ci from a 355 (461.5hp), and I can get 1.2 from a 383 (459.6hp), and maybe 1.15 from a 396 (455.4hp) then wouldn't building the 355 make more sense?

The cons to the smaller engine would most likely be that the engine is more 'peaky' and would develop less torque, but that shouldn't be too big of an issue (M6 car used for racing).

The pros to a bigger engine would be more torque and less rpm for the peak output.

What is the difference in output per cubic inch of each engine (355vs383vs396)? Is there a huge difference?

I do have to limit my $ expenditures. Like a $350 oil pan makes a difference if it dosn't get me down the track faster.

Thoughts?

BBB

 

To answer your first question, yes the 355 would make more sense. Any time you can keep piston speed to a minimum and make the same power, go for it.

Like you have hit on already, the smaller engine will be more "peaky". However the "hp per cubic inch" differences are very little when you are talking about 355 to 383 to 396, anywhere from 4-8% maybe, which you can make up for in another area of the engine.

However the longer stroke motors will make more torque on the lowend and throughout the powerband, but will be running a higher piston speed which can be an issue if you plan on revving this thing too high.

Piston Speed @ 6000 RPM
355 - 3480ft/min
383 - 3750ft/min

I don't remember the stroke on the 396, but just take it X 1000.

 

Ok, if everything is the same (heads, cam, headers, etc) how do you plan on getting more power from a smaller engine? It just doesnt happen...

Anyways the "rule of thumb" is to always build the biggest engine you can afford. Generally speaking you can count on at least another 1hp and 1lb/tq from each additional cube provided your top end is up to snuff.

Considering a cast Scat 383 crank runs ~$200 cubes are cheap power. Where else are you going to pick up ~30hp/ and 30lb/tq for $200 all other things equal?

I wouldnt worry much about piston velocity, not at the level you're talking about anyways. Plus given the same cam a 383 will peak lower than a 355 so you wont need to rev it as high (which will reduce piston velocity).

IMHO a 383 is the sweet spot on the "cost vs CID" curve. Cranks are cheap and plentyful plus i'm pretty sure you can get away with the stock oil pan after some massaging. For a 396 the only 3.875" cranks I've seen are forged so they're going to run 3x-4x more than the aforementioned Scat crank. I'm not sure a stock oil pan will work with the 396 either.

 

Thanks for the feedback. Here is some more food for thought.

First: I am not trying to get more power from a smaller engine (although that would be cool). I am just wondering how they respond to the same parts (like with heads you are limited on valve size because the bore diameter is the same for all three engines, therefore with heads that flow adequately for the cubic inches valve shrouding should be more of an issue for a 396 than a 355).

Second: I want as many cubes as I can afford (ther is no replacement for displacment.) , but there has to eventually be a downside to increasing inches by only increasing the stroke. Horsepower per cube has to fall off somewhere (like the 500ci NHRA Pro-Stock vs 800ci IHRA Pro-stock). If I can make more torque and even hp in a 396 vs a 383 then the 396 is the better choice if it dosn't cost a fortune; however, if the 396 makes less hp per cube than the 383 (say 1.2 vs 1.3) then the 383 is the better choice.

Third: Oil pans are a prime example of my concern. A stock oil pan won't fit a 396, but it will on a 383 (with work). If the 383 hangs right with the 396 then the extra expense is not worth it.

By the same token 355 parts are more readily avaliable than 383 or 396 parts. I would think you can get better rods, cranks, and pistons for a 355 than a 396 for less $$$

Am I thinking in the right direction?

BBB

 

I see, thats sorta what it sounded like with your talk of hp per ci though. My point was given the same heads and cam it would be very unlikely a 396 would make the same power as a 355.

How do you figure? Only two things shroud the valve, the combustion chamber and the bore itself. If you're running the same heads, and the bore is the same how does a 396 shroud the valves anymore than a 355?

I'm sure you're correct, but I doubt you'll reach that point on a production LT1 block. Well at least not before you run out of money Building anything larger than ~396 gets pretty expensive. That said the gain in cubes going from a 355 to a 383/396 would more than offset any loses incured with a longer stroke.

There are tons of "of the shelf" parts for 383's just waiting to be ordered. True you probably wont find them at the local Autozone store but you shouldnt be building a hi-po motor out of those parts anyways Stuff for a 396 (namely the pistons) are harder to come by but they are out there if you look around.

Cant say I agree with you, the only difference is the crank really. There isnt any difference in the rods. More than likely you'll end up running a 5.7" or 6.0" which will be the same on all 3 motors. 355 and 383 pistons are about the same price too. A 396 may require a custom pistons but that isnt much more expensive.

As far as cranks go you can get the 383 Scat crank I mentioned earlier for $200 and it will be more than strong enough for the combo you're building. For a 396 you'll need a forged crank (since they dont make cast ones in a 3.875" stroke) which is stronger but much more expensive.

 

a stroker shoudln't cost more than 355
just the machining for clearing the longer stroke is all...
maybe an oil pan? i know both myself and kmook used our stock oil pans...
there is no replacement for dispacement
396 = +.7L
or ~13% more displacement

cheers

 

Valve shrouding: I was thinking that valve shrouding would be more of an issue for a 396 solely from a 'feeding extra inches' issue. The bore would be the same so all the clearances would be the same etc. And thinking on it further it should be a cam issue rather than a valve issue. (That is why I asked, good responses make me think things through better.)


HP/CI effeciency: from any current responses nobody has said anything leading me to believe that there is much to be lost effeciency wise with the bigger engine (although I have seen some debate between 396 vs 383 in other threads before)

Thanks for the responses, they have helped clear up my decisions a little.

BBB

 

i'm emailing you jason
I have one question of my own-

what crank is used in a 396 and does it impair its revving capabilites significantly when compared to a 400 crank?
(is it a 427 crank )

mr G- Racenet dicked me over too

 

Now returning to our original programing already in progress...

A 396 uses a 3.875" stroke crank. Sorta like a 383 crank just with a little more stroke. To my knowledge its not used on any production motors.

Theoretically the longer the stoke the worse the rode/stroke ratio gets and the less it likes to rev. In reality I dont think its a problem unless taken to the extreme. For instance take a look at jimlab's dyno sheet for his 396:

http://home.earthlink.net/~jimlab/images/dyno1.jpg

7600rpm is plenty fast enough for me

 

Yes, and I understand that it's somewhat popular to use a 6" rod in a 396, but my friend told me he worries that the piston is then too high into the ring lands.

I think there is a 5.8x rod/piston combo.

I emailed JE about a .060 over 5 7/8" stroke piston and they said it would be a custom piston...

HP/cu. in is probably better at 302 even, eh?

treyZ28: they said to me that they had never had a problem with that combo, I have gotten 4 people now that say otherwise

 

I'm no expert with strokers, so maybe I shouldn't be in here but I'll post my thoughts as I'm starting to work on what I want to do when the 147k stock engine finally pumps it's last ounce of oil

I've read that as the rod/stroke ratio gets lower, the crank has a tendancy to pust the piston into the wall of the cylinder, and I think that's where your efficiency goes down. Also, (just a thought) you are "dragging" the piston/rings further, so you have more friction, losses of heat, etc... Also you are getting the piston away from the combustion chamber quicker, thereby reducing the pressure exerted on the piston top during the power stroke. If you have a big rod/stroke ratio, the piston "dwells" near TDC longer, allowing the pressure to push the piston down for a greater angular displacement.

Lately, Hot Rod has been praising high rod/stroke ratios advising that they make more power and spin faster.

Having said that, I'm looking into a NA 396 myself, but I'd want 6" rods, which some have said puts the piston ring too far into the ring lands, which = more blowby and weaker pistons.

The stock engine has 5.7" rod w/ 3.48" stoke, so you're looking at a r/s ratio of ~ 1.64.

A 383, with it's 3.75" stroke is as follows:
5.70" rod; r/s = 1.52
5.85" rod; r/s = 1.56
6.00" rod; r/s = 1.60

A 396 , with it's 3.875" stroke goes like this:
5.70" rod; r/s ~ 1.47
5.85" rod; r/s ~ 1.51
6.00" rod; r/s ~ 1.55

Here's an idea:
A 302, with a 3.00" stroke w/ impy 6.00" Rods; r/s = 2.00

I think you have to decide between cost/forced induction/rev capabilities.

I think a 383 can be built nicely both ways, cheap, NA, medium rev = cast crank, 6.00" rods, high compression

expensive, Blown/NOS, high rev = forged crank, 5.85" rods, low compression

Unfortunately, I don't think a 396 can be as versitle, because it's expensive any way you cut it.

Da*n it, I think I just talked myself into a 383


If I may ask one question though that I'm not clear of, how much hp/stroke can stock 2 bolt mains take? If I'm not mistaken you can upgrade the bolts to ARP, or are the mains held by studs? I notice that 4-bolt/splayed conversions are pretty expensive, and so is the required labor.

 

See now that is the kind of thinking that has started me to back away from the 396

Dwell time, angularity, sidewall loading, and piston ineffeciency all should contribute to small losses in hp production. The issues can be overcome by more expensive parts, but gee if that was no problem I would buy a 550ci prostock carbon fiber car.

The 355 with 6" rods has the best rod ratio (I have always heard that a 355 and 377 make great acceleration engines due in part to rod ratios).


What do the engine builders say(like Nu-tec/Nick Norris). Someone who regularly builds both and sees dyno time every week should be able to enlighten us.

BBB

 

OK.... I think I have pruned out the irrelevant material..... I apologise if anyones relevant post got cut... this was my first adventure with the new software..... But I think it was done correctly.

Fred

 

I went with the 355 6" rod set up why?
I was on a budget as well, instead me spending more money on a 383 or 396 crank i put that money towards the rods and pistons and stayed with a stock crank. And another reason for the 355 i thought there whould be alot less tuning like to the fuel curve, spark etc. that is needed for the 383 or the 396. i think i spent around a 1000 dollars for the rods, pistons, machine work and of the other little things gaskets, oil etc. And i think with the AFR LT4 195cc heads with the LT4 intake and a CompCams nitrous cam 224/236 534/555 114lsa w/ 1.6 rr's and all of the other bolt-ons like the Hooker LT's, 3" Y pipe, Mufflex 4",TH-350, and etc.. I think my car will make some where around 430 to 450rwhp and thats not bad for a little 355c.i w 6" rods
Just my .02 cents

 

My94RedZ28A4 and Soma07 is who I would listen to.....bigger isn't always better...if the rod/stroke ratio is no good the more problems you will have.