Hi,

I was wondering if you used a 350 4 bolt block and used a forged crank with the same stroke as the cranks in the chevy 302 and longer rods and a reversed dome piston and a 72 or 76 cc chamber head also intercooled. With the shorter stroke and long rod combination couldnt you run a whole lot of boost with out running into detonation problems? Ive read articles about this being true with a naturally aspirated 400 block with a crank from a 327 and 6.2 rods makeing over a 12 compression ratio.

 

Well its kind of a tradeoff ... you either have high compression and a low boost or you have low compression and alot of boost. If you had a 12;1 compression ratio and tried to run 15 psi I'm sure the engine would explode. Guys with the LS1 (around 11:1 compression I think? not sure) run 6-9 psi at the max .... and still make 500+ hp! So if you wanted high compression like 12:1, I wouldnt recommend anything higher than 6 psi (and it will still eventually blow up .. you can just prolong it). Some of the guys on the boards are leaving their compression ratio around 9:1 and running around 15 psi. That seems to be a solid configuration. The compression ratio on the engine in my SVO is 8:1 and they can take 17 psi no problem (I plan to push 25 psi with a reworked shortblock). It just depends on how you want to do it. For me I would rather leave my compression low and let the boost dictate how much power I want to make. Other people like to have the high compression and run a max of like 9 psi .... both ways work just depends on what you like. Another advantage of the low compression ratio's is you can run 87 octage gas!! woohoo!! hahahah.

 

I have been thinking about this alot. How to get teh most out of alot of boost. I have been doing alot of thinking on how to get low compression. The only thing I know that really works is the reverse dome pistons.

If anyone cna offer some guidence it would be greatly appreciated.

thanks:hail: :hail: :hail: :hail:

 

Dish pistons and larger chambers in the heads will both lower compression.

 

What about the bore of the block. How does that affect the compression?

thanks:hail: :hail: :hail:

 

Increasing the diameter of the bore, increases the static compression ratio, all else being equal.

 

thanks:hail: :hail: :hail:

How does the crank shaft size affect the compression?

like if I had a 400 crank in it (383 ci). How will that affect compression?


Thanks :hail: :hail: :hail:

 

Inreasing the stroke would increase compression, but it is usually not something that you would or could do. Without changing the length of the connecting rod and/or using pistons with the appropriate compression height, and/or changing the cylinder block's deck height, you would end up with piston to head contact. Normally, when you increase the stroke and/or upgrade to a longer rod, you will need pistons which have a shorter compression height.

 

Umm... so...umm...
Not sure how to ask it.

Ummm.... basically since you need lower compression pistons and shorter rods to make a stoker, it does not effect the compression that much?

thanks :hail: :hail: :hail:

 

Hi again,

Crazy Hawaian,

What I meant was to use larger chamber heads like 72cc and 76cc and dished pistons so I would have a lower compression ratio prefferably less than 9.1 if possible wouldnt the longer rods be better?

 

It depends on what pistons you get. If you get the correct pistons for the chamber size the comp ratio will be around the same as what it was before you stroked it.
The longer rods with everything else designed correctly would probably allow you to run a little more boost.

 

thanks for your help:hail: :hail: :hail:

 

A stock 350 is 9.3:1 CR with 64cc heads. If i kept that CR and went with forged pistons (stock crank and rods) how much boost would I be able to safely run? I was thinking around 8 PSI (intercooled), does that sound right?

 

if you are changing the pistons to a forged set why don't you get a dished blower piston. I am running a 22cc dish piston from J&E for my 355, it's a off the shelf piston, nothing custom.
Also, rod length has nothing to do with compression ratio. Just bore,stroke,head gasket thickness,piston dome + or -,how far the piston is below the deck at TDC affect compression ratio. Less compression and more boost is usually better than high compression ratio and less boost. don't go too low on the compression or you loose a little on the bottem before the boost comes in, usually. I'd probably shoot for 8:1 to 8:5:1 and 8-10lbs.

 

crap, i forgot the obvious, duh, volume of the head combustion chamber has a play at compression ratio. just didn't want someone saying I forgot somthing

 

does the loctaion of the piston rings change teh compression much?
I know you can have them custom made closer to the top or further down.

thanks:hail: :hail: :hail:

 

We know that. I'm not really sure of this, but I've always understood a longer rod (to a point) is better. I think it has something to do with the time near TDC, and the longer rod allows slightly more compression without detonation or more boost. One case of this I've seen people mention is a 400 destroked to a 350 w/ 6in rods that had over 11:1 compression and still ran on 87 octane.

 

I'm not real sure how long rods help in a blown engine. I am always hearing conflicting arguments about long vs. stock length rods. Some say helps in keeping it at TDC longer and helps against piston side loading. I have never had a long rod engine though so I am not real sure.
I do know though that when I bought my rotating assymbly I had heck of a time finding pistons for 6in rods with a reverse dome for a small block. I could get them custom, but I would have payed about double what a off the shelf piston would have been. I did look into it but it just seemed like for the price it just wasn't worth it. I finally went with a Eagle 4340 forged crank with a extra 1/4in keyway, Eagle 4340 5.7in H-beam rods, and J&E reverse dome fly cut blower pistons, and had it all balanced. To me it seemed strong enough for just about anything I would do. by the way I am running a Weiand 8-71

 

I have calculated and re-calculated and the 4.030" bore X 3.00" stroke (302 chevy) motor with a dished piston lets assume around 19-21cc dish and a 72-76cc head will net you around like 6:1 or less compression. even a maximum style .430" dome piston would only give you about 9.9 to 10.5 with those heads and that bore and stroke.

 

What is the best piston manufacturer out there? I have heard that TRW is good and that Speed pro is good, but what is the best, i mean something that will hold up beyond expectation.