RawAzzLT1

I was over at http://www.flatlanderracing.com trying to hand pick my rotating assembly. for a Budget 600rwhp boosted 383. The SCAT crank was the cheapest, and Skardom said that it should hold up fine. and he said that the Scat H beams would also work. but what I forgot to ask him was what length would I need. I want a compression ratio of about 8:5:1 or a little less that than ( I fear detonation badly) and I wasnt sure if the rod lenth affected the CR if rod length does in fact affect the CR I would like to know what Pistons to get for a 5.7 rod and 6.0 rod to get the Cr I am looking for? are there any advantages to having one over the other like weight or faster revolutions?

SSkindog

you need to select the rod length and pistons together, so that you get the proper deck height for the piston. Then you select piston style and cyl head volume to determine compr ratio that you desire.

Just looking at rod length alone wont do it for you.

Smokey Yunick used to say" put the longest damn rod you can fit in the engine"

RawAzzLT1

That being said. what will I need to get the CR I want?

SSkindog

the piston mfg will list C/R 's for their pistons when used with the popular sized chambers on heads, 76 cc, 64 cc, 58 cc, etc.

Try to stay with a flat top piston if you can, as that makes more horsepower via better flame travel. Am not sure what size chamber gets you to 8.5:1 C/R with a flat top piston but, the piston mfg will tell you.

And go w/ the longer rod, you will get a more favorable rod/stroke ratio. And will rev just fine

If your not going to use a blower, you could prolly get more C/R than 8.5:1 without detonation. Especially if you are using alum heads, and elec fuel injection. The combo of both will allow you to run near 11:1 on pump gas w/o detonation.

9.5:1 would be a safe ratio with even iron heads, if no blower

rskrause

I pretty much agree. But a well designed dish piston has a squish area and should work as well as a flat top.

Rich Krause

OldSStroker

Dished might even be better than flat tops, IMO.

With 8.5 CR and flat tops, I'm thinking the piston will be something like .150 down in the hole, which promotes detonation rather than limiting it. Keep the squish around .035-.045 inches which will require about a 32 cc dish for 8.5:1 with zero deck and 64 cc chambers.

I also go along with Smokey on rod length. Zero deck and 1.125 compression height on the piston gives a 6.0 inch rod with 3.75 stroke and 9.000 deck (.025 off stock for a cleanup).

Heads are important, even for a blower. What did you plan to use? For a budget iron-head engine, Vortec heads are my choice.

LT1 1980 malibu

Its not an opinion..You want to get the smallest chamber and dished slug for a forced car..N/a engines need the flat tops for faster flame travel and to aid in effeicency..Forced cars can get away without it and gain much more from having more slug surface to use the most amount of flame energy to shove the piston downward..

Dish piston zero deck .032-.038 head gasket..Head chamber ccs will determine static compression..

Injuneer

Just curious.... is 8.5:1 the best static CR for a blown LT1? I honestly don't know, but input from someone I respect on high boost blown LT1's was that there was no reason to go below 9.0:1. Like I said.... just curious.

LT1 1980 malibu

In the 80s when turbos and blowers blew up in popularity 8-1 wasnt unheard of..Mainly due to the fact of fuel managment kinda sucked..Haltech E4 was the big thing..Heads were crappy compared to today and so forth..Hell think about it ..Everything from GM was batch fire....

The roots blowers and screw compressorsd have a lot to do with that mith as well..They need the low static compression and anyone who has driven a car with one can tell you the way they pull..

Myself on an LT1 id run 9-1 or 9.5-1 with boost.I wouldnt run any more with an intercooler either..But thats a whole notha topic thea kimosobee.Hand shaped slugs, CNC chambers, and huge headers should help ward off problems from the rattle gremlins...The thing about the staic compression is when the boost is building you need some power..Its currently made too much of an issue from what i see because of older antiquated turbo wheels causing some serieus turbo lag..On a turbo car id probally run a lot less comp like 8.5-1..

rskrause

Ideal CR for a forced induction motor depends on a number of factors. Some of the main ones are maximum boost and fuel used. I chose 8.5:1 for my 14psi, pump premium unleaded optimized setup. I utilize a small cam (another important variable) and it works fine. My DCR (dynamic compression ratio) is ~6.5:1.

With a very small cam (214/224) and mediocre heads, this combo makes 613rwhp off juice. Next years I will be using a bit bigger cam and more boost. I am in the process of deciding what CR to use. Most likely I won't change it as the lower DCR and the higher boost should cancel each other out. I need to run some numbers to verify this though.

Rich Krause

RawAzzLT1

Rich, I have a burhead TT kit almost Identical to Bradys Minus the Intercoolers and some piping Steven ( zturboo) will be completing the kit for me. I want to do a 383 so I can make around 600rwhp on Pump gas So I guess it wouldnt see more than 12-14psi. I dont care to make more power than that. I am still stuck on what Compression ratio to go with. especially after what Injuneer said. I know he is talking about George Baxter.

WS6 TA

rskrause- can you define dynamic compression ratio and how you calculated it?

WRT to the question… what ‘the best compression for a ____’ is one of those questions that is impossible to answer, there are too many variables, and no one can assume that it’s a stock engine since you’re talking about changing the compression ratio. Good quench, well shaped chambers/dishes, any polishing, any effort to decrease the temp of the air/fuel charge, good engine controls, big headers/exhaust, loose converters, steep gears, low boost, cam overlap/egr… all help control detonation. High boost and almost anything that makes the engine work harder make it more detonation prone.

rskrause

The easiest way to explain dynamic compression ratio (DCR). Is to consider that compression doesn't start until the intake valve closes. Even with a short duration cam, this does not occur until the piston is already a considerable distance up the bore. My cam is a mild 214/224 duration. Even with this cam the intake doesn't close until 67 degrees ABDC. At this point the piston is already 0.9" up the bore! If you add 10 degrees duration the intake doesn't close until 72 degrees ABDC. Doesn't seem like much, but in this 5 degrees the piston moves another ~0.1" up the bore.

So, the main variables are static CR and intake duratiuon, with lesser contributions from the lobe separation and intake center line as well as the rod length. There are formulas you can use, but don't bother as there is a great downladable Javascript calculator at http://cochise.uia.net/pkelley2/DynamicCR.html, which also has the formulas if you prefer to do it the hard way.

Run a few numbers using the calculator. It's an eye opener and explains (among other things) why it's possible to run sky high static CR NA on pump gas (if a very big cam is used). DCR under boost may also be calculated, but isn't as straightforward.

I run ~6:1 DCR to accomodate both nitrous and the blower. Oh, and BTW, contrary to what many people say this does not make the car a "slug" off boost.

Rich Krause

Mindgame

The subject of dynamic compression ratio was brought up some time back and I believe Chuck Riddeck brought the term to a little better understanding than I ever had of it, so to give credit where credit is due, here's the thread.
http://web.camaross.com/forums/showt...threadid=46128

-Mindgame

WS6 TA

Thanks guys… interesting. I’ll read the second thread later…

Any thoughts on how this relates to boosted engines and other octane fuels?