FlamingChicken

Im having a hard time contacting my machine shop and head porter so figure Ill tap the minds here. The pistons I was looking at were the mahle powerpak but was also suggested the mahle pro-series. Im leaning toward the powerpak but a flat top -5cc would put me at 12.6 using the comp calc on this site. I believe dcr is gonna determine streetability but not sure. Any thoughts?

Setup is Lunati Sledgehammer 3.75, Pro-Mod 6" rods... all parts are bought except the pistons and bearings.

lethal93ta

12.6 is really high, if you have compression that high you better be running 110 race fuel. I wouldnt go much over 11-11.5 if you plan on running pump gas.

95fbformula

i think you might be putting something wrong in i got 11.9 to 1 with a 58cc head chamber and .039 gasket. when putting the piston cc put 5 instead of -5. the 5 is what the mahles are some just have them listed wrong.

With a 56cc chamber you get 12.2 to 1 which is still fine for the street as long as you run 93 and have a good tune and a DCR between 8-8.8, 8.5 being ideal.

The 12.6 would probably be ok to just pushing the limits.

Edit i got 12.6 to 1 with 54cc heads so i assume that what you are running but as you assumed DCR determines street ability as long as you can run 93 have a good tune, DCR and cooling system you should be fine. Also if your heads are already done you can bump up to the stock width lt1 gasket and drop the compression down some i believe to 12.4 to 1

1racerdude

Lt1 heads are 54CC's.
Your DCR is cam dependent.12.2 is not to high depending on your intake valve closing.
Max DCR is 9.0 on 92 octane.
I would try to stay around .040 with the quench.

FlamingChicken

I know dcr depends on the cam but im not sure on the specifics. ive read you want close to 9 dcr with 8-8.5 being typical street builds. im having a hard time getting a hold of ai so not sure what cam specs he will recommend. according to my spec sheet my chambers are 54.3cc avg which with a 5cc flat top and .039 gasket puts me at 12.6 scr, according to the calcs on this site. thats also with a 0 deck height and the 3.75" stroke. although when i put in a bore size of 4.030 it calculates it as 4", so not sure what the extra bore size will do to the comp ratio. i assume it would go even higher.

jimbob65

I run 9.0 DCR with a SCR of 12.48. Daily drivin with zero problems even on hot days. Make sure you have no oiling issue, good cooling, cold stat (160*) and real good tuning and .040 quench, and you'll be fine... I changed from a cam that gave me 8.77 DCR to the one now that puts me right at 9. Noted no change in timing requirement or fueling (thank god), and run the air conditioning when needed.

rskrause

DCR is best thought of as being related to detonation under high load/low engine speed conditions. A high load/high engine speed condition is another matter. A motor with a high VE is going to see higher cylinder pressures under these conditions. The same thing that allows it to live at low engine speeds - the big cam will give the motor a higher VE. I would look to sucessful combos running a similar cam in order to help determine the static CR you want. DCR is only part of the story.

A little knowledge is a dangerous thing!

Rich

WS6T3RROR

i've yet to see a ported head come back with the chamber at stock cc's. most of em tend to grow a little bit and end up in the 58-60cc range depending on what all is done. honestly you probably need to wait till you have the heads finished and in your hands before you select pistons. mid 12's is workable on pump gas but it takes a builder who is pretty experienced in doing this with ltx engines.

FlamingChicken

The heads are already off the car and sitting in my garage. I never cc'd them myself just was reading what Ai put down on the assembly sheet. Hopefully I can get ahold of Ron today.

Does this mean at higher rpms its likelihood to detonate increases more and the overlap at low rpms is what helps "bleed" off the compression? Any good reading someone can recommend on the subject? something like "how to build a strip/limited street motor without blowing it up for dummies"?

1racerdude

If you stay below 9 for DCR it should not detonate in the upper RPM'S. It all has to be perfect including the tune. If you are uncomfortable with 9 the drop back to 8.5.

I have talked to several people out west that are running 9.6DCR on the street on pump gas with no problems. I would not do it but it has been done.

FlamingChicken

just got off the phone with ron@ai and he said im stuck between a rock and a hard place. he said 12.6 is doable but everything would have to be perfect unless i want to run race gas for insurance. he also said a guy hes doing heads for is running a 10cc piston which according to my calcs would be 11.8 scr. anybody know what piston this might be? ive only found 4,5,6,16,28cc in a flat top. also what are your thoughts on buying a 55gal drum from vp (local distributor, can pickup) or using sunoco 100 from the pump? the only other options i can see would be the lower comp piston, 355 instead of 383, or new heads (not really an option). Could I just open the chambers a bit? I need to stop rambling....

95fbformula

10cc is probably a custom piston and yes you can port out the chambers a bit to lower the comp i think 58cc might be the max for LT1 casting but im not sure. Or if you want you can try to find a cheap set of bare cast trickflows and send them to AI or were ever the TF chambers are 62cc so you wouldn't have any problems then. Either option would be a lot less then running race gas all the time especially for a street car.

rskrause

There is no ready reference on this subject. And yes, detonation is a potential problem with high rpm/high load operation. Many race engines require some timing retard in the upper rpm range to avoid this, especially when they are running a marginal fuel due to a class rule or whatever. We normally only think of detonation as an issue under high load and relatively low rpm, because this is where most engines have a problem. Of course, you can avoid this altogether by judicious use of the Mark I throttle and gear shifter!

You lose very little (maybe 1%, or less) by dropping the compression back 0.5 or so. 12.6 is very high, IMHO. Look at the LS7 - it is a very high performance OHV engine with a 100K warranty, so they must really have done their homework. The CR is 11.0:1. Gives you something to think about, doesn't it?

Rich

RedLT4Mike

I was gonna go with a flat top and try and push
the limits, but when it came time to put my money
where my mouth is, I ordered my dished probe pistons.

I can afford to build a stroker with nice heads,
but I can't afford to do it twice!

FlamingChicken

im off today so ive been doing nothing but sitting in front of this damn computer weighing my options. sounds like a custom piston might be my best bet. ive already dropped way too much money into this car and i want it to perform the best it can so no compromising unless i have to. a 9cc piston volume would give me 12.1:1 with a .035 gasket which sounds decent. i think the reason my chambers are so small is because of how much they were milled. looking at the spec sheet it says they were decked .032". any recommendations on a company to do a custom set of pistons? preferably less than $1k to make, closer to $800. Ron also said they might be able to change valves and with a vj gain a few cc's