RSKrause: please advise on compression ratio: 8.3 or 9.5

I am building a stroker for supercharged use. Here is the run-down:

- 385 cid, 5.7 rod LT1
- AFR 210's flowed 280 in 218 ex at Thunder Racing
- Comp custom grind 276/286 @ .006, 224/230 @ .050, .567"/.565", 114 LSA, 0 deg adv.
- LT-4 intake port matched
- ASM 1 3/4" headers
- T-trim with 6.66 / 2.75 pullies (12 - 14 psi boost)
- Aftercooler

- TH-350 with '0' pump Vigilante (3300 actual stall) and 3.42's

With my 68 cc chamber heads and SRP pistons, I am given a choice between 16 and 31 cc dishes. One compression ratio is 8.3 and the other is 9.5. I'd really like something in the middle, but SRP doesn't make pistons with a 25 cc dish.

It is not a daily driver, but it is 70% street, 30% track use. I run 93 octane in it and I'm in BR, LA, where summer days get to 95+ degrees and the altitude is low.

Which compression ration should I choose?

Thanks,
Mike

I know im not even close to Rich but either one will work fine. I much rather have a little lower than higher. It decreases detonation factor a lot. Power loss will be little to none on a high boost motor. 9.5 to 1 is the highest i would go. Why not just spend the extra bucks and get the desired compression you want. One other thing to consider is the fuel system driving that monster.


Bill

If you want compression in between those numbers you can get the 8.3 and do some milling on the heads to get to where you need to be.

Originally posted by engineermike
I am building a stroker for supercharged use. Here is the run-down:

- 385 cid, 5.7 rod LT1
- AFR 210's flowed 280 in 218 ex at Thunder Racing
- Comp custom grind 276/286 @ .006, 224/230 @ .050, .567"/.565", 114 LSA, 0 deg adv.
- LT-4 intake port matched
- ASM 1 3/4" headers
- T-trim with 6.66 / 2.75 pullies (12 - 14 psi boost)
- Aftercooler

- TH-350 with '0' pump Vigilante (3300 actual stall) and 3.42's

With my 68 cc chamber heads and SRP pistons, I am given a choice between 16 and 31 cc dishes. One compression ratio is 8.3 and the other is 9.5. I'd really like something in the middle, but SRP doesn't make pistons with a 25 cc dish.

It is not a daily driver, but it is 70% street, 30% track use. I run 93 octane in it and I'm in BR, LA, where summer days get to 95+ degrees and the altitude is low.

Which compression ration should I choose?

Thanks,
Mike

I think you know what I will say. So, hopefully I am not "beating a dead horse" when I advise you that you will be much happier with 8.3:1. There is minimal hp loss with a blower app and changing the CR from 9.5 to 8.3:1. The mass of the air charge is greater due to the increased size of the combustion space and this just about compensates for the lower CR with the benefit of decreasing the tendency to detonate.

An NA setup fills the cylinder passively and the mass of the air charge is essentially the same no matter the CR. There is less power because the combustion process is less efficient at the lower CR. But with a blower and the resultant positive manfild pressure, it's a different ballgame.

Your combo should make good power, in excess of 650rwhp if everything is right. Tuning a combo like your is vital however. I would like to see a little more exhaust duration on the cam.

Good luck.

Rich Krause

OK, Rich, I've picked out some new lobes from Comp:

Intake: 276 adv, 224 @ .050, 152 @ .200, .567"
Exhaust: 288 adv, 236 @ .050, 162 @ .200, .585"

My question now is lobe centers. I had originally planned to use 114 lobe centers for intake and exhaust (114 LSA, 0 deg adv.).

What do you think?

Thanks,
Mike

Originally posted by engineermike
OK, Rich, I've picked out some new lobes from Comp:

Intake: 276 adv, 224 @ .050, 152 @ .200, .567"
Exhaust: 288 adv, 236 @ .050, 162 @ .200, .585"

My question now is lobe centers. I had originally planned to use 114 lobe centers for intake and exhaust (114 LSA, 0 deg adv.).

What do you think?

Thanks,
Mike

I have the same lobes on a 116LSA with a 114ICL.

Rich

Listen to Rich, BUT if you want to go for a 9:1 compression ratio then take a look at Probe pistons. They have one for a 383 stroker with a dish that is 20.4cc's. Wiht your heads and the usual Felpro gaskets that would come out to be 9.09:1. And if you really want a 25cc dish they will custom make a set for about $775. I was thinking about going that way but instead I am going to use the 31cc SRPs and get my head milled down to about 59cc's.

In the past, I've used TRW's, Wiseco's, and SRP's. I paid $600 for the Wiseco's and they knocked like a diesel and used a quart of oil every 500 miles. But, I was very happy with the SRP's. I ran .001 extra clearance on the bore and set the ring gap at .024 - .026, but oil consumption was less than one quart every 3000 miles and they were very quiet.

I'm not saying that the Probe's are bad, but I don't have any experience with them. I don't want to build another motor like the Wiseco motor.

The SRP's, however, have proven themselves so I wanted to stick with them.

Mike

Originally posted by engineermike
In the past, I've used TRW's, Wiseco's, and SRP's. I paid $600 for the Wiseco's and they knocked like a diesel and used a quart of oil every 500 miles. But, I was very happy with the SRP's. I ran .001 extra clearance on the bore and set the ring gap at .024 - .026, but oil consumption was less than one quart every 3000 miles and they were very quiet.

I'm not saying that the Probe's are bad, but I don't have any experience with them. I don't want to build another motor like the Wiseco motor.

The SRP's, however, have proven themselves so I wanted to stick with them.

Mike

:think:
the pistons caused the motor to knock:confused:
or was it the incorrect compression ratio with those pistons? my weiscos dont knock at all... or burn oil

the pistons caused the motor to knock

If oil is getting past the rings into the cyl, then detonation can occur from the oil. It sounds like an oil issue causing the knock. Or maybe piston slap?? too much clearance, which caused a piston slap noise and oil consumption??

The motor didn't knock from detonation. Trust me on that -- it only had 8.4/1 compression.

It was piston-slap.

Mike

Originally posted by engineermike
In the past, I've used TRW's, Wiseco's, and SRP's. I paid $600 for the Wiseco's and they knocked like a diesel and used a quart of oil every 500 miles. But, I was very happy with the SRP's. I ran .001 extra clearance on the bore and set the ring gap at .024 - .026, but oil consumption was less than one quart every 3000 miles and they were very quiet.

I'm not saying that the Probe's are bad, but I don't have any experience with them. I don't want to build another motor like the Wiseco motor.

The SRP's, however, have proven themselves so I wanted to stick with them.

Mike

There is a reason why diesels sound like there knocking. The injector psi is in the hundreds of thousands, and the piston just about hits the head. Compression is insane, which is why no spark is needed. Why would oil lead to detonation? Sounds like rings where not set right, and or wrist pins.


Bill

I file-fit the rings myself to .024 - .026", just like the SRP's.

The wrist pins are all OK. No excess clearance.

I tend to believe that I got a bad set of pistons that are either too small or improperly shaped.

Mike

I am not to enamored of Wiseco SBC pistons myself. When I used to mod air-cooled VW's they were the hot setup, FWIW.

Rich Krause

In "dirt-bike land" we call them Siezeco's because they tend to sieze more than the OEM pistons.

glad mine dont knock:confused:

The motor didn't knock from detonation. Trust me on that -- it only had 8.4/1 compression

Oil in the combustion process from possible bad rings,/piston problems could cause detonation. I mentioned this possibility due to the 1 quart every 500 miles. That could be enough to cause problems. But a piston slap is different than a "pinging" sound. :)

It wasn't pinging, it was knocking.

Mike