Boost Motor

I want to build a boost motor. My understanding of the basics is that I want a forged bottom end and lower compression than what I have now. I'm considering either a 355, 383 or a 396 stroker block. Here are my questions.

1. Other than the obvious, more low end torque and displacement is there an advantage between a 355 and 383 or 396 in a blower application.

2. My heads are a bit unknown. They're stock castings that have been locally ported and I'm unsure as to their exact configuration. Won't I need to know what the compression chamber volume is to calculate compression?. I was thinking about shooting for something in the 9:1 - 9.5:1 range this correct?

Thanks,

Mark

Bump

Originally posted by 93ZM6Tally
I want to build a boost motor. My understanding of the basics is that I want a forged bottom end and lower compression than what I have now. I'm considering either a 355, 383 or a 396 stroker block. Here are my questions.

1. Other than the obvious, more low end torque and displacement is there an advantage between a 355 and 383 or 396 in a blower application.

2. My heads are a bit unknown. They're stock castings that have been locally ported and I'm unsure as to their exact configuration. Won't I need to know what the compression chamber volume is to calculate compression?. I was thinking about shooting for something in the 9:1 - 9.5:1 range this correct?

Thanks,

Mark

1. Nothing else of significance.

2. It's easy to "CC" the chambers. You need to do this. Any good head porter will have the stuff, or you can buy a basic setup (which is really just an appropriately sized burtette and a piece of plexiglass with a little funnel hole in it) for cheap. OTOH, if they are stock castings without a lot of work on the chambers or milling of the deck you can reasonably assume the stock 58cc volume.

Desired CR depends on a number of factors, but I'd advise more in the 8.5:1 range unless you are going for a low boost setup or are going to run high octane fuel at all times.

Rich Krause

How streetable is 8.5:1 on a daily driver? I thought it would be tough to drive and keep emissions on anything below 9.5:1. Rich, you're the man and your packing a supercharger, so I trust what you have to say. Can you help with this?

Thanks in advance.

I don’t understand why people get so stressed about running lower CR’s. They’re all over the place, you’ve probably owned one.

Most 2500 or better full size GM’s built in the 80’s and 90’s were in the mid 8’s. Most factory forced induction engines are in the low 8’s, and some really responsive, smaller turbo engines have come with much lower CR’s (ex, the turbo sunbird came from the factory with 7.5:1…)

1. Other than the obvious, more low end torque and displacement is there an advantage between a 355 and 383 or 396 in a blower application.



If you are going to be replacing the crank, might as well go for a stroker crank, as they are around the same price. A 383 crank with 5.7 rods allows you to go with deep dished pistons to get around 9:1 CR with stock heads, without compromising piston strength (location of the ring package) where it could become an issue.

Originally posted by Wild1
How streetable is 8.5:1 on a daily driver? I thought it would be tough to drive and keep emissions on anything below 9.5:1. Rich, you're the man and your packing a supercharger, so I trust what you have to say. Can you help with this?

Thanks in advance.

I am not sure where the myth about performance comes from. The reality is that a properly set up SC car runs fine off boost. This is partially because even when not "under boost" in the sense that the manifold pressure is still negative, the SC is still spinning and improving VE (and thus dynamic CR). That may account for the seemingly paradoxical increase in fuel mileage you sometimes see under light load with a centrifugal.

As far as a tailpipe emission test goes, I am not very knowledgable. My understandig is that lower CR decrease nitrous oxide emissions and increase hydrocarbon emissions. I doubt the effect is nearly as large as the other variables (AF ratio, amount of overlap, catalytic converter efficiency, etc.).

Look at factory FI cars, as WS6TA suggests. They have very low CR.

Rich Krause

Originally posted by TimbrSS
1. Other than the obvious, more low end torque and displacement is there an advantage between a 355 and 383 or 396 in a blower application.

If you are going to be replacing the crank, might as well go for a stroker crank, as they are around the same price. A 383 crank with 5.7 rods allows you to go with deep dished pistons to get around 9:1 CR with stock heads, without compromising piston strength (location of the ring package) where it could become an issue.

What about with a 396? Can you build a 396 and still avoid the issue of wrist pin ending up in the ring lands? What is the best rod/stroke combo for a blown 396?

Thanks,
Rich

OH yeah, we need to talk...
I live in tally as well. Lets meet up some time this wknd.
Later,
STEW

Originally posted by stewmeistr
OH yeah, we need to talk...
I live in tally as well. Lets meet up some time this wknd.
Later,
STEW

I think you're talking to me. Do you by chance work at DOI or DBF, I believe I've seen your car.

Originally posted by TimbrSS
1. Other than the obvious, more low end torque and displacement is there an advantage between a 355 and 383 or 396 in a blower application.



If you are going to be replacing the crank, might as well go for a stroker crank, as they are around the same price. A 383 crank with 5.7 rods allows you to go with deep dished pistons to get around 9:1 CR with stock heads, without compromising piston strength (location of the ring package) where it could become an issue.


Not a good idea in all situations. I went 355 for reliability factor, and its just stronger. Runs cooler and is great for a daily driver. A 355 with good heads, and a nice cam with boost will yield all the HP you want. How many 383/396 do you see making 40k with out boost? One of my buds drives 20-25k a year on 8psi 355 and I think he is around 88k on the motor now.:eek: I would not worry about the compression to much 9 to 1 or 8.5 to 1 will work fine. I like it it low, less chance of detonating.

I agree with low compression. I was just concerned with emissions. Thanks for the input.