forged 10.4-1 383 + 9lb blower...can it be done...safely?

My factory motor gave up the ghost so I'm researching my options. One thing I'm tossing around is doing a 10.4 - 1 forged 383 plus the 9lb procharger I've had in the closet for the last 4 years. I don't really want a dedicated blower motor because if I decide to take the blower off I'd like to have something that will run pretty decent N/A. Would it be possible for me to do this safely and reliably and what precautions can I take to keep this a reliable daily driver? Would my current cam be adequate for the blower combo (222/230, .533/.544, 112 lsa).

It might just be me but thats to much compression for 9lbs. I would run 9 to 1. I went to 8.5 to 1 and never had any issues. Most ford engines run 9 to 1 stock its not that much of a loss.

When I bought the procharger, it was made for the factory 10.5 -1 motor. Maybe 9lbs is too high for the factory compression, probably why procharger dropped their boost to 8 psi after I bought mine, but I figure with the increase in cubic inches plus my more efficient heads, I may not even see all 9 lbs anyways. I may even lose boost to the cam overlap. Like I said, I really don't want to build a dedicated blower motor, but since the blower is just sitting here collecting dust, I'd like to see what it would do with this combo.

Put a good tune and quality 93 octane and you should have no problems.

When I rebuilt my motor I put in some forged 10.5/1 pistons. I had the same thought process as you. My tune is good and my stock motor lasted 114,000(95,000 s/ced) so I figuered It should work as long as I keep a eye on it. So far so good, but I only have 400+ miles on and and have not hammered on it since I am still breaking in the motor. I on the other hand just went with a quick rebuild as supposed to strokeing the motor, but with you got I would say its okay, just get the tune dead on or the motor will be DEAD. Good luck!

Did anyone happen to see that GM high tech a while back.

They had a 383 high compression (in the 10's) 23psi supercharged Camaro. Supposively the fastest streetable LT1

The deal with low compression pistions is that you can fit more of that compressed charged in with a larger combustion area than with a high compression small combustion area.

you'll have to bear with me, i'm not good with words but i read some theroy into this a while back....

the higher cyc pressure the more and more effecient burn so therefore on a NA motor you squeeze the charge more with a smaller combustion area

on a car with forced induction it becomes more benifecial to try to host the compressed/dense charge than to make one by squeezing.

hopefully you guys get what i'm trying to say.

Just keep it tuned, keep it fueled and away from detonation and you should be good

Running 9psi on a 10.4:1 CR motor will be iffy and depend on the cam and the fuel primarily. I think it's unwise. A blower car loses less hp, compared to an NA setup, when you drop the compression. This is due to the larger combustion space, which is filled due to the positive manifold pressure under boost. An NA car has the same mass of intake charge at lower static CR even when you enlarge the combustion space (total volume of combustion chamber plus the area if the piston dish). Hence the relatively large power loss. Not so with a blower. The larger space is filled by the positive pressure generated by the blower.

You will lose very little (if any) hp with 9:1 vs. 10.4:1 with the right cam choice and boost. The motor will be less fuel efficient and there wil be some loss of part throttle responsiveness, but a low CR ratio blower car is far from the "slug" off boost that some people make out. And it will last a lot longer without eating parts. Anyway, the boost is just a small movement of the throttle away, so who cares?

Rich Krause

If you do a search on N20Dave I believe he was running pretty good numbers out of a similar setup. :)

I'm doing a 383 right now and I just decided to lower my CR from 9.8 to 9.2 just to be on the safe side seeing as I'm just about ready to put the motor together and like Rich says you don't loose that much HP or Torque with a blown motor. I have it figured I will only loose 17 HP and 27 FT. LBS. torque between 9.8 & 9.2. at 7PSI of boost.

RCF925,
What pistons are you using? I am trying to find some for a 383 with AFR heads @ 64cc to get about 9.0 to 9.2 compression ratio. With a 9.00 deck height and 0.039" thick head gaskets I figure I need about a 22 to 24cc dished piston and it seems all I can find have too much or not enough dish.

I understand that it would be much safer going with the lower compression setup but I really don't want to build a strictly blower motor. If for whatever reason I decide that I don't want the blower anymore, I don't want to go through the problem of having to disassemble the motor just to put new pistons in to raise the compression. I actually have access to a 7# pulley if that will increase the reliability of the 10.4-1 setup. What are the main concerns when dealing with a high compression blower motor? I thought detonation was a primary concern and that can be dealt with by using 104 or higher octane and retarding the timing. What else should I be concerned about and what precautions can be taken to combat those problems?

I'm buiding my 383 to run on Ca. 91 octane on the street. Even without the blower I would be making 450 HP, so I guess it depends how much power you want. big dave, my last motor was a 355 at 10.25 & 8lbs boost and I had problems at higher revs with detonation. I was also using an aftercooler.

Why do you have this uncertainty about whether you are going to keep the blower that you already have? In what scenario do you envision abandoning the supercharger? Just seems to me that you are asking an odd question. I'd try to decide which way you really want to go and build accordingly. Who wants a built motor that's optimized for neither forced induction nor NA?

Originally posted by IDOXLR8
If you do a search on N20Dave I believe he was running pretty good numbers out of a similar setup. :)

Yep. 10.98 @ 125 w/ 10.5 cr, stock heads, 355cid, 12psi pullies, 3 core FMIC. His cam was on a 110 , so I think that helped the motor live. His timing was on the ragged edge w/ 36 degrees of total timing. I believe on the street he was taking timing out w/ the BTM and 94 octane. On the track, hello 116.

If you're gonna go w/ the higher compression, just make sure you have enough fuel.

Later,


Steve.

Thanks for the replies. My uncertainty comes from the fact that I lost both the tranny and motor within a month after I installed the blower. So you can probably understand why I'm a little gun shy. Several folks have inquired about purchasing the blower. It would be easier on the wallet if I decided to build a N/A motor and just sell the blower but I would hate to get rid of it then realize I would've been better off keeping it. The procharger was sent back to ATI when the motor was out of the car. It was completely rebuilt and has been in the box since it came back. It has the upgraded 3 core intercooler that mounts under the car. I really loved the way it sounded and performed. It ran 12.40 @ 112.8 with stock stall and stock gears. The only programming I had was a custom chip done by hypertech :mad: I just didn't know any better back then. Unfortunately I didn't have access to the internet back then. I do realize it would be better for emmissions if I decided to go the blower route. Lower compression and smaller cam, heck my fuel economy might get better. What would you guys do if you were in my shoes?

is running 116lsa with blower safer then running 112lsa? or is it the other way around? thanx guys, to me it makes sense that the 112lsa would be safer but not as efficient?

Originally posted by bunker
is running 116lsa with blower safer then running 112lsa? or is it the other way around? thanx guys, to me it makes sense that the 112lsa would be safer but not as efficient?

Good question. I'm interested to hear the answer as well. A narrower LSA tends to build more cylinder pressure (and therefore more peak torque) and is more likely to detonate than a wider LSA in an NA motor. In a supercharged motor a narrower LSA and the consequent increase in overlap is thought to let the pressurized charge escape during the increased overlap period. Seems like the narrower LSA cuts both ways. But hopefully someone who actually knows what they are talking about will chime in :)

Rich

Rich, it’s never that easy.

Really, LSA is just a convention that we use to give us some easy means for comparing cams without having to figure out what individual ivo, ivc, evo, evc angles mean.

For most applications the question is getting the timing events in the right place for the dynamic flow that you’re trying to establish with each individual event, and then adjusting those events to find the best compromise their relative effect on each other. Where the events happen and how much overlap you end up with is more important then the LSA in what the cam will do.

In the case of a cam for a typical blown engine, your bigger concerns are to close the intake later and open the exhaust later. Past that you pick durations that are appropriate for the rpm band that you want it to operate in (this is one place that the heads flow #’s, exhaust and intake flow really come into play), and then adjust all this to limit overlap if you’re trying to prevent the loss of any boost, or possibly increase overlap in extreme situations where you’re worried about detonation below the torque peak.

There have been some pretty good cam selection discussions in the last couple weeks, do a search…