Hey guys Im planing on putting on a B&M procharger from one of the cars to another.

Here is the problem. (I hope someone can help)

My 305 still has stock TBI 305 heads. Funny I dont seem to lose much on the street.

Anyways the problem is I have a 218@.05 duration cam at .450 lift, the separation however is 106 on it. Ive had my car confused many times with a healthy 350. (I love single pattern cams exagerated mid torque)

The blower is set up for 6-7psi on a 350 it should be more on a 305 (8-9psi... to high for me) however My separation angle should decrease that somewhat. I hope.

I wanted to put 76cc heads thinking that with 8-9psi it will be ok, but with this cam i might bleed a little off so i then thought about 64cc heads. (you guys still with me). The problem is a have variable duration lifters and they suposedly take away 10degrees of timing away below 3000rpms. Its true the car is very torquey and at 3000 rpms it give you an extra push.

So if anyone can understand the mess I just wrote please tell me what you think and what should I do.

If I didnt put the variable duration lifters then this choice would be easy, I would put 64cc heads and use the 8-9psi setting thinking it will bleed off around 2psi due to cam(making the boost around a safe 6-7psi).

But I have these variable lifters and I thought I should go with 76cc heads and leave the 8-9psi pulley on but then again the cam might bleed alot of it off. And being at a ultra low compression already might be a very bad thing.

The thing is that I have the stock 350 supercharger pulley and want 6psi of boost. I dont want to put on a 76cc head thinking im going to make lots of boost and then get 1.5psi at the guage???!?!?!

I have desktop dyno and the 305 with some 76cc heads(882) and my mods with the supercharger and the stock pulley should yield 390hp at 5500 and 406 at 6000rpms. With higher compression say 8.5to 1 comp I says 25more hp and 390lbs of torque at 3000rpms

Any advise would be most welcome....If you can understand my ramblings

 

PLEASE HELP!!!!

 

I am not sure what to tell you but can offer my advice on Desktop Dyno. It is good for studying trends on NA engines and actually can be ok in the accuracy department too. But on a forced induction motor, the numbers are completely wrong. Don't use it to guage your estimated power.

An LSA of 106 is absolutely terrible for a boosted car. With those lift specs and duration, you shuold be running a 12 or 14 degree LSA. Your cam is not at all blower friendly but i know you want to hear how to combat that.

Don't use the 76 cc heads. They will take your compression down horribly low. You need the 8.5 compression ratio or else, when you are driving around and not in the boost at all, your car is going to be a complete dog.

 

I put in my cam timing events and it says at .05 I have 6.0 of valve overlap. Im guessing thats around 50-60 overlap. But seriously My car has a slight lope sound good but not a nasty i want to shut down idle. It should be like that but is not due to the rhoads lifters. It idles at 700rpms with no problem


Man, I just got the 76cc heads today....oh boy......I was hopeful with desktop dyno telling me these figures. There was a huge drop from 8.5 to7.4 compression but at 7.4 compression it was still respectable. (and i plan on using these for the 350 engine)

I just did the same setup without the blower with 7.4to 1 compression and stock soon to be ported 882 heads and got more power than a stock TBI 305 :-) I could probably live with that.....DAMN 7.4 compression with 882 heads are better than a TBI 305...WOW If I port the 882 and keep the 7.4 compression its alot faster than a stock tbi305?!?!!??!

How much boost do you guys think ill have with this setup....Please be mercifull. IM guessing -44PSI :-(

If i stay like this how much horsepower do you guys expect!!!

 

Desktop Dyno may say you will make more peak hp but you don't want a compression ratio of 7.4:1. The car is going to be a complete dog except when under your full boost amount. If you are pullying the blower for 8-9 psi, then you won't get that much less than that since you aren't really improving the flow of the engine. WHat is measured as "boost" is simply the backpressure that is in the intake manifold. It doesn't matter what size the engine is provided that it has the same flow characteristics. The stock 350 heads you have probably flow a bit better than the 305's so you might lower it to 6-7 or 7-8 . You can run that amount of boost just fine on about 8.5:1 compression.

EDIT: Ah, i had forgotten about that angle on the cam. Your lifters hopefully do make up for it like you said and won't cause you to drop down in boost. You are deffinately not going to get your money's worth out of a blower with that cam though.

 

those heads (882) are a great improvement over 305 heads. You are right about everything though. I was hoping that somehow i would avoid the bullet!!!

The heads flow 206 intake 145ex stock.

Damn what if I port polish the heads. (Im pretty good at that I know i can pull another 15-20cfm out of them with ease). And then I change out the camshaft so i could have no overlap.???!!

Oh by the way I have a roots blower so most of the boost comes on quite quickly.


At 7.5 compression I get without boost at all
(This in not even including that I will port polish it and that I have Rhoads lifters for the bottom end)
(NO BOOST)
at 2000rpms i get 104hp 273tq
at 2500 133hp 280tq
at 3000 166hp 290tq
at 3500 203hp 304tq
at 4000 238hp 313tq
at 4500 268hp 313tq
at 5000 289hp 303tq
at 5500 296hp 283tq
at 6000 290hp 254tq

I dont know if this might help but rhoads lifters drop duration on the closing ramp of the camshaft. about 10 degress bellow 3000rpms

Please tell me again that desktop dyno is not optimistic about supercharged numbers....PLEASE...


Anyways thanks bEtUr4dRustang You seem to be the only one trying to help(probably the only one thats knows about this). I thank you much. Anything else you might have I would welcome..

Thanks...

 

Oh, i was in your shoes not too long ago... Trust me, these guys know a lot more than I do now. I am just trying to help out until they can get in here and add to it.

well i have some bad news about desktop dyno...the numbers they give you for boosted applications are really inflated

If you plan to use the 350 heads then you port job would be great. but take them to a machine shop as well and get them milled down so that you improve your compression atleast a little bit. The machine shop will now how much they can take off without having to do anything to the intake manifold. Also, run as thin a head gasket as you can find to help with your compression problem.

Roots is better with the low compression, but having compression as low as you will would only be ok if you were running massive amounts of boost. You aren't going to be running enough to make the low low compression work for you. Try get it up to atleast 8.5. You can run about 8 lbs of unintercooled boost before you run into terrible detonation problems and really need to back off the timing.


You have your 305 heads still right? Port and polish those yourself, run a slightly thicker head gasket than stock, and put that blower on it. Provided you can do a good enough job to get a few more cfm out of those 305 heads, there should be no reason for your boost to spike because you are running the blower on a 305 instead of a 350.

In short, you don't need to kill your compression with those 76cc heads. 6-7lbs of boost is perfectly safe to run on an engine seeing about 9:1 compression so dropping it to 7.4 and not increasing the boost because i assume you have a stock bottom end will only hurt you. Port those 305 heads if you like just to get any extra little gains and maybe run a slightly thicker head gasket.

 

You are right I have a set of 305 heads ON the 305.

According to holley the 6-7 pound pulley that I have will give me around 8-9psi on the 305.

I guess they said that because of the restrictive 305 heads??!!!

I thought about what you said using the 58cc heads with thick head gaskets. I believe whats in there are stock thin metal gaskets.

I could probably lower the compression to around 9 to one from the 9.3 this way but isnt this still high for the amount of "unintercooled boost" im going to run. BUT then again I have that 106 LCA that might bleed of some of that. :-)

Damn does anyone want to buy a good used set of 76cc heads??? Seriously!!!

Ive never had heads milled how much would that run?

Hey bEtUr4dRustang I thank you again for helping me out here. I NEED MORAL SUPPORT NOW!!!

 

I honestly don't remember what milling runs...i don't think its too bad though.

 

OK But what HP will the 305 bottom end handle? 400 RWHP, 500 RWHP ???

 

My $.02:

You won’t even see a significant difference in flow between the 305 heads and 350 heads till someplace over 5000rpm. Below that you just won’t have enough engine to breath more air, even if the heads can.

The 882’s shouldn’t hit that same point till about 6200rpm.

Basically, I don’t think that you’ll see a significant difference in boost between the 2 sets of heads until just before your redline.

Just slapping the 76cc heads on there without any other changes would net you about 7.5cr… which isn’t really that bad, and would probably make a fairly happy engine combination. If it was me though, I’d try to find gaskets in the .015” range (felpro makes some, and I think that there is a gm gasket in the .019” range) which would improve your quench distance, make the engine more responsive, make more power and at the same time give you even more detonation resistance. Oh, and it would raise your compression to about 7.9:1.

Really, that cam will probably do OK with the blower. It’s not nearly the optimum timing, but with the Rhodes lifters and positive displacement blower should give you an insane low and midrange. Really, with those lifters you’ll want significantly lower compressions then you would normally run, since they will up the cylinder pressure produced below 3000rpm and with a higher compression I suspect that you’d have problems controlling detonation below 3000rpm.

 

I just want to point out that Desktop Dyno gave me pretty accurate numbers on my blown 355 combo...

you just have to remember it reports at the engine, not at the wheels.

you also have to make sure you set up the pulley ratio right. just selecting a roots blower does not apply the correct ratio.

if you are running a 144 blower as i suspect you are (you should be) then you need to swap the pulley ratio for 1.9:1 instead of the 1:1 it starts with, the pulley that comes with the 144 / 142 is a 1.9:1 5-6 PSI pulley for a 350. also make sure you have the efficiency around 55% thats more realistic for those blowers, because they heat the air so much even when not in boost the thing can burn your hand off...

all in all im still happy with mine, but its going on my 383 now. im going to run an XE 270 comp cam, its got 110 LSA.. but im also running AFR195CC heads and a complete 9.5:1 forged bottom end. desktop dyno says about 600 Horses, i would guess around 550 on pump gas with 5-6 PSI. so its not terribly off... it told me my 355 had 500~ horsepower, and it made a simple 440 RWHP so thats pretty accurate....

good luck!