[HemmiGremmie]

Wel the time has come for me to buy my rebuild kit for my blown 355ci sbc. I'll tell you my plans and you tell me your thoughts.
350, 4 bolt, bored 030 over
Steel crank n rods
5 qt oil pan, steel 8 inch balancer, electric water pump n dual fans.
TRW forged dish blower pistons, moly rings, dbl roller chain, hi volume oil pump, fel pro gaskets.
Heres my cam choices. 488X292, [email protected] and 114 lobe seperation, or 452X284, [email protected] with 114 lobe seperation.(Not sure the rpm's, but Im using a PAW grind like Briansboy used).
Heads will be either ported big valve 882's with 76cc chambers, or ported 434's with 64cc chambers.
Should I run 7.5-1 comp or 8.5-1 comp on a street motor? I heard some say the lower compression is better with more boost, but Ive heard that a little higher comp with lower boost is better for the street????
2 Edelbrock 750's or 600's??? MSD ignition, I think I'll try the rear dump S-10 headers and I have a friend with a muffler shop that can hook me up with either 2.5 or 3 inch exhuast.
Let me know what ya think? HG

 

[gypsypoet]

Man do I want to drive your car!!
:drool:
Anyways,
The more compression you run, the less boost you can run. However, with a 7.5 compression ratio, you could run around 15-20 lbs of boost, more than most superchargers are ready to run. With the 8.5, you can run 8-10 lbs on the outside, comfort zone between 7-8 lbs.
Either way, it'll haul balls, but it'll last longer with the less boost/more compression mix. Good luck!!

 

[TurboS10]

You can run alot more than 10psi on pump gas with 8:1. The problem with going lower than that is that the engine will be harder to start and will be a dog off idle until RPM and boost come up.

The one thing that I dont like about your buildup is the 5 qt pan. I would get at least an extra quart especially with a hv pump. I think that either cam will work well, but I would go for the larger one just so you can make a little more power if you want to crank things up.

I think the smaller pair of carbs will be plenty. Heck, a pair of 450-500's would probably work well too. I would suggest a MSD-BTM while you are buying. You want to be able to pull a little bit of timing out on boost to keep detonation at bay.

Chris

 

[jimdavis]

Blower Stuff.....

I have extensive experience with blown sb Chevy engines. While every engine reacts to boost/compression ratio combinations differently, my experience has always been to run a lower compression ratio to avoid detonation. You can always play with the boost by changing pulleys but once the engine is built and in the car you can't do much about the compression ratio. My current engine is a 355 with a B&M 250 Powercharger. I am only running 7.6:1 c.r. with about 12 pounds of boost. The engine has Dart cast iron heads. I have a very mild cam because I wanted the engine to be totally streetable. It made an easy 510 hp on the dyno but has gobs of bottom end torque. I installed an MSD Boost Retard system in it to try and control detonation but have never had to use it. I am guessing that with the setup I have I could probably run up to 14 pounds of boost without any detonation. I am running two 650 Holleys but as someone else said, you could probably get by with smaller carbs. You only need about 900 cfm to make 500 hp. Even with the 650's I have excellent throttle response and super driveability.

A previous engine I was running had similar specs to my current one but had a bigger cam and Brodix aluminum heads. It made 560 hp and was generally streetable but the cam was a little too rough for my taste.

All my dyno work has been on Dick Landy's engine dyno. I was involved in a blower development program back in the 'eighties and probably made over 300 dyno pulls on various blown engine combinations. All on 350 small blocks. We did do one 383 and then later a 496 big block with a B&M Megablower that we squeezed 1,100 hp out of on pump gas. It had two Dominators on it.

One other thing. If you are running a really big cam with a lot of overlap you can run higher boost than normal because a certain percentage of your boost goes out the exhaust.

 

[HemmiGremmie]

Thanks for the replys guys.
Im running an oil cooler, so that will add some oil to my combo.
I think unless I find cheap aftermarket heads I'll go with some old big valve stockers. Still trying to figure out my cam. HG

 

[jimdavis]

Picking a cam

Stick with 114 degree lobe separation. The best cam for a blown engine should have more lift and duration on the exhaust valve than on the intake. Because the blower is cramming air and fuel into the cylinder under pressure, you don't need a lot of cam on the intake. But also because it is putting more air and fuel into the cylinder than a normally aspirated engine, you need more cam to get it out. That is also why you should run bigger tube headers than usual. I have 1-7/8" primaries on my smallblock. 1-3/4" should be minimum. However, if this is primarily a street engine that is going to be operated at rpms under 5,000, header size isn't that critical.

Cam selection really has a lot to do with what you are looking for out of the engine as far as overall performance and what you are going to be using it for. If bottom end torque in the ranges you drive the car in 95% of the time is more important than all out horsepower at 7,000 rpm, then you don't need a big cam. In fact you are better off with what they used to call an RV cam. Maybe they still do call them this. I don't recall off the top of my head what I am running for a cam because I am in my office and all that stuff is home. I will try to look it up tonight and post something here about it. I have done a lot of dyno testing with various cams and you don't start seeing the benefits of a bigger cam until about 4,500 to 5,000 rpm.

However if this is a race car and you want all out performance and maximum horsepower in the upper rpm ranges, then obviously a big cam is what you need.

 

[HemmiGremmie]

Heres the specs of the cams Im looking at. Im running old stockers with bigger valves and want power in the 6000 rpm range. My car is real light, so low end is not real needed, but I dont want a dog. Heres my choices. 2 are split designs with 112 lobe sep and 2 are straight up with 114 lobe sep.
1. 488X292, [email protected], 114 lobe sep. 1500-6500 rpm range

2. 452X284, [email protected], 114 lobe sep, 1500-6500 rpm range

3. 465-488X290-300, [email protected], 112 lobe sep, 1500-5800 rpm.

4. 488-510X300-310, [email protected], 112 lobe sep, 1700-6500 rpm range.
You like more lobe sep, but same pattern, or split pattern with less lobe seperation??? HG

 

[1BAD80]

# 1 cam would be the better of the 3.
LOL

 

[TurboS10]

I like 3 as well. I think I picked in earlier or in another post.

I am going back with a similiar cam in my turbo engine when it goes back together. I am adding about 10 degrees of duration and .03 lift, though, since it is a race engine.


Chris

 

[brainsboy]

Let me share a little bit of problems I had with my last blower and why I suggest going with 8.5:1 rather then 7.5:1. As I had mentioned to you in a personal post before, the main problem in street blowers is going to be heat. The last blower motor I had was making 6lbs boost at 14% underdriven it ran great on the street. When I wanted more power I switched up to 14%overdriven which gave me 10lbs boost. Although blowers only make boost at full throttle, the amount of power needed and heat generated from 6lbs going to 10lbs during normal driving was almost unstreetable, I had major heat problems. I also noticed that I had to re-adjust the carbs and run the idle much higher. Now I was running 8.3:1 compression, if I had ran 7.5:1 I would have to run my blower at almost 18-20% over driven just to get 10lbs boost. Thank god i didnt as it would of been completely unpractical and would not be a good option for the street. I would suggest if your set on 7.5:1 go with an 8-71 and keep it underdriven, or stay with 8.1:1+ to better suit streetability.

As far as the cams I would run a dual pattern that favors the exhaust.


Ben

 

[camaroman7d]

You have gotten some good advice. I would suggest not running the Edlebrock carbs in a blown application, if performance is your goal. I know you see a lot of "street rod" type cars with them but, my experience has been that Holleys tend to respond/tune better on a blower.

As far as carb size goes, this can be tricky or a matter of opinion. I run two 750DP blower carbs on my 388ci SBC. My compression ration is 8.47:1 and I am running 12lbs of boost on pump gas (91 octane). NO O-rings, I went with Cometic MLS head gaskets.

You should really re-think using the 882 casting heads. They are prone to cracking and in a blown application the cylinder temps will be higher than normal. You want a good set of valves in whatever heads you choose to run.

If I were in your shoes I would go with two 650 Holleys (I assume you are running a 6-71) cam choice is another thing that really depends on what you are looking for and what you plan to do with the car. If it is a street cruiser get something with a 114LSA and slightly more exhaust lift/duration. I prefer solid cams for performance/blown applications.

Compression wise, I would shoot for 8:1 this should be happy on pump gas without getting tricky and it shouldn't be too mushy on the bottom end. Don't forget to set it up with a nice tight quench to keep detonation away. I have quite a few details and pictures on my site about my last engine build you may be able to use some of the info.

Royce

Edit: I was typing at the same time as Ben above. He gave you excellent adcive. You really don't want to spin the blower too fast if you can help it. On pump gas detonation will be the result. Due to heat as he mentioned and this same heat robs you of power.

 

[jimdavis]

Blower cam

OK...in digging back through all my old B&M stuff, here is what I found.

These cam profiles were originally developed working with Jack Engle at Engle Cams and Dick Landy's dyno shop. We were looking for streetable blower cams and came up with the following. These are all for SB Chevy. These cam profiles were the result of very, very extensive testing programs.

Fairly mild blower cam for daily driver use with 6-7 pounds of boost:
Advertised Duration - 262 int. - 272 ex.
Overlap - 43 degrees
Duration @ .050 lift - 214 int. - 224 ex.
Overlap -5 degrees
Lift at cam - .295 int. - .310 ex.
Lift at valve - .442 int. - .465 ex.
Lobe centerline - 114 degrees

Stronger street blower cam for use with 9-10 pounds of boost:
Advertised Duration - 282 int. - 292 ex.
Overlap - 59 degrees
Duration @ .050 lift - 234 int. - 244 ex.
Overlap 11 degrees
Lift at cam - .325 int. - .340 ex.
Lift at valve - .488 int. - .510 ex.
Lobe centerline - 114 degrees

Stout street blower cam for use with 10 plus pounds of boost:
Advertised Duration - 292 int. - 302 ex.
Overlap - 69 degrees
Duration @ .050 lift - 244 int. - 254 ex.
Overlap 21 degrees
Lift at cam - .340 int. - .355 ex.
Lift at valve - .510 int. - .533 ex.
Lobe centerline - 110 degrees

I don't recall why we went with 110 degree lobe separation on the bigger cam. There was some technical reason but this was all done some time ago and I don't remember the details. We eventually ended up having Crane make these cams for us and I believe that all of these same grinds are still available from Crane although I don't know what their catalog numbers would be. Most cam companies would have something similar. Back when we were doing this, none of the cam grinders were really into street blower cams but I believe that is quite different now.

All of these cams were designed for hydraulic lifters.

 

[Cstraub]

Knowing a little bit about cams, I have always found that blowers like short duration and moderate to high lift. As pointed out since the blower is doing the work, you don't need to hang the valve open for a long time. You need to open quick and shut quick.

I woudl suggest something in the 212 range on the intake and around 226 on the exhaust. Lift in the .520 range intake and 470 in the exhaust. Why, this is a 355 CID enigne, it doesnt need a big cam. In comparison, a major marine engine builder that has a 572CID package with a 12-71 that makes around 1000HP only uses a flat tappet cam in the 240 range at .050".

Oil pan, you need a high capacity oil pan. If not you'll be picking up pieces.

Chris

 

[jimdavis]

Chris

I would have to disagree with you on recommending a higher lift on the intake than on the exhaust. Blower cams always should have a higher lift exhaust for the reasons stated in one of my earlier posts.....the blower is ramming air in on the intake...so intake duration and lift are not critical, but because the blower puts more air and fuel in the cylinder than a naturally aspirated engine, this creates more exhaust gases, so you need more lift and duration on the exhaust valve to get the stuff out.

 

[Tech @ BG]

Chris

I've got to agree with Jim. I've never used a cam in a Blower motor that had more lift on the Intake side. I've used a bunch that were straight up on lift, but nothing with more intake lift. All of the Marine guys I deal run more exhaust lift, or at least the same as the intake, and more exhaust duration.

 

[brainsboy]

I agree with Jimdavis.

 

[Cstraub]

Jim,
I understand what you are saying, but working with these PRO 5.0 cars and marine blower engines, I have found with my customers more power this way. Granted these are high flow cylinder heads, but once the exhaust valve open, the gases are drawn to the low pressure and with the assistance of the piston moving up, you just need to hang the valve open a little, not a lot.

If you want, I'll cam you next hotrod, you won't be diappointed!!

Chris

 

[jimdavis]

Chris....

Thanks for the offer but I suspect this current car I have is the last one I will be doing. These things take so much time and money that I think I will just stick with what I've got. When I did this last hot rod I had a lot of spare time on my hands but that's not the case today.

I can't comment on what you have to say about cams on blower engines. Your experience is more current than mine.

When you start addressing these kinds of issues it is difficult to generalize on anything because there are so many variables that enter into the mix. What you say regarding higher lift intakes may work with one combination but not others and same with higher lift exhausts.

 

[Cstraub]

Jim,
I agree 100%. The problem in our industry is that the neighbor used that cam 20 years ago and it ran. . . so it should work for me. That doesn't hold water with me. An engine is a combination of parts. . .parts that have to work together. The cam being the brain. Cams for me have always been 1 off's designed for the engine combination that that engine builder is building for his customer.

The public way over cams. . .bigger is better.. .but sometimes smaller is faster.

I try to "ball park" where someone should look. That's all. Hemi will go with what he is comfortable with, that's fine.

Chris

 

[HemmiGremmie]

Heres the 2 choices Ive come up. Let me know what ya like.
465-488X290-300 with 112 degree lobe seperation [email protected] with the rpm range of 1700-5800 rpm's. Or,
488-510X300-310 with 112 degree of lobe seperation [email protected] with the rpm range of 1500-6500 rpm's.
One of these are going to be it I think. Weiand lists there blower liking 112 or more degree of lobe sep so I should be good and either rpm range is good. Let me know. HG