Hot Rods ? For engine gurus, blower cam specs

My engine in my 32 is in need of a little more umph! It is a 355" sbc with flat top Pistons, a scat crank and rods. The heads are 461 casting double humps. They have 2.02 x 160 valves and I am running 6 lbs of boost theough a 4/71 blower. I have a matched set of edelbrock 750s on it, and a 4 speed. I'm thinking my cam is a little on the small side, what is a good cam for this combination? My car made just over 300 horse at the wheels and 400 ft pounds of torque on the dyno. It makes its peak power at 4700 rpm. I run it on vp110, super unleaded mix and it runs fine. Am I correct in thinking a cam could help it or should I just leave it be and be happy with what I got?

 

Blowers tend make any given cam behave less aggressive than in a NA engine, Much the same as adding cubic inches. In other words if you take a cam that's pretty mild in a NA 350 and stick it into a NA 327 its more aggressive and even more so In a 283. So an engine with a blower wants more cam than seems reasonable for equal NA cubic inches.

 

I believe that on a street-driven blower motor you also want to keep the cam overlap down, otherwise you're blowing boost out the exhaust...

 

Jim,
How much cam do you have now? If you like the characteristics of the engine now, but just want more "oomph", I'd be tempted to just add more boost. Cheaper than cams, too!

 

You also want a little more exhaust duration than intake as I recall. its easy to get mix into a blower motor and hard to clean one out.

Think this way say 240@.050 intake and 250@.050 exhaust say .500 lift intake and .561 exhaust. those are not exact numbers for your engine just an example and a fairly wide LSA like in the 114 to 116range.

 

is the blower underdriven now? and how much. want some more boost, buy some used pullys on ebay . id say turn yours over but that might go to far.

 

This is just a guess but I would think that with a 355 iron heads and flat tops unless the block is o ringed he is probably blowing about as much at it as is prudent at this point. Like I said just a guess.

 

It's straight driven. an old stacked belt set up.

 

This is true. I'm thinking I'm about
Maxed out on boost.

 

I've found that the cam needs to match the intend RPM range you plan to use the engine at...just like with an unblown engine.

You didn't list a lot of the specs of your vehicle, that a cam grinder normally wants to see to help someone pick a cam.

 

Jim,
A quick change of subject, this headers are nice.

Back to the question at hand I am going to agree with you on the cam yes you could use more cam. You are not going to change your HP with any other mods at this point. I don't know what you are running now but I would think that you would want your intake in the .5 range and duration to be above 220 @ .050 but still less that 250 on a street motor and your exhaust should be larger along the lines or ratio that I suggested. Perhaps if you chat with someone at the major cam grinder of your choice they will get you better ball parked.

You should be cranking out more than 300 and 400 at the wheels with the blower for sure. You can do that with a 355 and a carb.

 

Something is not right. I had 10 lbs boost with turbos on a 355 vortec heads and stock roller (1996 truck) cam Stopped making power at 6200. 430 hp 500 ft lbs at the wheels.

 

My car weighs 2200 with me in it. I would like an rpm range of 2500-5500, I'm running a 30 inch tire and I have 4.11 gears in the rearend.

 

I am thinking with flat tops your static compression ratio is too high for more boost without some charge cooling. The race gas may be ok with more boost but I have no experience with it. I had 7.8 to 1s 125 cc dish pistons but had an intercooler. Later ditched the intercooler and went with water meth inj. All on pump gas. The water meth was much better and less restriction without the extra piping. May be an option for you if you up the boost

Something is holding your motor back. 4700 is pretty low peak power RPM.
What are the cam specs?

 

Do you have the specs of the current cam?

sounds like you want something relatively mild, around 220-230 degrees at .050".

More power would come from more boost. Less compression would help there.

 

So here's a question: The bottom line on making power is the cylinder pressure that pushes the piston down. The goal is to increase combustion pressure, so why would you steer away from more boost due to a concern about the block not being o-ringed? Is it because of the additional power that has to be produced to drive the blower, whereas a cam change will not require any additional power to drive the blower (no increase in boost) but be more effective in filling the cylinders?

I've only messed with turbos, so need to be educated.

 

Blown head gaskets. I am going to guess that he is @ 9.5 or 10:1 static, without o ringing the head gaskets are not going to hold when he starts to make boost.

When we build blower motors we usually use a dished piston and try to keep our static around 8:1 and with that compression if we plan on running more boost that 8psi we O ring the block. Actually I usually suggest O ringing anyway because I know that huffing is like heroin and you always end up dialing in more boost to get off on it.

 

Tuning on a street driven blower is EVERYTHING. Too big on the carbs will kill your response and feel that you are looking for. beaner is right about the cam. A friend got me involved in tuning his 48 Anglia after he had changed the cam 4 times. We did some research and found that a stock HP 305 cam had the specs for the street. Some carb work (PVs) , ignition work and a cam change (no blower changes) made that engine come alive on the street.

 

I have put hundreds
Of
Miles
On it with no head gasket issues. I know it doesn't make huge power, but it does hold up
Quite well to abuse.

 

Hundreds of miles?

 

I can't figure out what that article is supposed to prove....

 

I deleted it, all better.

 

I didn't mean for you to delete it, just maybe explain it? Comparing a small cam to a big cam, yeah, I expect the big cam to make more power, whether it's blown or unblown, just because the engine will rev higher. Now, if they'd compared two cams with the same grind, but different LSA, then maybe we'd learn something.

 

That's easy Jim,

From the article.

The idea behind this test was to dispel the notion that mild cam timing is best for supercharged applications.

[ Insert Page filler ]

Just make sure to pick the right cam for your blower motor.



I've been accused of the following: Over thinking
Over complicating
Over analyzing
Over simplifying
Over estimating
Under estimating
Fully understanding the application yet lack in explanation. So take this for what it's worth, and have the salt handy.

Boost can be though of in 2 different and separate ways or a correctly as a combination. First is that in and of itself Boost will increase the effective compression ratio there by producing greater power thru greater expansion rate.

2nd is boost will increase the effective size of the air pump and pump more air compared to an equal sized NA engine - boosted engine will pump an equal amount of air as an engine that has greatly increased cubic inches.

So how do you take advantage of just high compression? Actually high compression is a by product of chasing high rpm power. There's loads of theory why but with a big cam that builds high rpm power, they also bleeds some cylinder pressure and gets great mid -high range power. The static compression is raised to compensate for the bleed off. Usually involves high rpms, big stall converters and numerically higher gear ratios to take advantage of a higher power band.

So how do you take advantage of more cubic inches - For instance a 327 vs a 383? Or a 396 vs 502, or 331 vs 392,417,421. With A bigger cam that will will increase flow in and out of the cylinders & Cylinders and valves that flow enough. Gets you great low, solid mid range and great high rpm power till the breathing stops. Usually involves less Rpm than above, less stall and less rear gear to take advantage of the greater torque curve and power band that will happen lower in the rpm range.


The normal recipe for boost is to start with a low static compression engine and allow the boost room to compensate and fill the void. This gives the boost a chance to increase the amount of air without over pressurizing the cylinders. A blend of increased compression and a bigger pump.

If something like mr sibly's ride which seems to start off with already moderate compression before the boost is added doesn't seem to be able to take advantage of much more air passing thru before the increased compression exceeds the limits of detonation. Im thinking this one can benefit greatly from a bigger cam that bleeds compression (and lots of it) and getting more air and fuel thru the engine.

With The flat top pistons and 64 cc chambers of the 461 heads, I'm almost thinking that engine in its current comfiguration could probably put more power to the ground without the added parasitic drag from the 471. That would be an interesting magazine article.

 

Yeah you'll blow a little, but on overlap you'll also be blowing the residual combustion gas out and filling with a more potent and pure mixture for the next cycle. If loosing some boost is a concern, generate a bit more to cover the loss. Running Roots blowers and carburetors sort of eliminates fuel economy from the top of the priority list.

 

I'm also still trying to figure out what Jim wants his car to be. Does he want it to go fast at the drags? does he want to drive it on the street a lot? does he want to be able to brag about how much power it makes on a dyno?

I like driving and drag racing, and I've found that boost is a good way to go quick with a car that you can drive on the street. The guys who aren't into "traditional" use a turbo, and they go faster and get better mileage and have fewer other issues than I do with my carbs and 54 year old hunk of aluminum.

But I can also see Jim's dilemma with wanting to run old heads, etc for the right look, while also being able to have it make power and be streetable. Since I'm running a big block, I can get "correct" looking open chamber heads, which helps a bunch.

 

With The flat top pistons and 64 cc chambers of the 461 heads, I'm almost thinking that engine in its current comfiguration could probably put more power to the ground without the added parasitic drag from the 471. That would be an interesting magazine article.
[/QUOTE]

I was thinking the same thing, based on our old dirt -track days when we were required to use "stock" parts, this was a very popular combination.
I have no blower experience, I mostly wanted to subscribe so I can learn what you guys decide. Good Luck!

 

I'm wanting a period correct car that is capable of running mid 11s in the quarter. It is street driven, but I'm not at all concerned about fuel
Milage.

 

I was thinking the same thing, based on our old dirt -track days when we were required to use "stock" parts, this was a very popular combination.
I have no blower experience, I mostly wanted to subscribe so I can learn what you guys decide. Good Luck![/QUOTE]
Interesting, I could do that and see.

 

Lobe centers matter. What is the lobe center measurement of the current cam?