Low Compression, Can I do it.

[StormBringer]

This may seem odd as I have my Cobra up for sale, so a little back story. I went out and bought the biggest stick that I could because thats what I wanted. Really a well sorted track car that looks like a Cobra.

After driving the car for about 2 months I decide that it was just not right for me. I end up using it more as a putter around car then getting off my butt and going to the track. (More a function of no time to do this). So I decided to sell the cobra and find another one that was a little more daily drivable. (+ I need better gas mileage out of the beast due to 3.00 a gallon 91 out here)

After sitting down I have come to the conculsion that maybe I should just really tone down the one that I have. I own it, I can sell off all the spare parts and make some extra cash, Its registered in the great state of California, and above all things I have grown attached to it in a wierd sort of way.

The adjustable suspesion can be softened up a bit for street driving, I already have power steering. I can get more squishy seats. Maybe make the doors functional.

The big thing is the powerplant. I do not want to swap that out, just to much hassle for me right now and I don't want to go though the trouble so I figured that I could just pull the supercharge off the 350 and drop in a basic manifold and much smaller carb on the thing.


So the question is, can I do this. I know that I can mechaically but here is what is holding me up. The engine was built around the supercharger system, beefed up internals, roller cam, and more importantly 7.2:1 compression. It was built to go to the track and take 12 pound sof pressure for hours on end.

"cam is a middle of the road deal nothing to hot, not specifically designed around the blower but more of a every day sort of thig.

Will a chevy 350 run at that compression ratio without a blower?

Do I risk damaging anything?

If I conservativly tune it what do you think I can pull for gas mileage in a a 220 pound cobra wthat turns ~2800rpm at 70 mph?

Other then the intake manifold and the carb what else to I need to swap it over?

[blykins]

Sure, a SB will run at 7.5:1....that's pretty much close to what most of the cars were running in the 70's when the big smog wars/gas wars hit.

Without being force fed, it's gonna be a feat to make horsepower at 7.5:1.

However, you shouldn't have any problem swapping everything over. Besides the intake and carb, I can't think of anything else you'll need.

You might wanna look at your cam specs. Some cams are ground specifically for forced induction applications. The cam may be hindering you a little bit too.

How much hp you looking to have when it's all said and done? 300? 400?

With that being said, I think a good streetable low horsepower SBC would get at least 15-18 mpg.....just my opinion...but I'm thinking back to the days where the Monte Carlos had 305's and 307's.....with a 4 bbl.....

[mylesdw]

If the supercharged powerplant is good, then why not see if someone will swap it for you? If the hassle factor is the only thing stopping you, why not see if someone will drop in a nicely build mild 383 and take the blower motor as whole or part payment.

That low CR means that you would end up with rather a good engine struggling to make decent horsepower which would be a waste.

[StormBringer]

First and formost thats what I was thinking, but what does a engine like that sell for right about now. I figure there is close to 8k in the engine as it stands for just the long block and blower setup. Its going to run me ~5K for another 350 that is built to the same specs. I have the mounts to bolt up a 454 but I don't want to go big block.

So I thought about that and the thing is that I know what went into this engine, and there is the money and time. It was built for the long haul and I hate to give it up. Plus pulling the blower and replacing it with a standard setup is real quick (~4 hours for me plus the tuning).

All I am shooting for is 250-300 hp do you think that I can make that with the compression ratio being where it is?

I would also like to run 87 ocatane is this possible? I know wh=ith that low of compression it should be very easy to tune to run on the low grade stuff.

California is getting crazy with the price of gas along with the rest of the country and I would prefer a milder motor that gets better gas mileage so I can drive the cobra more.

Right now I am getting ~7 mpg so anything is an improvement. I have seen some guys reporting 25 mpg on there slightly hot 351's which would be great. I would at least like to be in the high teen's.

Plus unlike a lot of cobras I don't have one of those drop gas tanks that you can see from behind the car, just a foam 10 gallon cell which with the accuslump and the battery, and the panel for the quick change take up 90% of the truck.

this is why I wanted to sell the cobra, but I have a feeling that once I make the changes I will be a very happy camper.

Plus selling the drag spares and the blower will give me the cash I need to do the EJ20 swap on the Riot which has been put off way to many time.

[SCOBRAC]

Yes. No.

How much for the car?

[Excaliber]

SB Chevs are not known for good mileage, with the exception being late model factory fuel injection on SOME applications. Generally speaking, the 350 is a dog for mileage.

7.5 should be OK for the street, certainly can run that low octane cheap gas! Loose the blower, get a small carb and forget about performance. I would consider a change in cam profile, look for a low rpm "motor home" or "truck" use application for max torque at low rpm.

If you got a solid roller lifter cam your asking for trouble down the road. The kind of cam is NOT recommended for low rpm or "street" use due to oiling of the roller on the lifter issues. 10K "street" miles is about all you can expect if it's a solid roller. Not to mention that cam is probably WHY your getting such bad mileage.

[mylesdw]

What about sticking with the blower, under-drive it to lower the boost, milder cam to improve driving and economy and EFI (for the same reasons). You get to keep your motor and there is no real reason why you should not get reasonable mileage. Maybe change the rear-end ratio too?

[mylesdw]

Another thought: what size heads are you running? You could maybe gain the compression you need by running a smaller chamber head.

[StormBringer]

Originally posted by Excaliber


SB Chevs are not known for good mileage, with the exception being late model factory fuel injection on SOME applications. Generally speaking, the 350 is a dog for mileage.

7.5 should be OK for the street, certainly can run that low octane cheap gas! Loose the blower, get a small carb and forget about performance. I would consider a change in cam profile, look for a low rpm "motor home" or "truck" use application for max torque at low rpm.

If you got a solid roller lifter cam your asking for trouble down the road. The kind of cam is NOT recommended for low rpm or "street" use due to oiling of the roller on the lifter issues. 10K "street" miles is about all you can expect if it's a solid roller. Not to mention that cam is probably WHY your getting such bad mileage.

Heres jsut a quick rundown on the motor, will have to dig up the notes to get more specific.


350 Racing Head Service SBC with forged crank, 4-bolt block, roller tip rockers, turbo TRW pistons, mild hydraulic cam, Mallory comp 9000 dist, HD oil pump, headers, oil cooler, flowmasters, Jacobs ignition box, B&M low profile blower, Holley 800CFM double pumper. New Richmond Gear super T-10 four speed with dual face Centerforce clutch, steel shatter-proof bellhousing. Franklin Grand National quickchange rear-end with Wilwood discs and calipers, gun drilled full floating axles, Torsen gear type posi [extra QC gears and Detroit Locker]. Front brakes are Corvette rotors and calipers with billet aluminum hubs and GM bearings, dual master cylinders with balance bar. Other features: 3 quart Accusump in trunk, gel battery with cut-off switch, Moroso extra capacity oil pan, Alden coilovers, 4 row radiator, auxiliary elect fan


The plan so far is to pull the blower, put a torqu'y manifold on the 350 and a mild carb. Pull the 3.33's that I think are in the car right now and drop a set of 3.13 or 2.98. (Cheap with the quick change). Soften the Aldan down about 2 notches and see what it gets me. I want to leave the cam thats in the car alone if the specs look right for a daily.


PS
I thought about going the fuel injection route, in fact I even talked to a few "experts" each said for a street car there really is nothing straight off the shelf that going to be able to support 600hp. 450 or so looks like the upper limit. This is mainly due to the fact that I have to run a carb like setup on top of my blower.

[Excaliber]

That double pumper 800 cfm Holley IS the problem!

[StormBringer]

Originally posted by Excaliber


That double pumper 800 cfm Holley IS the problem!

Tell me about it

Sorry about the lack of details guys this is car number 8-9 that I have bought this year so details off the top of my head are flaky and I don't want to give bad information.

Any body want to suggest a manifold and carb for this application. I think that I have seen around the net that the newer Demon carbs don't have main jets but adjustable on most of the settings which looks prommising. But do they make one small enough.

i am digging through my summit catalog as we speak but I need to get find a carb range.

Using the formula that Holley provides.

350 CID x 6000 RPM = 2,100,000 / 3456 = 608 CFM.

I rarely take it up that high and the motor is good to 7000, 6000 is whre I would like to be.

[MidOHasp]

decent sized cam, low compression = bad.

low compression and a very mild cam won't make any power, but they're at least kind of matched.

but i'm guessing your cam was at least moderately big. i'm a huge believer in matching parts. lots of duration (even moderate duration) NEEDS compression. Not 12:1, but at least 9:1.

Running 7.2:1 with a medium-hot cam, the thing is not going to idle, it will make NO bottom end torque, etc...

Seems like there is a better way to go at this goal!

[Edley Rondinone]

I say: New pistons at about a 10 to 1 CR...a "lumpy" cam, in the RPM range you want to use the car at...and probably a smaller carb...SBC stuff is very cheap. And I would get a bigger gas tank/fuel cell.
Also, if you are worried about MPG and the price of gasoline, are you SURE you want to keep the Cobra? Most people here would say Cobra ownership is about the SPG! (Smiles Per Gallon!) And to hell with the price of gasoline!!!

[StormBringer]

Originally posted by Edley Rondinone



Also, if you are worried about MPG and the price of gasoline, are you SURE you want to keep the Cobra? Most people here would say Cobra ownership is about the SPG! (Smiles Per Gallon!) And to hell with the price of gasoline!!!

Really all I want to do is go further on a tank, getting gas every 50-60 miles cuts into my driving time. Plus I can't really go any bigger on the tank there just in not room in the in the trunk because of the slump and battery and the cell. I might be able to swing a custom 15 gallon but its not really worth the cost.

After digging through the 6 inch stack of documents on the car I have the build sheet.

The cam is a B&M super 288 cam. I have the B&M manual so I will dig up the specs.

This is the cam spec's as far as I know it. I think someone made a copy of this cam at some point becase these numbers look similiar to others that I have seen.

Specifications ........Intake ......Exhaust

Duration at .050....x.234 ........x244

Lobe Lift ............... .3254 ...... .3400

Lobe Seperation ............................ 114° I think?

Seat Duration @ .004 .......288 .. .292

Gross Valve Lift .............. .488 ...........510

Degree Intake Lobe To ................102° ATDC ?

Cam is labeled as having reasonable idle plenty of vacuum and good mid to top range power.

Also got comformation that my is a shell valley stuck to the back of another paper is the original sales reciept, who ever did the body mods is crazy good then because I have never seen a car this clean if it was modified after the mold.

[Ron61]

Ernie (Excaliber), can tell you all about how to increase your gas mileage from around 3 MPG to around 10 MPG. Above that and he may have to do some research. He still hasn't figured out how I get 29 MPG on the highway.
But all jokes aside, the 350 Chevys have never been really good gas mileage engines. I know several people who have tried many things and I don't know of many that have really gained a lot. But if it can be done I am sure some of these guys can help. Also maybe you could check on the Corvette or one of the Chevy clubs as they would certainly know all the tricks.

Ron

[StormBringer]

I think we are paired with a the hotrodtalk forum right, they have a small block forum. Does my screen name and password work over there or am I do I need to re register.

[Excaliber]

The other Brent Mills "Hot Rod" site, there is also a Brent Mills "Harley" site, and there used to be a "Chess" site! Anyway, you need a new log in name for each site.

Strangely enough even in NASCAR the Ford engines get better mileage than the Chevy engines. And you know BOTH are built to the max. Bad mileage, yet another reason to avoid GM.

[StormBringer]

Just thinking along another line right now so maybe this would work.

using the calculator for static compression with boost at http://www.rbracing-rsr.com/compression.htm

It looks like I can run about 8:1 with ~2.2-2.5 pounds of boost. Lucky me I have a pulley around the 2lb's mark. Ordered a set of 3.13's for the rear and will more then likely replace the 800 with a 650 tamer carb. Does that float on a mechanical levbel. i can still use 87 at that ratio, I can use a smaller carb, and when I go the track just bolt up the 800 and the bigger pully and go run.

[StormBringer]

Just for those of you who are interested, I droped the boost down to about 4 pounds and picked up about 3 mpg on the freeway and aout the same in town.

If I drop it to 1 lb I should end up with another 3 mpg, plus leaning out the carb should pick me up another 2 mpg. For a gand total of about 8 mpg increase on the freeway. Which effectivly triples my freeway cruising distance to the tank. Not bad, ordered a 600 cfm supercharger cab from holley and hpeflly everything will run quite well and I can swap back to the 800 and the smaller pulley fro track days and tow the car.

[brainsboy]

First of all changing the cam is a great idea, but its not that easy. A mild cam is never going to work with the spring pressures you have now, so this means taking off the heads. That motor is setup for race in everyway, and a blower. The practical thing to do, is just pull the motor and sell it. But a motor you want and be done with it, get a normal 9:1 350 chevy and sell your old motor.