12:1 compression & a Blower

[fasterthanu]

Is it possible to put a blower on an engine with 12:1 compression? We are building a 350, looking ot be like 400+hp. I will be running tbi, or my dual quads again. Looking to run low 13's all day long. WIll be driven on the street. May go 11:1. High compression is the easiest and cheapest way to do this. MY question is, if I do go 11:1, or 12:1, am I shooting myself in the foot if I ever want to get a blower? My dad who was one of the head mechanics for the fastest streetcars from world renowned Niki Chevrolet back in the 60's. He said they use to blow cars even at 13:1. That was then, this is now. Blowers have changed, chargers have changed. I know this. New technology has come. Don't want to make the mistake of building this low 13 second car, with no blower in the future. On the other hand, don't want to screw myself and have it run 14's hoping for a blower, and never getting one. It needs to be fast without it, and even faster with it =)

 

[69MyWay]

I helped a friend set up a blower on a sbc with 11:1 compression. We had to run the smallest boost pulley they had, and he constantly fought predetonation problems. It ran very strong, but I am sure he would have done better to have dropped back to 8:1, then gone with an extreme amount of boost and a computer regulated timing curve.

You need to make your mind up right here and now. Either go radical high compression street insane normally breathing, or drop back and punt on the short block in full and complete preperation for the perfect blower design.

BTW, he blew the motor up twice on the above combo, and eventually sold the truck we put it in. It was no good for daily driving in summer time heat and pump gas.

 

[Edmond]

I was always...

under the assumption that if you have a blower, it's taken that you lower the compression or you end up with problems like 69MyWay's friend had.

Am I correct?

 

[sscam69]

That compression is to high, in my opinion, for a street car to begin with unless you have a regular source and pocket book for race gas and then some.

Even if you could pull it off without any problems, with the right top end you could run 13's heck you could break twelves with a streetable setup N/A all day long with a lower compression.

What kind of supercharger?

If you are going with a centrifugal type, then you can keep the compression at 9 or 9.5 without any problems and get the power you need to make it in the low 13's to mid-high 12's and then you still have room for the blower. You could run 10-12psi with an intercooler with some tunning. You would be deep in the 11's with the blower.

my .02 cents

Frank

 

[WhalePirot]

The shop that built my 406 took me down the same road that 69MyWay stated. I had the 'options open' fantasy, but after deciding what I'd really do with the car and that I had to choose whether I would blow it, somehow, I chose normal aspiration at 11:1 with aluminum dart heads. The aluminum dissipates heat faster than cast iron, thus another full point of compression is okay, syas them.

It drives just fine on super unleaded pump gas, and is all wheelspin (335/35x17 on Epsilons), anytime the throttle is floored in first gear. I have no ETs, but it is the type of ride I expect from a Corvette! My biggest problem will be the rather low torque capability of first gear on the Doug Nash/Super T-10 setup. My tires are my safety valve.

A real good engine shop should know how to build the right engine for you application. Speed-O-Motive, in SantaFe Springs, CA did it right and relatively cheaply. Probably why they've built engines for 20+ years.

My smile cannot fade upon full throttle, due to G-loading! Take your time, make your choice and do it right, with expert advice, as you seek.

 

[Rowdy1]

I Agree With The Whale

I'm running over 12:5 to 1 in my 327 bored .030 with Trick Flow aluminum heads on pump 94 octane with no problems. I have a friend with a 5.0 Mustang that runs consistant 12.2 @ 107 with slicks, and although I can't hole shot mine because of tire spin, from a roll of any speed above 15MPH I get him two to three cars.

 

[fasterthanu]

my buddies got a 89 mustang GT, just put a charger in it. He was running 14.1 at the track before. Made a hell of a difference. He still doesn't have anytime slip with the blower, but can't keep traction at all. Hell, he is in second gear *5 speed* at 10mph, and punches it, goes sideways. I am like wow. I believe he will be upper 12's if he grips. He has about 400 hp with the blower. Big deal. Build a typical chevy, and it will have close to that without a blower. Just don't want to screw myself =(

 

[Tom Bryant]

A real good engine shop should know how to build the right engine for you application. Speed-O-Motive, in SantaFe Springs, CA did it right and relatively cheaply. Probably why they've built engines for 20+ years.

I used to buy from Speed-O-Motive in the '60s. Good people.

Tom

 

[JohnZ]

No way in the world it'll stay together with 11:1 or 12:1 with a blower - the first sound you hear will be the detonation, the next sound will be the rods coming through the block and the counterweights going through the pan.

I rebuilt a 350 for a guy two years ago who had been telling everyone that his motor had run fine on pump gas at 12.5:1 for 20,000 miles; when I tore it down, it had flat-tops and measured out at 9.4:1. He was no longer impressed with his previous engine builder.

 

[69MyWay]

No way in the world it'll stay together with 11:1 or 12:1 with a blower - the first sound you hear will be the detonation, the next sound will be the rods coming through the block and the counterweights going through the pan.

I rebuilt a 350 for a guy two years ago who had been telling everyone that his motor had run fine on pump gas at 12.5:1 for 20,000 miles; when I tore it down, it had flat-tops and measured out at 9.4:1. He was no longer impressed with his previous engine builder.

How true. I can't tell you how tired I get of hearing about all these 400 hp 10:1 small blocks running around with the infamous 3/4 race cam. I think people start telling little white lies about what is inside their engines and it grows over time until they start believing it is true too.

I would venture to say the majority of rebuilt 350 engines out there in "performance" applications that were built either by the previous owner, or a speciality shop are lucky to have 9:1 and pump out more than 250 hsp.

I can't say for sure if that look on his face was one of real shock that he had been ripped off, or that he had been caught in his fantastic stories of horsepower grandure.

 

[JHL]

On my engine the 4 valve relief flat tops and 64cc heads only yeilds about 9.5 cr. You can buy domed pistons that will give the required 12.5 but you`d better buy a spare set as they will most likely end up in the oil pan with a blower on top. I`ve head it said that 10.5 is about the max you can safely run a n/a motor. Gearing down the blower so that it produces very little boost would seem to me to defeat the object unless you just want the looks. My car ran an easy 13.8 with a worn out clutch and not trying so 12`s is within reach with the right bit in an n/a smallblock.

It is possible to run a high static cr with a bigger cam which bleeds off a little compression, I think this is referred to a as dynamic cr, I`ve read a bit about it in some theads on Chevytalk. Alternatively you could always run it on Alcohol.

If you are going to build a blower motor you will be better of in the long run starting with the right foundation, good rods,crank and about 8.0 to 1 cr.

J.

 

[Nut]

I agree with John. I have heard guys running 8:1 or even lower with a blower.

............ Nut

 

[Rowdy1]

I KNOW What Mine IS!

With the help of two VERY experienced friends, I built my engine. What are we saying here, you can't run over a certain compression on pump gas, or you can't run over a certain compression and a blower?:confused

 

[Tom Bryant]

According to the Pro-Charger literature you can run a higher compression ratio, around 9-9.5, with less boost and get better results than with a lower ratio and higher boost. Of course they are talking about their kits for otherwise stock engines or mildly modified ones. I'm thinking that it may also have something to do with the amount of volumn you can pump through a centrifugal compressor as compared to a Roots type blower. The Weiand street blowers (6-71 ot 8-71) run underdrive to avoid too much pressure. A friend ordered his with a 25% overdrive and couldn't keep head gaskets on his small block. Switched to 10% underdrive and it was a great street engine.

The right combonation of parts will aid greatly in allowing more boost or higher static compression ratio on the same grade of fuel. Aluminum heads and thermal barrier coatings also help in this area.

Tom

 

[JHL]

The quote below is from a thread on ChevyTalk discussing the difference between Static CR and Dynamic CR and the link will take you to the page with this and several other discussions on this theme.

Personally I don`t know enough to comment properly and the figures I quoted were what I considered to be a general rule of thumb. I don`t doubt that with the right combination of parts and set-up of timing that high CR`s can be run on pump gas but I know a couple of guys who ran blower motors without taking CR into consideration and they had nothing but trouble and again if you are going to run the blower so underdriven what is the point except for "show" so I would say you can`t run a blower with 12.1 CR

ChevyTalk Threads

I really think working with DCR is a better way to go than SCR. With static CR, you don't really know if you can use pump gas with higher compression. You stuck with guessing if the cam will bleed off enough pressure. With DCR you can make a much better decision regarding the octane requirements. You can also make a much better decision about camming an engine. Since knowing what DCR to shoot for, a cam with the needed intake closing time can be selected to provide the desired DCR. Once the DCR is know, figuring out the static is easy. Selecting chamber volumes and pistons becomes a piece of cake. Of course, this is only part of cam and component selection process. The desired rpm range and output need to be considered when selecting duration and LSA. Plus having the proper heads and other componets to support that power level is necessary. However, knowing the DCR helps make these decisions much easier.

J.

 

[mgms_1968]

Hi

I have a 350 sbc with 8:1 and a 144 B&M blower. The rest of the engine is stock. even the exhaust. I run 91 octane fuel and I have no problems.

The engine is producing about 350 torque and 375 hp, with the blower producing 5 psi boost.

I and installing a set of modified camel hump heads (fuelie heads) and a set of headers. this should bring the car to about 425 hp and 425 torque (this is from dyno 2000 computer sim).

My 2 cents are that if you build the engine for a blower use only then you can easily set up the enge for 13's no problem. Get the engine set up with quality parts and buy a set of forged pistons. Also get a good cam from comp or crane, along with some head work to make them flow better and you can see 500 torque and 500 hp easy. (This is what I am doing for my car anyway)

The car is a 1968 vette with an m21 4-speed, I have the car up to 130+ with still throtle left to go, and she can turn 13's in 1/4 mile.

Well that is my imput.....

Matt