blower guys
 

what is the general rule of thumb for lowering compression ratios when adding intercooled boost? i.e. it seems as though engine builders tend to lower the compression ratio to about 8.1 : 1 for about 8 lb. boost. is there a formula?

Dean

Check with Marv at The Blower Shop (661) 299-5483 www.theblowershop.com

There is no rule of thumb.

The biggest factor in figuring how much boost you can put in is the amount of octane you get/afford to run CONSISTANTLY in your engine. Your static comp ratio is part of that equation.

I guess I'm wondering what you really need to know. There are an infinite number of variables to setting up a blower motor.
Going from blower with no cooler to blower with cooler????

Things like; the boost you can put in, which really translates to the more air you can move, the power you can make.

Using that theory you'd want to start at say...7:1 static. But pushing a boat and cruising in the 3-3500rpm range you're not in boost. So you're spending alot of fuel pushing a boat with (for example) 400hp @7:1 when you could have say... 475hp @8:1. When you get to the 4000rpm range and get into the boost, you'd be able to run more boost with the 7:1 but the idea is, how often to you spend above 4000 vs below it?

Now, I pulled those numbers out of the air I think you catch my drift.

I don't know maybe I'm making the question more complicated than it needs to be.:rolleyes:

If I did sorry
DAVE:o

The lower the compression (say 7-1) can run more boost on the same octane of fuel before detonation kicks in & build more HP than a motor with higher compressioin (say 8.75-1) with less boost.

thanks for your reply dave.

i guess what i am getting at; is around here, on most lakes fuel isn’t available above 89 octane, so to be on the safe side (as not to run the chance of detonation) i was looking at lowering the compression. my understanding is that was preferred over pulling out a few degrees of timing. i understand there are way too many variables to say this is the EXACT number you ALWAYS want. but i thought there might be some kind of guide lines. maybe i have the theory all wrong. but when the cr is lowered you are able to put a larger fuel/air charge into the cylinder without risking the chance of detonation.

ok, now, i’m trying to follow the example in your 5th paragraph. are you saying that by running a lower cr i’m going to be burning more fuel and working the engine harder to cruse at the same speed...until i get into a boost situation?

yeah Turbojack,
that’s what i was thinking. but where does one start? for example; say i know i can safely run 9.5:1 cr on 89 octane, i want to run 8 lb boost what cr do i need in order to safely run on 89 octane assuming all other factors remain the same?

I agree with rumrunner on getting expert advise as much as possible. I do like to try and get as much info in many ways and this board has been a wealth of information and many times I en dup knowing more than I ever wanted to. Sorry back to the subject at hand. :)

What you are looking for is your Final Compression Ratio.
(FCR) = [(Boost / 14.7) =1] x CR.
Depending on altitude.
Corrected Compression Ratio = FCR - [(altitude / 1000 x .2].

There are a lot of variables to take in to consideration like temerature of intake charge and cam shaft that effect octane requirements also. Innercooler will help. Be carefull it can get costly on mistakes. ;)

Something else to be concerned about is what type of blower you are going to go with. Take my motor 540, w/ stage 4 procharger. With a centrifical supercharger the boost I am getting at low rpm's (2000rpm, 3 lbs boost in box) is far less then at top rpm (5400 rpm, 12 lbs boost in box). I went 8.0 compression so I could run more boost & make more HP to try & keep from melting/breakting another piston. With this set up I do not have the torque at low rpms since I have low boost & low CR but I can live with this. Boat is a little slow comming off plane but easier on outdrive. If I had increased my CR I would have more low end torque with the low boost with this setup.

Whipple is a postive displacement blower. The motor sees close to the same amount of boost at low rpm (say 6lbs at 2000 rpm)as it does at high (say 6 lbs @ 5000 rpm) Thus this will have more low end torque.

What is safe CR with ??? boost & 89 octane gas. I do not know the answer to that question but if I was putting something like this together I would start with say 7.5-8.0 compression & 4 lbs of boost. It is easier to change pulleys to make more HP & torque than to change pistons.

Another variable comes into the mix is the intercooler. The larger the intercooler the greater the air charge temp is dropped & more boost or greater CR you can run safely before detonation. Always go with an intercooler. Intercooler=cheap insurance against detonation.

Turbojack,

Actually (at least in my installation) my Whipples don't develop noticeable boost until app. 4,000 RPM. It builds from there on to app. 7.5 lbs. at 5,500. This is mainly because Whipple has a bypass system until the higher RPMS. Getting on plane, which in this case is gradually bringing up the throttles to 3,000, I'll see 3 lbs. of boost which then goes to -0- once it's on plane.

This is a triple engine installation (500 EFI's with Whipples) in a 13,500 lb +/- 43' boat.

On the demo boats we had at the Miami show this year and last, we were running Merc 575's and didn't see boost until about 4,000 rpm and saw max boost of app. 4.5 lbs. at 5,300.

I'm guessing the differences you and I are experiencing is in large measure the hull type, engine(s) and weight?

Have you had any outdrive problems with 12 lbs. of boost? I baby mine, really baby them. (1.36 XRs') When I say I gradually accelerate to 3,000 rpm to get on plane, that's rrrrreeeeeaaaaalllllllllyyyyyyyyy gradual:)

Take care,

Steve

variance in boost, boat weight, drives?
 

Turbojack,

Actually (at least in my installation) my Whipples don't develop noticeable boost until app. 4,000 RPM. It builds from there on to app. 7.5 lbs. at 5,500. This is mainly because Whipple has a bypass system until the higher RPMS. Getting on plane, which in this case is gradually bringing up the throttles to 3,000, I'll see 3 lbs. of boost which then goes to -0- once it's on plane.

This is a triple engine installation (500 EFI's with Whipples) in a 13,500 lb +/- 43' boat.

On the demo boats we had at the Miami show this year and last, we were running Merc 575's and didn't see boost until about 4,000 rpm and saw max boost of app. 4.5 lbs. at 5,300.

I'm guessing the differences you and I are experiencing is in large measure the hull type, engine(s) and weight?

Have you had any outdrive problems with 12 lbs. of boost? I baby mine, really baby them. (1.36 XRs') When I say I gradually accelerate to 3,000 rpm to get on plane, that's rrrrreeeeeaaaaalllllllllyyyyyyyyy gradual:)

Take care,

Steve

As Turbo said, you do have to weigh the effects of a blower.
Two very popular brands ar Pro Charger and Whipple. There have been many, many posts and arguments over that subject.

Anyway, to answer your question; yes. You can lower your static cr too much to where you're "lugging" the engine at cruise. Where that point is, depends on your particular motor/boat combination.

I have no idea what that symbol is you're using. But if its 89 octane gas that you're going to use.
I would rely on the equation that was given;

effective cr = [(boost/14.7) + 1]static cr

The problem is 89 isn't good for very much of a comp ratio. 9.5:1 would be really pushing it in my opinion.

DAVE

Steve

In a PC motor boost in the box is not boost in the motor. there is approx a 2# dp across the carb.

Still. 10# isn't anything to sneeze at.

DAVE:D

Thanks Dave, and yes, 10lbs. in a boat connected to an outdrive is defintiely dancing with the devil.

Just for yucks, I carry spare pulleys in my boat that would bring our boost to something over 10 lbs. I doub't I'll ever use them, it's more a "mental sobriety" check, kind of like, when I get real real stupid, I'll bolt these on :)

Take care, thanks again for the info on the drop from PC box to intake plenum.

Steve

Total compression
 

at 8:1 with 6lbs.boost your total would be 11.3:1. I use the rule no more than 12.4:1 total comp. for 92 octane.anything beyond a total of 12.4 start buying Torco. My motor is 8.3:1 and 7lbs boost, I figure right under 12:1 total. Jay

I have never had a whipple but I thought they would build total boost at any rpm. Guess I am wrong. I thought that is what whipple advertises is instant boost. The motor I had in boat before this was twin turbo with wastegates. With that setup anything over 2000rpms If I got in throttles hard enought I could pull 10 lbs of boost. I know everyone says turbos are dogs & have lag but I saw more boost at lower rpm with my turbo setup then I do with this procharger setup.

As Dave F said the pressures I was talking about was in the box above the carbs. When I am crusing along at 35 mph -22000rpm's motor is at aprox. 5" vacuum & box has 3 lbs of boost. As soon as I nail the throttles motor raps to 4k 7 then starts pulling from there. I am not sure of boost at 4k.

As far as outdrive I have XY (I think, only difference between my drive & XR is top gears) & have no problem with drive as of yet. When I take off the most boost I can see is about 1-3 lbs. With the prop I have I almost always need to get in boost to get boat planed out.

8.5:1 and 6 lbs and 89 octane will be fine, just make sure yor fuel and timing is right and you run an intercooler. I run 8.75, 11-12 lbs, 91 pump gas and a huge intercooler with proper fuel and timing and it lives. Too low of CR will make your motor lazy on the bottom end. I would rather have an 8.5 and 5lbs than 7.0 and 9lbs.

I have also been of the opinion that Whipples would build boost at low revs. If the setup has a bypass, then this will definitely dump most, if not all, of the boost till it closes. Then, at whatever revs it closes, it should (in my thinking) provide near to its max boost. Granted, if the throttles aren't open very far when the bypass closes, then the indicated manifold pressure will be much lower than what the pump is actually producing.

I'm just guessing that if SteveDavid's motors make 7.5 psi at WOT, then at 3000 (or wherever the bypass closes) then it would make close to that same 7.5 psi IF THE THROTTLES WERE WIDE OPEN. Not recommending that he disintegrate his drives by trying this, but I'm thinking that the boost is there if he wants it...

Cobra- I feel if your blower was able to build say 5lbs boost at 2000rpms wide open throttle you would have more torque than with a higher compression & less boost. Am I wrong on this?

Yea but you're not at 2000rpm at WOT often. This rpm is usually part throttle. Boost is loaad and throttle position dependant.

This is a marine motor boost chart from B&M...Its on the conservative side but it shows a basic guide line.

Re: Total compression
 

Jay...I like your setup!
What is your total timing?
Im @ 8:1 looking at finishing up with 7# and 30* total.
This is non innercooled with alum. heads.

Mcollinston,

Yes you're correct. If you open the throttles at 3000 RPM or really any RPM above idle, the Whipples boost will go to MAX. I wouldn't do it in my boat, as the parts and pieces from the drives would make a mess of the water :(, but when we dyno these we pull boost from the mid 2000 rpm all the way to WOT, on power pulls. Alternatively, make it an easy run up and the boost doesn't come into play until the 4,000 or so range.

If you're ever in the South Florida area, give a call and we'll take the boat out.

Take care,

Steve

I also really don't get into the boost with my Whipple setup until 4000 rpm's. It's not that the blowers can't do it… it’s more of, they don't need to.

Even if I’m cruising at 3000 – 3200, as soon as I stab the throttle, the motors will almost instantly tach up to at least 4000 rpm’s where I will see about 5 psi of boost. I guess it’s more engine load than rpm’s that use boost. I can cruise all day long at 4000 rpm’s and see 0 vacuum or boost on the gauges. However, I did try one “burst” from 2600 rpm before and I did see about 4 – 5 psi by 3200 – 3300 rpm’s.

Also, the boost on any positive displacement blower (like Whipple) isn’t as constant as people say. I would bet that there is about 2 psi less boost at 3000 rpm WOT (I’ll never see it) than WOT at 5000 rpm. I’ve attached the dyno sheet from a run I did this past winter.

-Bayley

Interesting... the 530hp is a lot less than the 590hp they advertise for the 454mag efi.

"the 530hp is a lot less than the 590hp they advertise for the 454mag efi."

...Isn't it?

I've talked to Dustin in great detail about these results. We both agreed that the water temperature was too high for the intercooler (150 F). It should have been done around 110 F, where I normally run. Also, the run up on the dyno was pretty quick. Dustin thought that this system needed time to "stabilize" in order to make peak HP. Don't get me wrong, he was just as surprised as I was.

550 HP seems about right to me. I'm about 2 - 3 mph faster than the exact same boat with HP500's. However, I do have significantly more acceleration.

Hey Back4more :)

Nice chart-helpful. What happened with your drive? I hope it was somthing simple....

SteveDavid
 

how much mechanical cr are you running in your tipples with those whipples?

Back4more
 

Yep, thats pretty close to mine. 502 with Dart 360cc aluminum heads bowl ported intake and exhaust and the combustion chamber is ported and polished 124cc. I used the 168771 Crane cam (gen6 block) B&M 250 non intercooled supercharger. I bought new Holley supercharger carbs, 750's. 30* total timing and it made 700.0 hp at 5700, 697hp at 5800 and at 4300 it made 667 ft lb torque. at 5700 it made 644.9 ft lb. Jay

Yep, just a couple of gears.:rolleyes:
I thought there things where built better!?!?

Speedmaster project this winter??? :D

Bravo to ASD6 project;)
Comes in a kit.

SteveDavid

I do get to South FL on occasion and will never pass up an invitation to ride!

Yee Ha!