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.

If I did sorry
DAVE

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

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