How many CFM do I need? - The math looks out of line.

[115-1172523331]

Hi all, For an unknown reason, I just did a quick calculation and now have a question! My 6 ci engine is a 4 stroke, therefore requires 3 ci per revolution. I run to 9,000 rpm, therefore need 27,000 CIM. This is equal to 15.6 CFM. A totally stock head will flow 40 CFM, so am I to expect no HP improvement from modifying the head?

As a "sanity check", I took a quick look at a small block Chevy. It says 350 ci or 175 / rev. At 8000 rpm, the total flow is 1,400,000 CIM or 810 CFM. I don't know how much these (modified) heads flow, but I don't remember numbers that large.

Am I missing something here? Help!

Doug

[86rocco1]

What you're missing is the pressure difference. On a flow bench, it's usually a steady 28" WC or approximately 1 psi. On an engine at full throttle it's whole different ball game, maybe someone else here can speak to the air requirements of an engine, Tom V perhaps?

Any idea what the power output of that engine is? 40 cfm should support somewhere around 10-12 hp.

[larrycavan]

[115-1172523331]

Ed, Based on best speed to date and estimated drag, rolling resistance, etc, the engine is making in the 10+ HP range. I just need the 12HP to get the speed GOAL I set for the project.

Thanks Larry! This is a 1963 Honda 90 PUSHROD motor (they went to OHC around '64-65). It's a sure bet that 127% EFF is going to hard to get! The PRODUCTION class has to use a stock EXTERNAL bodied carb. Boring it out got me almost 1000 rpm increase and that run was best speed to date (at 8900 rpm so I use 9000 for "in my head" calcs). The head on my bike has matched port to runner and carb outlet to runner and is cleaned up slightly (opened up to make things match). No other changes in that area.

I was in contact with Larry Meaux last year and have pages of data that I don't fully understand. I was concerned that the small size would limit his computer program. A number of them on the market won't accept data for that size! He ran my specs as a test case. Now I am starting to "design" a MOD class engine and can change exhaust and inlet so will be picking up a copy to play with.

Thanks for the input! -- Doug

[bruce]

Doug, can you send me a picture of your carb? I've played with small carbs

[115-1172523331]

Bruce, I have used both Keihin and Mikuni as both were available on my (production) motor. They are 18 mm bore (stock), but I have modified the Mikuni by boring it to 20 mm. What pics do you want?

Doug

[larrycavan]

[115-1172523331]

Hi Larry, Thanks for the input! This is a C200. It has a manual clutch and 4 speeds. The CT200 has the same transmission but a centrifugal clutch. The later CT90's had a manual clutch but an OHC engine, also slightly different bore and stroke. There are hardly ANY parts available, I don't know of any high performance ones! Fortunately, land speed record racing is an old guys sport so the bike isn't older than most of the competitors! Rolling resistance is the main area of "work" this winter. I am thinking about the 1800 mile (one way) trip to the east coast for an early trial run in April to see what we have accomplished. Other than that the class restrictions are a pain! Inlet and exhaust have to look stock so I can't play with their lengths. I ran a straight pipe inside the muffler to fix some problems there but the exit diameter has to remain stock so I couldn't hide a stepped pipe. As far as revs, I just open the throttle as wide as it will go and hold it there as long as I am on the track! All of my problems are compounded as I am one of the inspectors so have to be extra cautious that I don't step over the line somewhere. As they say, "This year..." -- Doug

[larrycavan]

[115-1172523331]

Hi Larry, This photo is me on the bike as I was leaving for a record run in 2006. The "official" Bonneville Nationals photographer took the shot and made it part of the slide show on the web site. A friend told me it was there, so I copied it for my files. As I explained in my PM to you, I'm still trying to learn, and didn't realize how much I had to go! Thanks for all YOUR help! -- Doug

[115-1172523331]

Hmmm! Dropped the attachment! Here's another try!

Doug

[thomasvaught-1]

I missed 2006 and 2007 at "B-Ville" due to work.
I need to get back to that place to "chill" again.

So who is the new starter now that the Old Master is gone from our ranks?

Tom V.

[86rocco1]

The more I see and hear about that place, the more I think I gotta go some day. Hmmm, September road trip?

[106-1194218389]

I have a couple of friends that own "Powder Works" here in Salt Lake City, a powder coating facility. They are building a turbo charged 50cc Honda for a land speed attempt. Their names are Walt and Ray Bloomfield. Bonneville is a hoot. You see anything from very high dollar to very backyard, but everybody is friends and has fun.

John

[bruce]

I'm still amazed almost everyday with the diversity of the various venues of motorsports represented on this forum!

The other day a package came in the mail to me and it had a framed picture of a mini-sprint on the track racing with my logo on the side of the car. This picture showed up out of the clear blue!

I truly am blessed with everyone coming here and enjoying my forum!



Edited By bruce on 1202600451