More ponies from a 280

**RTech** · 06-30-2002, 10:54 PM

I am looking for suggestions on how to coax a little more out of my 1999, 280 sport.
What could I expect from a SVS intake? Are there any good options for tuners?
I would also like some suggestions regarding the rev limiter. I'm turning a 28 to the limiter now and really hate to lose my low and mid range with more pitch.
Any help would be great, Mike

**Prof. OB** · 07-01-2002, 12:11 AM

The 280 is a fairly formidable engine as it is, basically a drag motor with more restrictive port timing. This port timing gives you the bottom end & mid-range you presently enjoy but restricts the engine's ability to "breathe" on the big end. A SVS will help if it the engine wants more air, but once again you are restricted mainly by port timing. In this scenario, I would first remove the RPM limiter and then add some compression; since the porting is going to limit the dynamic compression of the engine I feel you could safely support 165-170 PSI cranking compression with this engine on a 50% pump premium / 50% Av Gas mix. If you ported the engine to produce more power you would once again, be required to use tighter heads to regain the bottom end & mid-range power you most probably will loose. One thing I think many fail to be truly cogitate in their attempt to make power is - regardless of what method you choose to make more power, it all centers around one point, i.e., you must create more B.M.E.P. (brake mean effective pressure) on top of the piston. This can be done with more elaborate porting which increases charge volume in the combustion chamber or by increasing compression. It makes little difference to the engine how this additional pressure is created, nor does one method of obtaining the additional power more favorable in regards to reliability. I always like to think of adding compression as a means by which we capitalize power on a "given" charge. While porting is a means by which we can increase the charge. Compression is desirable in that it is inexpensive to do and one of the few modifications that increases performance in every aspect. 2.5's love compression
and react most favorably to increases..............OB

**Prof. OB** · 07-01-2002, 12:14 AM

Give me the serial number of your engine so I can review it.....OB

**Laker** · 07-01-2002, 01:07 AM

Prof OB
Are you speaking in Specifics in regard to the Mercury V6 and directly the 2.5?
Or more in general about 2 strokes? Thank you again for All your knowledge!

**Prof. OB** · 07-02-2002, 12:17 AM

Laker, I have fooled arounded with many different engines types, both 2 & 4-stroke in competitive situations; I can not think of an instance in which optimizing the compression (within any reasonable limits) did not increase performance with a naturally-aspired engine. As you are aware, when compression is increased, naturally you have to increase the octane of the fuel and fuel delivery to compensate for the increased heat that will be generated by the increased compression. For each compression ratio increase (say moving from 8.5:1 to 9.5:1) I usually "bump" the octane by 2-3 numbers to compensate for that increase..................OB

**Laker** · 07-02-2002, 12:35 AM

Thank you Professor
with reguard to the 2.5s is their a limit at which more cylinder pressure hurts power? Or limits RPMS? Or does reliability become an issue before that point?
Best I could do in a two stroke was 230psi in a 38cc chain saw engine. This motor responded to each change quite well and if I could have built more compression into the design it would have made even more power. I just ran out of room, I could not get things tighter. I have always wondered if their was a limit. I also could not find anything better that 130 octain to deal with 230 psi compresion

**KeithRichards** · 07-02-2002, 09:04 PM

RTech,
A 280 Sport has the rev limiter set at 7750pms. You can get it raised to 8250. If you want to spin it faster than that you'll have to change the electronics to a 260 or a drag. So the bad thing about a 280 is you are limited to the amout of RR"S you can turn. If you can't spin you can't win. Now if you were unable to hit the rev limiter I would start looking at some motor mods. Just my 2 cents.

BTW I know Prof. OB and he has a line of BS as long as Texas.

Keith

**Prof. OB** · 07-03-2002, 11:21 PM

Laker,
Due to the apparent insight Keith R. has expressed in regards to engine performance - I will defer your question to him to explain what would be the factors that would determine the practical limits of compression for an engine, I'm sure he will wish to discuss "Static Compression", compression as calulated by the "Japanese Method" and of course "Dynamic Compression" . I feel this would be enlightening to us all about what the true nature of compression is, and what effect the "porting" of an engine (which he favors) would have on the use of compression ..........OB

P.S. I Have no problems with porting 280's. I am doing one at the present, however, it is both tedious & expensive to bring one up to par with a Drag motor

**Rickracer** · 07-04-2002, 12:59 AM

I believe the Prof. was indicating that the size of the ports is one of the limiting factors right along with the electronics. If you would be so kind, when you don't have something good to say, zip it. The Prof. is taking time to share knowledge with us that you obviously don't possess, so how about save the bull for your bench racing buddies.

**KeithRichards** · 07-04-2002, 10:30 AM

Let's back up here !

Prof. OB Is a friend of mine. He is very knowledgeable in making an outboard run beyond it's designed limits. If he tells me something I take it to the bank! And if I want my motor to run like a bat out of hell , he will be the one I send it to. I was poking fun at a friend not flaming an adversy. Sorry if it was taken wrong.

Keith

**Rickracer** · 07-04-2002, 11:50 AM

I know that controversy on the board cause the Prof. to stop sharing with us at one time, and I didn't want to see it happen again. Sorry if I took you wrong.

**Techno** · 07-04-2002, 12:56 PM

This may be a personal choice thing but I don't think raising compression is a cheap Mod. It may be from how much it costs you to do it but it isn't from the perspective of the fuel cost. If it was a drag boat then there isn't really a fuel cost associated but if its a pleasure boat sucking down 20 gallons or so then this starts to begin to be a very expensive thing. ( I dropped mine before buying the engine). Not to mention the hassle for people like me that don't have access to the fuel nearby.
Obviously I am always Awed at someone who wants this mod. I usually assume they're just dragging or rich.

Also it hasn't been mentioned yet but you'll lose your charging system if you want to spin at high Rs.

260 tronics and charging system I think is the route?

**Prof. OB** · 07-04-2002, 02:20 PM

Hey Fellows relax!!
I was just pokin' back at Keith, like him, I was just having a little fun! Keith's is a Good Guy. The worse things I could imagine is that each of us had the same point of view and when someone posted something there were 10 posts that followed, each saying what a great post it was! For those with a different view, please continue to express it - that's what we are all here for!! When we express our opinions with Good objective criticism hopefully we can get ALL the sides of a debate. Techno made some valid points concerning obtaining and having to carry a better fuel. I respect that! I have been blending fuels so long that it isn't even a option for me anymore and I really don't even considered when it when I'm determining what I'm going to do. That of course, doesn't mean that it would not represent a problem for someone else, and I'll admit, I some times lose sight of things like that. So keep firing away, I don't pretend to know it all - I'm sure there is something that each of you knows that I probably don't, so let's help each other........OB

**captcarb** · 07-04-2002, 04:23 PM

Have you ever tried the Klotz racing fuel concentrate, and what do you think of it?

By the way, I'm glad Rick spoke up even though he misunderstood the situation. We have had some pretty cool technical guys leave the board, and not because someone disagreed with them. They, or a friend were personally attacked. That has produced some sensitivity to this type of stuff.

Jim

**T-REX** · 07-04-2002, 07:00 PM

Glad U here so'z U kin share sum of that bottled up knowledge U been hidd'in...and also glad ya here, cuz now I ain't tha Ugliest one postin here...U iz

Yo goodlookin buddy