Topic: Highest HP - UNDER 750cc & Stock compression

[Sam Green Racing]

What drag racing class are you going to be racing in ?

See above. But I’d love some advice if you know of a category I should be aiming for.


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There are 2 classes you can race in with the motor basically untuned, the HP and compression ratio don't come into it for these classes.
These 2 classes are Street ET where a lot of newcomers start off and Pro ET where anything goes and you can race anything that you want and still win.
 We have a member Wilbur who runs a basically stock bike in Street ET and last year out of around 200 competitors finished in the top 10 I think.
Is this what you are looking for ?

Sam.

[Plzhalpimlost]

Well I was figuring 9.0:1, but if you have a more accurate way of describing the stock ratio(s) then I’m willing to adjust my parameters for this competition. I will say though that there is some slight wiggle room. No one should be cut out just because they needed to deck the head and thus changed the ration by a decimal.


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F0/F1 = 9.2:1. The 77/78 K has the same basic engine but I doubt the 9.2:1 as the pistons are not a full dome like the F0/F1 and have a relief around the circumference.
[/quote]
Hey, thanks for this info! Well again, this should be fun and I’d hate to kick people out. I’d say that personally I’m aiming for the highest hp out of the standard K 9.0:1 ratio, but if someone has an F model and has done fantastic work without RAISING your compression ratio any further, I think we can still all appreciate that.


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[dragracer]

Try using F2/3 stock pistons under that K head. It'll bump up compression.  I gave a set to George for comparison sake. I think one of the road racers he's working with was interested in using them due to bore limits in his class.

[Plzhalpimlost]

Try using F2/3 stock pistons under that K head. It'll bump up compression.  I gave a set to George for comparison sake. I think one of the road racers he's working with was interested in using them due to bore limits in his class.

I’m sure it would. I’m keeping stock “K” compression though. Part of the challenge. Great idea for the next build though


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[754]

I don't think you will see much competition.  A lot of people want higher compression  along with a hotter cam.,
Horsepower  does not always win dragraces, first a cross line usually wins..
 There us always someone with xx horsepower  getting beat by someone with less.

[Plzhalpimlost]

I don't think you will see much competition.  A lot of people want higher compression  along with a hotter cam.,

Great. Sounds like an easy win then . Trust me, I’m not trying to set such tight rules that I can be the leader in. I’m surprised so few people have tried this.

If I’m the first to set this as a goal, it sounds like we could gain some great knowledge from this test as a community. Whether anyone finds it valuable or not, I couldn’t care less.


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[MRieck]

 No milling or decking or tightening of squish. You will be behind the eight ball because the head and deck aren't flat anymore after a million heat cycles. If you can find a NOS head/gasket and cylinder I guess you can give it a go. But....as somebody else mentioned there are different chamber shapes(and it makes a 2cc difference). The 750 was lucky to have 55RWHP after a tuneup but maybe you can go full Smokey Yunick and get 75 RWHP using Kryptonite as fuel.

[Plzhalpimlost]

No milling or decking or tightening of squish. You will be behind the eight ball because the head and deck aren't flat anymore after a million heat cycles. If you can find a NOS head/gasket and cylinder I guess you can give it a go. But....as somebody else mentioned there are different chamber shapes(and it makes a 2cc difference). The 750 was lucky to have 55RWHP after a tuneup but maybe you can go full Smokey Yunick and get 75 RWHP using Kryptonite as fuel.

See, I think that the added compression from decking or similar things nearly required for a proper restoration is alright. No need to hold people back from doing the job right. But using a thin head gasket and buying High CR pistons are two different things in my book


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[seanbarney41]

I don't think you will see much competition.  A lot of people want higher compression  along with a hotter cam.,
Horsepower  does not always win dragraces, first a cross line usually wins..
 There us always someone with xx horsepower  getting beat by someone with less.

Exactly Frank...not to mention, no practical access to a dyno in my neck of the woods and I am sure many are in the same or worse availability

[Plzhalpimlost]

I don't think you will see much competition.  A lot of people want higher compression  along with a hotter cam.,
Horsepower  does not always win dragraces, first a cross line usually wins..
 There us always someone with xx horsepower  getting beat by someone with less.

Exactly Frank...not to mention, no practical access to a dyno in my neck of the woods and I am sure many are in the same or worse availability

And I’m sure there are people without access to prepped drag surfaces with bike much faster than others. Yes, access will always be an issue.

I (apparently) can’t say this enough. I’m not trying to build the fastest or quickest bike. I’m building an engine. It’s the beauty in assembling and modifying parts to create a machine reaching its highest potential within given parameters. Do you think the engineers at Honda we’re told “yeah, make it whatever compression you want, just make sure it has 50hp” or do you think they were given certain specifications for displacement, fuel type, and reliability standards that needed to be met, then from there attempted to fit in as much power as possible? I would guess the latter.


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[seanbarney41]

Honda engineers back in the day are not really even in the same universe so why even worry about them.  You are aware of a 250cc six cylinder that redlined at 17k rpm in the 1960's?  But i dig it and have been riding a machine with similar build goals for several years now and it still makes me smile every time...engine is stock K6 pistons and compression 1st oversize, home ported head, F2 camshaft, k0 ignition advancer, K1 rocker towers with the un-retained shafts, F1 carbs (086A I think?) and hand bent Yoshimura 4-1 exhaust.  After all that work and planning, I think my stock F makes more power and would be faster with a 17 tooth front sprocket

[scottly]

I (apparently) can’t say this enough. I’m not trying to build the fastest or quickest bike. I’m building an engine. It’s the beauty in assembling and modifying parts to create a machine reaching its highest potential within given parameters.

What is your objection to higher compression?? Are you trying to see how much power you can make with your stock pistons? If so, carry on, but it's not really going to be any kind of competition..

[Plzhalpimlost]

Are you trying to see how much power you can make with your stock pistons?
[/quote]

Yep. You got it. Oversize bore ok as long as under 750cc.


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[Plzhalpimlost]

Honda engineers back in the day are not really even in the same universe so why even worry about them.  You are aware of a 250cc six cylinder that redlined at 17k rpm in the 1960's?  But i dig it and have been riding a machine with similar build goals for several years now and it still makes me smile every time...engine is stock K6 pistons and compression 1st oversize, home ported head, F2 camshaft, k0 ignition advancer, K1 rocker towers with the un-retained shafts, F1 carbs (086A I think?) and hand bent Yoshimura 4-1 exhaust.  After all that work and planning, I think my stock F makes more power and would be faster with a 17 tooth front sprocket

Wow, that sounds like a fantastic build. Now I see your frustration with not having a Dyno around haha. I’d love to see the numbers.


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[754]

Honda built these to run at sustained high speeds in desert heat. I think that and being able to run on available fuels influenced their choosing the CR, if you are not constricted by that, then why worry about raising compression.
Btw how big is your motor at.5 oversize.. ?

[Plzhalpimlost]

Honda built these to run at sustained high speeds in desert heat. I think that and being able to run on available fuels influenced their choosing the CR, if you are not constricted by that, then why worry about raising compression.
Btw how big is your motor at.5 oversize.. ?

Exactly! I think that’s SUPER cool. I want to respect that in a way and constrict myself to the same challenges.

I believe .5mm is 748 or 749.


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[johno]

Ya know most blokes on this forum over 60 years old would have likely built hotted up stock piston bikes in the 70's, that's a lot of bikes, most never went near a dyno we would meet out the road to see who had the best bike  . 
I reckon it would take something to motivate us old farts to do it all over again, something like a prize on the table after 12 months 

yeh that's what used to motivate us back then to ,  $$

[slikwilli420]

My bike will meet those two qualifications while still having:
Balance crank and alternator, .5mm overbore pistons, HD rods, ported head with +2mm intake valves, kibblewhite springs and a Megacycle 125-NR cam.

You sure bet I’m taking it to the dyno and drag.


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So your main distinction is a bore at or under 750cc.? I believe Matt's road-race 750 dialed about 80 RWHP on the dyno?

That's correct. 760cc and 80hp at the wheel but my pistons are 12:1 and everything else in the engine has been worked over as well.

[DME]

I put mine on the dyno and netted 55 rwhp.
Stock everything, except 0.5mm overbore (749cc), podfilters and open 4-4 exhaust.
Just wanted to get a baseline...

[Plzhalpimlost]

I put mine on the dyno and netted 55 rwhp.
Stock everything, except 0.5mm overbore (749cc), podfilters and open 4-4 exhaust.
Just wanted to get a baseline...

Hey, this is awesome! Cool to see that overbore putting you back stock numbers. Though I’m curious why you stopped the pull at 7,xxx rpm? I bet you could have squeezed a little more out at 8,500.

With having forged rods and HD valve springs I’ll probably push it to 10k on the dyno, which power may taper off. But it looks like you were headed up until the moment he let off.


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[754]

10 K what are you talking about .
 My 836 was constantly shifted at 10, 5 or 11 k.. it had stock rods . At least 500 trips down the 1/8 mile strip..
 Poor thing only lasted 25 years.. at the end I was sort of trying to break it..

[Plzhalpimlost]

10 K what are you talking about .
 My 836 was constantly shifted at 10, 5 or 11 k.. it had stock rods . At least 500 trips down the 1/8 mile strip..
 Poor thing only lasted 25 years.. at the end I was sort of trying to break it..

I can't tell if you think 10k is too high or too low. Care to advise a max rpm for my setup?

[754]

10 k with springs won't hurt anything. Top rpms depend on the cam , once power drops off, no point in reviving higher.
 About 2 years ago I almost put forged pistons in a 750 motor, stock bore but encountered a ring issue.
 They were hi compression pistons.. the whole idea was to be able to run my weber carbs and a big cam..  .

[PeWe]

When my K6 had 836 I often let it rev to around 9500rpm. 160kmh on 3rd gear (17:48), time to shift. I remember I felt how power reduced.
Stock rods, race springs.

I could have geared 1000 rpm earlier.  Many years later dyno showed max hp at just before 9000 rpm with that setup.
As often written, not much more to get at higher rpm except for  expensive maintenance.

Dyno, a good thing for verifying where max power happen and keep rev in control.

[mark ward]

Good point Per. I still have a Wiseco paperweight, with valve stuck in the dome, from my original 836 build from '81. I learned about missed-gear,-hi-revs, and keepers that pull through the retainers. Took Mike Rieck's suggestion to set limiter to 9500, on my new 836 setup.