Topic: Highest HP - UNDER 750cc & Stock compression

[Plzhalpimlost]

Yes, yes. The classic HP question. But every time it comes up we get the big bore guys coming in. And if someone says under 750, then high compression enters the chat.

I am putting out a request for anyone to prove they have the highest HP cb750, under 750cc (slight overbores make the cut) with STOCK COMPRESSION.

Now, I say PROVE. Meaning no “my uncle’s friend had a 100 hp stock motor back in the 80s”. I want dyno numbers or at the very least a quarter mile slip.

I’m here to beat the record, but I need to know what the number to beat is.

Let’s stay civil.
Cheers.


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[754]

I can tell you it's not something folks put a lot of effort into.. sooner or later they start hopping things up..

[Plzhalpimlost]

I can tell you it's not something folks put a lot of effort into.. sooner or later they start hopping things up..

Everyone has their own goals — this is mine. It’s why there are so many categories for land speed records. I’ve set this as a specific challenge for myself and can’t be the only one.


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[754]

Well then look at the records for Bonneville. . Everywhere else almost us faster but less challenging..
 horsepower to most racers is not what they dwell on too much, results at the track counts the most....

[Alan F.]

Who'd run a stock bike at the dragstrip or pay for dyno time?

[seanbarney41]

I ain't got no numbers or proof...but I am gonna say, from what I have experienced, it's gonna be an F2 or a K0

[scottly]

The dyno charts I've seen show about 55 RWHP for a stock 750, and I recall one that was about 60 RWHP, but I suspect the head had some slight massaging? What modifications are allowed under your rules, other than displacement and compression? Quarter mile times are not a good measure of HP, as the weight of the rider can have a significant effect.

[Plzhalpimlost]

Who'd run a stock bike at the dragstrip or pay for dyno time?

1. I’m sure more than a few people.
2. Who said anything about a stock bike/motor? Just stock compression and cc.

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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[Plzhalpimlost]

The dyno charts I've seen show about 55 RWHP for a stock 750, and I recall one that was about 60 RWHP, but I suspect the head had some slight massaging? What modifications are allowed under your rules, other than displacement and compression? Quarter mile times are not a good measure of HP, as the weight of the rider can have a significant effect.

Great question. Only other “rule” would be N/A. No forced induction. See my reply above for what my general setup looks like.


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[Finnigan]

Great topic, will be following with interest

[scottly]

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?

[Plzhalpimlost]

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?

Yes, under 750 but I find that confirming standard compression to be just as important. Some of the guys out at Bonneville will keep it under 750 but raise compression.

But, if Matt’s is as stated above, wow! Happen to have a link?   


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[seanbarney41]

yeah, Matt's has pistons to raise compression...that are now available from CycleX

[seanbarney41]

This thread could be cool to see what can be had with razor sharp tuning.  It will need people to actually do dyno runs.  And since dyno's are not very accurate comparing from one to another, the interesting part is gonna be the amount of increase, not the total number.

[dragracer]

👀

[mark ward]

Perhaps Dennis from Wisconsin has some info to add here.   Frank, have you ever ventured down the quarter on a stock 750, for curiosity-sake?

[Sam Green Racing]

Plzhalpimlost, can I ask which drag racing class you will be running in that requires these specifications (low compression and under 750cc) ?

Sam.

[johno]

What your talking about was the classic hot up in the 70's , at least in Australia as Ozzies were big on head jobs and cams rather than the US big bore scene which stressed the rods and could get expensive. So Im curious, what is the standard compression ration your nominating as standard as there a 3 different head chambers, different piston combos etc so nominate the numbers.
cheers Johno

[dragracer]

Perhaps Dennis from Wisconsin has some info to add here.   Frank, have you ever ventured down the quarter on a stock 750, for curiosity-sake?

Mark,

I did some test passes on my stock K8 about 25 years ago. By stock I mean engine wise.  It did have a sidewinder for lowering purposes, a set of straight drag bars and a cut seat to keep me from sliding. As I recall my best 1/4 mile pass was a 13.22. The bike had a stock swingarm and I foot shifted it.  It was re-geared also.

I've been curious for several years now as to how quick i could get a bone stock engine to go on a perfectly set up, no bar extended chassis.  I may freshen up a stock engine with no internal modifications one day and swap it onto my mono shock chassis to see what numbers I can achieve.  I'm a total advocate for concentrating on proper chassis set up versus expensive engine modifications for drag racing. Its the best bang for the buck in order to get decent times from  out dated technology.

[Plzhalpimlost]

This thread could be cool to see what can be had with razor sharp tuning.  It will need people to actually do dyno runs.  And since dyno's are not very accurate comparing from one to another, the interesting part is gonna be the amount of increase, not the total number.

Glad you like the topic! But I may have to disagree on one point. While not all Dynos are created equal, I think it’s our best bet. Before the rebuild began, my CB ran like garbage and probably only had 30 some HP (oil starvation, terrible carb setup). No bragging rights for putting back the 20 hp. Yes, certain years were faster than others, but I think it’s a moot point after any performance rebuild.


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[Sam Green Racing]

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

[Plzhalpimlost]

Plzhalpimlost, can I ask which drag racing class you will be running in that requires these specifications (low compression and under 750cc) ?

Sam.

I’ve never hear of one, and I’m not assuming one exists. But I think you’d agree.

My main goal is HP. I brought up the quarter mile time as a supplement to a dyno run if we had to. Meaning, if someone didn’t have a dyno slip, but still had a claimed HP number, that with a total weight and quarter mile time that we could face check a little.


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[Plzhalpimlost]

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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[Plzhalpimlost]

What your talking about was the classic hot up in the 70's , at least in Australia as Ozzies were big on head jobs and cams rather than the US big bore scene which stressed the rods and could get expensive. So Im curious, what is the standard compression ration your nominating as standard as there a 3 different head chambers, different piston combos etc so nominate the numbers.
cheers Johno

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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[Jerry Rxman Griffin aka MuthaF'er]

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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[/quote]

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.