Piston Speeds

[Adam Lipcsey]

Originally Posted by billyekracer

I'm just looking for advice on safe peak piston speeds so I can put the appropriate parts and porting together for maximum power...without having a grenade under the hood...and gear it appropriately to that.

Billy, sorry for my naive remark here, but don't you think that you need to decide first how much power you want to get out the particular motor and then select the parts and tuning methods towards achieving it?

If you want to do the engine modifications first, then you have to dyno the engine (you can calculate the load how much to dial in) to figure out what you achieved, because running the engine by itself with no load on it, is a useless and sure way to destruct it.

[bumblebee]

Billy, if you insist that designing an engine based upon speed is what you would like to do then consider the following;
- the mechanical forces due to piston speed are directly related to rotating mass, therefore, run the lightest piston and pin combination that you can considering that you want to get more than 1 lap out of the motor, use someone else's experimental results by calling a reputable piston manufacturer like Wiseco or JE and ask them for the lightest piston/pin/ring combination for your engine that seals well, has some longevity, and has been proven because many other Honda engine builders use it for similar applications
- if you call 2 or 3 manufacturers you can then compare their results. Likely they will all be the same if you give them the same information, which gives you some assurance that the combination will work...if they are all different then no one has figured it out and they are still experimenting, in that case, err on the side of conservatism
- most of these manufacturers can then tell you the impact of changing stroke on piston weight and longevity and what the "hot" engine builders are doing, do not be surprised if you have to spring for a custom rod to manage piston acceleration/deceleration rates (rod/stroke ratio)

Most of my experience is with engines bigger than Honda's but our engine programs all seek a rod/stroke ratio of around 1.8. The deck height of the engine is always the limiting factor. Much below 1.8 and you have high accel/deccel forces which result in longevity issues (including friction) but the engines are easy to tune because the initial "suck" is really strong (you get lots of torque), much above 1.8 and cam and intake tuning becomes really critical, things like the "inertia supercharge" effect become really critical so all the small details matter, but, if you get it right the engine will create more hp across a narrower range.

I looked up some old data from one of our programs, a carbureted V-8 with a rod/stroke ratio of 1.71 (sbchev with a 6" rod and 3.5" stoke) compared to an identical engine (but cammed and intaked for short stroke) with a rod/stroke ratio of 1.88 (sbchev with a 6.125" rod and 3.25" stroke), the 3.5" stroke engine made almost 40 ft-lbs more across 4,500-6,000 rpm (integrated) but the 3.25" engine made almost 40hp more from 6,000-7,000 (integrated)...I set my personal best at Mosport with the 3.5" stroke engine, I am not Michael S. and it was a lot easier to drive...a better driver would like have gone faster with the 3.25" motor...so I sold the short stroke motor because for me it was slower...

Hope that helps...

[uptownguy]

The is some engine modeling software where you plug in the values and it will give you results based on all the parametors.
You supply valid airflow, compression, cam timing, rodlength etc. it will give you a dyno curve and you can even see the effect of cam timing and it will check your piston speed etc.

The trick is then to build the motor and match your entries.

GM uses the Bosch modeling it models combustion chamber and gives pressures, calculates gas flow etc. but I believe it licences for 30k with all the features.

If you check e-bay you will find engine dyno ??? software for less than the cost of a piston.

If you reach where piston speed is an issue, with a little engine, my guess is you will have somewhere in excess of $30k in the motor and you will of had to fix other problems like pulling studs out of the block, oiling issues, custom intake and exhaust solutions to name a few.

Peter Austin
Montreal
Solstice roadracer (300hp, 340ft/lb, max rpm 6,200, new engine cost $5,000)

[Steven Scala]

Originally Posted by bumblebee

so your argument is that you set your car up for special circumstances (the mother of all drafts) rather than steady state...with no tolerance for error (slow corner exit on to the long straight, etc.)...do you really think that you should do this because a race is more than one lap and humans make mistakes (even Michael S.)?

if I race against you please gear your car as you say...I will give you the "Queen's Wave" as I pass, probably on a corner exit...gear your car for lowest lap time, not just performance on one section of the track, gearing is always a compromise, the goal is lowest lap time, not fastest mph on the main straight...

Bumble, let me break it down for you again. 1) I was talking about top gear; 2) I was talking about top gear; and 3) I was talking about top gear. That's one out of four, five, maybe six gears. You following along okay, here? Assuming some kind of race box, we'd be getting into this cog, like, twice a lap at Mosport, for example. In the lower gears, again, as I'd already said, I'd go with you and Carroll Smith.

But in case I didn't mention it, I've been talking about top frickin' gear here. Again, no big loss being short a couple hundred revs for a total of maybe 10 seconds a lap in a speed range where the acceleration curve has already gone flat. (While you're in top gear, that is. In case you haven't picked up on that.)

Originally Posted by bumblebee

...since I will have a lap on him it will not be really necessary to teach him a better line...

With your attention span, I think I'll just wait for the mistake.

[billyekracer]

Thank you Bumblebee and Uptownguy...that helps.

I do have access to some engine modeling software through a local engine builder, who swears by the program. He also has a dyno and has modeled various engines before dynoing them with surprising accuracy.

Bumblebee brings up a good point about piston weight (for some reason i wasn't considering that).

Lighter pistons obviously have less impact on the rotating assembly.

[bumblebee]

Steven, I was talking about winning races, winning races, winning races...I could care a less about top speed, that is my argument...your argument is irrelevant because it is so narrow and way too special case oriented...better to invest in a good set of brakes and out-brake the other competitor...anyway, all good fun...best investment is putting a data system in a car and getting good training on how to interpret it, amazing how much you leave on the table every lap and amazing how inconsistent most of us are driving...

Billy, bottom line if you want the most power, build the engine with the biggest bore size that is reliable, sit around a rod/stroke ratio of 1.8, that will define the most that you can stroke it, and you will have a big engine that revs pretty well and is fairly easy to tune. "God" is on the side of big battalions and if the rules allow, go big, lots of torque and hp are usually the result. Watch the weight of the reciprocating components as balancing them properly can be an issue.

Steven, you will have to wait a long time for a mistake, and, when it happens, I will have the torque to stay ahead and the brakes to make sure it stays that way...good luck this year, unlike you young guys my first goal is not to go fast but to qualify for the medical each year...

Be careful of the programs on the market. Many of them are pretty bad. If you want to be really smart and have access to a community college or university go to their electronic library and you can search the SAE and other databases for articles on your subject. There are even great articles published as far back as 1970 about the engine development that Penske and Donahue did to get power out of the American Motors Javelins and Honda did to develop engines to win motorcycle GPs. Articles like these are peer reviewed which means other experts checked them so the quality is very good. You can even write your own models in EXCEL using the peer-reviewed stuff, all the equations are there. You will even pick up things that most engine builders forget because these people are researchers. Yeah, it takes time but if you want the extra results you have to put in the equity sweat.

[billyekracer]

That's great advise Bumblebee.

Thanks again.