What are the determining factors in RPM limits?

[dsm_ftw]

I was wondering what the factors are on the rev limt on a motor. Meaning what parts need to be replaced in order to safely achieve a higher RPM? I am not worried about cost or practicality at this point. Just trying to get a basic understanding of how these things work. To relate it to my project, I have a 1991 N/T that has two cracked pistons and 4 shattered valves . . . My dad f***ed up the timing. The car was given to me for free. Obviously I am looking to turbo it and am going forged pistons and probably a butchered stoker crank with aluminum flywheel and rods. The turbo will not be purchased for a while. So right now I am looking to get as much performance as I can out of a N/T. This is where the RPM question comes in. I know lifters and valve springs have to keep up, is there anything else? Such as limitations to the coil pack?

 

[DSMReviver]

The determine factor are... as follows....

crank
rods,
pistons(basically the whole rotating assembly, if they are balanced, their weight),
valves springs,
port work,
or air velocity.

All this is depended upon how high you want to rev your motor. if you are going to raise the rpm by 500 or 1000 at a time or the higher you set it. The more you have to pay attention to the list i've stated above. Generally for our DSM's 4g63 platform, its safe to take it up to 8k without a problem on stock motors. People have done it and there is no problems. But, anything further then 8k i would look into a good set of valve springs, rework bottom end etc... hope this answers your questions bud.

 

[dsm_ftw]

Is there a good site where I can look at the prices of "the good stuff" so I can have an idea of my goals? I know of ffwd and maperformance but they seem to want to sell lingblocks and shortblock instead of internals.

 

[DSMReviver]

Is there a good site where I can look at the prices of "the good stuff" so I can have an idea of my goals? I know of ffwd and maperformance but they seem to want to sell lingblocks and shortblock instead of internals.

They sell them all bud, you got to look. Just go through the vendors on here and look through them. Before you buy it, look at the review for that product and make a purchase.

 

[dsm_ftw]

ok thanks!

 

[Absit]

I'm going to have to disagree with DSMReviver here and say the most important factors in redline are rod ratio and the valvetrain. The rest is far less important.

 

[my dreamt dsm]

For me it's if you make power up there. There is no need to take my little turbo up there if it's not making power.
Think of honda's.(no honda jokes please). They tend to rev higher because they make more power up in the higher rpm band, example: type r motors!
Kinda get an understanding now!
The valve train will be the weak spot imo.

 

[Absit]

For me it's if you make power up there. There is no need to take my little turbo up there if it's not making power.
Think of honda's.(no honda jokes please). They tend to rev higher because they make more power up in the higher rpm band, example: type r motors!
Kinda get an understanding now!

Not even remotely related to the question, great job.

 

[my dreamt dsm]

Thank You! Just examples and your criticism isn't beneficial to op.
Out of curiosity why would op want to rev higher? Planning on a n/t beast?

 

[PieEyedPiper]

Thank You! Just examples and your criticism isn't beneficial to op.
Out of curiosity why would op want to rev higher? Planning on a n/t beast?

I think more of what he was getting at was the "hondas rev higher cause they make power up der" nonsense you posted.

OP, when I rev high I fear of putting a valve through a piston or at least tapping one just enough to ruin my day.

 

[dsm_ftw]

I am planning on building the motor from the ground up. The RPM question *is* because of the hondas vtec and was, quite frankly, interested in building a N/T beast until the turbo is purchased. Again, I am new to DSMs and what better way to learn to swim than to jump in the deep end?? Thanks to all for the quick posts. As for the unrelated comment, It wasnt the answer I was looking for, but it reminded me of a couple things. Like will the car be able to make power that high and would it even be worth it? So let me rephrase the question now, what determines whether or not you make power that high. I know airflow and the ability of the lifters to keep up, but is there anything else?

 

[slow8033]

Well there is one more rather important thing you need to plan/work out. A better "beast" Naturally aspirated engine is quite a bit different from the build you would have to do in order to have a very good turbo engine. High compression ratios will help you with a Non turbo application, but under boost its less forgiving and more likely to result in damage.

The only reason I bring this up, is you seem to want to build it for naturally aspirated use now, but have a good ideal future turbo motor. Generally if you used Forged internals on a 420a engine, you go with a lower compression ratio. This in turn actually makes your car SLOWER until you get your turbo set up.

Just a thought. Make sure you have a plan.

 

[evo1rice]

Agreed with rod ratio. In rod ratio you have many choices available. See magnus' chart below

http://magnusmotorsports.com/wp-content/uploads/2010/02/DetailedEngineList.pdf

Shows everything you need to buy & what are the outcomes: potential redline, displacement, rod lenght etc e tc.

 

[dsm_ftw]

@slow8033, so higher comprerssion pistons is better? Even if the car is turboed, higher compression produces more power, but the offset is risk damaging your motor if not tuned right? If thats the case why not go high compression and take it to be porfessionally tuned?

@evo1rice, thanks for the link. Now just makes me wonder what formula they use to calculate the recommended rpm limit.

This is getting slightly off topic. Does this mean I need to start a new thread?

 

[talonDSMerr]

High compression pistons while running boost will affect the knocking threshold. Using pistons with, say, 10:1 compression ratio will severely limit the amount of timing advance or boost you'll be able to run on a tune by a professional or amateur. Using pistons with slightly lower compression ratio, like 9.0:1, will allow more timing advance and more boost, which will generally make more power than higher comp. ratio and less timing advance and boost.

 

[slow8033]

Its true, and I didn't mean to get your thread off its original topic. Just making sure you are thinking about that.

Yes a higher compression motor will yeild more power, and some guys do go with 10 to 1 compression even on the 4g63 platform. But your tune needs to be much better. When I was looking into turboing my old 420a, the compression ratio that was most used on forged internals was 8:5:1. That just allows for easier tuning. And this set up, when tuned right, allows for more timing which in turn gives more power than the higher compression alternative.

EDIT: Talondsmerr beat me to it.

 

[talonDSMerr]

A lot more goes into determining RPM limit. Higher rod ratios will give you lower mean piston speeds and less side loading on the cylinder walls at high rpms than low rod ratios. The weight of valvetrain parts also plays a huge factor when determining at what RPM valve float occurs. There are lots of complicated inertia, moment, etc. formulas that go into it. Lighter is what you want for valves, springs, retainers, rockers. Keeping them strong and durable while being light is why these parts are expensive!

 

[dsm_ftw]

Thanks you guys have helped so much! And cudos for the quick replies!

 

[tkelly27]

There are a few things that determine how high you can rev your engine. As far as I know, even with current mechanical technology, the ECU and spark system are not the weak link. A stand alone and coil on plug setups never hurt, but I don't think it is needed.

In the short block it is based on a lot of factors. The acceleration and deceleration of the piston is what puts stress on the rod bolts, which are the weak link of the rods when it comes to RPMs. The most stress put on the rod bolts is at the end of the exhaust stroke. A longer stroke will cause the piston to have to move faster to get through the stroke in the same amount of crank degrees. A longer rod will allow the piston to move slower and transmit less force through the rod. When it comes to power, the rod buckling is where things go bad.

Then you come to the cylinder head. At higher RPM the stock valve springs are unable to force the valves closed in the amount of time given. I think they are also thrown open faster, especially with a bigger cam. You'll need stiffer valve springs to help with this. Then comes the camshaft.

A pretty basic camshaft primer. As RPMs go up the amount of time available for air to move goes down. Bigger camshafts have a longer amount of time the valves are open to allow more air in/out at higher RPM.

Slowboy seems to know a lot about 2.0 long rod engines, and they say they can rev out to 10 or 11k. You'll need some pretty crazy gear in the head for that. For me it sounds like a lot of fun to build an engine like that. If you want a stroker, you can forget high revs. They typically max out at 7-8k.

 

[TunaTalon]

Good general advice above. If you want to get into the technical details of piston dynamics give http://www.kidzuku.com/StrokeOrNot.pdf a scan. It's written for the 4G63 but the math is the same for any piston engine.

 

[dsm_ftw]

Thanks guys, and the PDF is AWESOME! I think my engine iq just doubled

 

[cliffkemp]

Thanks guys, and the PDF is AWESOME! I think my engine iq just doubled

It is really simple, if you want lots of power with an NA motor, you have to have higher compression. IF you want more power potential for the addition of a turbo, you will have to use lower compression.

With higher compression drivability is great at just about any rpm with a good NA set up.

With a turbo setup, the lower the compression, the more 'room for error' you have with timing and boost without ill effects.

The down side of having a boosted engine with lower compression is that you will not have much power....drivability..... off boost meaning that you will have to stay in boost or somewhat in boost to get moving.

The opposite is true with higher compression, you will have less room for error in tuning in boost and timing but, drivability off boost will better.

This is where you have to decide what you really want in the car. If you want to turbo it, many go with a 8.5 to 9.0 compression. If you want to just go all motor, most will go with 12 to 1 or slightly higher.

To get more revs, balance the engine well and use, if you have the money, titanium parts as they are light, strong, and can stand the heat. They are also very expensive. The less weight the engine has to rotate, the faster it can go with supporting parts.

the titanium parts are for the head. The pistons should be an aluminum alloy.

 

[dsm_ftw]

When you say room for error, what do you mean? How much more difficult is a lower compression to tune than higher compression?

 

[cliffkemp]

When you say room for error, what do you mean? How much more difficult is a lower compression to tune than higher compression?

With lower compression, the air is heated less as the piston comes up. Remember you physics. If you compress air, it heats up as you compress it. The more you compress it, the hotter it gets. Under boost, the same applies. Given most factors equal, a 10/1 compression engine will heat up the air more as it compresses the air than a 7.8 (stock 1g compression)

For tuning purposes, the same holds true. Higher compression will heat the air up more so, this means that tuning for power is harder due more to density and humidity.

Since air temp is never constant all the time, if you were to tune your car on a cool day, for example, and it doesnt knock. If you were to drive to a warmer climate, there is a chance it may knock due to the humidity and temp difference.

A shop or tuner will usually tune their cars rich to start and lean it out til it starts to knock slightly then change fuel/timing so that there is room for a temp change. When you run into problems is if you tune any car whether boosted or not and dont allow for a decent change in the tune to compensate for changing variables within reason.

The main thing to remember is to go with a decent compression ratio that will allow for decent acceleration off boost. Most will use 8.5 to 9 so that drivability is still good off boost or you will have to stay in boost or have very little get up and go until you are in boost at decent rpms.

Hope this helps.

 

[dsm_ftw]

With lower compression, the air is heated less as the piston comes up. Remember you physics. If you compress air, it heats up as you compress it. The more you compress it, the hotter it gets. Under boost, the same applies. Given most factors equal, a 10/1 compression engine will heat up the air more as it compresses the air than a 7.8 (stock 1g compression)

For tuning purposes, the same holds true. Higher compression will heat the air up more so, this means that tuning for power is harder due more to density and humidity.

Since air temp is never constant all the time, if you were to tune your car on a cool day, for example, and it doesnt knock. If you were to drive to a warmer climate, there is a chance it may knock due to the humidity and temp difference.

A shop or tuner will usually tune their cars rich to start and lean it out til it starts to knock slightly then change fuel/timing so that there is room for a temp change. When you run into problems is if you tune any car whether boosted or not and dont allow for a decent change in the tune to compensate for changing variables within reason.

The main thing to remember is to go with a decent compression ratio that will allow for decent acceleration off boost. Most will use 8.5 to 9 so that drivability is still good off boost or you will have to stay in boost or have very little get up and go until you are in boost at decent rpms.

Hope this helps.

It helps a lot. Thanks.