TheSamba.com :: View topic - Rules of Thumb: How does Displacement relate to HP

Wed Aug 25, 2010 9:12 am

Hello, I'm quite new to VW engine performance and just trying to get a sense of the power associated with boring out a VW engine. I have a 1600 DP right now and I am wondering how much horsepower I would gain but increasing to a 1776 or a 1914....etc. This is assuming everything else is left stock, just talking about the engine here.

Is there a rule of thumb for how much more power you get as compared to the percentage of the displacement increase? I would like to be able to get some sort of idea what power these larger displacement engines put out.

Sorry for asking kind of a newbie question but it is difficult to find this information and I think a lot of people have the same question.

Wed Aug 25, 2010 9:45 am

It helps if you think in terms of torque instead of HP. The more displacement you have the more torque you can generate. Increasing bore is good but when combined with increased stroke the torque is greatly increased. This is because the motor has longer leverage against the weight of the car.

HP is a function of torque, dependent on how high you rev the motor which in turn is dependent on how well the motor can breathe - carbs, heads, exhaust and cam.

HP is a selling point for cars, torque is what wins races and provides good drivability in a road car. There are HP calculators that you can play with if you know what you want in your engine combo, they'll get you close so you have a general idea of what you may achieve.

Wed Aug 25, 2010 9:58 am

brokengun wrote: Hello, I'm quite new to VW engine performance and just trying to get a sense of the power associated with boring out a VW engine. I have a 1600 DP right now and I am wondering how much horsepower I would gain but increasing to a 1776 or a 1914....etc. This is assuming everything else is left stock

Just to twist things on there head the 1600cc of a stock DP engine is already very restricted in power by the small carb/cam/head flow/and exhaust.

So simply adding more cc is going to make an even more restricted engine. So for the cc increase you get little gains.

I would rather change carbs/cam/heads and exhaust on a 1600 than boost the cc to 1914 and leave all the other parts stock.

Have a read of these articles
http://www.aircooled.net/gnrlsite/resource/articles.htm

:D

Wed Aug 25, 2010 9:59 am

While displacement can dictate hp, heads also have a lot to do with it. You can get 150 hp out of a stock stroke motor (1750, 1835, 1915) with a big cam, carbs and heads, you can also build a 2180 with 85 hp by putting a mild cam and stock heads on it. People tend to go more radical when they build bigger motors, hence the giant hp figures.

As a very rough guide:

90 hp- stock dual port heads
125 hp- ported stock heads
170-180- ported 40x35 heads
180 +- 42x37 heads (and bigger)

A 1776 or 1914 with all components stock would have hp increases of about (roughly) the same percentage as the displacement increase; a stock 1600 putting out 50 hp would have about a 4-5 hp increase as a 1776, and about 8 -10 hp increase as a 1915. The big increases come when you increase displacement and efficiency; more flow through the motor with porting, more carburetion, more cam timing and lift, and an efficient exhaust. It is not hard to almost double the stock hp output with an engine the size you are talking about. Dual carbs, a mild cam, some port work to the heads and a 1 3/8" header can be a very reliable (and fun! :twisted: ) combination.

Hope this helps. Al

Wed Aug 25, 2010 10:17 am

Two extreme examples I can think of would be Jimmy111's 2332 that makes 90hp at 2,500rpm, and Jake Raby's 1500 that made something like 186hp at 8,500rpm. Totally depends on what you put into it and how high or low you want to spin it.

Wed Aug 25, 2010 10:24 am

brokengun wrote: This is assuming everything else is left stock, just talking about the engine here.
That's just it - you can't leave everything else stock. An engine is a system; all these parts work together. When you increase displacement, you also increase the air and fuel requirements of the engine, and the exhaust requirements. In order to realize the performance potential of a larger displacement, you need to flow more air and more fuel, which generally means going to a larger carb (or dual carbs). This in turn necessitates the replacement of the exhaust, with a less restrictive setup.

Sigurd wrote: Totally depends on what you put into it and how high or low you want to spin it.
Exactly, and also how long you want it to last. An 8500 rpm 1500 isn't a 100,000 mile motor...

In summary, there is no generic formula, it all depends on the specific build and tune of the engine in question. Like the saying goes, Speed costs Money; how fast do you want to go?

Wed Aug 25, 2010 10:56 am

RPM = Ruins People's Motors LOL

Wed Aug 25, 2010 2:06 pm

You all bring up some good points. You could turn more RPMs and make more HP but the life of the engine is going to be significantly decreased. Makes sense. So basically what I should be looking for then is a good combination of reliability, lifetime and performance. Is there some chart or easy way of gauging what an engine setup will do physically... for example will a 2275cc setup with the correctly sized (not restrictive carbs) pull wheelies but still be alright to drive in town? Will I have to have the cam so high that it won't idle without feathering the gas?

I just want to get a sense of where these engines with appropriate setups fall in actual performance. I guess I was trying to use horsepower to do that but as many of you pointed out that isn't the best way to tell. I will continue to read and try to get a sense of the type of performance they put out.... maybe I'll make a chart.

Thanks so much for the helpful words though. I'm still learning and much of the conversation I've read is a bit further along than I am so I find that I'm not understanding the whole picture. Turbos have a lot of terms that I am still trying to fully understand in my head too.

Wed Aug 25, 2010 2:21 pm

There is no relationship at all when you're boosted... :twisted:

Wed Aug 25, 2010 2:21 pm

brokengun wrote:
I just want to get a sense of where these engines with appropriate setups fall in actual performance. I guess I was trying to use horsepower to do that but as many of you pointed out that isn't the best way to tell. I will continue to read and try to get a sense of the type of performance they put out.... maybe I'll make a chart.

Trying to comprehend every possible variable and combination and reconcile that into hp numbers seems a bit silly, unless you are a professional engine builder.

Why don't you just state what your performance goals are, and then have folks suggest appropriate builds to meet those goals.

Wed Aug 25, 2010 3:13 pm

As everyone pointed out.....you can't JUST make it bigger and gage it. SOOOOO....with the increase in size you port the stock haeds and get a good 3 or more angle valve job, put some dual 34 carbs on, add a small header muffler combo, keep the cam stock....you end up with an engine that makes about 100 hp has a power range of 4500-5000 tops and drives like a stocker and even pulls down some real good mileage figures, doesn't overheat and lasts 50,000+ miles easily.

RC

Wed Aug 25, 2010 4:42 pm

lupin..the..3rd wrote:
Trying to comprehend every possible variable and combination and reconcile that into hp numbers seems a bit silly, unless you are a professional engine builder.

Why don't you just state what your performance goals are, and then have folks suggest appropriate builds to meet those goals.

I'm not trying to make some formula or something to gauge these engines by HP. I'm just trying to understand what the differences between them are, how significant a change it is from stock, and what I want.

I've got multiple goals, so I am trying to learn this from the ground up to have a better understanding. I'm sure I'll read a lot of builds, probably lots of them will interest me as I have a couple of different projects.

I am actually interested in modifying a type 2 engine with dual carbs to perform well on the highway (probably will get a freeway flier transmission). Also interested in turbo charging and or stroking a 1600DP for my beetle.

Not saying it wouldn't be helpful to see some builds of the above mentioned stuff but I'd just like to have a general understanding too. That way I can apply it to more than one VW.

Wed Aug 25, 2010 5:35 pm

The best rule of thumb is probably that torque is just VE times Displacement and nothing more. Generally an engine can make 1 footpound for each cubic inch, and that's probably at some 85 or 90% VE.
There are 16.4 cc's in a cubic inch.
Power of course is just torque X rpm, so it's a question of how the torque drops off as rpms rise. If torque doesen't drop off HP just keeps on rising!

Another general rule of thumb is headflow in CFM is almost directly related one to one with HP potential on a 4 cylinder engine. If your stock heads only flow 90CFM on the intakes you should be able to make 90hp before they become the limit.

Every rule of thumb can be broken ya know, but they are helpful.

Wed Aug 25, 2010 5:36 pm

horsepower increase is linear with displacement increase, and horsepower increase is linear with RPM increase, until cam or heads or carbs limit the power.

A single carb seems to severely limit horsepower at higher RPM's on an air cooled VW engine due to the length of the intake.

5000 RPM seems to be a point where horsepower fades off due to single carb resttrictions.

here is a simple calculator from bowling.

http://www.bgsoflex.com/roughhp.html

Wed Aug 25, 2010 6:03 pm

modok wrote: Every rule of thumb can be broken ya know, but they are helpful.
That's coz ever body has a different size thumb....but they all work close to the same.....hahaha :lol:

Wed Aug 25, 2010 6:06 pm

Haha, heck yea.
That's why I like inches, because my thumb IS about 1" wide.
My feet are much longer than a dozen thumbs though, that doesen't work so well for me.

Thu Aug 26, 2010 8:10 am

CFM is directly related to CCR and Displacement .
CFM - Cubic feet per minute .
CCR - Cylinder compression ratio .
Add displacement bore and stroke .
All 3 play a part in H.P. out put and Torque .
Think of you Engine like a Big Carbon emmission's Pump that work's in efficiently .
Generally on Pump gas 9-1 CCR is max I actually prefer 8.7-1 CCR
Usually the higher the CCR less service higher the wear .
But I understand what your askin in Layman term's and the answer is ?
1776cc runn average of 70 H.P. too 100 H.P.
Just depend's on where you set CCR and how well you can flow it CFM
Valves , Head Configuration , Carbs to suck the air , Exhaust to Disperse it .
It's a Gas in Gas out game !
Then you have to also realize here that Combustion motor's are only about 25% efficient .
For reliability / longevity I like the 90.5 bore / 1776cc after that your rapidly loseing longevity .
Sean

Thu Aug 26, 2010 9:20 pm

simple cc's increase, power increases

however it is NOT linear and has many variables, despite what the PRA wants you to think with the useless lbs/ci rules

Fri Aug 27, 2010 4:56 pm

2stroke wrote: simple cc's increase, power increases

however it is NOT linear and has many variables, despite what the PRA wants you to think with the useless lbs/ci rules

RPM also increases horspower, and compression ratio increases horsepower.

after that, it is indeed linear until the cam/valves/intake/jetting screw it up.

Fri Aug 27, 2010 11:11 pm

Wow.....that's some Heavy Duty s**t man! :shock: