restrictor displacement

[bsfc9]

we are building a restrictor plate motor and have a rules limit on displacement of 358cid, any thoughts on building this motor at a smaller displacement(300-330) because of its restricted nature?

[speedtalk]

What are the rest of the engine rules?

[EngineTech1]

I would recommend you build the thing as close to the limit as you can while leaving yourself some room for freshens without going over the limit. The size of your restrictors is going to limit the total airflow of the engine and therefore power so your best bet for getting as much power to the ground as possible is to maximize the efficiency of the engine. Brake Specifics get better as engine size gets bigger indicating a more efficient engine. The smaller cube engine may get close in power output to the larger engine but will probably not match its output and it will make peak power at a higher rpm and probably be down on average power as well. It will also require a deeper rear gear which is less efficient. I know alot of people think that this smaller engine thing is the key to a restrictor race, but I've seen it tried by some very skilled engine builders and it hasn't worked yet. We tried it at the school once and the smaller engine peaked at a higher rpm, made less power and torque, and needed a deeper gear than the 358's we had and was slower on the track so it ended up being made a back up engine. And this engine had the best of everything, CFE restrictor heads, winberg crank, trick dyers rods, and custom JE pistons, versus some 2 and 3 year old engines. If the 358 engines would have had the benefit of all the higher end parts used on the baby engine the gap would probably have been a little bigger.

[Racer7088]

Yep most restrictor specific racing like the 24 hour stuff always favors large engines more with restrictors as they suffer less losses than the smaller higher rpm engines do. Most classes even allow larger restrictors for the smaller engines to let them compete or even less weight. The 500 inch Vipers were much faster than everything when they all had the same restrictors and now they had to give the smaller engined stuff bigger restrictors to get even and let them weigh less too so I would also say you hurt restricted engines by making them smaller.

[bsfc9]

rules include 650 holley 4-barrel, no sheetmetal manifold, 12:1 compression, any cam(roller, etc.) most 18 degree heads(for Chev.), 358cid and i think the restrctors are .825".

[SW]

What about spacers? What are the rules there? We have found as much as 40hp in the correct spacer on a restricter plate engine.
Shawn

[bsfc9]

rules allow 2 inch open spacer.

[speedtalk]

Can the spacer protrude into the manifold?

[bsfc9]

protrude into the manifold? do you mean not match or overhang the carb flange of the manifold?

[bunner engines]

throttle bore extension tubes is one method . mostly useful with big plenums (like a tunnel ram) imagine a one inch four hole spacer but with extended sleeves protruding from the four holes about 1 inch below the bottom of the base of the spacer . we have made some different configurations to test ,didnt show much on the dyno but did launch the car better at the track ! alot more to be learned and tried here . grumpy did some reaserch on them and said that , the extensions seem to bring the throttle bore exits closer to the runner entries without reducing plenum vol.it gets the accelerator-pump fuel into the runners very quickly and minimizes the dispersion that occures when the fuel enters the turbulance in a large plenum . i think a restricted engine (if rules allow ) could benifit with some reaserch on this . ahh more testing ,i love it !

[Racer7088]

Most of these restricted classes tend to have open spacers with a sharp edged orifice plate with gaskets that DO NOT go near the holes at all or another words open spacer gaskets. These are the rules so that the restrictor does work. That way the restrictor effectively limits airflow as much as it can and you must simply make more efficient use of what air DOES come through these orifice plates as you can't hardly get any MORE air to go through them than anyone else with the same plates.

[bsfc9]

Brake Specifics get better as engine size gets bigger indicating a more efficient engine. .

would anyone be able to argue for or against this statement?

[Racer7088]

Brake Specifics get better as engine size gets bigger indicating a more efficient engine. .

would anyone be able to argue for or against this statement?

bsfc9,

You should know! it's in your name! Seriously though small high rpm engines tend to have poor efficiency as compared to really large low rpm engines. That's just physics.

HP/inch however favors the smaller higher rpm engines and frowns on the big low rpm stuff.

Efficiency is how much power you make out of a given amount of fuel and air. The rotary for instance makes great HP/inch also and is usually terribly inefficient as well due to the huge surface to volume ratio of it's combustion space among many other reasons. The most efficient engines tend to be in the 1,000,000+ CID range with BSFC in the .25-.30 range.

If you're trying to make max HP/inch you usually end up with a smaller valvetrain (rpm) limited engine compared to your heads and if you are trying to make HP you usually end up with a larger airflow limited engine. A comp eliminator 270 inch engine can make around 3 HP/inch but is still down over 900HP from an 800+ CID IHRA Mountain Motor Pro-Stocker that only make a little over 2 HP/inch. The IHRA engine is much more efficient and has much more power as compared to the fuel running through it. It may see BSFCs in the low .30 range even.

The comp motor has tons more working against it in many areas but due to the high rpm makes way more HP/inch and gets to take weight off as a reward for running the smaller engine as compared to someone else in their class having to weigh more with a bigger engine. Now in Pro-Stock you won't get any more weight off your car for building a smaller engine so you don't. Even though they do not make as much HP/inch as some Comp cars again they are much more powerful and go faster.

Keep in mind that some people also call HP/inch efficiency but in engineering terms that's wrong. It's called specific output or HP/inch in our world. In the end it's power/weight and power/drag that wins races not power/inch. It's something the ricers will never understand. Who cares if you make 10 HP/inch when you only have 1 cubic inch! Now put three or four in a car and they may keep up with that 540 inch BBC!