Thread:

Compression for Gas / Alky blower motors

Originally Posted by tkriley13

1.Instead of running a bigger blower why not run more compression & spin the blower less?





2. Say this works & you have 8.1 compression in a 468 and you want to run alky now but you need / or want around 10 1/2.1 compression, couldn`t you just deck your heads to bump up the compression.


I just ordered new pistions for next season but I was thinking How come I just couldn`t deck the heads & bolt on injection instead of spending more money on pistions & a blower.....

1. Depends on if this is a max effort deal. If you are trying to run a class and can get there by doing that OK. But the more air you move through that engine the more hp you'll make. So more blower = more power.
2. To many variables. How far are the pistons in the hole, head gasket thickness, PV clearance, which heads and how much can be removed before causing yourself other issues?

Originally Posted by wagspe208:1531053

This will be a great thread.
Wags

X2 lol, low compression will work

1. A low compression high boost gas engine will kick ass on a higher compression lower boost engine every single time. This isn't even dabatable. Any one that thinks it is, should surrender their blower. The ONLY reason for building a higher compression GAS blower engine is if your looking for part throttle, no or low boost power.

2 You can never machine the heads enough to raise a 8.1 engine to anything near what a alky engine wants. You can raise the boost enough to make it work, but it will be lazy off boost. But 60psi with 8 to 1 has been done hundreds of times. It was a pretty standard set up at Indy in the Offy years.

Yes lower compression and more boost makes more power.

You end up with a very narrow tune up window with the higher compression boosted engine. The higher the compression the narrower the window. Constant adjustments to changes in conditions.

Plus there generally is not much selection between 8:1 and 12:1 for shelf pistons for a 468.

Originally Posted by gn7

1. A low compression high boost gas engine will kick ass on a higher compression lower boost engine every single time. This isn't even dabatable. Any one that thinks it is, should surrender their blower. The ONLY reason for building a higher compression GAS blower engine is if your looking for part throttle, no or low boost power.

The OP said Gas/Alky and he probably should have seperated them. I do know he's gonna run alky. Never ran/tuned blown gas, but alky likes compression. Probably pretty obvious but higher compression will make more HP at the same boost. My suggestion to the OP was leave everything alone, pull the carbs and put an alky hat on it and hang on.

Originally Posted by gn7

2 You can never machine the heads enough to raise a 8.1 engine to anything near what a alky engine wants. You can raise the boost enough to make it work, but it will be lazy off boost. But 60psi with 8 to 1 has been done hundreds of times. It was a pretty standard set up at Indy in the Offy years.

Yup

Originally Posted by Infomaniac

Yes lower compression and more boost makes more power.

You end up with a very narrow tune up window with the higher compression boosted engine. The higher the compression the narrower the window. Constant adjustments to changes in conditions.

Plus there generally is not much selection between 8:1 and 12:1 for shelf pistons for a 468.

Havent built a little motor in a while, but I know JE makes flat tops and that ends up around 10:1 on 4.25/4.5" strokes... (to lazy to punch in the numbers to check a 468). Also, 12:1 isn't so high alky would become a tuning nightmare... Plus you can make the HP you need without a bunch of boost (we're bracket racers, not class racers)

When you say the blower shop and BDS recommend higher compresion, are you talking about alky? Thats pretty much a given. A low compression alky motor with low to no boost is a real loser. But if you run enough boost and do it all the time, like the old F1 and Indy stuff, the compression isn't nearly as high as a A/FC or a TAF or H

On gas? They do recommend higher compression on street deals with less boost because a unboosted low compression engine on the street is gutless. There is no difference between a low compression blower motor and a unboosted blower motor except your dragging the blower along like a over grown A/C compressor, and the cam in the unboosted blower motor with low compression just makes things even worse.

Its like was said earlier. A blower motor without the blower on it is a pig. Having the blower, but at part throttle/no boost is no better. If you put some compression to it, its aleast decent when its not under boost.

Its not that they don't know what they are talking about, its more like the use.
We have to run a limited blower speed so our compression is pretty high for a blower gas deal. But I would trade 3 full points of compression in a heart beat for twice the boost.

Yeah, there is definitely a point where boost over compression falls down. And I would say below 8 to 1 is getting into that territory. Lower compression engines lose there cylinder pressure at an accelerated rate compared to higher compression engine. The cylinder pressure degrades much quicker from its peak.
But I would love to see a 7 to 1 max effort, max boost gas deal against a 9 to 1. My money says the 7 to 1 engine would take it. Below that I would be come more doubtfull.
I looked up the compression ratios for alky Indy turbo deals in the late 60's and 70s. They were higher than I first thought. They were up around 9 to 1 which is pretty high considering the boost they ran for a race that long.

Jim, on anything less than a max effort deal I will trade compression for boost. The closer to a max effort deal, the less compression and the more boost. Like I said, I will give you 3 points of compression is you can convince the powers that be to let me have 12-15 pounds more boost. I would much perfer to squeeze twice the air into a slightly larger area, than squeeze a half air into a slightly smaller area,