What is the displacement of your motor? If it is a 350 and you run 10.5% underdriven, you are going to be making about 9 pounds of boost. With your static CR, you can only run about 2-3 pounds of boost on 93 octane gas. Take a look at this tech write-up on Weiand's website...

Weiand blower info

 

What is the displacement of your motor? If it is a 350 and you run 10.5% underdriven, you are going to be making about 9 pounds of boost. With your static CR, you can only run about 2-3 pounds of boost on 93 octane gas. Take a look at this tech write-up on Weiand's website...

Weiand blower info

Sorry my motor is a 383.

 

What is the displacement of your motor? If it is a 350 and you run 10.5% underdriven, you are going to be making about 9 pounds of boost. With your static CR, you can only run about 2-3 pounds of boost on 93 octane gas. Take a look at this tech write-up on Weiand's website...

Weiand blower info

Exactly what XNT said, you're effective c.r. with 9 pounds of boost will be 16.2:1.

Whoever gave you the go ahead, telling you your c.r. is good to go, either didn't have all the facts or is misinformed, you need to do some homework.

 

Curious, did the " reputable ' engine shop talk to you about the single key way on the crankshaft snout? There are some very good reputable engine shops that build a very good N/A or Naturally Aspirated engine, but should not ever touch someones blower engine. So far we have no less than 3 RED FLAGS. I know it's not what you want to hear, but we are only trying to help.

 

Exactly what XNT said, you're effective c.r. with 9 pounds of boost will be 16.2:1.

Whoever gave you the go ahead, telling you your c.r. is good to go, either didn't have all the facts or is misinformed, you need to do some homework.

I told the builder that it would be more show than go. With boost probably only in the 5-7lbs range.

 

That still puts you at or just over 10:1, tune wisely, it doesn't take long for detonation to take it's toll, mine took 10 seconds on what I thought was a mild tune. The damage was significant, 7 pistons and cylinder wall damage, the rings wouldn't pull out of their grooves (the ones without broken lands) with pliers. Be careful.

 

That still puts you at or just over 10:1, tune wisely, it doesn't take long for detonation to take it's toll, mine took 10 seconds on what I thought was a mild tune. The damage was significant, 7 pistons and cylinder wall damage, the rings wouldn't pull out of their grooves (the ones without broken lands) with pliers. Be careful.

You are so right, detonation under boost happens violently and very quickly. Your story is a common one, our dyno is never run without 2 knock sensors, one for each bank of a V configuration engine. It's always the same sad story, I hope you have your set up sorted out now.

 

You are so right, detonation under boost happens violently and very quickly. Your story is a common one, our dyno is never run without 2 knock sensors, one for each bank of a V configuration engine. It's always the same sad story, I hope you have your set up sorted out now.

I do. My BTM was in El Paso for repairs, I bought a 6AL for the weekend, with aggressive timing. I've since installed a full time Innovate A/F monitor, a methanol injection system, along with the BTM, and I was able to increase boost to 12-15 #'s, with 8:1 static.

You're right about things happening quickly, I was out 1,000 feet, hard run, when it nosed over, I never heard the detonation, and that was through the mufflers, Borla's with 3". It only takes one of those days...

 

Not to pile on, but your cam probably isn't blower friendly either.... I am sure that the LSA was a typo... but it needs to be 112 or 114 to work well with the blower.

 

Not to pile on, but your cam probably isn't blower friendly either.... I am sure that the LSA was a typo... but it needs to be 112 or 114 to work well with the blower.

According to Blower Drive Service, 110 works best with gasoline...read here....
http://www.blowerdriveservice.com/recommend.php

 

My advice: listen to these posts. I ran a 144 single carb blower atop a 350 and currently run a 4:71 blower with twin 600 cfm Holleys on top. I had nightmarish tuning issues with the change and the same experts who have commented on your inquiry, capably led me to a combination that works. I currently run a street/strip 355 c.i., 7.5:1 static compression, which runs 10.93 in the quarter mile, and runs (honestly so) as a grocery getter on the street.
Based on my own personal experience (age 64 with 50 years experience with hot rods and drag racing), your static compression is about two points above what is rational with a street blower motor. Unless, and some trailer queens do this, you are going to remove the rotors and simply use the blower case as a tunnel ram carb manifold, your current combination is grenade-ready !

 

After you grenade this motor and $$ wise up $$, build a blower friendly 355 CID sbc with a 7.5:1 cr using -22cc dished pistons and 76cc cylinder heads.

The motor you have now , will require 110+ octane race gas or methanol fuel to run @ WOT under boost and make any real horsepower, without eating itself.

The compression ratio is way too high for usable boost and pump gas.

The doubled keyed crank snout is a good idea.

This is all great info. I think I get the picture. I prefer not to replace my piston so if I'm going to replace my heads with higher cc chambers what is everyone recommendation for heads.......brand, size of chamber, intake runner, etc.

Thanks

 

I ran a 9:1 compression ratio with a low boost Weiand 144 stop a 355 using 64 cc heads. When I went instead to a 4:71 Weiand blower atop that 355, I opted for static compression of 7.5:1 by a combination of dished Ross pistons and Dart aluminum Pro-1 72cc heads, 230 cc intake runner. The new combination is very tame on the street, and runs 10.93 at 120.43 mph on weekends at the strip. The fun is HUGE !

 

Without knowing your deck height and what pistons you have now, along with gasket thickness, bore if not stock, we can't tell give you a good answer. How to you come up with your c.r. in the first post without that data?

Using stock bore of 4.0, piston .025 in the hole, .041 gasket, 64 cc heads and flat pistons with 6 cc's of valve relief, I come up with 9.57:1. With only a head change to 72 cc's you will net about 8.82:1, still on the high side for a blower motor.

My 383 with a 31 cc dish piston, 3.75 stroke, 4.03 bore, 64cc heads is 8.2:1.

If you're going to stay with that c.r., at least go with aluminum heads to help with detonation. I have Dart Pro I, 230/64 with an 8-71.

 

After you grenade this motor and $$ wise up $$, build a blower friendly 355 CID sbc with a 7.5:1 cr using -22cc dished pistons and 76cc cylinder heads.

The motor you have now , will require 110+ octane race gas or methanol fuel to run @ WOT under boost and make any real horsepower, without eating itself.

The compression ratio is way too high for usable boost and pump gas.

The doubled keyed crank snout is a good idea.

This is all great info. I think I get the picture. I prefer not to replace my piston so if I'm going to replace my heads with higher cc chambers what is everyone recommendation for heads.......brand, size of chamber, intake runner, etc.

Thanks

 

Well now that all the guys have hit the cr problem I'll only say the same thing...too much cr as stated. 68...suggested larger chamber heads. That may get you there but you will have to measure everything as noted to get the exact number. Larger chamber heads will be better anyway. 72-75 cc.

There are any number of good heads for this so pick your best deal. 100 % street is the key. You don't need the ultimate flow heads, just something that will last made of aluminum. I use Performer RPM heads. So far thay have been very good. No problems. I'd limit the motor to about 5500 rpm. Don't be too hasty and toss the vac advance. You need full mech adv by 2000-2200. 10 deg of vac advance will smooth the motor out even more and get a couple mpg improvement.

The real get out of jail card will be to install a water/alcohol injection system. You aren't going to cool 85 pounds of blower or even the air/fuel charge but you will improve the octane rating of the charge which will save the motor life. haha There are about 6 major mfg of these and any of them are pretty good. Most important will be to get the optional solenoid valve. This will prevent the blower from sucking all the W/A from the tank. Don't let them talk you into just a check valve. These mfg all deal with the turbo and centrifugal blowers and not too much with 6-71 so you have to be sure to state your combo. The turbo guys all run a lot more comp ratio but they have inter coolers plus the W/A so they can get away with it because the charge is cooled so much.

 

Without knowing your deck height and what pistons you have now, along with gasket thickness, bore if not stock, we can't tell give you a good answer. How to you come up with your c.r. in the first post without that data?

Using stock bore of 4.0, piston .025 in the hole, .041 gasket, 64 cc heads and flat pistons with 6 cc's of valve relief, I come up with 9.57:1. With only a head change to 72 cc's you will net about 8.82:1, still on the high side for a blower motor.

My 383 with a 31 cc dish piston, 3.75 stroke, 4.03 bore, 64cc heads is 8.2:1.

If you're going to stay with that c.r., at least go with aluminum heads to help with detonation. I have Dart Pro I, 230/64 with an 8-71.

Here's more info:

Pistons: Probe SRS Sportmans race series. Dish -12.8cc
Bore: 4.00
Stroke: 3.75
Deck Height: .017
Gasket: .039" (felpro 1014)

I came up with the estimated compression using Probes table for this piston. With a 76cc head and these pistons the estimated compression is ~8.79.

The race engine builder I spoke with advised not bringing it down this low for what I want to do with the car because he felt that I would have to really turn the blower to get boost and lose bottom end. Again weekend driver with very minimal WOT passes........not enough places to drive like that in LA.

He said for my street/show car he would target 9.0 to 9.25:1.

He recommended a 1:1 ratio for blower to crank. H0For hads he recommended 200cc or 210cc with about a 70cc chamber if i decided to swap heads.

I'm trying to do all the research possible. I understand and have read many posts on what everyone is trying to tell me.....I totally understand. Just trying to make the best decision for what I plan to use the car for and take everyones input.

 

Sorry my motor is a 383....

To begin with, if you're bore is 4.00 you do not have a 383.

With what you provided, with a 4.0 bore, about 9.74:1.

If you have a 383 (4.030 bore), approximately 9.86:1.

 

To begin with, if you're bore is 4.00 you do not have a 383.

With what you provided, with a 4.0 bore, about 9.74:1.

If you have a 383 (4.030 bore), approximately 9.86:1.

Yes ut's a 383. The Probe table say the my current compression is 9.86 which matches your calculation. On this same table if I use 76cc heads then I should be at 8.79?

Based on eveyones input I will swap heads to be more on the safe side since I'll already be working on the motor. Just gives me more room before things go really bad.

Any recommendations on heads, runner size, cc, etc if I keep bottom end as is?

Thanks!