How much HP will I gain?

[Tepot]

Hi,

Just wondering. I've baught a few upgrades for my 1974 and was wondering about how much Horse Power I might gain will those upgrades. Do you think I'll feel a difference in the seat?

Right now I have the stock L-48 engine with Edelbrock performer intake, running on points ignition with an accell performance coil and headers, true dual exaust on flowmasters 40's. everything else is stock.oh and they've removed air conditioning before they sold it to me too.

In a few week I will get rid of the point system and replace by a breakerless SE electronic ignition kit, upgrade the stock rochester quadra jet carburator for an Edelbrock 600 cfm carburator and also swap the original cam shaft for a Comp Cam XE268H.

What do you guy's think. i don't know how much HP I have right now. They say the car came with 195 HP stock. Some say it might of come with more because GM at some point were saying the HP was lower to fool the insurance company and with the upgrades I already had , I don't know how much HP I had to begin with.

But if you had to guess. The new upgrades, how much you think I can get for HP gain? Will I feel a difference in the seat? are they all worth it?

As well. can anyone guess how much HP I have with everything I've mentioned? Stock engine, edelbrock performer intake, electronic ignition conversion, headers, true dual exaust NO CAT on flowmaster 40's, comp cam xe268h, edlelbrock 600 cfm carb, accell performance coil, removed Air conditioning.

Thanks
Daniel

 

[Vettehead Mikey]

Hi,


In a few week I will get rid of the point system and replace by a breakerless SE electronic ignition kit, upgrade the stock rochester quadra jet carburator for an Edelbrock 600 cfm carburator and also swap the original cam shaft for a Comp Cam XE268H.

The ignition system will give you exactly zero increase, contrary to what you might hear. The carburetor is a downgrade (750 Qjet to 600 cfm Edelbrock). I don't know the cam, others can advise.

Change the ignition for ease of mtce or reliability reasons but not performance.

Use the money from the carburetor and put it towards a decent set of heads and pistons. Low compression is your problem, not spark or gas.

 

[Evolution1980]

What you feel is torque, not horsepower. Just for clarification purposes.
So basically you are adding the digital ignition and a cam.
It will probably sound a bit different with the cam. You may feel a bit more pull in the midrange.

Why are you swapping out your OEM Qjet (750cfm) for a smaller Holley? Get your QJet running correctly and you'll likely get better performance than the Holley. (My 2¢)

With regards to underrating HP, that was only in the late 60's as far as I can recall.

 

[fine69]

"... I will get rid of the point system and replace by a breakerless SE electronic ignition kit, upgrade the stock rochester quadra jet carburator for an Edelbrock 600 cfm carburator and also swap the original cam shaft for a Comp Cam XE268H..."

With respect to your plans to swap the Q-jet for an Edelbrock.... been there - done that. I first swapped it for the 600 cfm as you described, then again with the Edelbrock AVS Thunder Series --- which cost me close to $400! Neither performed like the Q-Jet and I am now in the process of going back to the Q-jet... I'm just waiting for the coveted GF-432 filter to be delivered!

Learn from other's mistakes and experiences (like me!).... don't over-cam or over-carburate your engine -- you will surely be disappointed. Take the extra time to do your research to determine the proper specs for your needs (i.e., race track or street driving). Keep in mind, that some of the cool hi-performance upgrades are great --- just as long as you always drive 80 mph or above -- all the time.

 

[Curtis]

you would be lucky to get 275 Hp with those heads. That cam won't help much without some heads that will breathe and step up the compression a point or two. If you want to build torque, the XE268 is too much cam. The XE262 makes a lot more torque and only loses 10hp on the top. This is with good heads like vortecs. If you should go to vortecs, and the 262 cam with the other stuff you will be bumping 400hp and 440 lb ft of torque, and your tires are gonna hurt from the thrashing. I've been there. For about a $1000 you can get the heads, intake, gaskets bolts and all, and have one nice street performer. Do you want to sell your 600cfm Edelborck, I might be interested.

 

[Evolution1980]

Tepot, when is the last time, if ever, that you've had the car tuned up with all the basics checked: Carb adjustments, timing, etc?
I would suggest based on your initial post and the suggestions here that you make sure you get a good tune up first with whatcha got. Then go get the car on a dyno to be sure that you've got everything optimized. THEN add in the new parts. The "butt dyno" can be tricked by the brain. The chassis dyno doesn't lie. Why ask for somewhat educated guesses when you can have the actual answers presented to you? Forking over $75 for three dyno pulls could actually save you more money on upgrades that won't benefit your game plan.

 

[Tepot]

750 cfm !!!. Are you guy's serious? Every speed shop told me that my rochester carb was probably around 500 cfm and jets are restricted and all that.

Everyone told me that I would be way better with an Edelbrock 600 cfm. But you guy's are telling me that it's actually a downgrade. wow. I'm in shock !!!

I wanted to upgrade not down grade. I'm very surprise that it's a 750. normally you would have to go with big carbs like holley to get in the 750 cfm range. I wasn't expecting my stock rochester to be a 750 cfm. I'm very surprised.

before I use the Edelbrock, I think I'll leave the Rochester on my car and go make a pass on the dyno. then swap the carb and try another run on the dyno. that way I'll know for sure.

Now to answer some of your questions. I'm changing the ignition point system to electronic for reliablity. I go through 2 sets of points every summer and got tired of all that b*ll sh*t. changing points on the side of the road during a trip is not fun at all. specially if it's raining or dark. The only reason I've mentioned it is because all the people I know that changed from points to electronic told me that they felt an improvement.

I'm installing all of the upgrades in 2 weeks from now. Now I'm disapointed about the Edelbrock 600 cfm. All I keep hearing is that the Rochester carbs are somewhat restricted and they don't perform like a performance carb like edelbrock or Holley. I guess the dyno will have to show me the difference and then I'll be able to make a decision.

thanks for the info. if you want to add anything . go ahead I can use all the feedback possible.

Thanks
Daniel

 

[Curtis]

You have some mixed info to deal with. First your Quadrajet probably is 750 cfm, but your stock engine will never use 750 cfm. If your engine was 100% efficient about getting air into and out of itself, it could never pump more that maybe 620 cfm at 6000 rpm. That is why the smaller Elelbrock will work just fine, but since the primary venturies are slightly larger than your quadrajet, it may not give as good of gas mileage and throttle response may be affected to a small degree. Your stock carb has rather small front venturies and huge secondaries. Your car runs on the small front ones until throttle position and engine demand (meaning the engine is turning over fast enough to suck the bottom spring loaded throttle blades open) opens the secondaries. The engine speed and load determines how far open it sucks the bottom throttle blades . Since your engine will never pump this much air in stock form, it will never open the lower blades 100%. The carb only gives the engine as much air as it asks for. This is how the quad gets such good mileage and can produce great power. Your 600 cfm Edlebrock will support 400hp.

 

[Evolution1980]

Daniel, first of all, if your Qjet is the original carb, I'd be VERY leary of taking anymore advice from these so called speedshops in your area.

You have some mixed info to deal with. First your Quadrajet probably is 750 cfm, but your stock engine will never use 750 cfm. If your engine was 100% efficient about getting air into and out of itself, it could never pump more that maybe 620 cfm at 6000 rpm. ... Your car runs on the small front ones until throttle position and engine demand (meaning the engine is turning over fast enough to suck the bottom spring loaded throttle blades open) opens the secondaries. The engine speed and load determines how far open it sucks the bottom throttle blades . Since your engine will never pump this much air in stock form, it will never open the lower blades 100%.

I'm not one to typically disagree with you Curtis, nor with many people for that matter because I'm not 100% intimate with all things automotive tech. However, I have to question what you said.
The cfm's of the carb is it's MAX flow. While his engine may never require the max flow of the carb, at WOT, having the headroom I don't think is a problem.
The secondaries operate on throttle position and manifold vacuum pressure. At WOT, the throttle position is such that it allows the secondaries to activate. Along with the the throttle position, the manifold pressure drops enough that the force of the piston spring is able to raise the metering rods out, thus allowing more fuel to enter. At this point, both sets of butterflies (Primary and secondary) should be at or damn near 100% vertical. If they aren't vertical at sustained WOT, there is a problem with the carb that needs to be corrected. This is pretty much independent of how much air his engine actually requires. This is why we change the jets and metering rods. To strike that balance between the amount of air coming in and the amount of fuel needed to meet the ideal stoichiometric mixture of 14.7:1 (give or take)


If anyone is interested, I have a video clip of my QJet running WOT (sans aircleaner) while on the dyno. You can see exactly what is taking place with the carb. It's kinda interesting, as most people don't get to see this.

 

[Curtis]

Evo, I would like very much to see your video. That would be very interesting. I did not mean to make it sound like his 750 is too big, just that his engine doesn't need that much and a 600 will work. If the bottom butterflies are 100% open even if the engine is not at full rpm, then I stand corrected. I understood that since there is nothing attached to the bottom butterflies except a return spring, that they would not fully open until the engine was asking for all the air it could use which would be at WOT and max RPM. I also understood that the idea behind the spring on the butterflies was to ease the carburetor secondaries into operation so when you just stomp it, the secondaries don't just dump wide open and your engine bucks and farts and doesn't handle the added flow. So if it is demand for flo that opens the secondaries and they are spring regulated I would say it is safe to say they are not always wide open at full throttle. And I agree the metering rods are independent of the butterflies. Since I skipped carburetors 101, I am ready to be corrected and I will never (I swear to God) tell anybody else any thing else I have been led to believe as being correct and will not offer any advice whatsoever no matter how minute or simple.. Happy?

 

[Curtis]

understanding quadrajet tuning:http://www.73-87.com/7387garage/drivetrain/myqjet.htm

 

[Vette79]

Evolution1980, I would like to see that clip.

 

[Evolution1980]

The video clip

OK, gimme a couple days to load the clip into my computer and get it compressed down to a manageable size for emailing.

 

[Vettehead Mikey]

If the bottom butterflies are 100% open even if the engine is not at full rpm, then I stand corrected.

I believe that the bottom butterflies are opened and closed purely by mechnical linkage. The upper plate is controlled by vacuum.

 

[fine69]

OK, gimme a couple days to load the clip into my computer and get it compressed down to a manageable size for emailing.

Hey Andre,

I loaded a couple video clips of my son's hockey game for shared viewing using Google video (via Gmail). It worked great and it's free. I converted my video to WMV format.

If you need an invite to Gmail, lemme know and I'll send you one. If you already have a Google/Gmail account, just go to http://video.google.com/, then login, and on the top menu, click on "New! Upload your videos", upload your clip, and then your video should be ready to share!

Ralph

 

[Tepot]

OK, gimme a couple days to load the clip into my computer and get it compressed down to a manageable size for emailing.

Hey Evolution1980. If you want you can send me the file and i'll post it on our website for everybody to be able to access the file without rezizing.

let me know

you have my e-mail already in your Private message.

Daniel

 

[Koop]

Rejetted Qjet feeds my 383 just fine, I don't think even with the stroker that I use the full capacity of 750cfm.

A cam is fun, they can sound pretty cool plus you can get some more low end or top end, what ever you want. Heads would get you plenty more power to go with it but they're expensive. Even with heads and a cam the Qjet should more than git 'er done.

Carefull with the cam selection, in the early C3 to much cam affects the vacuum operation of EVERYTHING. I don't know how significant this would be in your year. There are plenty of guys that have done the cam thng (292 in my L48 when I had that motor) I really didn't like it. Who needs tons of top end in a C3? It was nothing but scary when that cam came on with a bang at high RPM! Of course I dropped that motor and went with more power .

I would have to wonder how you could burn up points every summer, what's up with that? You might want to find out what is doing that before you put in the electronic ignition since that might just burn up as well. I like my electronic ignition but it's just window dressing for the lazy man.

 

[Evolution1980]

Who needs tons of top end in a C3? It was nothing but scary when that cam came on with a bang at high RPM!

confused
I'm rather fond of my 155mph top speed runs...:cool

 

[Tepot]

I would have to wonder how you could burn up points every summer, what's up with that? You might want to find out what is doing that before you put in the electronic ignition since that might just burn up as well. I like my electronic ignition but it's just window dressing for the lazy man.

I didn't know why it was burning points until a mecanic told me that with the accell performance coil I would need heavy duty points instead of the regular ones. So I said the hell with that. swap to electronic ignition.

 

[Evolution1980]

I didn't know why it was burning points until a mecanic told me that with the accell performance coil I would need heavy duty points instead of the regular ones. So I said the hell with that. swap to electronic ignition.

This coming from the same guys that told you your QJet was a 500 cfm??? (Sorry... )