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Old 12-21-2011, 06:24 PM   #41
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Quote:
Originally Posted by chpmnsws6 View Post
Milling the head will increase cylinder pressures which will increase torque.

Welding washers was a joke. Old timers used to try it when they couldn't order custom pistons.
errr..,. my head has been worked over already and "decked" if that's what you mean by milling. otherwize i have no idea what youi;re talking aboutj.
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Old 12-21-2011, 09:11 PM   #42
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"decked" insures its flat, just taking off the minimum. Milling removes actual material, and will raise compression.
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Old 12-22-2011, 07:46 AM   #43
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My head was decked once before and I did it again to remove some detonation damage. At this point it's already past the factory limit and with the motor assembled I can't move my cams more than 2-3*. So yea, I'm not taking any more off.

If I feel like it one day I may put some higher compressions slugs in it. It's running 8.7:1 supertechs which isn't helping.
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Old 12-22-2011, 12:18 PM   #44
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Quote:
Originally Posted by miata2fast View Post
Squeezing the exhaust down gives you more torque at the lower rpms and part throttle. Smaller exhaust is a compromise that pays off on the road coarse, but penalizes the drag racer.
Can you show a dyno plot of a 4-valve head with streetable cams comparing a big bore vs smaller exhaust showing gains in midrange torque?
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Old 12-22-2011, 12:22 PM   #45
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falcon

DO you have the boost curve of your dyno plot?

If you have the data, then generate this plot in Excel, vs RPM:

torque divided by MAP
or
torque * 14.7 / (boost_in_psi + 14.7)

This gives you a number proportional to VE and cylinder pressure (assuming the timing is near MBT). It will show you at what RPM your engine is breathing well. If it's low around 4000 RPM and highest around 6500 RPM then you should go back to the stock cams.

Last edited by JasonC SBB; 12-22-2011 at 12:39 PM.
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Old 12-22-2011, 01:11 PM   #46
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Quote:
Originally Posted by JasonC SBB View Post
Can you show a dyno plot of a 4-valve head with streetable cams comparing a big bore vs smaller exhaust showing gains in midrange torque?
No I do not have access to such dyno plots. I am not speculating however, I have read about this many times over, and thought it was pretty much common knowledge.

Don't forget, I am talking about part throttle not full throttle applications.
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Old 12-22-2011, 04:01 PM   #47
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What's the use of having more torque at part throttle? Why not just step on the gas more if you need more torque?
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Old 12-22-2011, 05:41 PM   #48
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Jason, I have no road race experience what so ever. So I offer little explanation to the subject of on and off throttle transitions on the road coarse.

I do specifically recall a motorcycle team that put butterflies in the collector in an attempt to have just a tad more punch coming out of the corners. This was a few years back.
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Old 12-22-2011, 06:11 PM   #49
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Quote:
Originally Posted by miata2fast View Post
Jason, I have no road race experience what so ever. So I offer little explanation to the subject of on and off throttle transitions on the road coarse.

I do specifically recall a motorcycle team that put butterflies in the collector in an attempt to have just a tad more punch coming out of the corners. This was a few years back.
It has nothing to do with throttle application and everything to do with exhaust velocity. Nobody sizes exhaust/intake/anything for part-throttle power.
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Old 12-22-2011, 06:29 PM   #50
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Quote:
Originally Posted by Savington View Post
It has nothing to do with throttle application and everything to do with exhaust velocity. Nobody sizes exhaust/intake/anything for part-throttle power.
My statement about why road race cars typically having slightly smaller exhaust was a bit speculative and maybe a bit misguided.

Would you agree or disagree with the statement that road race cars have smaller exhaust tubing than drag race cars for a reason?
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Old 12-22-2011, 06:33 PM   #51
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No, I won't agree with that. It's far too broad a generalization to be true or false.
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Old 12-22-2011, 07:23 PM   #52
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I think the answer to Aaron's problem has already been answered and probably the easiest solution. A 6 speed with a 3.9 rear end. I just made the switch this year and the change is quite drastic.

5 speed 4.1 225/45/15 50mph 4000 rpm
6 speed 4.1 225/45/15 50mph 4940 rpm
6 speed 3.9 225/45/15 50mph 4700 rpm

This will put Aaron back into his powerband and the tighter gearing will improve his acceleration.
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Old 12-22-2011, 08:00 PM   #53
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Probibly a stupid suggestion since no one else has suggested it. If your not on MBT and are knock / temp limited why not give water/meth injection a try. I would have thought the easy answer is more timing, and add what u need to to allow that. i could be totally wrong though.

I got my aquamist HFS3 kit for under AUD$500 so its not a super expensive thing to test and resell. Woudl would likly need something like the hfs ( scales water flow on fuel injector duty cycle )though so u could control the delivert better since a simple boost activated switch without liniar flow will just dump in water later than were u want it then not provide what u need in upper rpms. Worse case u would be getting some cooler intake temp and in cylinder temps.
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Old 12-22-2011, 08:07 PM   #54
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What is your valve angle?
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Old 12-22-2011, 08:42 PM   #55
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Sav is being intentionally short I think.

Essentially its the exhaust gas velocity that needs to be kept up for good scavenging.

If your exhaust is bigger than you need to eliminate a restriction then the cross sectional area of the exhaust is too large and the speed of given exhaust is slowed down, we all know that momentum is mass x velocity, and so if we slow it down we lose momentum.

Less momentum = less scavenging.

A slightly smaller at lower rpm helps because at lower rpm the car produces less exhaust gases, therefore the smaller exhaust is not a restriction to power, however at high rpm we need an exhaust big enough to not restrict the engines breathing. Too big however and we lose exhaust gas velocity/momentum and lose scavenging.

So essentially while a 2.5 inch exhaust might be perfect for 5-7000rpm on a given engine, it will be too large for max power at 3-5000rpm on the same engine.

Dann
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Old 12-22-2011, 08:43 PM   #56
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Wow that wasnt very well written..

Also Ill add that drag cars use huge exhausts often with no collector in an effort to reduce torque at lower rpm and get traction out of the hole.

Dann
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Old 12-22-2011, 11:12 PM   #57
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Quote:
Originally Posted by Oni View Post
If your not on MBT and are knock / temp limited why not give water/meth injection a try.
If he's not on MBT from idle to redline with 8.7:1 slugs and a Rotrex, there's something wrong.
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Old 12-23-2011, 11:01 AM   #58
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Quote:
Originally Posted by nitrodann View Post
Sav is being intentionally short I think.

Essentially its the exhaust gas velocity that needs to be kept up for good scavenging.

If your exhaust is bigger than you need to eliminate a restriction then the cross sectional area of the exhaust is too large and the speed of given exhaust is slowed down, we all know that momentum is mass x velocity, and so if we slow it down we lose momentum.

Less momentum = less scavenging.

A slightly smaller at lower rpm helps because at lower rpm the car produces less exhaust gases, therefore the smaller exhaust is not a restriction to power, however at high rpm we need an exhaust big enough to not restrict the engines breathing. Too big however and we lose exhaust gas velocity/momentum and lose scavenging.

So essentially while a 2.5 inch exhaust might be perfect for 5-7000rpm on a given engine, it will be too large for max power at 3-5000rpm on the same engine.

Dann
Finally someone who makes sense of all this. And yes, I thought it was very well worded.

However, as a drag racer myself, you want all the torque possible off the line. don't forget, you are not only racing to the finish line, but to the first 60' as well. And a very well prepared drag car will have the traction issues sorted out. The more torque, the better 60' time.

The difference between the drag car and the road race car, is that the drag car with the rpm range you posted above, will always be in the 5000 - 7000 rpm range, and never in the 3000 - 5000 rpm range. This is the reason you will typically see a slightly larger exhaust on a drag car with the same motor configuration as a road race car.
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Old 12-25-2011, 11:00 PM   #59
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Analysis/Regurgitation Alert:

If this was a poll I would vote gearing change first, cam change second, higher comp pistons third, outright setup change fourth.

I know 5 speeds can handle 250WTQ, but 8000RPM shifts and 200WTQ is not exactly easy on one. May as well get ready to replace it and upgrade at the same time with shorter gears. Which the OP is doing.
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Old 12-25-2011, 11:48 PM   #60
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The 6sp will help a lot. +1 to sparetire's list.
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